U.S. patent application number 10/736116 was filed with the patent office on 2005-06-16 for liquid sensor and ice detector.
Invention is credited to Maatuk, Josef.
Application Number | 20050126282 10/736116 |
Document ID | / |
Family ID | 34653790 |
Filed Date | 2005-06-16 |
United States Patent
Application |
20050126282 |
Kind Code |
A1 |
Maatuk, Josef |
June 16, 2005 |
Liquid sensor and ice detector
Abstract
An improved apparatus and a method of measuring and interpreting
reliably, simply and accurately the information on continuous
liquid level, liquid temperature and other liquid properties within
a vessel. The apparatus could be made of a powered heater element
and temperature sensors can be screen-printed, vacuum deposited,
etched, welded, soldered or plated on one or both sides of a single
rigid or a flexible substrate. The geometry of the heater
determines the curve shape, such as steepness or shallowness of a
temperature profile along a heater. Various parallel and serial
configurations of thermocouples or temperature sensors can be used
to measure the temperature along a heater. Simultaneous
measurements from all the temperature sensors, before and after
heat is applied, are used to generate accurate temperature profiles
for the entire heater. Different features of the temperature
profiles will determine accurately the liquid level, liquid
temperature and other liquid properties. Apparatus of the invention
may also be used to detect ice formation.
Inventors: |
Maatuk, Josef; (Los Angeles,
CA) |
Correspondence
Address: |
JOSEF MAATUK
1607 S. SHERBOURNE Dr.
LOS ANGELES
CA
90035
US
|
Family ID: |
34653790 |
Appl. No.: |
10/736116 |
Filed: |
December 16, 2003 |
Current U.S.
Class: |
73/295 |
Current CPC
Class: |
G01F 23/246 20130101;
G01F 23/247 20130101 |
Class at
Publication: |
073/295 |
International
Class: |
G01F 023/00 |
Claims
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16. Apparatus for determining accurately liquid level in a
container by providing a signal and/or signals indicative of the
level to which said sensor is submerged in a liquid, said apparatus
comprising: a substrate having a longitudinal axis; a common heater
wire secured along the longitudinal axis of one face of said
substrate; said common heater having uniform or non-uniform
cross-section provided on either side of said substrate; a
plurality of thermocouples or temperature sensors provided on one
side of said substrate in longitudinally spaced relationship; said
plurality of thermocouples located at strategic vertically spaced
points and located very close to the common heater on one side of
said substrate; said temperature sensors are plurality of
thermocouples with hot and cold junctions connected serially or in
parallel configuration; a single cold thermocouple junction for
said parallel thermocouple configuration and/or a single common hot
thermocouple junction provided on one face of said substrate; a
plurality of cold thermocouple junctions for serially connected
thermocouples provided on one side of said substrate in
longitudinally spaced and positioned in a laterally spaced
relationship to said plurality of hot thermocouples junctions;
isothermal block means for keeping said cold junction of serially
connected thermocouple at the same temperature; coating of said
heater, said temperature sensors and two sides of said substrate is
thermally conductive, electrically insulating, chemically inert to
the operating liquid, slippery and liquid impermeable; means for
applying electrical power to heat said common heater wire,
controlled by a power control switch and power circuitry, wherein
both ends of said common heater wire are connected to said
electrical power applying means for heating by continuous or
discrete electrical pulses of said common heater; said apparatus is
being adapted to be positioned within a vessel containing a volume
of liquid with said coated substrate partially immersed in said
liquid such that said hot and cold junctions of plurality of
thermocouples will cooperate to generate a signal indicative of the
continuous level of liquid within said vessel; display means for
indicating the liquid level in said liquid container; a data
acquisition means comprising a microprocessor and or a signal
conditioning circuit connected to said thermocouples and display
means for indicating said liquid level from one of voltage and
temperature sensed by said thermocouples; and said power and signal
conditioning circuitry are provided on said substrate.
17. A liquid level apparatus as set forth in claim 16, wherein said
hot and cold plurality of thermocouple junctions generate a signal
of opposite plurality.
18. A liquid level apparatus as set forth in claim 16, wherein said
apparatus thermocouples operate to generate a signal indicative of
a pressure within said vessel;
19. A liquid level apparatus as set forth in claim 16, further
comprising a regulated power source for supplying power to said
common heater.
20. A liquid level apparatus as set forth in claim 16, wherein said
signal from said thermocouples is supplied to signal conditioning
circuitry.
21. A liquid level apparatus as set forth in claim 16, wherein
thermocouple junctions are positioned along a line extending
generally parallel to the surface of said liquid.
22. A method for accurately determining the density and pressure
change of compressible fluid and the liquid parameters of level,
absolute temperature and viscosity degradation in a three
dimensional liquid container, said method comprising of three
components on a monotonic profile namely: a line along most of the
lineal dimension of the sensor that is immersed in liquid; a line
along a significant portion of the lineal dimension of the sensor
that is in air or other medium above the liquid; a steep or shallow
curved line connecting said two lines; reading of a point or a few
points along each of said three components will be used to
construct said profile; adding, averaging or interpolation of said
readings from said points determine with different accuracy the
said density and pressure changes, liquid level, absolute
temperature and viscosity degradation.
23. The method recited in claim 17, wherein said monotonic profile
is a voltage or temperature profile, each of its three components
is constructed from reading of one or more hot thermocouple
junctions after power is applied to the heater and after zeroing
one of the voltage and the temperature reading from all of the hot
thermocouple junctions while the cold junctions temperature remains
equal and constant for all of the cold thermocouple junctions.
24. The apparatus recited in claim 16, wherein said common heater
is an electrically pulsed heater and a common wire for the hot and
cold junction of parallel configuration of thermocouples.
25. The method recited in claim 17, wherein the determining step
includes detecting the presence of two or more different stratified
liquids, such as oil and water, and determining the level of each
liquid.
26. The method recited in claim 17, wherein the determining step
includes detecting the level at pre-set points at one of the bottom
and top of liquid containers such as oil pans, fuel tanks and
coolant reservoir.
27. A data acquisition system comprising: an analog power circuitry
connected to the heater and an analog signal conditioning circuitry
with self-calibrated differential amplifier is connected to all
possible configuration of thermocouples; a first and second
multiplexers for odd and even thermocouples connected to said
configuration with parallel thermocouples; wiring of said
multiplexers to an analog signal conditioning circuitry and a
single differential amplifier and analog to digital converter; said
analog to digital filter wired to a microprocessor; said
microprocessor wired to the apparatus recited in claim 1, and a
display through a serial port and digital to analog input/output;
said microprocessor connected to a health monitoring electronic
circuitry for the apparatus in claim 1; said microprocessor
connected to a power supply circuitry capable of having a power
switched on and off; and said microprocessor comprise of algorithms
for signal conditioning and signal processing.
28. A data acquisition system as recited in claim 27, further
comprising software used with the microprocessor of said data
acquisition system, capable of eliminating non-random electronic
hardware errors and minimizing random errors in differential
voltage reading of said thermocouples.
29. A data acquisition system as recited in claim 27, further
comprising of software capable of determining absolute temperature
of each said thermocouples using said differential voltages.
30. A data acquisition system as recited in claim 27, further
comprising of software determining the kind of liquid based on
curvature of the temperature profile and the rise of temperature of
the thermocouples in liquid, and thermocouples in air.
