U.S. patent number 3,665,748 [Application Number 05/051,856] was granted by the patent office on 1972-05-30 for portable trace moisture generator for calibration of moisture analyzers.
This patent grant is currently assigned to Gulf Research & Development Company. Invention is credited to Richard T. Mator.
United States Patent |
3,665,748 |
Mator |
May 30, 1972 |
PORTABLE TRACE MOISTURE GENERATOR FOR CALIBRATION OF MOISTURE
ANALYZERS
Abstract
An improved calibrator for moisture analyzers is provided. A
flow of clean dry carrier gas into two streams, one of which passes
through an improved saturator bomb, wherein it is completely
saturated with water, and then this saturated stream is mixed with
the other dry stream to provide a controlled wet stream for
calibration. A capillary restrictor is used to control one of the
streams. The invention utilizes a simple mathematical relationship
for water concentration, means to vary each of the parameters in
the relationship independently of the others, and means to simply
determine values for the parameters, whichever are varied during a
calibration.
Inventors: |
Mator; Richard T. (Pittsburgh,
PA) |
Assignee: |
Gulf Research & Development
Company (Pittsburgh, PA)
|
Family
ID: |
21973784 |
Appl.
No.: |
05/051,856 |
Filed: |
July 2, 1970 |
Current U.S.
Class: |
73/29.01; 261/94;
73/1.05 |
Current CPC
Class: |
G01N
33/0006 (20130101) |
Current International
Class: |
G01N
33/00 (20060101); G01m 019/00 () |
Field of
Search: |
;73/1A ;261/94 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Wexler, U.S. Dept. Commerce, Nat. Bur. Stds. Research paper No.
RP1894, Vol. 40, June, 1948, pp. 479-486. .
Wexler, Journal of Research of the Nat. Bur. Stds. Vol. 48, No. 4,
April, 1952. .
Hedlin, Materials & Research Standards Jan., 1966, pp. 25-29.
.
Schnelle, I.S.A. Journal April, 1957, pp. 128-133. .
Cram, Journal of Scientific Instruments Vol. 33, July, 1956 pp.
273-276..
|
Primary Examiner: Swisher; S. Clement
Claims
I claim:
1. A method of forming a mixture having a known concentration of a
first fluid in a second fluid, comprising the steps of supplying a
first flow of said second fluid to a mixing chamber, maintaining
said mixing chamber at a first known pressure, supplying a second
flow of said second fluid to saturating means containing said first
fluid, maintaining said saturating means at a known temperature and
a second known pressure whereby said second flow of said second
fluid is saturated with the first fluid, flowing the saturated flow
from said saturating means to said mixing chamber, flowing a
selected one flow of said first flow of said second fluid and said
saturated flow through pressure restrictor means prior to flowing
said selected one flow into said mixing chamber, wherein said
temeperature and said first and second pressures are selected for
said first and second fluids so that said first and second fluids
will mix in said mixing chamber in the gaseous state, restricting
the mixed flow out of said mixing chamber to thereby control the
volume of said mixed flow out of said mixing chamber, changing the
pressure of said temperature one flow flowing to said pressure
restrictor means to thereby control the amount of said selected one
flow passing through said pressure restrictor means, whereby the
amount of said selected one flow flowing into said mixing chamber
is changed while the mixed flow flowing out of said mixing chamber
remains unchanged, and determining the amounts of said selected one
flow and of said mixed flow to thereby determine the concentrations
of said first fluid in said second fluid.
2. The method of claim 1, wherein said selected one flow is said
saturated flow.
3. The method of claim 2, wherein said first fluid is water and the
concentration of water in said second fluid is determined according
to the relationship:
where P.sub.water is the vapor pressure of water at said
temperature of the saturating means; wet flow is the amount of the
flow out of the saturating means; P.sub.total is said second
pressure of said saturating means; and total flow is the amount of
the mixed flow leaving the mixing chamber.
4. The method of claim 2, wherein said second known pressure in
said saturating means is greater than said first known pressure in
said mixing chamber.
5. The method of claim 2, wherein the mixed flow out of said mixing
chamber is supplied to a device for calibrating the ability of said
device to detect the concentration of said first fluid, and wherein
the amount of the saturated flow generated is determined by
simulating the effect of the device being calibrated, stopping said
first flow of said second fluid, diverting the saturated flow only
through the simulating means and to flow measuring means,
reproducing the different changing pressures of said saturated flow
going to said pressure restrictor means, and measuring the amount
of the saturated flow at each of the different saturated flow
pressures.
