U.S. patent number 3,631,436 [Application Number 05/054,743] was granted by the patent office on 1971-12-28 for gas-detecting device.
Invention is credited to Naoyoshi Taguchi.
United States Patent |
3,631,436 |
Taguchi |
December 28, 1971 |
GAS-DETECTING DEVICE
Abstract
A gas-detecting device having a gas responsive semiconductor, a
pair of electrodes in contact with the semiconductor, means for
heating the semiconductor to stabilize its resistance at a
predetermined value, time delay switching means and an alarm
circuit and an impedance circuit interconnected with said
electrodes and said switching means so that the impedance circuit
will be connected with the semiconductor electrodes during the
heating period and upon stabilization of the semiconductor the
switching means will automatically and effectively disconnect the
impedance circuit and substitute the alarm circuit.
Inventors: |
Taguchi; Naoyoshi (Kobe,
JA) |
Family
ID: |
21993216 |
Appl.
No.: |
05/054,743 |
Filed: |
July 14, 1970 |
Current U.S.
Class: |
340/528; 340/634;
422/98 |
Current CPC
Class: |
G01N
27/12 (20130101); G08B 17/117 (20130101) |
Current International
Class: |
G01N
27/12 (20060101); G08B 17/10 (20060101); G08B
17/117 (20060101); G08b 021/00 (); G01n
031/06 () |
Field of
Search: |
;340/237R
;23/254R,254E,232R,232E ;73/23,25-27 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Caldwell; John W.
Assistant Examiner: Myer; Daniel
Claims
What is claimed is:
1. A gas-detecting device comprising a metal oxide semiconductor
element having a resistance characteristic which changes in the
presence of a contaminating gas in the ambient air, a pair of
electrodes in contact with said semiconductor including means for
producing a current flow through said semiconductor, means for
heating said semiconductor to a predetermined temperature, an alarm
circuit having a predetermined impedance, an impedance circuit
having the same impedance as the alarm circuit and time delayed
switching means interconnected with said alarm circuit, said
impedance circuit and said electrodes, said switching means
normally connecting said impedance circuit with said electrodes and
maintaining the last said connection for a predetermined time after
energizing said heating means and then automatically disconnecting
said impedance circuit and substituting said alarm circuit whereby
said alarm circuit will be activated upon a predetermined
resistance change of said semiconductor.
2. A gas-detecting device according to claim 1 wherein said time
delay switching means includes a temperature-sensing element
responsive to the temperature of said semiconductor element and a
double throw switch actuated by said temperature changing element
and connections between the last said switch and said impedance
circuit and said alarm circuit.
3. A gas-detecting device according to claim 2 wherein said time
delayed switching means includes a bimetallic element.
4. A gas-detecting devices according to claim 2 wherein said
semiconductor element is enclosed within a perforated metal cap and
said temperature-sensing element is thermally coupled to said
cap.
5. A gas-detecting device according to claim 1 wherein said time
delay switching means includes a first resistor having a positive
temperature characteristic, a second resistor having a negative
temperature characteristic, means thermally coupling said resistors
to said semiconductor element and connections between said first
resistor and said impedance circuit and between said second
resistor and said alarm circuit.
6. A gas-detecting element according to claim 5 wherein said
thermal coupling means comprises a metal cap enclosing said
semiconductor element.
Description
This invention relates to a detector for gas, smoke and other
air-contaminating vapors and more specifically to a detecting
device utilizing a metal oxide semiconductor element which varies
in impedance when exposed to reducing gases such as hydrogen,
carbon monoxide, alcohol vapor, gasoline vapor or smoke and
embodies means for preventing the production of an erroneous alarm
during initial periods of operation when the resistance of the
metal oxide semiconductor is unstable.
Detecting devices for gas, smoke, and other similar air
contaminants utilizing metal oxide semiconductor elements must be
heated to a predetermined temperature in order to increase their
sensitivity. While the desired operating temperature depends on
various characteristics of the semiconductor element such as its
geometry, and the desired detecting sensitivity, it has been
determined that a temperature in the range of 170.degree. to
230.degree. C. is generally preferable.