Description
REFERENCE CITED
[0001]
1 U.S. PATENT DOCUMENTS 2279043 April 1942 Harrington 073/295
3279252 October 1966 Barlow 073/295 4969749 November 1990 Hasselman
073/295 2702476 February 1955 Boisblanc 073/295 3360990 January
1968 Greene 073/295 4785665 November 1988 McCulloch 073/295 4603580
August 1986 Waring 073/295 5521584 May 1996 Ortolano 340/581
4819480 April 1989 Sabin 340/581 4570230 February 1986 Wilson
073/295X 4573128 February 1986 Mazur 073/295X 6546796 April 2003
Zimmermann 073/290R
[0002]
2 FOREIGN PATENT DOCUMENTS 14926 October 1991 WO 073/295 44923
March 1980 Japan 073/295 158522 September 1982 Japan 073/295 6116
January 1981 Japan 073/295 281167 October 1993 Japan 374/016
2035154 December 1977 Germany 374/016
DESCRIPTION OF THE PRIOR ART
[0003] The invention relates to methods and devices used to measure
liquid level as well as temperature and other liquid parameters,
within a vessel or container. The devices of this invention are
made of a powered heater that can be used with various
configurations of parallel and serial thermocouples or other
temperature sensors. The power and signal conditioning circuitry,
the powered heater and the thermocouples are easy to manufacture on
one a single substrate. The power application and the sampling of
the temperature along the heater will not cause the temperature
measurements along the heater to be susceptible to errors from
extended or extraneous electrical sources. The method of the
invention can also be used with various technologies and not just
powered heater and temperature sensors.
[0004] There are various applications where liquid level and other
liquid parameters are required. For example, in a vehicle, the
liquid level or height of fuel, coolant and oil is required.
Moreover, the engine oil viscosity degradation is required too. In
a boat, the fuel and water level need to be checked prior to a boat
ride. Monitoring of oil level within a pump or compressors is
required. In those applications, the liquid level sensor need to
give a reliable, accurate measurement of the liquid level over an
extended period of operation. Furthermore, the monitoring of the
health of the sensor operation and its hardware, will require
maintenance only when the sensor fails or its performance is
degraded below an acceptable low threshold level of accuracy.
[0005] Different sensors have been used to determine liquid level
in a container. Those sensors include a float, a single or multiple
capacitors, hot wires, temperature sensors, ultra-sonic and others.
When these sensors are used to measure continuous liquid level,
they require the liquid properties to be uniform as well as the
properties of the medium above it to have uniform properties. Most
of those devices are susceptible to outside electrical and magnetic
noises.
[0006] The present invention precludes the shortcoming inherent in
existing liquid level measuring devices and methods. Moreover,
unlike existing thermal devices that use resistive probes to
measure liquid level, in the device of this invention, the actual
temperature at a strategically located points along the heater are
used and processed, and the temperatures measured are dependent on
heat transfer mechanisms rather than change in resistivity of the
probe material. Furthermore, the invention is capable of not only
determining liquid level at discrete points where the temperature
sensors are located, but can also determine liquid levels at
intermediate points between two temperature sensor locations, which
the resistive type device is incapable of doing.
[0007] The power circuitry apply current to the common heater. The
heat transfer mechanism creates a voltage gradient or a temperature
profile along a heater wire or a strip. This profile is used to
determine the discrete and continuous liquid level as well as other
liquid parameters. In the prior art, U.S. Pat. No. 2,279,043
Harrington used heated liquids in a container to determine the
discrete liquid levels with a set of discrete thermocouples. In
U.S. Pat. No. 3,279,252 Barlow used heated cylinder to determine
the discrete liquid levels. In U.S. Pat. No. 6,546,796 B2, the
proposed configuration of discrete heaters or continuous heater
with serially connected thermocouples use hot junctions that read
the temperature at points close to the heater or the end of Copper
traces between discrete heaters. The temperature of the hot
junctions are controlled by heat profile along the copper traces
between the discrete heaters. The heat transfer mechanism that
generates the temperature profile is not the radiation but the
convection of heat to the liquid and conduction of heat along the
heater. The configuration proposed in U.S. Pat. No. 6,546,796 B2
will work only by proper selection of the single heater wire
cross-section or the cross-section of the Copper traces between the
discrete heaters. Moreover, the sum of voltages from the
thermocouples will indicate reliably the continuous liquid level
only when the entire liquid volume, except of the liquid boundary
layer around the heater, has a uniform or close to a uniform
temperature and simultaneously the temperature of all of the cold
junctions are identical. Another limitation of the device described
in U.S. Pat. No. 6,546,796 B2 is the fact that it can measure
reliably the continuous liquid level only when linear thermocouples
are used.
[0008] In this invention, a method is presented to overcome those
limitation by creating a desired profile along either a separate
heater and discrete thermocouples or a heater that is also used as
a common wire for a set of discrete thermocouples. In other words,
the separate heater is eliminated and instead, the common wire of
the thermocouple set is also used as the heater.
[0009] In U.S. Pat. No. 4,573,128 Wilson and U.S. Pat. No.
4,573,128 Mazur used a poured molted liquid in a container to
obtain a profile and measure the liquid level in a container. In
this invention, I apply heat to the surface of a heated wire to
obtain a profile along the wire.
[0010] This invention also detects ice on a surface by looking at a
phase change effect ("Igloo") and temperature profile. In U.S. Pat.
No. 5,521,584 Ortolano detect ice by measuring heat flow and heat
measurement.
SUMMARY OF THE INVENTION
[0011] The invention described herein is a means of measuring the
level of a liquid in a liquid container such as a fuel tank by
means of a probe to which heat is applied and the temperature along
the length of the probe is measured. This invention makes use of
the difference in cooling efficiency between liquid and gas such as
air, or between two different liquids, such as water and oil. When
heat is applied to the probe, the temperature of the portion of the
probe submerged in liquid is significantly lower than the
temperature of that portion of the probe outside of the liquid and
typically exposed to air. This is because the liquid removes heat
at faster rate than air, so that the temperature difference between
the surface of the probe is much lower in liquid than it is in air.
This is also true between a liquid that removes heat more
efficiently, such as water, and a liquid that does not remove heat
as efficiently, such as oil. Temperature sensors, such as
thermocouples or thermistors that are attached to various points on
the probe to measure the temperatures at those respective locations
on the probe. This invention is not only capable of determining
accurately where the liquid level is at discrete points where the
temperature sensors are attached. It can also determine where the
liquid level is between two discrete points to within a fraction of
centimeter accuracy, when precision temperature measurement devices
and electronic circuitry are used in conjunction with suitable
microprocessor, which collects and process the signals received
from the temperature sensors.
[0012] The purpose of this invention is to provide a device that
can measure liquid levels, such as that of fuel in an automobile
fuel tank or lubrication oil level in an automobile engine
compartment fairly accurately and with minimal effort, such as
simply pushing a button on an instrument panel, as shown in FIG. 1.
The advantages of this invention are (1) It can measure continuous
and discrete liquid levels accurately, within a fraction of a
centimeter; (2) The power and signal conditioning circuitry, the
powered heater and the thermocouples are easy to manufacture on one
a single substrate; (3) the device can monitor the health of the
probe, the electronic hardware and software and thus eliminate the
need for periodic maintenance (4) It requires a very small amount
of power to operate; (5) It is compact and light weight and can be
installed in relatively small liquid containers if necessary; (6)
It is reliable since it has no moving parts; (7) because it
requires a very small power for operation, it does not generate any
significant amount of electromagnetic energy which could interfere
with the performance of other electrical/electronic equipment; and
(8) with certain modifications to the device, it can be used to
measure other important liquid parameters such as viscosity and
density. This device can also be adopted for the detection of ice
formation on the external surface of a road or an aircraft, such as
the external surface of an aircraft wing or fuel tank.