6. The method of claim 2, wherein said first fluid is water and
said second fluid is a water insoluble substance which is gaseous
at atmospheric temperature and pressure, and said saturating means
saturates said second flow of said second fluid with water by
forcing gaseous second fluid through a confined water soaked
sponge.
7. The method of claim 1, wherein all of said second fluid is
supplied from a single source and said first and second flows are
produced by dividing the outflow of said single source into said
first and second flows.
8. The method of claim 7, and removing any of said first fluid
which may be in said second fluid before dividing said source
outflow into said first and second flows.
9. The method of claim 1, wherein said first fluid is water and
said second fluid is a water insoluble gas.
10. The method of claim 1, wherein said second fluid is nitrogen
and said first fluid is water.
11. The method of claim 1, wherein said second fluid is hydrogen
and said first fluid is water.
12. The method of claim 1, wherein said second fluid is air and
said first fluid is water.
13. The method of claim 1, wherein the mixed flow out of said
mixing chamber is supplied to a device for calibrating the ability
of said device to detect the concentration of said first fluid.
14. The method of claim 13, bypassing a portion of said mixed flow
away from said device, and determining the amount so bypassed in
order to supply a small enough volume of the mixed first and second
fluids suitable for used by said device and yet large enough to
permit efficient generation of known concentrations of said first
fluid in said second fluid, and wherein the amount of mixed flow
generated is determined by determining the amount of the mixed flow
taken by the device and the amount of the mixed flow bypassed and
by the addition of said last mentioned two amounts.
15. The method of claim 1, said pressure restrictor means
comprising a capillary tube formed of stainless steel, having a
length of about 9 feet, and an inside diameter of about 0.010
inch.
16. Apparatus for generating a mixture of fluids containing a known
concentration of a first fluid in a second fluid comprising a
mixing chamber, means for holding said mixing chamber at a first
known pressure, means to flow second fluid to saturating means
containing said first fluid to produce a stream of said second
fluid saturated with said first fluid out of said saturating means,
means for holding said saturating means at a known temperature and
a second known pressure, pressure restrictor means for reducing the
pressure of a stream flowing therethrough, means to flow a selected
stream of second fluid or said saturated stream to said mixing
chamber via said pressure restrictor means and to flow the other of
second fluid or said saturated stream directly to said mixing
chamber, wherein said temperature and said first and second
pressures are selected for said first and second fluids so that
said first and second fluids will mix in said mixing chamber in the
gaseous state, means for restricting the mixed flow out of said
mixing chamber to thereby control the amount of the mixed flow out
of said mixing chamber, and means for determining the amounts of
said selected stream and of said mixed flow to thereby determine
the concentration of said first fluid in said second fluid.
17. The apparatus of claim 16, wherein said selected stream is said
saturated stream.
18. The apparatus of claim 17, wherein said first fluid is water
and the concentration of water in said second fluid is determined
according to the relationship:
where P.sub.water is the vapor pressure of water at said
temperature of the saturating means; wet flow is the amount of the
flow out of the saturating means; P.sub.total is said second
pressure of said saturating means; and total flow is the amount of
the mixed flow leaving the mixing chamber.
19. The apparatus of claim 17, a source of said second fluid, said
means to flow second fluid to said mixing chamber and to said
saturating means comprising means to divide the outflow from said
source, wherein said first fluid is water and said second fluid is
a substance which is gaseous at atmospheric pressure and
temperature, said source comprising a pressurized container of said
second fluid, pressure regulator means to reduce said supply
pressure to said first known pressure which is less than the supply
pressure, and pressure regulator means to control the pressure of
said saturated stream within a range more than said first known
pressure and less than or equal to supply pressure.
20. The apparatus of claim 16, wherein the mixed flow out of said
mixing chamber is supplied to a device for calibrating the ability
of said device to detect the concentration of said first fluid,
means for bypassing a portion of said mixed flow away from said
device, and means for determining the amount so bypassed in order
to supply a small enough volume of the mixed first and second
fluids suitable for use by said device and yet large enough to
permit efficient generation of known concentrations of said first
fluid in said second fluid, and wherein the amount of mixed flow
generated is determined by determining the amount of the mixed flow
taken by the device and the amount of the mixed flow bypassed and
by the addition of said last mentioned two amounts.
21. The apparatus of claim 20, said means for determining the
amount of said mixed flow which is bypassed comprising a
rotometer.