Assuming that the metal oxide semiconductor element of the detector
has a geometry such that the interelectrode resistance is of the
order to 50,000 ohms when heated to a temperature in the range of
170.degree. to 230.degree. C. and when the ambient air does not
contain any contaminant such as gas or smoke, such semiconductor
will vary in resistance from an exceedingly high value of the order
of hundreds of thousands of ohms to values as low as 5,000 ohms
during a period of approximately four minutes required to bring the
semiconductor up to its stable operating temperature. Under these
conditions the semiconductor detector will produce an alarm during
the initial heating period even though smoke or gas does not
contaminate the ambient air.
This invention overcomes the aforementioned disadvantages and
utilizes switching means associated with the alarm circuit to
prevent the generation of an alarm until after the semiconductor
has attained a predetermined temperature. Semiconductor elements do
not reach a stable state until after it has been heated to a
temperature that will permit a predetermined current to flow
between the electrodes. In prior gas-detecting devices thermisters
have been inserted with the alarm circuit to delay initiation of
the alarm circuit. While this procedure has certain advantages, it
does not provide the desired reliability and dependability for an
alarm device.
Accordingly one object of the invention resides in the provision of
a novel and improved circuit for gas detectors having metal oxide
semiconductor elements which will provide stable operation
throughout the entire temperature range. This may be obtained in
accordance with the invention by utilizing an auxiliary circuit
having an impedance substantially equivalent to he capacitance of
the alarm circuit and means for interconnecting the auxiliary
circuit with the semiconductor element until the latter reaches a
predetermined temperature and operation is stabilized. After the
semiconductor element has become stabilized, the alarm circuit is
automatically substituted for the auxiliary circuit to place the
detector in an operational condition.
The gas detector in accordance with the invention includes a metal
oxide semiconductor element having a resistance which varies
materially when exposed to a reducing has or smoke and a pair of
electrodes in contact with the semiconductor element to provide for
the flow of current through the semiconductor. The device further
includes a heater for heating the semiconductor, an alarm circuit,
an auxiliary impedance circuit having substantially the same
impedance as the alarm circuit and a delayed switching means for
connecting the auxiliary circuit to one of the electrodes for a
predetermined time period after energizing the heating element and
at the conclusion of the time period at which the semiconductor
element reaches a predetermined temperature or its so-called steady
state, disconnecting the auxiliary circuit and substituting the
alarm circuit therefor.
The above and other objects and advantages of the invention will
become more apparent from the following description and
accompanying drawings forming a part of the application.
In the drawings:
FIG. 1 is a graph showing the change in resistance of a metal oxide
semiconductor element during the heating period;
FIG. 2 is a cross-sectional view of one embodiment of a
gas-detecting device in accordance with the invention together with
a schematic circuit diagram;
FIG. 3 is a plan view in partial section of a gas-detecting device
of FIG. 2; and
FIG. 4 is a cross-sectional view of a modified embodiment of the
detector and circuit shown in FIG. 2.
The gas-detecting device in accordance with the invention utilizes
a metal oxide semiconductor element that is heated to he
temperature of the order of 170.degree. to 230.degree. C. to
stabilize its detecting sensitivity. It has been found that this
temperature range affords a smooth and continuous change of
resistance of the semiconductor element in response to changes in
concentration of gases and smoke in the ambient air and thus is
suitable for practical applications. Assuming that the metal oxide
semiconductor element has a geometry such that its interelectrode
resistance is of the order of 50,000 ohms when heated to a
temperature of the order of 170.degree. to 230.degree. C. Such a
semiconductor element will overcome resistance variations during
the warmup period as illustrated in FIG. 1. The initial resistance
of the semiconductor prior to heating is of the order of hundreds
of thousands of ohms and may be deemed to be an insulator at normal
temperatures. When the semiconductor is heated, the resistance
initially drops to as low as 5,000 ohms after the first thirty
seconds, and this condition continues for approximately 21/2
minutes. Thereafter the resistance gradually increases until it
reaches the inherent value of the material, namely, approximately
50,000 ohms after a period of 4 minutes.
The change in resistance of the semiconductor element during the
initial heating period as illustrated in FIG. 1 is believed to
result from the fact that the semiconductor absorbs water and gases
while it is cold and then during the heating step, the water and
gas react with the semiconductor material to reduce its resistance.
After a predetermined period of heating, the water and has are
expelled from the element and the resistance then approaches the
inherent resistance of the material which in the present case is of
the order of 50,000 ohms.
When a reducing gas or smoke contacts a heated semiconductor
element, its resistance will vary materially. With semiconductors
of the reduction type, the resistance drops from approximately
50,000 ohms to approximately 10,000 ohms as shown in FIG. 1 after
the time t.sub.1. With semiconductors of the oxidation type, the
resistance increases to approximately 100,000 ohms as shown in FIG.