[0013] Additional features and advantages of the invention will be
revealed in the following description, appended claims and
drawings. The invention covers all new characteristics, which maybe
inferred therefrom even if they are not expressly stated in the
claims. The invention is depicted in a plurality of exemplary
embodiments in the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a schematics of a typical liquid level measuring
device application;
[0015] FIG. 2 is a schematic diagram of one embodiment of the
invention;
[0016] FIG. 3 Probe temperature profile;
[0017] FIG. 3a Generic Probe temperature profile;
[0018] FIG. 4 Division of space thermocouples 43 and 44 into the
equal increments for intermediate level reading;
[0019] FIG. 5 Temperature difference between thermocouples 43 and
44 versus liquid level position between 43 and 44;
[0020] FIG. 6 Another embodiment of invention employing a strip
probe;
[0021] FIG. 7 Temperature behavior of 43 when initially immersed in
water and then exposed to air;
[0022] FIG. 8 Temperature behavior of 44, 45 and 46 in water while
41, 42 and 43 in oil.
[0023] FIG. 9 Another embodiment of invention employing a common
strip and heater probe;
[0024] FIG. 10 Data Acquisition;
[0025] FIG. 11 Schematic Diagram of Ice Detection Sensor;
[0026] FIG. 12 Temperature rise of Thermocouples 103 and 107 after
power is applied for no ice condition;
[0027] FIG. 13 Temperature rise of thermocouples 103 and 107 with
ice formed over the sensor;
DETAILED DESCRIPTION OF THE INVENTION
[0028] The present invention describes a new method and a new
sensor that measure accurately the liquid level, absolute
temperature, viscosity degradation and density and pressure of
compressible fluid in a three dimensional liquid container. The
method requires the development and use of a profile or a mode
shape along an axis. The profile has three components. First, a
horizontal line for the section of the sensor that is in liquid.
Second, a horizontal line for the sensor section that is in the
medium above the liquid. Third, a curved line that connects the two
horizontal line. This curved line can be viewed as a wave shape.
The curved line length is a fraction of the height of the liquid
container. The point of intersection between the wave and the first
horizontal line tracks the motion of the liquid level. The height
of the first horizontal line, height of the second horizontal line
or the changes in the slope and curvature of the curved line are
used to determine accurately the liquid level, absolute
temperature, viscosity degradation and the density and pressure of
the compressible (gas) fluid in a three dimensional liquid
container.
[0029] The present invention also describes a new method and a
sensor to detect ice on surfaces. This method requires the
development and use of the parameters of the "Igloo Effects". Those
parameters include the time delay due to latent heat for a phase
change from ice to liquid, steady-state response inside the "Igloo"
and the response time inside the "Igloo".
[0030] One possible way of developing a profile is to use a sensor
made of a heater element, temperature sensors that measure the
temperature at a few points along the heater element and a data
acquisition and processing system. First, the temperature sensors
measure the temperature along the heater at a few points prior to
applying power. Second, The temperature sensors measure the
temperature along the heater at the same few points after power is
applied to the heater. The difference between the first and second
temperature measurements give the amplitude of the temperature rise
at the points along the heater. Those amplitudes will be used to
construct the three components of the profile. Each of the
horizontal lines in the profile are obtained by using the amplitude
of one or more temperature sensors. The curved line of the profile
is obtained by using the amplitude of two or more temperature
sensors located immediately above the liquid level.
[0031] The heater element (maybe wire, ribbon etc.) has uniform,
linear or non-uniform geometry (cross-section). This geometry will
determine the amplitude characteristics (distribution) in the,
three components of the profile. The temperature sensors can be
resistor type, thermistor, thermocouples and others.
[0032] The heater element can be separate from the temperature
sensors or part of the temperature sensors. If the selected
temperature sensors are thermocouples connected in parallel with a
common wire, then the common wire of the thermocouples can also be
used as the heater. When the common wire of the thermocouples is
used as a heater, it can be heated directly (e.g. flowing a current
through the common wire when not measuring the thermopotentials) or
the common wire of the thermocouples can also be heated indirectly
(e.g. laminating an electrically isolated heater over the
temperature sensing common wire to form independent heating and
measuring processes) to image the heat flow characteristics along
the heater.
[0033] The packaging and materials, and the locations of the
thermal sensing taps, are engineered to properly sample the thermal
gradients along the heater element. An analog (spatial) profile of
the amplitudes of temperature rise along the heater is
reconstructed from these samples, and this analog profile may be
processed to accurately discern levels, layers, properties (like
viscosity, kind of liquid), etc. in the strip's boundary layer
environment.
[0034] The instrumentation electronics: need only be capable of
microvolt measurements, plus analog or digital processing as
appropriate to the application. Scanning the taps in sequence may
be used to transform the information of the analog spatial profile
into the time domain for simple analog filtering to reconstruct the
profile, and thresholding circuits for decisions. Equally,
digitizing the data from the sensing taps allows digital
processing, possibly for more elaborate signature analysis, for
ease of recalibration, etc. Output circuits and format may be
whatever is appropriate to the application (e.g. digital dashboard,
analog level meters, warning lights, etc.).
[0035] Using the reading from all the point sensors to make
discrete decisions as to (for instance) a liquid level are easy to
make based simply on comparing the individual measurements to each
other. For example, this comparison can be done by looking at the
amplitude of the voltage of different individual thermocouples or
amplitude of the difference between two adjacent thermocouples. For
an environment where the fluids (or whatever is being tested) have
substantially dissimilar properties (e.g. air/water), the data also
readily supports simple interpolation between points, with
increasing accuracy requiring the shaping of the heater geometry,
the thermocouples configuration as well as increasing the accuracy
of resolving the end-points and increasing the complexity of the
interpolation algorithm. For example, three thermocouples
configurations can be used. First, serially connected with hot
junctions along the heater and cold junctions are connected with
isothermal blocks away from the heater. Second, parallel
thermocouples with common with hot junctions along the heater and a
single cold junction away from the heater. Third, similar parallel
configuration but all of the hot junctions are connected to a
single point. The third configuration will average the
thermopotential reading from all of the hot junctions.
[0036] Using a microprocessor in addition to signal conditioning
circuitry, give substantial improvement in the decision accuracy,
by incorporating the data from multiple points, rather than just
the two points on either side of the fluid boundary or just the
total sum of the thermopotentials of all of the thermocouples or
the average value of all of the thermopotentials. By using many
data points to fit the profile along the strip, the accuracy of the
overall curve is improved beyond the accuracy of single
measurements, the individual measurement uncertainties tend to
average out. Further, the fitting of multiple points to a model
that incorporates the effects of different fluid characteristics
and heat inputs allows substantial improvements in discerning
boundaries between fluids that are more closely matched in
properties (e.g. water/oil) and in discerning the properties of the
fluids themselves (e.g. viscosity).