22. The apparatus of claim 17, wherein the mixed flow out of said
mixing chamber is supplied to a device for calibrating the ability
of said device to detect the concentration of said first fluid,
means for diverting the mixed flow away from the device to be
calibrated including means to simulate the restricting effect of
said device, on/off valve means for shutting off the direct flow of
first fluid to said mixing chamber, and means for measuring the
saturated flow passing through said diverting means.
23. The apparatus of claim 22, said simulating means comprising
needle valve means.
24. The apparatus of claim 22, said measuring means comprising a
bubble tube.
25. The combination of claim 16, and means for changing the
pressure of said selected stream flowing to said pressure
restrictor means to thereby control the amount of said selected
stream flowing through said pressure restrictor means.
26. The apparatus of claim 16, said pressure restrictor means
comprising a capillary tube.
27. The apparatus of claim 16, a source of said second fluid, said
means to flow second fluid to said mixing chamber and to said
saturating means comprising means to divide the outflow from said
source.
28. The apparatus of claim 27, and means for removing any of said
first fluid which might be in said source outflow.
29. The apparatus of claim 28, said first fluid being water and
said removing means comprising a molecular sieve.
30. The apparatus of claim 28, said first fluid being water and
said removing means comprising a drier.
31. The apparatus of claim 16, said pressure restrictor means
comprising a capillary tube formed of stainless steel, having a
length of about 9 feet, and an inside diameter of about 0.010
inch.
32. The apparatus of claim 16, wherein said first fluid is water
and said second fluid is a water insoluble substance which is
gaseous at atmospheric temperature and pressure.
33. The apparatus of claim 32, said saturating means comprising a
saturator bomb having a water soaked sponge located mediately the
ends thereof, and means to flow said gaseous second fluid in one
end of said bomb, through said sponge, and out the other end of
said bomb to thereby produce said saturated stream.
34. The apparatus of claim 33, said means to hold said bomb at a
known temperature comprising a container of ice water in which said
bomb is located.
35. The apparatus of claim 33, said means to hold said bomb at a
known temperature comprising ambient air.
Description
This invention pertains to method and apparatus for producing a
stream of two gases containing a known concentration of one gas in
the second gas. More in particular, the invention discloses an
improved apparatus for generating a stream of a gas or other
substance containing precisely known quantities of another
substance, usually water, for use in calibrating other instruments,
such as moisture analyzers.
In its most general sense, the invention can be used anywhere it is
desired to produce a mixed fluid stream which consists of two
fluids and which contains known concentrations of one of the fluids
in the other fluid. One important element of the invention is that
some of the carrier fluid is caused to become saturated, at some
controlled temperature and pressure, with the second fluid. The
saturated and non-saturated fluids are mixed together while
gaseous. In the environment of the moisture analyzer described
below, the carrier fluid is usually nitrogen and the second fluid
is water. However, since the invention is not so limited, the term
"saturated" shall be understood to mean: "being the most
concentrated solution that can remain in the presence of an excess
of the dissolved substance". The word "solution" in the above
definition shall be understood to mean: "an act or the process by
which a solid, liquid, or gaseous substance is homogeneously mixed
with a liquid or sometimes a gas or a solid, or the mixture so
formed". These definitions were taken from a standard dictionary,
"Webster's Seventh New Collegiate Dictionary", published by G &
C Merriam Co., copyright 1965. However, as will be appreciated from
a reading of the detailed description below, the invention is most
practically used and achieves all of its versatile capability in
mixing two gases together. It will be understood that the
substances must be gaseous when mixing takes place, and that they
must mix rather than form a solution or otherwise combine, but that
other substances, such as water which is normally liquid, can be
used so long as they will gasify under the temperature and pressure
conditions imposed. The invention provides means to adjust the
temperature and pressure conditions in order to cause such
gasification.
More particularly, in regard to calibrating moisture analyzers, the
invention uses a stream of carrier gas which has been previously
dried and cleaned. The initial dry flow is divided into two
streams, one of which is kept in the dry state, and the second of
which is completely saturated with water. The two streams are
recombined, in varying proportions, to thus produce a flow
containing known concentrations of water in the carrier gas. In
other situations, generally, the carrier must be initially free of
the other substance.
The invention provides several advantages over the best moisture
calibrators heretofore available. The device of the invention is
portable, where as prior devices were fixed in one place. Another
advantage is that the invention is highly versatile in several if
its facets, that is, it can supply a wide range of total volume
flows, and it can supply, within its various total flows, a wide
range of moisture concentrations. Another advantage is that no
physical weighing is ever done. The various concentrations are
achieved in accordance with a very simple pressure, temperature and
volume relationship. In prior devices it was often necessary to
weigh some flow or substance somewhere in the system, and this
weighing introduced another source of inaccuracy and of human
error. The invention has versatility in still another aspect, that
is, moisture concentration in the carrier gas can be varied in
several different ways, and this feature cooperates with several
others in achieving the advantages of the invention.
