Note: Descriptions are shown in the official language in which they were submitted.
"MONITOR FOR DETERMINI ~ fi
PROPERTY OF A GAS OR VAPOR SAMPLE
BACKGROUND OF THE INVENTION
This invention relates to the determination of the
characteristics of substances in gaseou~ form, and more specifi-
cally to determination of heating value, or heat of combustion, density,
and humidity or moisture content of a gas or vaporized liquid.
HEATING YALUE
Fuels in liquid or gaseous farm are burned to produce heat
for a plethora of applications. These fuels may vary in composition
from primarily single carbon hydrocarbons to hydrocarbons having
many carbon atoms arranged in branched chain or ring structures or
may be mixtures of many hydrocarbons. Often a fuel contains
compounds which are inert with respect to normal combustion. It is
useful to know the heating value of a fuel, that is, the amount of
heat which a certain quantit~ of a fuel will produce when i~ is
burned under a certain set of conditions. While the heating values
of most pure substances caoable of being used as fue7s are readily
available in the literature, that a myriad number of mixtures of
compounds are used as fuels results in a continuing need for making
heating value determinations. Apparatus and methods for determining
; heating values are used both in laboratories and in industrial
operations outside the laboratory. It is often desirable to monitor
heating value of a flowing stream on a corltinuous basis. Following
are several exemplary applications for heating value m3nitoring.
Since the value of a fuel depends in large part upon the
amount of heat ~t is capabl~ of producing, it is more appropriate to
set fuel price in accordance with heating value and quantity~ rather
than quantity alone. Natural gas ia a prime example9 today it is
almost always sold on ~he basis of dollars per thousand ~TU instead
~s~
of the previously used basis of dollars per SCF. This is primarily
$he result of ~he price increases of recent years. Imprecision in
the number of BTU's transferred is now too expensive to tolerate.
Another factor necessitating transfer of custody on the basis of
heat quantity is that natural gas heating values tend to vary more,
as gases from different locations are pipelined around the country
and gas is imported.
A gas stream which is a by-product of operation of a
factory~ or plant, is often piped to a nearby plant to be burned as
fuel. As in the above natural gas example, the payment made for the
gas will probably be based on its heating value as well as the
quantity burned. Average heatin~ value may be determined by
periodic laboratory analysis or the heating value may be continu-
cusly measured as the gas enters the user's plant. Further, heating
values of the by-product gas must be determined, before its actual
use begins, for reasons other than pricing~ Design and control of
the burner, furnace, and other equipment involved in handling and
burning the gas depends in part on the range of heating values which
can be expected. Heatin~ value of a by-product gas would normally
vary over a fairly large range, compared to natural gases, and the
average heating value would be different from that ~f natural gases.
In certain manufacturing processes~ t2mperature and/or
furnace atmosphere ~ust be maintained in a relatively narrow range
in order to assure product quality. Changes in heating value of the
fuel supplied to the furnace may necessitate corrective action to
avoid an excursion from the acceptable range. An increase in
heating value of a fuel indicates that more oxygen is required to
combine with it. Where a ~urnace atmosphere is required to be rich
~sg~
in oxygen, an increase in rate of oxygen depletion in the ~urnace
caused by an increased heating value may create quality problems.
The solution is often to increase oxygen flow as soon as an increase
in heating value is detected by a heating value monitor and thereby
avoid significant depletion.
Fuel savings can be reali7ed by using a heating value
monitor in a combustîon zone control system. The amount of air
supplied to the combustion zone can be adjusted by reference to the
heating value monitor so that the excess air quantity is small, thus
saving fuel used for heating unneeded air and so that use of extra
fuel as a result of incomplete combustion is avoided.
There are many applications, such as mentioned above,
where an apparatus and method for determining heating value on an
instantaneous and cont,nuoùs basis is required. The most usual
method of determining heating value in a laboratory is by use of a
calorimeter in which the fuel is burned under precisely controlled
oonditions and rise in temperature of a water bath heated by the
burned fuel îS measureda While accurate, this method is time-
consuming and cannQt be adapted to provide a eontinuous read-out of
heating value for a continuous flow o~ sample to the calorimeter.
Also, as mentioned above, there are many applica$ions where a series
of laboratory determinations of heating value need to be made
quickly and not necessarily with the accura~y of a primary standard.
The instant ~nvention is expected to be significant in meeting these
applications~
For purposes of comparison to the present invention, a
continous reading on-line calorimeter device available from Fluid
Data, Inc. of Merrick, New York is described. In this instrument,
variations in heat produced by a test burner are of~set by
adjustment of air flow to the burner, in a null balance fashion,
and air flow rate is related to heating value. A sample of gas is
piped from a stream to be tested to the test burner and gas flow
rate is held constant. The flame heats a thermal expansion element
whose movement adjusts air flow to the burner through a mechanical
and pneumatic linkage. Air flow is independently measured by means
of an orifice meter and displayed on a scale which is marked in
terms of heating value.
Several recently issued patents show the interest in
methods for determining heating value. In U.S. Patent 4,337,654, a
fixed amount of gas is burned with a ~easured ~uantity of air and
hydrogen or oxygen supplied by an electrolytic cell. The amount of
hydrogen or oxygen added is controlled by an oxyg n sensor and
related to the heating value of the gas burned. U.S. Patent
4,355,533 describes a method of determining heating value where
information developed by use o~ a gas chromatograph is correlated
with heating value. U.S. Patent Nos. 4932g,873 and 4,329,874
describe another calorimeter in which gas is oxidized~
A recent article by Yan Rossum which points up the need
~or the present invention can b~ found in Oil and Gas Journal of
January 3, 1983 (p~ 71, Part 1) and January 10, 1983 ~p. 85, Part 2).
For background information on diFferent gases and liquids
used as fuels and on combustion, reference may be made to the Fuels
section of Perry's Chemical Engineers Handbook9 published by McGraw-
Hil1, and in particular to p~geS 9-1 to 9~33 of the fourth edition.
5~
DENSITY
It is important to know the density of a gas in many
industries, in particular, in the area of pe~roleum and petrochemical
processing. A typical application is a mass flow meter, where volu-
metric flow rate is combined with the density of the flowing stream
to produce mass flow rate. One seeking to measure density, particu-
larly on a continuous on-line basis, has a limited choice o~ apparatus.