[0037] A typical application of the liquid level measuring
apparatus is measuring the liquid level of fuel in an automobile
fuel tank or lubrication oil level in an automobile engine
compartment. A schematic diagram of such an application is shown in
FIG. 1. The schematic diagram in FIG. 1 depicts an automobile
engine oil pan 10 containing lubrication oil 11. The liquid level
sensor probe 20 is installed inside the oil pan 10. A plurality of
electrical wires 21 connect the probe 20 to data acquisition and
processing system that can be connected to a microprocessor which
may be located in the engine compartment of the automobile or
behind the automobile instrument panel 30 or very close to the
probe. The data acquisition and processing circuit is in turn
connected by one or more wires 23 to an analog or digital display
31 located on the automobile instrument panel 30. The data
acquisition circuit and the microprocessor is situated with respect
to the probe as appropriate for the noise and cabling constraints
of the application environment. A pulsed power supply 24 made of
one or two batteries or AC is located either in the engine
compartment, behind the automobile instrument panel or close to the
probe and electrically connected to the data acquisition and
processing system 22, the probe 20 and an activation button or
switch 33 located on the automobile control panel, provides pulsed
electrical energy to the liquid level sensing system. Sensing of
the lubrication oil level is accomplished by activating the button
or switch 33, sending pulsed electrical energy to the probe 20 and
the microprocessor 22. The lubrication oil level is displayed on
the display 31.
[0038] The apparatus makes use of the cooling efficiency between
liquid and gas, such as air, or between two different liquids such
as water and oil. One embodiment of the invention is depicted
schematically in FIG. 2. The embodiment depicted schematically in
FIG. 2 is comprised of a probe 40 made from 0.002 inch diameter
Nichrome heater wire three inches in length but maybe of any
suitable lengths, a data acquisition circuitry and a microprocessor
50, a display 51, an electrical power source 52, a switch 53,
electrical wires 54 and, wires 56. Six thermocouples beads 41, 42,
43, 44, 45 and 46 from 0.008 inch diameter or Copper-Constantine
pairs of wires are attached to the probe 40 by wrapping the probe
Nichrome heater wire around the thermocouples beads 41 through 46.
The number of thermocouples beads may be varied depending on the
length of the probe and the accuracy desired. The thermocouples 41
through 46 are electrically connected to the data acquisition
circuitry by a Copper-Constantine wires 47 of suitable size and
length. The probe 40 is coated with an insulative material to
electrically isolate it from the thermocouples beads 41 through 46.
This coating is also chemically inert with respect to the
liquid.
[0039] Referring again to FIG. 2, when the switch 53 is in the open
position and no power is applied to the probe heater 40, the
temperature of the thermocouples 41 through 46 will measure the
same temperature as the media which surrounds the probe, either air
or liquid or both. When the switch 53 is in the closed position, a
pulsed current flows through the circuit including the probe and
heat is generated at the probe 40 in the form of --I.sup.2R--
losses. The heat generated at the probe 40 is dissipated to the
surrounding medium. In order for heat to be dissipated to the
surrounding medium the temperature of the probe has to be higher
than that of the surrounding medium. At steady-state condition,
that is when the temperatures have stabilized some time after the
switch 53 is closed, usually several seconds, the characteristics
temperature difference between the medium and the probe 40 is
established. For example, if 6.0 milliwatts of power is applied to
the probe and the entire probe is in air which is maintained at a
constant temperature of 20 degrees C., the temperature at the
thermocouple location 41 through 46 are approximately 35 degrees
C., or approximately 15 degrees C. higher than the temperature of
the surrounding air when steady-state condition is reached. If the
entire probe is immersed in water, also maintained at 20 degrees
C., the temperature of the probe at the thermocouples 41-46
locations will only be slightly above 20 degrees C. The actual
temperatures at the thermocouples locations are found in Table 1.
This is because water can remove heat from the probe at much faster
rate than air.
[0040] So the water requires only a small temperature difference
(less than 1 degree C.) to remove the same heating rate as the air
has to remove. In FIG. 3, the temperature profile of the probe is
shown for three conditions: (1) where the entire probe with 6.0
milliwatts power is in air whose temperature is 20 degrees C. (55),
(2) where the probe is completely immersed in water whose
temperature is 20 degrees C. (56), and (3) where the probe is
immersed in water from thermocouples location 43 to 46, with both
air and water maintained at 20 degrees C.
[0041] FIG. 3a shows a typical temperature profile of a 6 inch
probe 20 with 0.5 inch spacing between the thermocouples. This
profile is made of three sections: (1) section 111, which is the
temperature profile of the probe section that is immersed in liquid
below the liquid level point. (2) section 112 which is the
temperature profile of the portion of probe 20 that is in air (or
other medium) but some distance above the liquid level. (3) section
110, which is the curved portion of the temperature profile of
probe 20. By proper design of the geometry of the probe heater 40,
this curved section of the probe profile can be designed to be
shallow or steep. Similarly by proper design of the shape and
geometry of the heater, the curved section of the profile could
also become linear or non-linear curve.
[0042] One of the main features of this invention is the use of a
sliding profile along the heater of a sensor as a method to
determine accurately the liquid level and other liquid parameters.
The control of the shape and geometry of a probe will result in a
desired profile that among other things, will simplify the data
processing needed to determine liquid level and other liquid
parameters. A desired profile along the probe, can be achieved not
only with a heater but also with other technologies. For example,
controlling the shape and geometry of the reflection and refraction
surfaces of optical technology will control the desired profile
(i.e. linear profile) along an optical probe. Similarly, in
capacitor technology for measuring liquid level and other liquid
properties, the electrodes can be shaped as thin and narrow
(instead of uniform) to shape the profile of the dielectric field
along the capacitate probe.
[0043] In this example heat is transferred from the surface of the
probe to the surrounding medium by free convection. The basic
convection heat transfer equation (applicable to both free and
forced convection) is
Q=HA(Tp-Tm) (1)
[0044] Where Q is the heat transfer rate
[0045] H is the convection (free convection in this case) heat
transfer coefficient.
[0046] A is the area of the probe exposed to the medium
[0047] Tp is the temperature of the probe surface exposed to the
medium
[0048] Tm is the temperature of the medium (air or water in this
example)
[0049] The temperature difference between the probe surface and the
medium is expressed as DT or
DT=Tp-Tm=Q/(HA) (2)
[0050] In this example the values of Q and A in equations 1 and 2
are held constant. Only H, which is a measure of the heat transfer
coefficient or heat removal efficiency, is varied. The higher H is
the lower DT is. Water, which is a good heat transfer liquid,
usually orders of magnitude better than air in removing heat from
the probe both by free convection and forced convection. Therefore
it requires a very small DT compared to that required by air in
removing the same amount of heating rate or power.
[0051] When only liquid levels at discrete locations are desired,
such as where the six thermocouples 41-46 are located, the
processing of the temperature data becomes relatively simple. The
points (thermocouple locations) that are completely immersed in
water will indicate a much smaller DT. For example, if
thermocouples 44, 45 and 46 are completely immersed in water and
thermocouples 41, 42 and 43 are in air, the temperature of the six
thermocouples 41-46 will not be constant. The DT's of the
thermocouples immersed in water will be much lower. The temperature
distribution along the probe when the thermocouples 41, 42 and 43
are in air and when thermocouples 44, 45 and 46 are immersed in
water are shown as 57 in FIG. 3. From comparison of the difference
in temperatures of the six thermocouples 41-46 to each other, it
can be determined which thermocouples or discrete points are
immersed in water.
[0052] The invention can also be used to determine the liquid
levels at intermediate points between the thermocouple locations.