1.
Reduction-type metal oxide semiconductors will generate an alarm
when the resistance falls below 50,000 ohms, and accordingly,
during the heating time period of 30 seconds to 3 minutes, the
semiconductor resistance will drop sufficiently to create erroneous
alarms. In the case of an oxidation-type semiconductor material,
the erroneous alarm would be generated within the first 30 seconds
of the heating period.
One embodiment of a structure in accordance with the invention is
illustrated in FIGS. 2 and 3. This structure includes a
gas-detecting element generally denoted by the numeral 2 and a
surrounding metal cap generally denoted by the numeral 4. The
gas-detecting element 2 includes a cup-shaped electrode 6 open at
the bottom end and having a number of openings therein to provide
for the flow of air. A metal oxide semiconductor material 8 fills
the cup-shaped electrode 6 which in the case of a reduction type
semiconductor would be SnO.sub. 2. A second electrode 10 which is
in the form of a flat or cylindrical rod is embedded in the
semiconductor material 8 and extends downwardly into an insulating
plate 12 which is secured and closes the bottom end of the
electrode 6. The electrode 10 may be formed of a nickel-chromium
alloy and further includes a heating wire 14 which is wound about
and insulated from the electrode 10. One end of the heating wire 14
is connected to a conductor 16 which extends downwardly through the
insulating plate 12. The other end of the heating wire is connected
to a conductor 20 which also extends downwardly through the
insulating plate 12. The junction 18 of the heating wire 14 and the
conductor 20 is also electrically connected to the electrode 10 so
that the conductor 20 also serves as a connection to one of the
electrodes of the gas-detecting element.
The cap 4 has a plurality of holes 22 extending therethrough to
permit smoke to enter the cap. The cap 4 is also electrically
connected to the electrode 6 by conductors 24 and is fixed to an
insulating plate 26.
A bimetallic switch 28 includes a bimetallic element 30 which is
electrically and mechanically coupled to the top of the cap 4. The
switch 28 has one contact 32 connected through an alarm device 34
and then to the terminal 52 of a secondary winding 46 of the
transformer 40. The primary winding 42 is connected by means of a
plug 44 to a suitable source of alternating current. A second
contact 36 is connected through a resistor 38 having an impedance
substantially equivalent to the alarm circuit 34 and thence to the
terminal 52 of winding 46. The conductor 16 of the heater winding
14 is connected to the terminal 48 of the secondary winding 46
while the conductor 20 is connected to the terminal 50 of the
winding 46.
With the aforementioned structure that portion of the secondary
winding between the terminals 48 and 50 may provide approximately
1.5 volts for supplying energy to the heater 14. The portion of the
secondary winding 46 between the terminals 50 and 52 provides
approximately 100 volts for producing a current flow through either
the resistor 38 or the alarm circuit 34 as the case may be and
thence through the semiconductor 8.
When the metal oxide semiconductor element 8 is not heated, the
contact carried by the bimetallic element 30 will be engaged with
the contact 36. When energy is supplied to the transformer 40, the
bimetallic switch 28 will be in the position as illustrated in
solid lines placing the resistor 38 in circuit with the
semiconductor 8. Under these conditions an erroneous alarm cannot
occur. After a period of time which permits the semiconductor 8 to
attain a predetermined temperature and thus maintain a stable
resistance, the bimetallic switch 30 will be operated and moved to
the dotted line position shown in FIG. 2 to disconnect the resistor
38 and connect the alarm circuit 34 with the electrodes 6 and 10.
In the case of a reduction type semiconductor 8 the bimetallic
switch will prevent an erroneous alarm even though the
semiconductor resistance may drop to as low as 5,000 ohms. It will
be observed that the temperature of the outer cap 4 is spaced from
the electrode 6 and thus its temperature will increase very slowly
as it is not in intimate contact with the gas-detecting element 2
and also has a fairly large thermal capacity. Thus the bimetallic
switch will operate only after the semiconductor element 8 reaches
a stable value of the order of 50,000 ohms. Therefore, by selecting
a heater 14 of appropriate heating capacity and utilizing a cap 4
of predetermined size, a delay of approximately 4 minutes can be
obtained before the bimetallic switch 28 functions to switch the
system from the resistor 38 to the alarm system 34 at which time
the semiconductor 8 will be sufficiently stabilized.
