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Description  |
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BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to instruments and methods for determining properties of a gas and, more particularly, to a transducer apparatus and a related method for substantially simultaneously determining pressure and one other property (or
property combination) of a flowing gas of varying pressure and composition.
2. Description of the Prior Art
The heating value of a gaseous substance is of significant interest because it forms one basis for determining the commercial value of that substance as a fuel. Techniques for measuring the quality of gaseous fuels to ascertain the amount of
heat available therefrom are already being used in practice for numerous purposes. One particularly novel technique for determining the heating value of a gaseous fuel is described in co-filed U.S. Patent Application entitled "On-Line Combustionless
Measurement and/or Regulation of Gaseous Fuels Fed to Gas Consumption Devices," Ser. No. 07/781,598. In addition to determining heating value of a gaseous fuel based upon parameters such as gas density, thermal conductivity, specific heat, molecular
weight, viscosity, etc., proper heating value determination normally requires contemporaneous pressure and temperature sensing for correction of the calculated value to standard conditions. Most, if not all, known techniques for determining such gas
parameters as pressure and temperature separately measure each desired property. Further, existing sensing devices are often expensive and complex, requiring extensive electronic support equipment and thus warranting only limited use in complex systems
where cost is less critical. Fuel gas quality measurement is further complicated by the fact that combustion gases, and particularly natural gases, are typically distributed together notwithstanding separate origin, composition and properties that
differ to a greater or lesser extent from each other.
As an example of the most relevant art, it has been known for some time that a vibratory element, such as a quartz crystal, when exposed to a gas will change its frequency of vibration as the gas pressure changes (see, for example, U.S. Pat.
No. 4,644,803 and references cited therein). In addition, references exist in the open literature for determining gas density through frequency shift of an oscillator exposed to a test gas (see, e.g., U.S. Pat. No. 4,734,609). However, along with
being inoperable in a changing gas pressure and/or changing gas composition environment, most or all of these devices determine only one gas property such as density or pressure.
Thus, a novel, noncomplex approach to the simultaneous measurement of multiple fuel gas properties has clear advantages over the known art, particularly when implemented in a heating value measurement device such as that described in the
referenced co-pending application.
SUMMARY OF THE INVENTION
Accordingly, a primary object of the invention is to provide a transducer apparatus and method for determining multiple properties of a gas of varying pressure and composition.
Another object of the present invention is to provide such an apparatus and method which are capable of determining multiple gas properties within the same measurement cycle.
Yet another object of the present invention is to provide such an apparatus and method which are less complex and costly to implement than presently available instruments for determining gas properties.
But another object of the present invention is to provide such an apparatus and method which can be readily incorporated into a heat content measuring apparatus and method.
A further object of the present invention is to provide such an apparatus and method which can be implemented in an on-line manner.
A still further object of the present invention is to provide such an apparatus and method which are capable of determining gas pressure and the property combination (molecular weight.times.viscosity) substantially simultaneously.
These and other objects of the present invention are accomplished in one aspect by a transducer apparatus which determines pressure and at least one other gas property or property combination of a test gas of varying pressure, density and
viscosity. The apparatus includes a reference vibrator sealed within a chamber having a fixed gas pressure and density, and a detector vibrator exposed to the test gas surrounding the transducer. The frequencies of the reference and detector
oscillators are compared by a first means which produces an output signal proportional to the difference in the frequencies of the oscillators. The series resistances of the reference and detector vibrators are compared by a second means which similarly
produces an output signal proportional to the difference in the series resistances of the vibrators. Lastly, the transducer apparatus includes computational means for deriving signals representive of test gas pressure and one other gas property based
upon the proportional differential frequency signal and the proportional differential series resistance signal obtained from the reference and detector vibrators. In a specific embodiment, the test gas comprises natural gas and the apparatus
simultaneously determines pressure and at least one other gas property, which may consist of the property combination (molecular weight.times.viscosity).