Supposing the liquid level is somewhere between thermocouple 43 and
thermocouple 44 and it is desired to determine the location of the
liquid level within 1.3 millimeter. The space between thermocouple
43 and thermocouple 44 of the probe heater 40 in the embodiment
depicted in FIG. 2 is 12.7 millimeters. If the space is divided
into ten equal spaces as shown in FIG. 4, the distance between each
intermediate mark is 1.27 millimeters, within the 1.3 millimeter
accuracy desired. Referring to FIG. 2 and FIG. 4, as the level of
the water is varied from thermocouple 43 (o distance from
thermocouple 43) one intermediate mark at a time to thermocouple 44
(12.7 millimeter distance from thermocouple 43), the actual
temperature of thermocouple 43 and thermocouple 44 and the
difference between the two temperatures will vary, as shown in
Table 2 and FIG. 5. These data can be collected and processed by
the microprocessor to where the actual liquid level is. The
thermocouples or equivalent temperature sensors used to measure the
temperatures at the various locations will have to be able to
provide much more accurate readings than when only discrete
temperature levels are being measured. This can be accomplished by
using the entire spatial profile of differential rather than
absolute thermocouple readings. Using proper data acquisition and
processing circuitry together with a proper algorithm in a
microprocessor, the non-random electronic errors can be eliminated
and the amplitude of the random errors can be minimized. In the
present invention, the use of averaging of individual thermocouple
reading together with a correlation function for a simultaneous few
thermocouples has minimized the impact of error amplitude in
individual thermocouples on the accuracy of calculating liquid
level and other liquid parameters.
[0053] The performance characteristics of the invention with other
liquids such as gasoline fuel or engine lubrication oil will be
similar to that of water. However, in the case of liquids like
lubrication oil, which have lower heat removal efficiency than
water, but much higher than air, either more precision temperature
measuring devices or more sophisticated signal conditioning or
both, may be required to achieve the same overall performance of
signal to noise ratio, because the temperature difference between
thermal sensors may not be as pronounced as that when water is
used. Instead of keeping the same signal to noise ratio for liquids
with different heat removal efficiency by using more accurate
temperature sensor (reduce the noise), it is possible to keep the
same overall performance by driving the sensor to a given
temperature response. However, for some applications like oil/water
or liquid/vapor. The choice of driving the sensor to a given
temperature might be constrained and temperature sensors with
smaller uncertainty (random error amplitude) need to be used to
achieve the same overall performance.
[0054] The invention describe herein is one configuration. Other
configurations, such as probe heater in the shape of very thin
metallic strips together with serial or parallel thermocouple
configurations, can be deposited on a printed circuit board. The
data acquisition and processing circuitry together with a
microprocessor can also be mounted on the same circuit board. The
principle of operation of the probe heater and the use of the
resulting profile to perform accurate measurement and calculations
of liquid level together with other liquid parameters, is the novel
feature of this invention, and it applies to other heater and
temperature sensors configurations.
[0055] Another embodiment of the apparatus claimed in this
invention is metallic strip or a wire version of the probe heater
of FIG. 2 and parallel thermocouple configuration, which is
depicted in FIG. 6. The probe heater is comprised of a strip or
wire of Constantan attached to a fiberglass or a film substrate or
a plastic sheet, or an equivalent printed wiring board material 61,
a series of Copper pads (taps) 71 through 80 also attached to board
61 and electrically connected to a Constantan strip 60 to form the
hot junctions of a parallel thermocouple connections herein
referred to as the thermocouple network, a heater strip 62 also
attached to board 61 but electrically isolated from said
thermocouples network with a thin dielectric film 63, and a second
thin dielectric film 64 electrically isolating the heater strip 63
from the air or liquid to which the probe is exposed. A DC or AC
power source 90, provides pulsed electrical energy to heat the
heater strip 62. The thermocouple networks senses the probe
temperature at various points along the probe and sends the
appropriate electrical differential voltage signals from the
thermocouples to the data acquisition and processing with a
microprocessor 81. The precise liquid level location can be
calculated by processing the signals and determining the
corresponding temperature profile along the probe.
[0056] Another variation of the apparatus claimed is a modified
strip design of FIG. 6 as shown in FIG. 9, whereby the Constantine
strip (60) is used as the heater that is heated by power supply 90
as well as the common wire for the thermocouples. Such a design
will eliminate the heater and the thin layer that electrically
insulates the heater from the thermocouple junctions. This modified
design of FIG. 9 will also have smaller thermal mass and thus
faster response time than the probe of FIG. 6. In other words, for
the same power, the signal of FIG. 9 will be bigger than the signal
in FIG. 6. The power to the probe of FIG. 9, will be applied to the
Constantan strip as a set (cycles) of many short on and off pulses.
The duration of the pulses is very small compared to the time
constant of the heater. For example, if the heater has a time
constant (response) of 1 second, we can apply the heat cycles as
equal or non-equal pulses of a few milliseconds. During the on
portion of the heating power cycle there will be no measurement
taken by the thermocouples. During the off portion of the heating
cycle, the temperature measurement from all of the thermocouples
along the probe will be taken. Those measurements can be taken a
few times (during the off portion of the power cycle) to minimize
the random errors by using time-averaging of the reading from each
thermocouple.
[0057] The heater and the parallel thermocouples of FIG. 9 can be
easily manufactured and mounted on a single board or layer. The
electronic circuitry of power circuit, data acquisition and the
microprocessor can also be mounted on a single board.
[0058] Instead of measuring the individual temperature along a
heater to determine the continuous liquid level an average of those
reading can be used. Using the average reading from all of the hot
junctions in FIG. 9, by connecting all of the Copper traces of the
hot thermocouples junctions (i.e. 71 to 80) to a single point will
simplify the electronic hardware and eliminate the need for a
microprocessor for the embodiment of the sensor that uses parallel
thermocouple configurations. However, such an embodiment is
dependent (sensitive) to the variations in the electrical
resistance of the copper traces as well as other factors like the
uniformity of the properties of the liquid and the medium above
it.
[0059] The continuous liquid level can also be determined by adding
the voltage from the individual temperature sensors along the
heater. Selecting thermocouples that are serially connected can do
this addition of voltage or temperature. While such an embodiment
is easier to manufacture and has lower cost electronics, such
embodiment has limitations and drawbacks. For this embodiment to
give any level of accurate continuous liquid level reading, it
needs to have all of its cold junctions connected by isothermal
blocks and the liquid and the medium above it must have uniform
properties. Measuring of any other liquid property (i.e. individual
temperatures along the heater) will require additional separate
circuit for each liquid parameter.
[0060] It must be emphasized that the use of the heater temperature
profile is the reason that the signal to calculate continuous
liquid level is generated. The various thermocouples configurations
allow different steps in the signal processing and not in the
signal generation.
[0061] In all of the embodiements of thermocouples configurations,
the thermocouple junctions are formed between the leads (traces) of
both legs, with suitable thermal, electrical and chemical
insulations to keep measurement data from each thermocouple
junction clean and the probe stable.
[0062] The two elements of the thermocouple in each embodiment can
be comprised of Constantan as one element and the Copper as another
element. However, other type of elements for thermocouple can also
be used to form the thermocouples in FIG. 6 or FIG. 9. For example,
exotic material like Zinc-Antimony (to replace the Constantan) can
be used to increase the voltage reading from the thermocouples. The
thermocouple that uses Zinc-Antimony as one of the thermocouple
elements, will approximately give a signal that is 25 times the
signal from the thermocouple that uses Copper and Constantan. One
of the big advantage of using a thermocouple that uses Constantan
and Copper is the fact that the voltage readings from such a
thermocouple is linear for a wide range of operating temperature.
This linearity of the thermocouple over a wide range of operating
temperature range, is one of the requirements for making the sensor
accurate and self-calibrated.