In regard to the advantage of portability, the invention provides a
compact instrument and a separate water saturator bomb which is
removably attached to the instrument. The bomb is very simple in
construction and provides the advantage of a long useful life
between regenerations. Regeneration of the bomb is a simple
procedure. The user need only supply a constant temperature bath
for the bomb, which may be simply a bucket of ice water, or
atmospheric air, or the like, and a source of clean and dry carrier
gas, which may be simply a commercially available cylinder of
nitrogen, hydrogen, air, or other gas, and suitable dryers. Thus,
the invention provides an inexpensive, simple and highly reliable
instrument which may be brought to the various analyzers to be
serviced, thus assuring users of more reliable analyzer operation
while reducing the cost of calibrating.
The above and other advantages of the invention will be pointed out
or will become evident in the following detailed description and
claims, and in the accompanying drawing also forming a part of the
disclosure, in which:
FIG. 1 is a schematic drawing of an analyzer embodying the
invention;
FIG. 1a is a view similar to part of FIG. 1 showing a modified form
of the invention; and
FIGS. 2, 3 and 4 are front, left side, and right side views,
respectively, of an instrument embodying the invention which has
been built and successfully experimentally used.
Referring now in detail to the drawing, reference numeral 10
generally designates a preferred form of the apparatus of the
invention. The dotted line box in FIG. 1 indicates the main
instrument housing, but a saturator bomb 12, physically separate
from the main instrument housing, is also part of the invention.
Where a line crosses dotted line box 10 in FIG. 1, that is an
indication that some other outside connection may be made.
Accordingly, a fitting 14 on the instrument 10 is used to connect a
line 16 to any suitable user supplied source of dried carrier gas.
Most commonly the source will comprise a cylinder of nitrogen and a
suitable dryer such as a molecular sieve type or chemical absorbent
type, or the like. Line 16 includes a micron size filter 18 and a
pressure gauge 20. The filter is provided as insurance that no
particles enter the system. After gauge 20, line 16 branches into a
pair of lines 22 and 24. Line 24 includes a pressure regulator 26,
an on/off valve 28, and terminates at a mixing chamber 30. Line 22
includes an on/off valve 32 and ends at a fitting 34. Dry gas from
the user's source is delivered via fitting 34 and a line 36 to the
saturator bomb.
The invention requires means to saturate one fluid with the second
fluid, and means to hold the saturating means at some constant
temperature. In the moisture analyzer being described, the constant
temperature is usually 0.degree. C., or ambient room temperature,
because of the ease of obtaining and holding these two
temperatures. The bomb 12 shown in the drawing is the preferred
form of apparatus for achieving this end, but it will be understood
that other forms, such as a bubble chamber, a packed column, or the
like, could be used, Bomb 12 comprises a body 38 see FIG. 2, to
which is screwed a cap 40 in an air and water tight manner. Within
the body 38 there is provided a clean sponge 42, see FIG. 1 and
means such as springs 44, to hold the sponge located intermediate
the ends of the assembled bomb. The sponge may be of the ordinary
plastic variety such as is used in the home, and is formed with a
small hole through its center through which the line 36 is forced
for a tight fit. Line 36 delivers dry gas to the space in the bomb
to the right of the sponge. The dry gas is then driven through the
wet sponge, and a line 46 delivers the now water saturated carrier
gas to a fitting 48 on the instrument. The pressure drop across the
sponge is less than the inherent instrument error of the
components, and is therefore ignored. The bomb 12 is made ready for
use by simply removing the cap 40, filling the body 38, with the
sponge 42 located therein, with water, and then pouring out the
excess water and reassembling the cap and lines. For periodic
reconditioning, water can be simply run through the disconnected
lines 36 and 46. Calculations have shown that in typical usage the
amount of water in the sponge will be sufficient for 100's of
calibrations of typical moisture analyzers in the 0-100 PPM range.
To assure no failure at this point, it is a simple matter to
infrequently and routinely, once every few months for example,
refill the sponge with water. When the instrument is not being
used, to prevent evaporation, the lines 36 and 46 are preferably
capped by means of a protecting loop 50 on the fittings 34 and
48.