- One commercially available density meter utilizes an oscillating
element in the fluid whose density is measured. Qscillation is caused
by an electromagnetic field~ The frequency of oscillation depends
on the density of the fluid. The sensin~ element is contained in a
housing havlng one-inch flanges for installation in a pipeline. A
standard reference, Process Instruments and Controls Handbook, 2nd
- ed., 1974~ edited by Considine, lists only three techniques for measur-
ing density, none o~ which are well suited for use outside the
lS laboratory. The listed methods ~p. 6 152~ are as f~llows~
In a gas specific gravity balance, a tall column of gas is
measured by a floating bottom fitted to the gas containment vessel.
A mechan~cal linkage displays movement of the bottom on a scale. A
buoyancy gas balance consists of a vessel containing a displacer
2Q mounted on a balance beam and with a manometer connected to it.
Displacer balance is established with the vessel filled with air and
then filled with gas, the pressure required to do so being noted
from the manometer in both cases. The pressure ratio is the density
:~L2~?S~
of the gas relative to air. In a viscous drag density instrument,
an air stream and a s~ream of the gas under test are passed through
separate identical chambers, each containing a rotating impeller.
The two streams are acted upon by the rotating impellers and in turn
each acts upon a non-rotating impeller mounted in the opposite end
of the chamber. The non-rotating impellers are coupled together by
alinkage and measure the relati~e drag shown by the tendency of the
impellers to rotate, which ~s a function of relative density.
HUMIDITY
This inYention also relates to determination of humldity, or
moisture content, of a gas or vaporized liquld. It ls pr~marily
useful for analyzing Qases where the moisture content is large and
there is a smdll difference between the molecular weight of water
and the average molecular weight of the other components of the gas
~rwhere there is a large difference between the molecular weight of
water and the average mNlecular we~ght of the other components.
There are a variety of methods for measur~ng water con
tent, each of whlch involves at least one significant dlsadvantage
which disqualif~es it for use in certain applications. Thus the
cholce of a method must be made in light of the appl~cation. A sur-
vey of methods and apparatus can be found in Process Instruments and
Controls Handbook~ edited by ConsidineD 2nd ed., MoGraw-Hill, 19749
p. 10-3 and following. The applications for which the instant inven-
tlon is suited will beco~e apparent upon reading this specification,
as will the gap in the area of humidity measurement which is filled
by the instant invention.
5~
STATEMENT OF ART
In an article in Oil and Gas Journal of April 5, 1982
entitled "Acoustic Measurement for Gas BTU Content", Watson and White
suggest a method and apparatus which utilize the dependence of sound
speed and BTU content on molecular weight and which utiliz2 some of
the same basic scientific principles as this inYention. LeRoy and
Gorland have explored the use of a fluidic oscillator as a molecular
weight sensor of gases and reported their work in an article entitled
"Molecular Weight Sensor" published in Instruments and Control Systems
of January, 1971, ~nd in National Aeronautics and Space Administration
Technical Memoranda TMX-527~0 (circa 1~70) and TMX-1939
(January 1970). In Fossil Ener~ C Briefs~ NOY~ 1g81, prepared
~or the U.S. Dept. of Energy by Jet Propulsion Laboratory of California
Institute of Technology, Sutton of The Garrett Corp., referred to
the use of a f1u1d~c oscillator to measure gas composltions, mass flow
and the heating value of natural gas.
In a paper ~ntitled "Thermal Energy Measurem~nts",
presented at the 55th International School of Hydrocarbon Measurement
in 1980 at the Universi~y of Oklahoma~ ~. A. Fox of Consolidated Gas
Supply Corp. of Clarksburg, West Virginia, suggests that specific
gravity methods may be used for determining heating values. The use
of a fluidic oscillator in measuring composition in a methanol-water
system is discussed in an article on page 407 of In 9. Chem.
Fundam., Vol. 11, No. 3, 1972. U.S. Patent No. 3,273~377 (Testerman)
shows the use of two fluidic oscillators in analyzing fluid streams.
A fluidic device for measuring the rativ by volume of two known gases
is disclosed in U.S. Patent No. 3~554,004 (Rauch et al~ In U.5.
Patent No. 4,1509561, Zupanick claims a method of Jetermining the
l~C5~6
constituent gas proportions of a gas mixture which u~ilizes a
fluidic oscillator.
In National Aeronautics and Space Administration Technical
Memorandum TMX~1269 (August 1966)~ Prokopius reports on the use of a
fluidic oscillator in a humidity sensor developed for studying a
hydrogen-oxygen fuel cell system. In NASA TMX-3068 (June 1974~,
Riddlebaugh describes investi~ations into the use of a fluidic
oscillator in measuring fuel-air ratios in hydrocarbon combustion
processes. NASA Report No. L0341 (April 16~ 1976), written by Roe
and Wright of McDonnell Douglas under Contract No. NAS 10-8764 at
the Kennedy Space Center, reports on work done to develop a fluidic
oscillator as a detector for hydrogen leaks from liguid hydrogen
transfer systems. U.S. Patent No. 3,~56,068 (Villarroel et al.)
deals with a device using two fluidic oscillators to determine the
lS percen~ concentration of a particular gas relatiYe to a carri2r gas.
Previously cited U.S. Patent Nos. ~337D654 (Austin et
al.), 4,329,873 (Maeda), 4,323,874 ~Maeda), and 4,355~533 (Muldoon),
disclose methods of determining heating value. The previously cited
artic1e in the Oil and Gas Journal (January ~ and 10, 1983~ presents a
survey of methods used in EuropeO
BRIEF SUMMARY OF THE INVENTION
It is an object of this invention to provide methods and
apparatus for determining unknown properties of gases and liqulds, which
are capable of use both in the laboratory and in the field. Also,
it is an object that such apparatus be relatively inexpensive9 have
a minimum of moving mechanical parts, and be compact9 so as to
faoilitate transportation and installation. It is a further object
-8
~ 2~S ~ 6
of this invention that such methods and apparatus have high accuracy
and reliability while providing resul~s essen~ially instantaneously.
In one of its broad embodiments, the invention comprises
(a) a fluidic oscillator; (b) means for establishing flow of the
sample through said oscillator (c3 means for measuring or control1-
ing the pressure at which the sample passes through said oscillator
and for providing a signal representative of the pressure when pressure is not
controlled in a preYiously established range; (d) means for measuring
the temperature of the sample at said osc;llator and for providing
a signal representative of the temperature; (e) means for measuring
the frequency of osc~llation at said oscillator and for providing a
signal representative of the frequency; (f) computing means for read-
ing said signals and for calculating the unknown property of the sample
using equatlons and data stored in said computing means and data supplied
by said means for providing a pressure signal when pressure is not con-
trolled in a previously estabtished range; and, (g) means for communicating
information contained in said computing means.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic diagram of a fluidic oscillator.