In another aspect, the present invention comprises a related method for determining two properties of a test gas having varying pressure and composition. The method includes the steps of: providing a reference vibrator sealed within a chamber
having a fixed gas pressure and density; providing a detector vibrator exposed to the test gas; causing the reference vibrator and the detector vibrator to vibrate at a resonant frequency, the frequency of the detector oscillator varying with variations
in test gas pressure and composition; providing signals corresponding to the frequencies of oscillation of the reference and detector oscillators; comparing the frequencies of the corresponding signals and producing an output signal proportional to the
difference in said frequencies; determining the series resistance of the reference vibrator and the series resistance of the detector vibrator; comparing the series resistances of the reference and detector vibrators and producing an output signal
proportional to the difference in the series resistances; and deriving signals representative of two gas properties based upon the proportional differential frequency signal and the proportional differential series resistance signal produced from the
reference and detector vibrators.
A further feature of the method includes the substantially simultaneous comparison of the frequencies and series resistances of the vibrators to determine the two test gas properties within the same measurement cycle. Again, in one typical
implementation the test gas comprises natural gas and the two properties determined are pressure and the property combination (molecular weight.times.viscosity).
BRIEF DESCRIPTION OF THE DRAWINGS
The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of practice, together with
further objects and advantages thereof, may best be understood by reference to the following detailed description taken in connection with the accompanying drawings in which:
FIG. 1A is a schematic diagram of a tuning fork type crystal vibrator and a basic drive circuit;
FIG. 1B is a schematic diagram of an equivalent circuit for the tuning fork crystal of FIG. 1A;
FIG. 2 is a block diagram of a gas property transducer apparatus according to the present invention;
FIG. 3 is a schematic diagram of a preferred drive circuit for the detector and reference vibrators of FIG. 2;
FIG. 4 is a schematic diagram of an equivalent circuit for the frequency mixer of FIG. 2; and
FIG. 5 is a schematic diagram of the ratioing circuits of FIG. 2.
DETAILED DESCRIPTION OF THE INVENTION
As briefly noted above, the heating value of a gaseous substance is important to determination of the commercial value of that substance as a fuel. In industrial heating processes, it is frequently necessary to feed a well defined amount of heat
per unit of time to a furnace in order to obtain optimum results. In other cases, it is desirable to optimize the consumption of fuel, i.e., to feed only the amount of heat actually required by a process even if supplying a larger amount of heat does
not adversely affect the process or product. For accounting purposes, billing on the basis of the amount of heat supplied has also been preferred to billing on a volume basis.
A principal goal of the referenced co-filed U.S. patent application entitled "On-Line Combustionless Measurement and/or Regulation of Gaseous Fuels Fed to Gas Consumption Devices" comprises the production of a low cost, reliable and inexpensive
meter for determining heating value of gaseous fuel, and particularly natural gas. Such a low cost meter could be installed on-site with most industrial and commercial consumers, and possibly even residential consumers, to more accurately determine heat
content of the gas. Due to the complexities involved with available equipment, gas properties and heating value (e.g., BTU/ft.sup.3) are presently evaluated only at gas transfer stations.
In one preferred embodiment of the referenced copending application a heating value determination method and apparatus is described which utilizes the surprising discovery that heat content of a gaseous fuel can be readily and accurately
determined from an empirical expression, for example, of the form:
where:
.mu.=gas heat content;
a.sub.o, b.sub.1, c.sub.1, c.sub.2, d.sub.1 =constants;
o1, m1, m2, p1=exponents;
n=viscosity;
z=molecular weight, M, or density, of the fuel gas;
k.sub.t1 =thermal conductivity at a first temperature, t1;
k.sub.t2 =thermal conductivity at a second temperature, t2; and
c.sub.pt1 =specific heat at the first temperature, t1.