Internally, a line 52 extends from fitting 48 to, in sequence, an
on/off valve 54, a pressure regulator 56, a pressure gauge 58, a
capillary tube 60, a three-way atmospheric vent valve 62, and
terminates within mixing chamber 30. In accordance with good fluid
handling practice, the lines 24 and 52 are preferably arranged in
chamber 30 in the manner shown in the drawing to prevent vortexing
and other undesirable effects, and to achieve proper mixing. A vent
line 61 connects to the third leg of three-way valve 62 and
terminates at a fitting 63.
After mixing, the combined saturated and dry flows in the mixing
chamber 30 are carried away in and are delivered by a line 64,
which contains a pressure gauge 66, and ends at a three-way valve
68. The second leg of valve 68 is connected to a line 70 which
contains a needle valve 72 and terminates at a fitting 74. The
third leg of three-way valve 68 is connected by a branching line 76
to a pair of needle valves 78 and 80. A line 82 connects valve 78
to a fitting 84. A line 86, containing a rotometer 88, connects
valve 80 to a fitting 90. Suitable lines connect one or both of the
fittings 84 and 90 to the instrument or analyzer to be calibrated,
the selection depending upon the requirements of the particular
calibration as will appear in detail in the Operation section
below.
Referring to the other drawings, as an indication of the
compactness of the invention and not as a limitation, the main
apparatus 10 shown was built in a cabinet having approximate
external dimensions of 15 inches high, 9 inches wide and 7 inches
deep. A carrying handle 92 was provided for convenience. The bomb
12 was made of stainless steel tubing and measured about 11 inches
long, 2 inches inside diameter, and 2 1/8 inches outside diameter.
The instrument weighs about 28 pounds and the bomb about 6
pounds.
In regard to constructional details, the capillary 60 is critical.
For general use a capillary made of stainless steel having a length
of about 9 feet with an about 0.010 inch inside diameter has been
successfully used. Other lengths and/or capillary diameters may be
needed for other flows, and/or concentrations. In certain
applications it may be desirable to use glass for the capillary,
despite the disadvantage of possible breakage, because glass may be
needed when handling certain highly corrosive fluids. Virtually all
of the other conduits are one-eighth inch outside diameter grade
316 stainless steel tubing with a 0.012 inch wall thickness. The
valves, pressure regulators and gauges are all standard items for
the pressures used, with the exception, possibly, of needle valves
78 and 80 which must function as both needle valves and on/off
valves. For this reason it may be desirable to use both a separate
needle valve and a separate on/off valve in lieu of each of the
items 78 and 80 shown in FIG. 1. Also, needle valve 72 is
preferably of fine multi-turn construction because fine adjustment
is often required of it in use. Similarly, it may be desirable to
use a test meter in lieu of gauge 20 for improved accuracy.
The user may supply any suitable container C to house the bomb 12,
such as an ordinary bucket, and this container may be filled with
ice and water when a 0.degree. C. saturation temperature is to be
used. Under other circumstances, the bomb could be housed in any
other sort of a bath, including ambient air, to hold it at any
desired temperature at or below ambient temperature. If the
particular fluids being handled require high temperature, the
entire apparatus could be housed in a modified oven.
OPERATION
The theory of and various modes of operation of the invention tie
together. They can probably be best understood by means of the
following three equations. Logically, the final output water
concentration, parts of water per million parts of total flow, is
equal to the parts of water per million parts of saturated wet gas,
times the wet flow rate, divided by the total flow rate. The
conversion factor of 10.sup.6 is used to convert the parts per part
relationship to parts per million parts or PPM units. The above can
be expressed as Equation I as follows:
It is fact of nature that a given volume of a gas can hold a
certain amount of water at any given temperature, with only a
slight deviation at elevated pressures in accordance with the
Clausius-Clapeyron equation.
Thus, the wet gas water concentration term is equal to the partial
pressure of water divided by the pressure in the system and this
term in Equation I expands out as follows:
Combining Equations I and II the following results:
with the following definitions:
Conc. water is the concentration of water in the total diluted gas
flow and is in PPM. P.sub. water is the vapor pressure of water at
the temperature of the bath. Units given in PSI. P.sub. total is
the pressure at the water saturator as read by the gauge 20 after
the dryer. Units given in PSIA (absolute). Wet flow is the flow
going through the restrictor and is found using the technique
described below. Units given in milliliters/minute. Total flow is
the flow measured at the analyzer exit and at the by-pass exit.
10.sup.6 is the conversion factor for PPm units.