Figure 2 is a schematic diagram of an embodiment of the in~en-
tion comprising a heating value monitor wherein the heating value of yas
flo~.~ing in a pipeline is measured on a continuous basis and d;splayed
in a remote location.
Figure 3 is a schematic diagram of an embodiment of the inven-
tion comprising a density monitor wherein the density of gas flowing in
a pipeline is measured on a continuous basis and displayed in a remote
location.
~5~6
Figure 4 ls a schematic diagram o~ an embodiment of the
invention comprising a humidity monitor using two oscillators in
parallel wherein the moisture content of gas flowing in a pipeline
is measured on a continuous basis and displayed in a remote location.
Figure 5 is an expansion9 in block diagram form, of the
portions of Figures 2, 3 and 4 labelled electronics.
DETAILED DESCRIPTION OF THE INVENTION
A device known as a fluidic oscillator is used in this
invention. ~his is one nf a class of deYices which are utilized in
--10
5916
the field of fluidics. A fluidiç oscillator may have any of a
number o~ different configura~ions in addition to that depicted in
FIGURE 1. The publications men~ioned under the heading "Statement
of Art" describe fluidic oscillators and their governing principles
in detail and therefore it is unnecessary to present herein more
than the follow~ng simple description.
A fluidic oscillator may be described as a set of
passageways, in a solid b10ck of material, which are configured in
particular manner. If the passa~eways are centered in the block and
the block is cut ~n half in the appropriate place, a view of the cut
surface would appear as the schematic diagr~ of FIGURE 1.
Referring to FIGURE 1, a gas stream enters the inlet~ flows through
; nozzle 109, and "attaches" itself to one of two stre~m attachment
walls 105 and 106 in accordance with the principle known as the
Coanda effect. 6as flows through either exit passage lOJ or exit
passage 103, depending on whether the stre~m is attaehed to wall 105
or wall 106. Exit passages 1~7 and 108 can be cons~dcred as
extending to the outside of the block of materîal in a direction
perpendicular to the plane in ~hich ~he other passages lie.
Consider a gas stream which attaches to wall 105 and flows through
exit passage 107. A pressure pulse is produced that passes through
delay line 104. The pressure pulse ~mpinges on ~he gas s~ream a~
the outleg of nozzle 109, forcing it to "attach" to wall 106 and
fl3w through exlt passage 108. A putse passing through delay line
103 then causes the stream to switch back to wall 105. It is in
$his manner that an oscillation is e tabtished. The frequency of
the oscillation is a function of the pressure propagativn time
through the delay tine and time lag involYed in the s~ream switching
from one attachment wall to the other. For a delay line of given
length, the pressure propagation time is a ~unction of the
characteristics of the gas, as shown in the above mentioned
publications and also by the equations which are presented herein.
The frequency of oscillation can be sensed by a pressure sensor or
microphone located in one of the passages, such as shown by sensing
port 102. A differential sensing device connec~ed ~o both passages
can also be used. Sensing port 101 is shown to indicate one
potential location for a tempera~ure sensor.
The invention can be most easily described by initial r~ference
to F~gures 2, 3~ 4 and S, wh~ch represent particular embod~ments
of the invention. R~ference will also be made to a particular proto-
type heating value monitor which was fabrioated and tested. Referring
to Fi3ures 2, 3 and 4, gas is flowing through pipeline 50. A sample flow loop
51 is formed by means of conduit, surh ~s 3/4-inch diameter pipe,
connected to pipeline 50 upstream and downstream of pressure drop
element 53. The purpose of pressure drop element 53 is`to cause a
loss of pressure in pipeline 50 which is the same as the pressure
drop in ~low loop S1 when a sufficient 2mount of gas is passing
thrcugh flo~ loop 51~ 6as flow through flow loop ~1 is suffioient
when gas composition at sample point 54 is sub~tanti211y the same as
that in pipeline 50 at any yiven instank. Normally pressure drop
el~ent 53 is ~ device present ~n the pipeline for a primary purpose
unrelated to taking a s~mplez ~or example9 a control valve. A
suff~cient length ~f pipeline ~0 can serve as pr~ssure drop element
53 or an orifice plate oan be installed in pipeline 50 to serve the
purpose. Valves 52 are used to isolate flow loop 51 from pip~line
~0.
~Z~59~6
With regard to Figure 3 only, pressure and temperature of the
gas flowingin pipeline 50 are provided by pressure ~ransm;tter 75 and
temperature transmitter 76. These are located close to pipeline 50,
so that differences in pressure and temperature between their loca-
tions and pipeline 50 are not significant. Pipeline 50 is covered
with thermal insulation of a type commonly used on pipelines. The loca-
tion shown in Fi~ure 3 has the advantage of allowir.g the density moni-
tor to be a self~contained package. However, If the pressure and tempera-
ture differences are significant, transmi~ters 75 and 76 can be located
directly on pipeline 50. The measured pressure and temperature are
referred to hereln as Tl and Pl.
Figure 4 represents one alternative embodiment of the present
invention wherein it is desired to determine the moisture oontent of a
~as sample. Flow of a gas sample is provided in parallel through first
fluidic oscillator 56 and second fluidic oscillator 78 with means for
adjusting the water content of a portion of the sample before it passes
through the seoond oscillator 78.
In all three embodiments as depicted in Figures 2, 3 and 4,
sample lines 5~ carry samples of gas from sample points 54 to fluidic
oscillators 5S. Sample line 77 branches off to supply a sample ofgas
to fluidic oscillator 78 in the embodi~ent of Fi~ure 4. Filters 57
are provided to remove particles which might be present in the sample,
so ~at the narrow passages of fluidic oscillators 56 and 78 or other flow
paths will not become plugged. Pressure regulators 58~ of the self-con-
2~ tained type with an in~egral gauge, are prov~ded so that gas flvwing
through oscillators 56 and that flowing through osci11ator 78 is at a
substantially constant pressure. The frequency of oscillation at the
osoillators may vary w;th pressure, depending on the particular
oscillators used and the actual pressure at the osci11ators. As
will be seen, frequencies are correlated with humidity
~oisture content and density9 so var~ation
-13-
~L2~S~6
for any other rcason is unacceptable. Any pressure regulating means
capable of maintaining flnw through the oscillators at a substantially
constant value may be used. Under certain circumstancesl sufficient
pressure regulation will exist by v;rtue of system configuration and
pressure level, so that no separate pressure regulation device is
needed.