When gas or smoke is not present in the ambient air or the
concentration is below a specific value, the interelectrode
resistance of the semiconductor 8 when at its stable operating
temperature will be about 50,000 ohms and the current flowing
through the alarm circuit 34 will be too small to activate the
alarm. However, if the air becomes contaminated with a gas such as
hydrogen, carbon monoxide, or a vapor of an organic fuel such as
alcohol or gasoline or if the air becomes contaminated with smoke
at a specific concentration, then such gas, vapor or smoke will
pass through the porous electrode 6 and penetrate the metal oxide
semiconductor 8. This will cause the resistance in the case of a
reduction type semiconductor to abruptly decrease as shown by the
curve in FIG. 1 following the time t.sub.1. The decrease in
resistance will increase the current flowing through the alarm 34
and thus activate it. When the concentration of gas, vapor or smoke
in the ambient air falls below a specific concentration, the
resistance of the semiconductor will return to its normal or
original value within a time of several seconds to several
minutes.
Should the resistance of the metal oxide semiconductor 8 require
more than 4 minutes to become stabilized then a bimetallic switch
having a greater time delay would be utilized or in the alternative
the size of the cap 4 may be increased to introduce additional
delay. In the alternative should the semiconductor element become
stabilized in less than 4 minutes, a more sensitive bimetallic
switch may be employed. In addition to the utilization of SnO.sub.2
as the semiconductor 8, such materials as ZnO, Fe.sub.2 O.sub.3,
TiO.sub.2, V.sub.2 O.sub.5, MnO.sub.2, WO.sub.3, ThO.sub.2,
MoO.sub.3, CdO, and PbCrO.sub.4 may also be utilized.
A modified embodiment of the invention is illustrated in FIG. 4,
and this embodiment of the invention utilizes a semiconductor
element in place of the bimetallic switch 28 of the embodiment
shown in FIG. 2 Since the detecting element of FIG. 2 is identical
to the detecting element of FIG. 4, only the switch operation will
be discussed.
In the embodiment of FIG. 4, a temperature-responsive resistor 62
is affixed to the cap 4 and has a resistance characteristic which
decreases with an increase in temperature. A second temperature
responsive resistor 66 is also fixed to the cap 4, and its
resistance increases with an increase in temperature. For example,
a thermister or a critesistor may be utilized as the resistor
having a negative temperature characteristic while a posistor or
semistor may be utilized as the resistor having positive
temperature characteristics.
When the heater 14 is first energized and the cap 4 has not become
heated, the resistance of the resistor 66 will be very much below
that of the resistor 62. Accordingly, during the initial heating
period when the resistance of the semiconductor 8 is unstable,
current will flow through the resistor 66 and the resistor 38 as in
the case of the embodiment illustrated in FIG. 2. At the same time
insufficient current will flow through the alarm 34 to activate it.
Thus an erroneous alarm cannot be produced during the initial
heating period. When the resistance of the semiconductor 8 is
stabilized and the temperature of the cap 4 is raised to a
predetermined level after a specific time period, the resistor 66
will greatly increase in value while the resistor 62 will
materially decrease in value. This action affectively connects the
alarm circuit 34 to the detecting device and disconnects the
resistor 38. Under this condition the device is operable to detect
contamination in the ambient air.
The embodiments of the invention as described above afford a number
of advantages. For instance through the utilization of an impedance
circuit having the same impedance as the alarm circuit and by
interconnecting the impedance circuit with the semiconductor during
the heating period, stabilization of the semiconductor 8 is
accomplished under normal operating conditions so that the
substitution of the semiconductor 8 is accomplished under normal
operating conditions so that the substitution of the alarm circuit
for the impedance circuit will not adversely affect the detector.
Furthermore, a smooth and rapid transfer from an unstable state
during the initial heating period to the normal operating state is
accomplished without any chance of the production of an erroneous
alarm. Moreover, since the entire operation is automatic, human
errors that may occur in preparing the detector for operation will
be eliminated.
While the time delay switch in each embodiment of the invention has
been described as a thermal switch, it is evident that a timing
switch which functions after a predetermined time period
corresponding to the time required for stabilization of the
semiconductor element 8 may also be utilized.
While only certain embodiments of the invention have been
illustrated and described, it is apparent that alterations,
modifications and changes may be made without departing from the
true scope and spirit thereof as defined by the appended
claims.
* * * * *