From Equation III it can be seen that the concentration of water
delivered by the invention to the device being serviced is
dependent on the four terms on the right hand side. The invention
provides means to vary each of these four parameters independently
of the other three, and it is this ability which is instrumental in
giving the invention its great versatility. The water vapor term
P.sub.water is changed by simply changing the bath temperature of
the bomb 12. The system pressure P.sub.total is changed by
adjustment within the user supplied gas supply system as read on
gauge 20. For example, all other factors being constant, a change
in bath temperature, the P.sub.water term, from 32.degree. F. to a
typical room temperature of 70.degree. F. will increase the water
concentration by about 300 percent. Similarly, a change in system
supply pressure, the P.sub.total term, from 100 PSIG to 10 PSIG,
which are two typical pressures used, will cause an increase in PPM
water concentration of about 500 percent.
Another way of obtaining changes in water concentration is by
making changes in the "total flow" term in Equation III. This is
accomplished by changing the flow in branching line 76. From FIG. 1
it can be seen that the flow out of the branches of line 76 are
controlled by the two needle valves 78 and 80. Changing one or both
of these needle valve settings will change total flow accordingly.
For a reason which will be explained in more detail below, changes
in total flow so obtained are usually made up entirely by dry gas
from line 24. Thus, increasing total flow will cause a decrease in
water concentration and, conversely, a decrease in total flow by
manipulation of valve 78 and/or 80 will cause an increase in water
concentration.
Another facet of the versatility of the invention is provided by
the structure above, i.e., the apparatus starting from line 76 and
to the right thereof on FIG. 1. This versatility concerns the
ability to bypass and vent varying portions of the combined wet and
dry flow in line 64. In use, fitting 84, called "flow 2", is
connected to the instrument being serviced and "flow 1" out of
fitting 90 is connected to a suitable vent. By manipulating the two
valves 78 and 80, any desired portion of the total flow from the
mixing chamber can be vented, and the amount so vented and not
taken by the instrument being serviced determined by the reading on
rotometer 88. Thus, the total flow term in Equation III is given a
numerical value by adding together the flow out of fitting 90 as
read on rotometer 88, and the flow out of fitting 84 as read by the
appropriate components on the instrument being serviced. This
versatility is important, for example, where the instrument being
serviced is small and/or requires an extremely small volume flow
rate. The calibrator of the invention can thus be operated at a
higher flow rate comfortably within its operating range, and any
desired fraction, including a very large portion, vented out of
fitting 90. Thus, the small flow used for calibration accurately
contains the desired water concentration.
Another adjustment of water concentration, the fourth mode of
operation, which is often used during the making of a calibration,
is provided by making changes in the wet flow term in Equation III,
i.e., the flow in line 52 from the capillary to the mixing
chamber.
This last mode of operation will be explained in conjunction with
an overall explanation of the manner of operation of all modes,
since there are several preliminary steps which are common. First,
the bomb 12, properly loaded with water, is allowed to stabilize at
the bath temperature. For example, this may be simply accomplished
by permitting the bomb to stabilize in an ice bath for several
hours or overnight. The wet and dry portions of the apparatus of
the invention must be conditioned prior to use. This is
accomplished by opening valves 28 and 32, adjusting regulators 26
and 56 to some desired relatively low pressure to efficiently
condition the system, and moving three-way valve 62 to divert the
wet flow to atmosphere. Three-way valve 68 is operated to direct
the dry flow through lines 76, 82 and 86 and out to both the
instrument and atmosphere to condition all lines and the
instrument. This procedure may be simultaneously allowed to
continue overnight prior to further calibration to assure that the
dry portion, including the instrument to be serviced, is dry and
that the wet system, between line 46 and valve 62, is purged of dry
gas. The zero water concentration calibration point is, of course,
automatically obtained after conditioning is complete and before
three-way vent valve 62 is operated to supply wet gas to the flow
feeding the instrument via fitting 84. The analyzer can be
conditioned together with the calibrator of the invention, and in
any case, the analyzer must be equilibrated before calibration
proceeds.
After conditioning is complete, some system pressure from the
user's supply system is selected and registered on gauge 20, thus
determining the "P.sub.total " term, and valves 78 and 80 adjusted
to some setting, thus determining the "Total Flow" term in Equation
III. Regulator 26 is operated to produce some operating pressure on
its downstream side and in mixing chamber 30. At the same time a
value for the "P.sub.water " term is determined since a bath
temperature was previously selected, see Equation II. After vent
line 61 is shut off by valve 62, and because all lines are large
with respect to capillary 60, the pressure in the entire system,
from the output side of capillary 60 through mixing chamber 30,
including the part of line 24 after regulator 26, and lines 64 and
76, is all controlled by the setting on pressure regulator 26. The
invention utilizes the fact that capillary tube 60 is a highly
stable, almost a fixed restrictor to flow passing therethrough.