Orifices 60 are provided for the purpose, in conjunction
with pressure regulators 58, of maintaining a constant flow of gas
through each oscillator. Pressure gauges 59 indicate the pressures
lC downstream of orifices 60. Normally it is not necessary to installorifices 60, as the sample lines or the inlet ports of the oscillators serve
the same purpose. Conduits 71 (Figures 2, 3, 4) and 79 (Figure 4) carry the
samples away from oscillators 56 (Figu~e 2, 3, 4) and 78 (Figure 4), to the
atmosphere in a location where discharge of the gas will cause no harm ar to a
process vessel where it can be utilized. However, the quantity of gas is suffi-
ciently small that it may not be economical to do more than discharge it to
the atmosphere. Pressure transmitters 61 are switch devices which
provide signals for actuation of alarms if the pressures do not remain
in previously established ranges. Thus oommunication that inaccurate results
may be obtalned is accomplished. With reference to Figure 4 onlyg ~ryer 80
is provided to remove substantially all water from th~ gas which passes through
oscillator 78. There are many commercially available devices to
accomplish this. A typical device contains two beds of a desiccant
material so that gas to be desiccated passes through one bed while
?5 the other bed is being regenerated by applied heat.
Obtaining a representative sample stream fram a pipeline,
providing it to the inlet port of a fluidic oscillator, removing it
~rom the outlet port of the oscillator, and maintaining a substan-
tially constant pressure drop across the ~scillator can be accom-
plished by a variety of different means and methods for each given
~ 6
set of conditions, such as desired flow rate through the oscillator
and pipeline pressure. These means and methods, which can be applied
as alternatives to those shown in Figures 2, 3 and 4, are well known
to those skilled in the art.
A fluidic oscillator can be designed and fabricated upon
reference to the literature, such as that mentioned under the heading
"Statement of Art" or may be purchased. In test work applicable to
this invention, an oscillator supplied by &arrett Pneumatic Systems
Division af Phoenix, Arizona was used. This oscillator is of a
di~ferent configuration than that shown in FIGURE 1 in that the
"loops" formed by delay lines 10~ and 104 are open such that the
"loops" define cav;ties and in that there is only one exit passage.
Drawings of this configuration can be found in the cited references.
The flow rate through ~his oscillator when testing natural gas is
approximately 250 em3/min when upstream pressure is approximately 20
psig and the oscillator is vented directly to atmosphere. A f10w
rate range of 200 to 500 cm3~min is considered to be reasonable for
cummercial use and sufficient to provide aeceptable humidity results.
Te~perature transmitters 67 ~Figures 2, 3, 4) and 8l (Figure 4) pro-
vide the temperature of the gas at each oscillator. Any of the well known means
of sensing temperature may be used, such as a the~mister, thermocouple, or solidstate semiconductor sensor. The sensor may be located in a passage
of the oscillator, such as shown in FI~URE 1 ~sensing port 101), or
in the sample line or conduit ad3acent to the oscillator. Microphones 66
(Figures 2, 3, 4) and 82 (Figure 4) sense the frequency of oscillation at each
oscillator~ A microphone is located in a position to sense when the
gas stream a~taches itself to one o~ the walls, such as the position
shown in FIGURE 1 (sensing port 102). There are a wide variety of
~ 9~Lt~
sensors which can be used, ~or example, a piezoceramic transducer,
in which pressure induces a voltage change, or a piezo-resistance
transducer, in which pressure induoes a resistance change. Used in
test work applicable to this invention was a Series EA 1934 micro-
phone supplied by Knowles Electronics of Franklin Park, Ill.
Signals from miorophones 66 and 82, temperature transmit-
ters 67 and 81, and pressure transmitters 61 are processed by equip-
ment denoted field electronics 68 and control room electronics 69.
Field electronics are located adjacent to the oscillators while
control room electronics are in a central control room some distance
away from the oscillators. This equipment processes the signals to
obtain humidities of the gas and performs other functions wh~ch will
be described herein. Display unit 70 receives signals from control
room electronics 69 and communicates humidities of the sample gas
and other information in human-readable form. It may be, for example,
a liquid crystal ~isplay. The information may be communicated to
other equipment, such as a strip chart recorder for making a
permanent reccrd or a computer for further manipulation.
Two containers of oalibration gas, 64 and 653 are provided
to check that ~he monitor is operating properly. Normally one of
the calibration gases has prop~r~ies in the lower part of the range
of values expected of the gas flowing in pipeline 50 and one has p~operties
in the hlgher part o~ that range. The monitor ~s placed in the appropr~ate
cal~bration mode by means of one of lnput switches 18 ~Fl~ure 5). By mani-
pulating valves 63s 72 and 73, the calibration g~ses are allowed to flow, in turn,
,
through calibration conduit 62 and sample line 55 to oscillator 56.
The monitor may be arranged so that properties of the calibration
gases are displayed and a human technician must, if necessary,
adjust the monitor to the known calibra~ion gas property values, or
- `~
3~2 ~
may be arranged so that the monitor is capable of adjusting itself.
For example, the mon~tor could re-calculate the values of constants
stored in it which are used in calculating sample humidlties or densities
or heating values. Periodic calibratiorl must be accomplished to check
for malfunctions and changes which might take place in the apparatus
such as electronic drift, corrosion, and substances accumulating
in the apparatus.
Since the pressure and temperature of the calibration
gases will vaPy as conditions such as ambient temperature change,
the calibration gas densities calsula~ed by the monitor must be adjusted
to a pressure and t~mperature at which the calibration gas densities are
known. For example, if pressure transmitter 61 measures a pressure of
20 psig (140 kPa gauge) and temperature transmitter 67 measures a tempera-
ture of 30F (--1.1C~ when cal~bration gas from container 64 is flow~ng
and the density of container ~4 gas is known to be 0.0448 lb/f~3
~0.718 kg/m3) at 0C and 1 atmosphere ~101 kPa gauge~, the density
conmunicated by the monitor must be at 0C and 1.0 atmosphere (101 kPa).
If the commun~cated density ~s significan~ly different from 0.0448 ~
(0.718 kg/m3), the monitor is not operating properly. Adjustment of a
density value frDm one pressure and tempera~ure to another is easily
accomplished by means of the equat~on of state presented herein.
The monitor may be arranged so
-1 ~
~ -
that densities of the calibration gases are displayed and a human
technician must, if necessary, adjust the monitor to the known
calibration gas densities, or may be arranged so that the monitor
is capable of adjusting itself. For example, as was done in the
prototype device, the monitor could re-calculate the values of con-
stants stored in it which are used in calculat~ng sample densities.