[0063] The basic embodiment of a simple two-metal strip (e.g.
Constantan with Copper taps) eliminates the need for specialized
thermocouple electronics. Traditional systems that employ
thermocouples use "cold junction compensation" to yield an absolute
temperature measurement, then process that data. The tapped strip
approach simplifies this to a data set that is entirely composed of
differential temperature measurements. The absolute temperature
along the strip is not needed for the basic level-sensing
applications, but is readily added with a discrete sensor at a
single location along the strip if desired. Further, by reducing
the probe to an entirely copper interface, the complications and
cost of bringing out a dissimilar metal lead is avoided.
[0064] The top and bottom layers of the probe of FIG. 6 or 9 can be
made of thermoplastic material and those two layers together with a
Copper trace pattern and a strip of Constantine can be clamped
together and put in a thermal chamber for a short time and at this
way make the thermocouple junctions without using soldering or
ultrasonic welding as well as eliminate the need to bond the two
layers with adhesive that most likely will dissolve in fuel or
other liquids.
[0065] In addition to using lamination technology to produce the
probe in FIG. 6 or 9, this probe can also be produced using the
production methods of vacuum deposition, screen printing, molding
or a combination of those methods. For example, the Copper and
Constantan can be screen printed on a substrate. The electronic
data acquisition system can also be mounted on the same substrate.
Then the entire sensor can be coated by depositing a thin coating
material that is hydrophobic (or oil phobic) and slippery as well
as give electrical insulation. Another method of coating such a
sensor is to screen print the electronic hardware with a buffer
coating and then screen print the entire sensor with a thin coating
that is hydrophobic, slippery, chemically inert and electrically
insulating.
[0066] The invention can also be used to determine the kind of
liquid from a set of liquids. For example, determining the kind of
fuel in a fuel tank from a set of fuels. The method of determining
the kind of fuel will be based on three parameters that are shown
in FIG. 3. The first parameter is the height or rise of line 56 in
FIG. 3 above the liquid temperature after a given amount of power
is applied to the liquid for a given amount of time. The second
parameter is the difference between line 55 and 56 in FIG. 3 after
a given amount of power is applied to the probe after a given
amount of time. The third parameter is the curvature of line 57. If
we use only the first parameter, it will be hard to determine which
liquid to select since two different liquids with the same thermal
convection will raise the reading from the thermocouples that are
in liquid by the same amount. However, the transfer of heat across
the boundary between the liquid and air or two other liquid mediums
will be different and thus the second and third parameters (for two
different liquids that have the same thermal convection) will also
be different.
[0067] The invention can also be used to determine the density and
pressure of the compressible fluid above the incompressible liquid.
For example, measuring the pressure variation in a sealed
compressor. As the pressure changes, the density of the
compressible fluid changes in a direct relationship. When a probe
with a heater is inserted inside a container with compressed fluid
above he incompressible liquid, the section of the heater that is
in the compressed fluid will have a higher temperature rise than
the section in the incompressible liquid. This temperature rise is
shown as line 55 in FIG. 3a. As the density of the compressed fluid
changes, the height of line 55 will change. The increase in the
pressure of the compressible fluid will increase the density of the
compressible fluid, which in turn will decrease the height of line
55 in FIG. 3a. Moreover, The lowering of the height of line 55 in
FIG. 3a will also change the curvature of the curved line 110 in
FIG. 3a. In other words, the changes in the pressure of the
compressible fluid can be determined by either of two portions of
the temperature profile (i.e. height of line 55 and curvature
variation in curved line 110).
[0068] Experiments performed on a prototype similar to the
configuration described in FIG. 2 indicated similar temperature
profile trends as those predicted analytically, although the
precision was not close to analytically predicted precision. This
is because the sensors used in the prototype did not have the
accuracy required for such precision.
[0069] The same experiments also indicated that the probe
temperature momentarily dips in temperature when exposed to air
after being immersed in water, is shown in FIG. 7. This is because
the small amount of water entrained on the probe is evaporated,
causing the temperature to dip. As the entrained water has been
evaporated, the temperature of the probe then rises to the level of
that when it is in air. These characteristics of the probe could be
employed in the detection of ice formation on the external surface
of an aircraft, because when the sensor is in ice, it is normally
insulated thermally from the surrounding air or water, and the
probe temperature is expected to rise. The heat required to change
the ice to water will appear as a time delay on this sensor. This
time delay will be one of three parameters that will be used to
detect ice with this sensor.
[0070] FIG. 10 shows the Data Acquisition that was developed for
this probe. In this figure, 40 is the probe with the thermocouples
and 22 is the microprocessor, which controls the entire data
acquisition as well as the software and the electronic circuitry
for monitoring the health of the probe and the entire electronic
hardware The microprocessor, 22, commands the power supply 95 to
apply power to the probe 40. Subsequently, the microprocessor
commands the multiplexers 93 and 94 to scan and measure the analog
thermocouple voltages after the power is applied. To eliminate
non-random errors in the thermocouple readings, that are caused by
the electronic hardware and the connections, the scanning of the
thermocouples is not done sequentially. For example, let us say
that we have a probe with 11 thermocouple junctions. One of the
thermocouple junctions (i.e. the Copper wire of the first junction
between this Copper wire and the Constantan) or the Copper trace
from another point on the probe that is not a hot thermocouple
junction (but is a junction between the Constantan and a Copper
trace), can be used as a reference point 0. The wiring between the
probe and the multiplexers are such that a Copper wire goes from
the reference point 0 to multiplexer 93 and another Copper wire
goes from the same reference point to the second multiplexer 94.
The Copper wires from all of the odd thermocouple junctions will go
to multiplexer 93 and all of the Copper wires from the even
thermocouple junctions will go to multiplexer 94. If thermocouple
junction 1 is selected as the reference point 0, Then all 5 Copper
wires from the odd thermocouple (3,5,7,9,11) junctions and the one
wire from the reference point 0 will go to multiplexer 93.
Similarly the multiplexer 94 will also have 6 wires. 5 from the
Copper traces of the even thermocouples and one wire from the
reference point 0. The first measurement that is done is the
voltage difference between the Copper trace of the reference point
0 (V0) on multiplexer 93 minus the voltage reading of junction 2
(V2) on multiplexer 94. Assuming the total non-random (bias, slow
drift etc.) on the lines and connections leading to multiplexer 93
is e1 and for multiplexer 94, the total non-random error is e2. The
differential voltage reading for each thermocouples junction will
come through differential amplifier 96 whose non-random errors are
self-calibrated. The sequence of sampled voltage differences for
each thermocouple junction will go from the differential amplifier
96 through Analog to Digital Converter 98 which is powered by 99.
The first differential voltage reading is equal to:
V0+e1-(V2+e2) (3
[0071] The next reading is the differential reading between
thermocouple junction 3 on multiplexer 93 and the reference point 0
on multiplexer 94. If we define V3 and V0 as the voltage signal
(without bias or random noise) from thermocouple junction 3 and
reference point 0 respectively, then this reading will be equal
to:
V3+e1-(V0+e2) (4)
[0072] The next reading will be taken between thermocouple junction
4 (V4) and the reference point 0 (V0) and it is equal to:
V0+e1-(V4+e2) (5)
[0073] By subtracting (in the microprocessor) the voltage reading
of equation (3), the voltage reading of equation (5) we get the
accurate differential reading of thermocouple 4 relative to
thermocouple junction 2 (i.e. the errors e1 and e2 are eliminated).