However, the restriction provided by a capillary is dependent on
temperature, and therefore the calibrator should not be subjected
to temperature changes during use. After valve 62 is operated, some
mixture of dry and saturated gas is supplied to the instrument via
the mixing chamber. Pressure regulator 56 will be in some initial
position to supply some initial inlet pressure to the capillary 60,
causing some initial saturated flow to the mixing chamber, which in
turn causes some initial concentration of water to be supplied to
the analyzer being serviced. The operator notes the capillary inlet
pressure on gauge 58, and then manipulates regulator 56 to change
the capillary inlet pressure. Because the capillary 60 is a very
stable restrictor, the capillary outlet pressure will stay at the
pressure determined by the setting on pressure regulator 26. If
capillary inlet pressure were increased, then the pressure drop
across the capillary will have been increased, and the water
concentration will have increased at the expense of a decrease in
the dry flow in line 24 because total flow stays constant.
Conversely, if it is desired to decrease water concentration, the
capillary inlet pressure will have been reduced by operation of
regulator 56 thus causing the pressure drop across the capillary to
decrease, which in turn causes wet flow to decrease while dry flow
increases. In both cases, total flow remains unchanged and at the
value determined by valves 78 and 80. Because total flow does not
change any increase in wet flow is made up by a corresponding
decrease in dry flow, and vice versa.
The calibration procedure continues in this manner, changing
capillary inlet pressure using regulator 56, and recording the
corresponding pressure readings on gauge 58, and the corresponding
concentration readings on the instrument being serviced, until any
desired number of calibration points have been so obtained. Having
obtained sufficient calibration points, four points are usually
adequate for most moisture analyzers, the operator need now only
determine a numerical value for each wet flow used at each
calibration point. Since the other three terms in Equation III have
been constant therethrough, these numerical values for wet flow
will be inserted in the Equation, and a simple multiplication
performed to obtain the actual water concentration which was
supplied at each point. These calculated water concentrations are
then compared to the corresponding concentration readings
registered on the instrument, and suitable corrections made, i.e.,
a calibration curve supplied for use with the instrument, or
suitable adjustments made in the instrument. To determine the
values of the wet flows used, the operator connects a bubble tube
or other suitable flow measuring device to fitting 74, shuts off
the dry gas by closing valve 28, operates valve 68 to divert all
flow away from line 76 and into line 70, opens needle valve 72, and
adjusts valve 56 to produce a pressure on gauge 58 equivalent to
that pressure used in obtaining one calibration point. The system
is now changed to a configuration wherein the wet gas flow only
moves through the mixing chamber 30 and through line 64, and out to
atmosphere via valve 68, line 70, valve 72 and the measuring device
connected to fitting 74 wherein it is measured. Valve 72 is now
operated to produce the same pressure in the mixing chamber and
associated parts as was used in obtaining the calibration points.
Thus, it can be seen that the restriction provided by needle valve
72 in this step simulates the total restriction that was on the
system when the calibration points were taken, that is, the setting
on valve 72 during the measurement of the wet flows reproduces the
pressure on the outlet side of restrictor 60 that was present when
the calibration point was taken. This simulation step is readily
accomplished by simply observing gauge 66 and operating valve 72
until the same calibration pressure is shown on gauge 66. Now valve
56 is again operated so as to reproduce the series of calibration
point pressures on gauge 58 which were previously recorded. Valve
72 is used to hold the constant pressure at gauge 66. At each
restrictor inlet pressure the wet flow is measured by the device
attached to fitting 74 and recorded for use in Equation III to
calculate the actual concentrations that were supplied.
During all of these steps, the taking of the calibration points and
the reproduction of the calibration point wet flows, it may be
necessary to make very minor "trimming" type adjustments in the
settings of regulator 26, and valves 78 and 80 to hold pressures
constant, in order to accommodate for slight inaccuracies in the
components.
As explained above, it can be seen that at no time is it necessary
or even desirable to physically weigh any flow. The values of all
parameters are determined by reproduction, by flow measurement, and
by simple calculation, thus substituting an effectively zero
instrument error for an otherwise substantial human error in making
a calibration. At the same time, inexpensive, simple, and highly
reliable standard items are used as components in building the
apparatus of the invention, as opposed to many prior calibrators
which need much more sophisticated, expensive, and more highly
prone to error components, such as relatively large baths, and dual
stage compressors.