The procedure just described does not accomplish calibration of
pressure transmitter 7~ and temperature transmltter 7S (see F~gure 3). These
items can be calibrated separately by standard means. If desired,
the calibration gases can be introduced into flow loop 51 upstream
of these items in order to include them in the calibration. It is
also possible to compare a value determined by the monitor to the
density of a calibration gas by manual means. Pressure~ temp2rature,
~nd density could be sommunicated by the monitor and an operator
1~ could refer to a standard chart or tables to compare the communicated
results to the actual dçnsity of the calibration gas. Another method
~s to provide apparatus in line 55 to adjust pressure and temperature
of calibration gas entering the oscillator to particular pre-
established values. However~ this me~hod would be used only in rare
circumstances, since it is less costly to manipulate numbers than to
manipulate the physic~l condition of the calibration gases.
-18- .
~2~59~ -
Partial calibrations, or operation checks, can be
accomplished in a number of different ways. Use of a calibration
gas can be combined with operation checks accomplished electronically.
A totally electronic operational check can be made. For example,means for
generating appropriate oscillating tones can be provided at mlcrophones
66 (Figures 2,3-4? and 82 (Figure 4) so that new values of Kl and K2 can be
calculated. Of course, this procedure checks only the electronics
and not the oscillator. In another simple check, tuning forks are
used to generate tones at microphones 66 and 82 and the synthetic
"value" resulting from the tone inputs is compared to the expected
proper value in computing means. Qperational checks can be performed by
switchingflow from one oscillator to ~he other in ~he embod~men~ of Figure 4.
Temperature changes can be used to perform operational checks. This can be doneby using heating means, such as electrical resistance coils, to heat
gas flowing into the oscillators and comparing v~lues of properties for
heated and unheated gas. If the gas used in the check is from a
changing process source, provision must be made to prevent changes
during the checking period. This can be accomplished by providing a
container to collect a sufficient quantity of gas to do th~ check or
recycling gas ~rom the outlet of the oscillators back through the system
6iven a particular objective to be accomplished, other checks will
become apparent.
19-
An assembly of electronics devices for processing signals
from the transmitters and microphones (variables) and providing signals
to the display unit can be fabricated from standard components by
one skilled in the art. FIGURE 5 shows one such design in simplified
~orm. Line 19 indicates which items are located in the field and
which are located in the control room. For ease of understanding, Figure 5
is drawn for the cases in which only one oscillator is used. It can
easily be seen that certain items would need to be duplicated so data
relat~ng to two osc~llators can be provided to the computing means.
lQ Though the following description mentions only oscillators 56 and associ-
ated items, operation of osc~llator 7~ (Figure 4~ and associated items
is the same as for oscillators 56. A signal from microphone 66 is
provided to amplifier 1, passed through filter 2~ and converted to a
square wave pulse 1n square wave shaper 3. The output of square
wave shaper 3 is provided to counter 6 by means of transmitter 4 and
receiver 5. Counter 6 counts the number of cycles occurring in
oscillator 5b in a unit of time, thus generating frequency information.
The signals from pressure transmitter 61 and temperature transmitter
67 are selected one at a time by analog switching device 7 and sent
2Q sequentially to analog-to-digital converter 89 where they are
converted to digital form. Serial input/output devioe 9 converts
the output of analog~to-digital converter 8 to a serial pulse train,
; which is provided by means of transmitter 10 and receiver 11 toserial input/output device 12, located in the control room.
Memory device 1~, a random access memory chip (RAM), is
used to store the variables. A progr~m ~or control G~ the
electronics devices and performing computations is st~red in memory
-20-
-
59~ E;
device 14, a programmable read-only memory chip (PROM). Constants
needed for the computation are storecl in memory device 16, an
electronically erasable programnable read-only memory chip (EEPROM).
Central processing unit 13 performs the necessary computations and provides
output signals to display unit 70 (Figures 2, 3,4). Input switches 18 are
used to provide human input to the electronic components. These are
rotary click-stop switches which can be set to any digit from O to
9. One of the switches is the mode switch and the others are used
to enter numerical values. The position of the mode switch
"instructs" the apparatus what to do. In the calculate mode, the
apparatus displays the humidity of a sample. When the mode switch
is placed in the "constant load" position, numeric~l values of
constants can be ~anually set on the other switches and loaded into
the system by depressing a button. Another position of the mode
switch allows values of variables to be displayed in sequence on
display 70. When it is desired to calibrate the apparatus, still
other positions are used~ Additional positions are used as required.
Parallel input/output device 17 provides a means of transmitting
inFormation from input switches 18 and also controlling counter 6.
It will be clear to one skilled in the art that certain of the
electronic devices may be collectively referred to as a computer or
computing means or may be contained within a computer or computing
means.
The basic equation used in the practice of this invention
which describes the operation of a fluidic osçillator is
KlGT
M = _ ~ K2 , where
F2
M = molecular weight of the gas flowing through oscillator,
G = specific heat ratio of the gas flowing through oscillator,
T = temperature of the gas flowing through oscillator,
F = frequency of oscillator output signal, and
K1 and K2 = constants.
The quantity G can be provided as a constant stored in
computer memory or can be calculated by means o~ a correlation, such
as the equation
6 = K3 ~ K4M + KsM2 ~ ~6M3 , where
K3, K4, Ks and K6 are constants.
The computer is programmed to solve these equations for
each oscillator, using values of F and T provided as described above,
and values of constants which exist in computer memory. lt can be
; readily seen that these molecular weights can be used to obtain the
moisture content of the sample by ~eans of the equations
Ms = Xw Mw + Xb Mb and Xw + Xb ~ l , where
X = weight fraction,
Xw = X of water present in the s~mple,
Xb = X of all components of the s~mple other than water9
Ms = M of the sample ~efore water content adjustment,
Mb - M of the sample components other than water ~average), and
Mw = M of water.
M~ is calculated by means of the bas k equation applied to data ~rom
oscillator 56 ~nd Mb is derived from data ~rom oscillator 78 in the
same manner. Thus there are two equations and two unknowns9 so Xw
ean be calculated in ~he computer.
-~2-
. The heating value of the gas can be calculated by use of
an equation such as
H ~ C1M ~ C2 , where
Cl and C2 are constants and H ~ heating value.
The computer is programmed to solve these equations to
obtain H, using values of F and T provided as described above, and
values of constants which exist in computer memory.
The density of the gas can be calculated by use of the
equation
D _ m = MPI , where
V ZRT
D = density,
m = mass,
V = volume,
Pl - pressure at the point of density measurement~
Tl = temperature at the point of density measurement,
~ = compressibil~ty factor, and
R ~ universal gas constant.
This equation is derived from the familiar equation of
state
m
PV - ZnRT = Z - RT 9 where
: M
n - number o~ moles. Z can be easily expressed by means of
equations which depend on M and data available in the l~terature, as
explained herein.