This differential reading of thermocouple 4 relative to 2 could
have been measured directly instead of being calculated. Similarly,
the voltage reading of the odd junctions relative to the first odd
junction (junction 3) will also eliminate the non-random errors. It
is to be noted that by placing accurate absolute temperature sensor
close to the reference point 0, (i.e. coupling thermally junction 0
and the absolute temperature sensor) it will be possible to
determine accurately the absolute temperature of each thermocouple
junctions. Using thermocouple junctions of Constantine and Copper
each one degree C. correspond to 40 microvolt voltage difference
between the reference point 0 and a thermocouple junction on the
probe. The data acquisition has a reference absolute temperature
sensor 97 and the microprocessor 22 can send out the absolute
temperature from each junction either as Analog or digital signal
100. In addition to using the microprocessor to calibrate the
non-random error of the electronic hardware and the wiring, the
microprocessor will also be used to average the differential time
samples reading of each thermocouple and at this way to minimize
the magnitude of the random error. In addition, the algorithm will
further reduce the remaining errors from each thermocouple
measurement by using a correlation technique which spread the
remaining positive and negative error from each thermocouple
junction over a set of thermocouples that include thermocouples
that are in liquid and thermocouples that are above the liquid. The
software of the microprocessor uses the readings with minimal
errors from all of thermocouples to complete the calculation of the
liquid level, or determine the kind of liquid or other liquid
parameters like viscosity, from the profile of the temperatures(or
voltage) along the probe, the microprocessor then will send those
parameters to a digital or analog display through a serial and
digital-to-analog converter input/output 100. It is to be noted
that the elimination of the non-random electronic hardware errors,
minimization by averaging of random errors and the use of
correlation function in the voltage reading from the thermocouples
as well as the pulsed heating of the probe will be done with
software in the microprocessor.
[0074] If instead of using two multi-plexers one uses a single
multi-plexer then the bias and drift (non-random) errors can be
minimizes or eliminated by taking two readings from the single
multiplexer. First a reading when the multiplexer is shortened.
Then a reading from the multi-plexer when a cold and a hot
thermocouple junctions are connected to the multi-plexer. Taking
the difference between those two readings will give the voltage
(temperature) of the hot junction without the non-random errors of
the hardware.
[0075] The data acquisition will include a circuitry and the
microprocessor will include an algorithm which will monitor the
health of the sensor. For example, when a thermocouple junction is
electrically disconnected or shorted, the voltage reading from such
disconnected thermocouple junction will be a large number and the
microprocessor with its software determines which junction is
disconnected. Similarly, the health monitoring circuitry can send a
reference signal (voltage) and monitor which component in the
electronic circuitry of the data acquisition is not working
properly and need to be replaced.
[0076] The liquid level sensor invention described (the strip is
most useful design to control the character of the temperature
profile) herein can also be adopted to detect ice formation on the
external surfaces of an aircraft, roads, roofs and bridges by
characterizing the signature (detail in the profile) associated
with the "Igloo effect" and the various forms of water/ice.
Disturbing the heat flow. When the sensor is covered with ice, it
is normally insulated thermally from the surrounding air or water.
When the probe is heated, and the amount of power applied to the
probe is not too high then at the beginning of the power
application, the probe temperature will not rise since the ice
needs power to overcome its latent heat and change the phase of a
thin layer of ice to water. The volume of the melted ice is smaller
than the volume of the ice and there will be an air gap between the
melted water and the remaining ice. The additional heat that is
applied to the probe will raise the air gap temperature to the
level expected when a surrounding wall of ice, not in contact with
the probe, insulates thermally the melted thin layer of ice from
the environment. The transient and steady-state data will be used
to infer what is there (i.e. ice, water, air etc) on any section of
the probe.
[0077] One version of the sensor for detecting ice is illustrated
in FIGS. 11, 12 and 13. FIG. 11 is a schematic diagram of the ice
detection sensor attached to the surface of an aircraft wing 102 or
other structure. The sensor is comprised of an insulating material
or insulation 110 attached to the wing surface, a heater strip 105
attached to the thermal insulation 110, a power supply 109 for
providing electrical power to the heater strip, two sections of
film type insulator 104, attached to the heater strip to
electrically isolate the thermocouples from the heater, a first
thermocouple 103 attached to the first film insulator and exposed
to the outside air, a second thermocouple 107 attached to the
second film insulation, an insulated dome 106 enclosing 107 within
which a volume of air 108 is also enclosed, and a data acquisition
system 101 or similar device to read and/or record the temperatures
or equivalent voltages measured by thermocouples 103 and 107. The
heater strip has active heating areas only immediately under
thermocouples 103 and 107.
[0078] In FIG. 11 when there is no ice forming over the wing
surface, the ice detection device is exposed to air that flows over
the surface of the wing. When no power is applied to the heater
strip, the temperature of thermocouples 103 and 107 are essentially
equal to the temperature of the air flowing over the wing surface.
When a certain amount of power is applied to the heater strip, the
temperature of thermocouples 103 and 107 will rise and level off to
their steady-state values. This steady-state temperature rise is
expressed by the following equation:
DT=QR (6)
[0079] Where DT is the steady-state temperature rise,
[0080] Q is the power dissipated in the vicinity of the
thermocouple,
[0081] R is the overall thermal resistance between the sensor and
the surface in the vicinity of the thermocouple and the air flowing
over the surface.
[0082] The transient temperature is the rise as a function of time
of thermocouple 103 and 107 and is expressed by the following
equation:
DT(t)=DT(!-exp(-Bt)) (7)
[0083] Where DT(t) is the temperature rise as a function of
time,
[0084] exp is a natural logarithmic function,
[0085] B is the inverse of the system time constant (response)
which is in turn a function of the overall system thermal
resistance and the overall system thermal capacitance,
[0086] t is the time variable.
[0087] As can be seen in equation 6 and 7, DT is a constant while
DT(t) is an exponential function. The characteristic plots of DT(t)
for thermocouples 103 and 107 as a function of time are shown in
FIG. 12, for a case where there is no ice formation on the surface
of the wing (see FIG. 11). After the heater power has been applied
for sometime (at least 4 time constant of the thermocouple response
or other combination of power application scheme that will reduce
the time to reach steady-state), DT(t) for each thermocouple
reaches its maximum (i.e. steady-state) value. The maximum value is
equal to DT, the steady-state temperature rise expressed in
equation 6.
[0088] When there is ice formation on the wing surface of the
aircraft, the temperature rise profile of thermocouple 103 and 107
are altered somewhat. When ice covers the dome of thermocouple 107
and thermocouple 103 and power is applied to the heater strip, a
thin layer of ice from the thicker ice that covers thermocouple 103
will be melted and since the volume of water is smaller than the
volume of ice, there will be an air gap between the melted water
and the ice above it. This is sometimes referred to as the "Igloo
Effect". The temperature rise profiles as a function of time DT(t)
are shown in FIG. 13. Thermocouple 107 has essentially the same
temperature rise shape as that when there is no ice formation on
the dome 106 except that DT (the steady-state temperature rise) is
somewhat higher, because of the additional thermal resistance
induced by the layer of ice over the insulated dome. The DT)t)
shape (profile) of thermocouple 103 has been more drastically
altered however. In this profile, after power application, there is
no temperature rise for a short duration (for the example of FIG.