Further, as discussed in said co-filed application, where the fuel gas comprises natural gas the preferred algorithm for calculating heat content of the fuel gas reduces to:
where:
c.sub.p =specific heat of the fuel gas;
k=gas thermal conductivity; and
Mn=(molecular weight of the gas).times.(viscosity of the gas).
The term molecular weight multiplied by viscosity, Mn, or its alternate expression density multiplied by viscosity, .rho.n, is capable of being determined by those skilled in the art using a combination of available technologies. However, as
noted above, all of these technologies have associated drawbacks, for example: requiring trained personnel to operate, producing time delayed results, lacking repeatability, destroying the sample, being cumbersome or expensive to implement, being
incapable of implementation in an on-line manner, and lacking sufficient accuracy due to an inability to completely distinguish constituents. The present invention is designed to avoid these drawbacks by providing a more efficient, inexpensive, reliable
and accurate means than any known technique for determining properties of gases, such as gas pressure and the property combination (molecular weight.times.viscosity), and thereby facilitate the determination of gas heating value.
In a preferred embodiment, the present transducer apparatus and method use a tuning fork type quartz crystal vibrator. The properties of such a mechanically oscillating member depend in part on the viscosity and density of the ambient gas
surrounding the member. In particular, the adjacent mass of the ambient gas affects the total mass of the oscillating member, and thereby it s oscillation frequency. Gas density and the viscosity of the gas will affect the Q or equivalent series
resistance of the oscillating member. At resonance, its series resistance and frequency are found to be uniquely dependent on the pressure, density, and viscosity of several test gases, such as natural gas, methane, air and others. The apparatus and
method of the present invention make use of this relationship.
Referring now to FIG. 1A, a tuning fork quartz crystal 10 is schematically shown along with a basic drive circuit 12 configured to sustain crystal 10 in oscillation at the crystal resonant frequency. Any available oscillating means, including
noncrystalline members such as vibrating plates or membranes, could be substituted for the tuning fork type crystal 10. Ideally, however, a tuning fork oscillator is used because of its low cost, reliability, ready availability and relative
insensitivity to temperature variations. Further, although any one of longitudinal, transverse and shear modes of deformation are acceptable for coupling the mechanical oscillator to the test gas, results obtained with shear mode coupling appear
superior, particularly when viscosity is one of the gas properties desired. Also, as a result of extensive experimentation, a fundamental oscillation frequency of 160 kHz seems to provide superior coupling of energy between the tuning fork oscillator
and the surrounding natural gas, and therefore provides greater accuracy.
Basic drive circuitry 12 includes: an operational amplifier 14, configured with gain; an inductor 16, a capacitor 18 and a resistor 20, all designed to drive crystal 10 with a sine wave; and a load resistance 22 at the input to amplifier 14. The
output voltage E.sub.o from crystal 10 is provided as positive feedback to circuit 12 as shown. Voltage E.sub.o varies in value as a function of gas pressure and gas composition surrounding crystal 10.
FIG. 1B is a schematic diagram of an equivalent electrical circuit for tuning fork type crystal 10. The circuit includes an inductor 24 in series with a capacitance 26 and a resistance 28, all of which are shunted by a capacitance 30. The
motional resistance of the test gas to the motion of the tuning fork while oscillating is represented by series resistor 28, R.sub.s, which comprises a variable resistance. A small part of R.sub.s represents internal resistive losses within the quartz
material that the tuning fork is made of. Determination of series resistance R.sub.s is important to the present apparatus and method.
As described below, two substantially identical tuning fork crystals are used in implementing the preferred transducer apparatus. One crystal is directly exposed to the test gas ambient (i.e., the crystal in the detector oscillator) and the
other crystal is sealed in a fixed ambient reference chamber, which is exposed to the test gas ambient (i.e., the crystal in the reference oscillator). The reference oscillator is used to account for effects of temperature variations on detector
oscillator readings. The reference chamber is preferably substantially evacuated. The damping component, or series resistance of each tuning fork can be obtained by dividing the voltage across the tuning fork by the current through it at series
resonance. Lastly, each tuning fork will control the frequency of its respective oscillator circuit.