While the above exemplary explanation of the operation of the
invention concerns itself with varying the wet flow term in
Equation III, it will be appreciated by those skilled in the art
that any of the other three parameters may be varied, either during
a single calibration or from one calibration to another, and the
effect of such other changes duly recorded and then properly
reproduced when determining the values for the parameter which has
been varied after all the calibration points are taken. For
example, P.sub.water is changed by changing bath temperature,
P.sub.total is changed by adjustment within the user supplied gas
supply system, and Total Flow is changed by adjustment of needle
valves 78 and/or 80.
As can now be appreciated, the constant restriction provided by
capillary 60 is an important part of the invention. Different
diameter and/or length capillaries can be used in different
applications. However, the invention is not limited to use with a
capillary, a relatively constant pressure restrictor means is all
that is functionally required. A high quality needle valve, or a
packed column, or other means, could be used in lieu of the
capillary. Many other structural changes will be evident to those
skilled in the art. For example, a suitable packed column could be
substituted for both the saturator bomb and the capillary. Another
modification, which might be useful in other circumstances and/or
when dealing with other fluids, might be a separate source feeding
the "wet" and "dry" paths, as described above. With such
separation, different pressures could be used for each path,
thereby permitting greater flexibility as to final concentrations
produced. In such a form, another change might be to replace
regulator 56 with a pressure intensifier or pump or the like, to
thereby still further increase the pressure drop across the
pressure restricting means 60. In the application described, as
mentioned above, the pressure drop across the sponge 42 in the bomb
12 is small enough to be ignored. In other applications, at higher
pressures, with other saturator configurations, or for other
reasons, it may be desirable to relocate gauge 20 closer to the
saturator, perhaps directly on the bomb. The disadvantage of such a
change in the constructed form of the invention is that the bomb is
handled considerably more than the main instrument thus increasing
the possibility of damage to gauge 20. However, for calibrating
moisture analyzers, the preferred form of the invention is that
shown in the drawings.
Because moisture calibrators generally operate with relatively low
water concentrations, the ratio of dry to wet saturated flow into
the mixing chamber is usually high. This fortuitous circumstance
enhances the ability of pressure regulator 26 to hold the entire
system, between the capillary and the needle valves 78 and 80, at
the constant pressure at which it is set. It will be understood by
those skilled in the art that final water concentrations produced
and dry to wet flows produced will depend upon the physical
configurations of various parts, particularly capillary 60, and
that changing the size and shapes of such components is another
facet of the versatility of the invention. With the parts as shown
and as described herein, experiments have produced a 5,000 PPM
water in hydrogen concentration at a bath temperature of 22.degree.
C. With no dry gas flow, valve 28 closed, a concentration in the
range of 8,500 to 9,000 PPM can be obtained. If very high
concentrations are required the capillary 60 can be replaced by a
straight length of tubing and concentrations up to 20,000 PPM can
be obtained.
In the event a situation should arise wherein very high
concentrations, in the range of 15,000 to 20,000 PPM, are needed,
the parts of the invention could be rearranged as shown in FIG. 1a
to restrict the dry flow and permit unrestricted wet flow. This
would be accomplished by replacing capillary 60 in line 52 with
tubing, and placing a capillary 60a in line 24a between valve 28a
and the mixing chamber 30a. In FIG. 1a the parts are indicated by
the same reference numerals used in FIG. 1 followed by a. This
modification has not been tested because such high water
concentrations are not usually required in calibrating moisture
analyzers.
Tests have shown that the instruments built in successfully testing
the invention have capacities of total flow in the range of about
100 to about 1,500 ccs per minute. As an indication of other
physical parameters, which are recited here to enhance the teaching
and not as a limitation, the mixing chamber 30 was made of 316
stainless steel and had a capacity of about 20ccs, the saturator
pressure on gauge 20 may be about 10 to 100 PSIG, the pressure in
the mixing chamber has been tested in the range of about 5 PSIG to
10 PSIG, and amounts of total flow bypassed have been in the range
of about 100 to almost 1,500 ccs per minute. The invention has been
operated at 0.degree. C. and 20.degree. C., with the capillary
described above, and wet flows from 5 to 70 ccs per minute have
been used.
While the invention has been described in detail above, it is to be
understood that this detailed description is by way of example
only, and the protection granted is to be limited only within the
spirit of the invention and the scope of the following claims.
* * * * *