-23-
9~
The computer is programmed to solve these equations to
obtain D, using values o~ F, T, Tl, and Pl provided as described
: above, and values of constants which exist in computer memory.
The equation for G used in the prototype unit was
S developed by a standard curve-fitting method usiny values of G
available in the literature h r gases such as methane, ethane, etc.
As can be appreciated by those skilled in the art, there are
other ways to develop and express G and to store it in the computer.
The most appropriate method is dependent on the particular applica-
tion.
An approach to developing a basic oscillator equation on a
theoretical basls is as follows. Reference is made to Figure 1 as
-~4-
an exanple. A pressure pulse which passes through delay linP 103 or
104, described above, travels at the local speed of sound, u.
Denoting the length of each delay line as L, the time required for
the pulse to traverse a delay line is L/u. The time for a oomplete
oycle of oscillation includes that required for a pulse to travel
through each delay line. An equation for the local speed of sound
is
~ GgRT ~ l/2
u = _ ) , where
M
u = speed of snund,
g = gravitational constant, and
R = universal gas constant.
Thus the time required for the pulse to traverse the two delay lines
is 2 L/u or
f GgRT ~ l/2
2 L / ~ _ J
As explained above9 the total time for a cyc7~ of oscillation also
depends on switching time, the time required for switohing of the
stream from one attachment wall to another, or the period between
arrival o~ a pulse propagated through a delay llne at nazzle 109 and
the start of a pulse through the oth2r delay line. Switching time
oan be expressed as inversely propor~io~al ~o w, ~hat is as
-25-
~2~5i9~
~ GgRr ~ I/2
constant / ~ J
\ M
Since L is a constant for any given oscillator and the inverse of
time is frequency, t~e following equation can be wri~ten
/GgRT~1/2 ~GgRT~1/2
F = ~ - ) / cons~ant ~ ~ ~ / 2 L
Solving the equation ~or M and making 9~ L, and R a part of the
constant, the equation becomes
constant x 6T
M = 2
: : F
: If the above constant is designated as Kl, and K2 is added to the
right-hand side~ the basic equation presented above is obtairled~ It
: has been found necessary to ~dd the constant K2 to the equation in
order to accurately describe the oscillator. It is not possible to
use a purely theoretical equation, in part as a result o~ the imper-
fections o~ hardware and measuring equipment, For example, no two
fluidic oscillators will perform in an ident~cal manner. In ~ par-
ticular oscillator, which was used in a natural gas applic~tisn, K1
and K2 were empirirally established by flowing gases such as methane,
~thane, propane, butane, and pentane through the mon~tor. The values
of Kl ~nd K2 thus established were 7.53B x 10~ and 1.~8, respectively,
2~ This calibration procedure mus~ be followed for each ~onitor which
is ~abrica~ed, using gases similar to ~he gas for which the monitor
is ts be used. However, only two calibr~tion gases are required to
define K 1 ~nd K2.
_?6_
~ S 9 ~ ~i
The compressibility fac~or, Z, from the equation
of state to calculate density, is a measure of the deviation
of the sample gas from ideality and is added to ~he expression
commonly known as the ideal gas law in order to make the ideal
gas law applicable to real gases. Since compressibility factors
are covered by a vast quantity of literature wllich includes a
number of different methods of computing them, there is no
need to explain the basic theory herein. For further
information and references to the litera~ure,
.
27-
refer to Basic Prin~ples and Calculations in Chem~cal En~ineerinq,
2nd edition, 1967, Prentice-Hall, Inc.9 by Himmelblaul p. 149 and
following. Also useful are Chemical Process PrinciEles, 2nd edition,
1954~ John Wiley & Sons, by Hougen et al, p. ~7, and Perry's Chemical
Engineers' Handbook, 4th edition, McGraw-Hill, p. 4-49.
In the prototype device, Z is calculated by means of the
equation
Z~
Z
S~
where
S = (I ~ 3.444 x 105 P1 loO~062 M ) 1
T13.82
M between 16 and 21.75, or
S ~ 9.16 x 105 P1 ~0.041 M ~ 1/2
.
T13-82~ J
M between 21O76 and ?7,55, and
~B = 0.999287 ~ 9.25222 x 10-5 M - 1.06605 x 10-5 M2, where
Z~ = 7 at partieular base conditions,
S - supercompressibility factor,
P1 ~ psig, and
T1 ~ R.
The equations ~or S are empirically derived. These and the equation
for Z can be found in Princi~les_and Prac~ices of Flow Meter
-28-
~2~S91f~
Engineerin~, 9th edition9 1967, by Spink, published by Foxboro Co.
and Plimpton Pre~s of Norwood, Massachusetts. The expressi~n for ZB
was derived by ~eans of correlating values of Z~ for gases of
different molecular weights. This was done by converting values o~
base temperatures and pressures for various gases~ using critical
temperatures and pressures ubkained from the literature, to reduced
pressure and temperature and then using charts prepared by Nelson
and Obert to obtain Zg.
The equation for heating value presented above can be found
in Report No~ 5 of the Transmission Measurement Committee of the
American Gas Association (Arlington, Virginia, Catalog No. XQ 0776).
When H is expressed in BTU per standard cubic feet o~ gas, C1 =
54.257 and C2 = 144. If it is desired to express H in BTU per pound
of fuel gas9 Report No. 5 indicates that Cl and C2 assume different
values and l/M is substituted for M. Of course, it is possible to
use other correlations for calculating H of natural gas ~n the
practic~ ~f this invention. And a ~ifferent correlation is needed
for determining H of substances other than hydrocarbons having one
to approximately six carbon atoms. This correlation would likely be
developed by empirical methods.
It is possible to present information derived from the
practice of this invention in several different forms. For example,
H may be provided in metric units by appropriately programming the
computer or the Wobbe Index of the sample gas may be presentedO
Wobbe Index is a parameter used in ~he yas industry. One method ~f
expressing ~t is
Wobbe Index = kH / M1/2 , where
k - the square root of the ~olecular weight of air.
The samp1e gas may contain compounds which are non-combust-
ible. The concentrations and mo1ecular weights of these compounds
must be provided to the eomputer in order to produce an accurate
heating Yalue~ This may be done by means of an analyzer through
-29~
`;
~2~5~
which the sample gas ~s passed and which is arra~ged to automatical~y
provide appropriate signals to the computer, A variety of analyzer
apparatus is available for use, such as a gas chromatograph.