Applicant has discovered that with such oscillators a simple relation exists between crystal series resistance and oscillator frequency shift parameters on the one hand and absolute pressure, density or molecular weight, and viscosity on the
other. By applying linear progression analysis to experimental results, the following formulas are obtained:
where:
P=gas pressure;
Zn=gas (Z).multidot.(viscosity), where Z=density or molecular weight;
A, B=coefficients;
a, b, c and d=exponents;
g=bc-ad
R.sub.sr =series resistance of reference vibrator;
R.sub.s =series resistance of detector vibrator;
f.sub.r =frequency of reference oscillator;
f=frequency of detector oscillator.
Thus, from two simultaneous measurements of a pair of quartz crystals it is possible to determine both gas pressure and the property combination (molecular weight.times.viscosity) or the property combination (density.times.viscosity). Series
resonance resistance R.sub.s of the detector crystal and series resonance resistance R.sub.sr of the reference crystal can be determined from equations of the form:
where:
R.sub.l =crystal load resistance of oscillator circuit;
E.sub.o =output voltage of detector crystal;
E.sub.i =oscillator input voltage to detector crystal;
R.sub.lr =crystal load resistance of reference oscillator;
E.sub.or =output voltage of reference crystal;
E.sub.ir =oscillator input voltage to reference crystal.
If the test gas under evaluation comprises natural gas and the crystals used in the transducer apparatus have series resonant frequencies of approximately 160 kHz, then specific values for the coefficients and exponents of equations (3) and (4)
are:
A=0.0038987
B=-1.1992.times.10.sup.-6
a=0.64677
b=1.9498
c=1
d=0.8794
g=1.38103.
Once determined, the property combination Mn can be used in a heating value algorithm such as equation (2) to calculate heat content of the fuel gas, while the pressure of the gas can be used as a conversion factor to translate the calculated
heat content to a corresponding value at standard pressure.
One preferred implementation of the present transducer apparatus is schematically depicted in FIG. 2. In this embodiment, a first tuning fork quartz crystal 10 (herein referred to as the detector crystal) is exposed to the test gas and a second,
identical tuning fork quartz crystal 10' (herein referred to as the reference crystal) is positioned within a sealed chamber 11. Sealed chamber 11 is itself exposed to the test gas. The crystals (and chamber 11) preferably reside in a sensor chamber
filled with the test gas, such as that described in the referenced co-pending application. Crystals 10 and 10' are sustained in oscillation by detector circuit 12 and reference circuit 12', respectively. The voltages from crystal 10, i.e., output
voltage E.sub.0 and input voltage E.sub.i are fed through a servo ratioing type of A/D converter, which converts the ac E.sub.0 and E.sub.i signals to digital signals and outputs the ratio E.sub.o /E.sub.i 40 to computer 44, (discussed further below with
reference to FIG. 5). Computer 44 uses the ratio E.sub.o /E.sub.i to calculate the series resistance R.sub.s of crystal 10, by means of equation (5). Similarly, the output and input voltages E.sub.or and E.sub.ir from reference crystal 10' are fed to
A/D converter 42 for conversion to digital format and determination of the ratio E.sub.or /E.sub.ir. Computer 44 uses the ratio E.sub.or /E.sub.ir to calculate the series resistance R.sub.sr of crystal 10' by means of equation (6).