13, it is the first 0.2 seconds) because the temperature of 103
does not change while the change of phase from solid (ice) to
liquid (water), i.e. the melting of the ice, is taking place. After
the melting of the thin layer of ice has essentially ceased,
because the heated area above 103 is too remote from the remaining
ice, the temperature of 103 begins to rise. As 103 temperature
rises it assumes a similar shape as that where there is no ice
formation but reaches a higher steady-state value because of the
ice surrounding thermocouple 103. Based on FIGS. 13 and 12, it can
be seen that the profile of thermocouple 103 with ice cover has
three parameters that are different than the temperature profile
without ice. The three parameters are time delay at the beginning
of applying power (i.e. no rise in the temperature of thermocouple
103), higher steady-state value and a response time that is closer
to the response time of thermocouple 107 of FIG. 12 or 13. The
calculation of the three parameters will be done with the software
of a microprocessor. The same invention can be used for a probe
with a single dome type of a thermocouple and a lot of
thermocouples like thermocouple 103 to detect ice at different
locations over the wing of an aircraft. The sensor of FIG. 11 can
be mounted on a horizontal or vertical surfaces of aircraft. A
modified version of the ice-detector shown in FIG. 11, is one that
has a constant strip that act as the heater (when the heat pulses
have a duration that is much smaller than the thermal response time
of the Constantan strip) as well as the common wire for the
thermocouples. The data acquisition of FIG. 10 together with a
probe of FIG. 9 and a dome of FIG. 11 were used in various icing
tunnel tests. In those tests, the readings from thermocouple 107
and 103 are differential relative to the reference point 0. The
temperature rise in equations 6 and 7 are the differential voltages
of thermocouples 103 and 107 relative to the reference pint 0.
[0089] Further tests of the behavior of the probe when immersed in
a body of fluids containing water and oil, wherein the lighter oil
stratifies above the water. There is a difference in temperature
between that when the probe is in water and when it is in oil, as
seen in FIG. 8, where the probe is moved up and down during the
various time periods of the experiment.
[0090] The principle of operation described in this invention can
also be applied to the measurement of the viscosity of a liquid,
because viscosity is a key parameter that determines the convective
heat transfer efficiency of the liquid. The lower the viscosity of
the liquid the more efficiently it can transfer the heat, and the
smaller the temperature difference between the heated surface and
the liquid (DT) will be. An increase in viscosity of the liquid,
which would result in a higher DT, generally indicates that the
lubricating quality of the liquid has deteriorated to some degree.
As such, a probe that work on the principle of this invention, can
be used to determine whether it is time to replace a liquid, such
as lubricating oil in an automotive engine.
[0091] The invention described herein can also be used to determine
the density of incompressible liquid. By measuring the temperature
of the liquid and its pressure (with an appropriate
pressure-measuring device such as pressure transducer) at the same
location, it is possible to compute, with a suitable
microprocessor, the density of the liquid.
[0092] The invention described herein can also be used to determine
accurately the liquid or gas temperature at the thermal junctions
of the probe. Since the probe responds to the temperature
differential between any two thermal junctions along the common
strip (Constantan strip in FIG. 9), a reference accurate
temperature sensor may be located at a convenient point (or the
strip extended to such a point) and the temperature at any other
point along the probe is resolvable. The calculation of the
absolute temperature of each thermocouple location will be done
with software using the thermocouple voltage differential whose
non-random errors have been eliminated and random errors have been
minimized using filtering which includes averaging and correlation
functions.
[0093] The measurement process for this sensor relies on acquiring
data from several temperature sensors and reconstructing an analog
thermal profile along the heater of a liquid sensor as sampled by
those temperature sensors at strategically located points along a
heater. The signal from the temperature sensors can be digitized in
a number of ways, from simply dwelling on each tap in turn until
the signal is adequately resolved to briefly reading each tap and
increasing the signal resolution as needed through accumulating the
results of multiple reads of a temperature sensors set. Likewise,
the commutation sequence need not follow any specific order;
however, noise reduction and ease of data processing are likely to
dictate the optimal sampling approach for any given applications.
Equally, a simultaneous sampling of all taps, using either multiple
parallel converters or multiple sample/hold amplifiers, is workable
(although this is likely to be the least cost-effective
approach).
Liquid Sensor and Ice Detector
[0094] An improved apparatus and a method of measuring and
interpreting reliably, simply and accurately the information on
continuous liquid level, liquid temperature and other liquid
properties within a vessel. The apparatus could be made of a
powered heater element and temperature sensors can be
screen-printed, vacuum deposited, etched, welded, soldered or
plated on one or both sides of a single rigid or a flexible
substrate. The powered heater element used in this invention is
similar to a hot wire. While in a hot wire technology the
electrical resistance of the wire is used to determine the liquid
level, in this invention the heater geometry is used to obtain a
temperature profile or a wave like temperature distribution that
moves up and down the heater. Due to non-uniformity and localized
effects in the liquid and the medium above it, the hot wire
technology gives large errors in its reading of the liquid level.
In contrast, in the method and apparatus of this invention,
localized and non-uniform effects within the liquid or the medium
above it will be distributed over the entire temperature profile
and will not significantly change the shape of the temperature
profile. The geometry of the heater determines the curve shape,
such as steepness or shallowness of a temperature profile along a
heater. The geometry of the heater will control the spacing of the
temperature sensors. The temperature sensors measure the actual
temperature of the heater at a few points along the heater. The
temperature sensors must be located close to the heater. When
thermocouples are used to measure the temperature along a heater
the thermocouples can be configured in parallel with a common cold
junction or in a series with a common temperature for all of the
cold junctions. The hot junctions of thermocouples configured in
parallel or in series are located close to the heater. Cold
junctions of thermocouples are located at a point away from the
heater. Various parallel and serial configurations of thermocouples
or temperature sensors can be used to measure the temperature along
a heater. Simultaneous measurements from all the temperature
sensors, before and after heat is applied, are used to generate
accurate temperature profiles for the entire heater and not just
from two adjacent temperature sensors. Different features of the
temperature profiles will determine accurately the liquid level,
liquid temperature and other liquid properties. Apparatus of the
invention may also be used to detect ice formation.
3TABLE 1 TEMPERATURES AT 6 THERMOCOUPLE LOCATIONS LIQUID LEVEL
DEVICE TEMPERATURES (DEG C.) THERMO- ALL ALL TC 4, 5 & 6 COUPLE
NO IN AIR IN WATER IN WATER 1 34.493 20.145 33.207 2 34.493 20.145
31.914 3 34.493 20.145 28.035 4 34.493 20.145 20.52 5 34.493 20.145
20.149 6 34.493 20.145 20.145 NOTE: BOTH AIR AND WATER TEMPATURES =
20 DEG C.
[0095]
4TABLE 2 PROBE TEMPERATURE VS POSITION OF LIQUID LEVEL POSITION
BETWEEN TC3 AND TC4 DISTANCE TC3 TC4 FROM TC3* TEMP TEMP TC3 - TC4
(MM) DEG C. DEG C. DEG C. 0.00 20.6583 20.1445 0.5138 1.27 21.6055
21.1451 1.4604 2.54 22.5432 20.1454 2.3978 3.81 23.4567 20.1460
3.3107 5.08 24.3350 20.1477 4.1873 7.62 25.9577 20.1644 5.7933 8.89
26.6949 20.1971 6.4978 10.16 27.3829 20.2842 7.0987 11.43 28.0347
20.5194 7.5153 12.70 28.7412 21.0100 7.7312 *SEE FIG. 3 NOTE: AIR
AND WATER TEMP = 20 DEG C.
* * * * *