Alternatively, average values of concen~rations and molecular
S weights of the non-combustible components may be manually entered
into the computer. For example, natural gas often contains carbon
dioxide and nitrogen and their concentrations do not vary greatly
from hour to hour. It will often be satisfactory to analy~e for
these once a day and enter values by use of the input switches
mentioned above. The eguation ~or H must be modified to account for
these constituents which add to the volwme of gas buk not the
heating value. For example, if there are two ron-combustible
constituents whose concentrations are expressed by Yolume fractions
Xl and X2 and having molecular weights M1 and M2~ the equation
1~ presented above becomes
~1 (M - XlMl - X2M;~) -
Il s , _ + C? (1 - Xl - X2)
1 - Xl - X2 ~
The derivation of this and similar forms is easily accomplished by
algebraic manipulation.
In some appl ka~ions it ~ay be desirable to provide to the
computer concen~rations and ~olecular weights of combustible constitu
ents in the same manner as non~combustible const~tuents in order to
improve ~ccuracy. The equation used to calculate H can easily be
modified for these applications. An example is the measurement of
heating value of off-gas from a hydrogen-producing hydrogen recycle
process, such as catalytic reforming or dehydrsgenation~ For
-3~-
. _ ,
~a 2~ 9
background in this area, U.S. Patent No. 3,974,~64 (Baiek et al.)
may be consulted. ThP o~f-gas is often used in whole or part as a
fuel. It is comprised of both hydrogen and various hydrocarbon com-
pounds. Since the heating value of hydrogen is not accurately reprç-
sented by many correlations used for hydrocarbons~ it can be seen that
use of exact hydrogen concentrations and a correlation for hydrocarbons
yields greater accuracy than use of a correlation which accounts for both
hydrogen and hydrocarbons. Also, because hydrogen roncentration in hydro-
gen recycle processes is often meas~red for other purposes, the improve-
ment in accuracy may be available without purchase of another analyzer.
Use of a heating value monitor in control of a combustion
zone may be highly desirable or necessary to ~chi2ve acceptabl2
control. Consider a process in which temperature in a ~urnace must
be mnintained in a relatively narrow range. A typic~l control
arrangement is to ~easure furnace temperature and adjust fuel flow
to maintain it constant. When the amount of heat absorbed by the
process incre~ses, the temperature drops and more fuel is burned to
increase temperature to the proper Yalue. Also, changes in fuel
heating value will cause furnace temperature changes for wh~ch the
control system must compensate. Since the performance of 2 control
system degrades as the number of factors for which it must compensate
increases, it is desirable to eliminate fluctuations in temperature
resulting from ohanges in fuel heating value. This can be ~ccomplished
by m~asuring fuel flow and heating value~ establishing a signal
representative of their produ~t, and adjusting fuel flow by reference
to this product. The product is representative of rate of heat flow
to the process. The rate of hea~ flow is adjusted ~ith reference to
process temperature. Expressing the system in terms of standard
-31-
~S91~
analog control apparatus9 a temperature controller receiYing a signal
representativè of furnace temperature would supply the set pointj in
cascade fashion, to a controller which receives a signal representa-
tive of the heat content of the fuel and adjusts the fuel flow control
valve.
A heating value ~onitor may be applied to improve fuel
economy. Consider a combustion ~one where fuel ~low is adjusted to
maintain a constant zone temperature. Combustion air flo~ rate is
normally established by measuring fuel flow and combining a signal
representatiYe of fuel flow with a previously established ratio value
to obtain a signal used ~o adjust air rate. This control method is
incapable of responding to changes in fuel hea~ing valu , so normal
practice is to set up the system so that excess air is supplied to
the combustion zone. Fxces5 air is that quantity o~ ~ir which is
not needed to combine with the fuel~ It is desirable to keep excess
air at a minimum as the amount of fuel used to heat it represents a
total loss. As the fuel heating value increases, more combustion
a~r is required. If insufficient combustion air is supplied~ fuel
i~ wasted as a result of incomplete combustion. A signal representa-
tive of fuel gas heating value can be used to ad~ust air ~low rate9usually by means of adjusting the ra~io value, SQ tha~ the excess
~ir quantity is small, thus saving fuel for heat~ng unnPeded air and
avoiding use of extra fuel.
In the simple examples above, reference is made to
objectives of close contrQl, or control in a narrow range~ and
control to improve fuel economy. Of course, these objectives are
not mutually exclusive. Control systenls can be designed to achieve
both objectives by adjusting both ~uel and air flows. Th~se systems
-32-
5~
may utilize standard analog control instrumentation or more
sophisticated apparatus, such as that incorporatin~ digital
computing devices. Further, there are other objec~ives, such as
mentioned herein~ which may need to be achieved in control of a
5 particular combustion ~one. While it is not possible to present
herein all of the variations in objeotives and methods of achieving
same, the usefulness of the present invention in doing so will be
seen by those skilled in the art upon consideration of particular
situations.
FIGURE 2 shows an embodiment of the invention where a
continuous flow of sample through the occillator is established in
order to obtain a continuous heating value for gas flowing in a
process pipeline. An embodiment of the invention for use in a
1aboratory would not require the flow loop shown in FIGURE 2.
Sample can be collected in an evacuated pressure-resistant container~
commonly called a sample bomb, which is then connected to sample
line 55. ~n applications where ~he heating values of liquids are to
be determined, a means for vaporizing the liqu~ds is required. This
can be accomplished; for example, by use of electric resistance heat-
ing elements surrounding a portion o~ conduit through which thesample passes. The term "gas" is frequently used hereinp it should
be understood to include vapors result~ng from fuels whioh are
lnitially in liquid form. For example~ it may be desired to deter-
mine the heating value of a sample of No. 2 fuel oil, whioh ~s liquid
at normal ambient temperatures.
,.
In a relatively simple embodiment of the invention, the
sample loop shown in Figure 3 omitted~ Sample is collected in an
evacuated pressure-resistant oontainer, which is then connected to
sample line 55, either upstream or downstream of filter 57. The
density communicated by the apparatus is that at the temperature and
the pressure measured by pressure transmitter 61 and temperature
transmitter 67. There is no need to divide the electronics into two
packages at two different locations. This embodiment might be used
in a laboratory. It might be desired to add to this embodiment the
feature that the apparatus is oapable of calculating a density value
for sample gas at pressures anJ temperatures different from those
measured by transmitters 61 and 67 and which are proYided to the
apparatus as follows. A tempPrature and a pressure can be manually
entered into the apparatus by means such as input switches 18 or
l~ they can be provided by apparatus which measures kemperature and
pressure at some point of interest and transmits appropriate signals
to the oomputing ~eans of the i m ention.
F~ure 3 shows a more complQx embodiment of the invention
where a continuous flow of sample through the oscillator (at
temperature T~ is established in order to obtain a con~inuous
density value for gas flowing ~n a process pipeline (at temperature
- -34- -
~ ~q~S 9
T1 and press~re P1). In thls embodiment, the apparatus is arranged
to provide a density representative of the sample gas at a point
upstream of the pressure controllinq means represented by item 58 of
Fiyure 3 further arranged so that the upstream point is
representative of the main stream from which the sample is taken.
As noted earlier~ a variation in the pressure at which gas
passes throu~h the oscillator may affeot the accuracy of ~he monitor.
This is true even though the pressure is a variable used in calculating
density; that is, a oalculated density value may be in~orrect i~ the
presswre value used in the calculation is correct but outside a
particular range. Thereforeg it is desirable to monitor the pressure
and communicate any departure ~r~m a previously established range.
This can be aocomplished by seYeral means, including adding a primary
sersor9 suoh as a pressure switoh, in ~he appropria~e location9 such as
line 55 nf Figure 3, or adding the appropriate means in the elec-
tronics portion of the apparatus to utilize the pressure signal
provided for use ~n the equation, suoh as the signal transmitted by
pressure transmitter 61 of Figure 3. This monitoring provision is
not depicted in Figure 3.
The presen~ invention may be embod1ed in apparatus ~or
determining the mass flow rate of gas in a pipeline~ This can be
donP by combinirg apparatus such as that shown in Figure 3 with
apparatus for measuring the volumetr~c flow rate of the gas in the
~ pipelire and multiplying density times volumetric flow rate in
apparatus such as the computing means of Fi5ure 3. Xf the apparatus
for measuriny volumetric flow rate comprlses a calibrated obstruction
to flow, such ~s an orifice plate, and means to measure the pressure
drop across the obstruotion, such as a differ2ntial pressure cell,
-3~-
~L2~59~
the pressure drop can be provided to the computing means for calcula-
tion of mass flow rate instead of calculating the volumetric rate
outside the computing means.
An alterna~ive to the use of dryer sn of Figure 4 is to use apparatus
to saturate the sample portion passing through oscillator 7&~ This
apparatus is readily available. For example, saturating apparatus
may comprise a small chamber into which a fine spray of water is
introduced through a nozzle. After gas pas~es through this saturating
chamber, it is passed through another çhamber for removal of any
water droplets which might exîst in the stream. The equations used
in practicing this embodiment of the invention are similar to those
presented above. An example is as follows. For the oscillator
through which sample is flowing before adjustment of water content
Ms ~ XwMw ~ ~bMb and Xw + X~ = 1.
For the oscillator through which saturated sample is flowing
Ma ~ XaWMw ~ XabMb and ~aw + Xab = 1.
Previously undefined terms are
Ma = M of sample after satura~ion,
Xaw = X of water in sample ~ter saturation,
Xab = X of all components of the sample other than water after
saturation.
It can be seen that there are five unknowns and only four equations~
so that it is necessary to know one morP quantity whPn practicing
this embodimen~ o~ the invention than when using drying apparatus as
described ~bove. However, ~his informatlon is of~en available.
-36- ~
~2~S9~l Ei
Equations ~or other cases can easily be written.
~ igure 4 shows an embodimen~ of the invention when a
oontinuous flow of sample through the oscillators is established in
order to obt~in a continuous humidity value for gas flowing in a
pipeline. An emb~diment of the invention for use in a laboratory
would not require the sample loop shown in Figure 4. Sample could
be collected ~n an evacuated pressure-resistant container, commonly
called a sample bomb, which is then connected to sample line 55. In
applications where the moisture csntents of liquids are to be deter-
mined, a means for vaporizing the liquids is required. This can beaccomplished, for example, by use of electric resistance heating
elements surrounding a portion o~ the conduit through Nhich the
sample passes. The term "gas" is frequently used herein; ~t should
be understood to include vapors resulting from substances which are
initially in liquid form.
In the parallel flow arr~ngement shown in Figure 4, the
sample is spllt into two portions and each portion is passed through
a different oscillator. The water content of one of the portions is
adjusted before passage thrcugh the oscillator and the humidity of
2n the sample is calculated by reference to di N erences in signals
obtained from the transmitters associated with ~ach oscillator. An
alternate flDw arrangement involves series flow, wherè the entire
sample is passed thrnugh one oscillator and then through another.
The means for moisture adjustment is located such that ~he ~ample
2~ passes thrsugh the first oscillat~r, has 1~s moisture content
~djusted9 and then passes through the second oscillator. This can
easily be Yisualized by altering F~gure 4-so that sample llne 77
connects to vent line 71 instead of sample line 55, thus the flow
37-
~L~ S ~L 6
sequence would be oscillatdr 56 to dryer 80 to oscillator 78. In
this embodiment of the invention, the moisture content o~ the sample
is calcul~ted in the same manner, that is, by reference to the
differences at each oscillator. However, it should ~e no~ed that
when a continuous flow of sample is provided, a rapidly changing
sample humidity could result in inaccuracies, since there is a time
lag between measurement of a "particle" of sample in the first
oscillator and measurement of the same moisture adjusted "particle"
in the second oscillator. Compensation for this time lag can easily
be a~complished in the electronics portion of a monitQr to remove
any inaccuracy. One of the methods of compensating inYolves simply
placing the same time lag in the s~gnal path associated with the
appropriate oscillator just before the signal differences are noted.
In another embodiment of the invention, only one oscillator
is used. Means for adjusting thc water content of the sample are
provided along with means for periodically bypassing the sample flow
around the water content adjustment means. For example, if a dryer
is used, the stream continuously passing through the oscillator
alternately co~tains water and does not contain water. This san be
: 20 easily visualized by altering Figure 4 to eliminate the sample line
branch for oscillator 56, placing a three-way va1ve in sample line
77 just ahead of dryer 80~ and placing a length of conduit between
the valve and sample line 77 just downs$ream o~ dryer B0; then the
three-way valve ~s periodically cycled to route sample flow "around"
dryer 80. The moisture content o~ the s~mple is calculated by
reference to differences in signals received from the transmitters
associated ~ith the oscillator for each oondition; that is, when
dried sample is flowing and when non-dried sample is flowingO The
-~8-
same time lag problem as noted above exists when the sample humidity
is rapidly changing. Compensation can be accomplished in the same
manner.
The use of the examples set forth herein are not intended
as a limitation on the broad scope of the invention as set forth in
the claims. It is also intended that further applications of the
principles of the invention as would normally occur ta one skilled
in the art to which the invention relates be included within the
claims. Mixtures of gases not including water can be analy2ed by
applications of the principles of this inven$ion. The term "gas"
is frequently used herein; it should be understood to include
vapors.
,
-39-
.