Simultaneous with this signal processing, frequency signals are fed from detector oscillator circuit 12 and reference oscillator circuit 12' to a frequency mixer 46 (discussed below) which is configured to output the sum and difference
frequencies between oscillator circuits 12 and 12'. Alternatively, frequency signals from the oscillators could be fed, with subsequent appropriate conversion, directly to computer 44 for direct computer calculation of the difference in the oscillator
circuit frequencies (f.sub.r -f). From the output of mixer 46, the signals are fed through a low pass filter 48, which eliminates the unwanted summation frequency; thereafter, the difference in frequencies (f.sub.r -f) is fed to a divide by N operation
50. Operation 50 comprises an optional and arbitrary division of the frequency difference signal f.sub.r -f by a preselected number N to reduce the frequency of the difference signal and improve its compatibility with other system components. The
reduced difference signal is then fed to a counter 52 which determines, for example, the number of pulses from a clock 54 that occur within one cycle of the difference signal. A representative signal is output from counter 52 to computer 44 for
determination of the desired multiple gas properties, e.g., pursuant to equations (3) and (4).
FIG. 3 is a schematic diagram of a preferred embodiment of detector oscillator circuit 12 and reference oscillator circuit 12'. (Since the implementation is identical for both the detector and reference circuits, only the detector circuit 12 is
described in detail herein.) As shown, input voltage E.sub.i is fed to tuning fork type crystal 10. Output voltage E.sub.o from crystal 10 is fed back to the drive circuitry at the input of an amplifier 60, which comprises a cascode amplifier configured
with gain. In normal operation, amplifier 60 is overloaded such that an approximate square wave signal appears at its output. This square wave signal is fed through a first series resistor 62 to a pair of shunt diodes 63 and 64, which are configured as
an amplitude clamp to provide a constant amplitude for driving the crystal. A second series resistor 66 is disposed between the amplitude clamp and an LC resonant circuit comprised as an inductor 68 and a capacitor 70. The LC resonant circuit is tuned
to the same frequency as the oscillator and functions to convert the square wave signal from amplifier 60 to a sine wave signal. A sine wave is preferred for driving crystal 10 to facilitate accurate determination of the equivalent series resistance
R.sub.s pursuant to equation (5), i.e., since E.sub.o is a sine wave, preferably E.sub.i is also. Subsequent the LC resonant circuitry, a second amplifier 72, this one without gain, is used as an impedance conversion device. Amplifier 72 comprises an
emitter follower with a low output impedance and high input impedance. Similarly, an amplifier 74 is disposed at the output of crystal 10 for measuring output voltage E.sub.o across the load resistor R.sub.l without loading the resistor. Output voltage
E.sub.o and input voltage E.sub.i are separately fed to ratio E.sub.o /E.sub.i 40 circuitry (FIG. 2).
Since the frequencies of both input voltage E.sub.i and output voltage E.sub.o are the same, the frequencies fed to frequency mixer 46 (FIG. 2) can be derived from either voltage signal. An equivalent circuit representation for frequency mixer
46 is depicted in FIG. 4. This circuit comprises a balanced frequency mixer (such as those available in the open literature) which obtains, in part, a difference between the detector oscillator frequency f and the reference oscillator frequency f.sub.r. The detector oscillator frequency f is fed to the primary windings of a first transformer 80 which has a center tapped secondary winding. Because crystal 10 is exposed to the test gas, e.g., natural gas, the exact frequency of detector oscillator
circuit 12 will vary with the pressure and composition of the surrounding gas such that it will be slightly off from 160 kHz (i.e., the resonant frequency of the reference oscillator). The voltages at opposite sides of the secondary winding of
transformer 80 are 180.degree. out of phase. Switches 82, for example, field effect transistors, are closed and opened in synchronism with the phase of the reference oscillator frequency f.sub. r. Frequency f.sub.r is fed into the primary winding of a
second transformer 84 which also has a center tapped secondary winding. The effect of such a circuit is to multiply the two input frequencies together such that a sum and a difference signal are attained as outputs. Since the unwanted sum signal
comprises a much higher frequency than the difference signal, it is filtered out by a subsequent low pass filter 48 (FIG. 2).
One implementation for the ratio E.sub.o /E.sub.i 40 circuitry is depicted in FIG. 5. (Again ratio E.sub.o /E.sub.i 42 circuit would be identical.) This circuit comprises a servo ratioing type A/D converter, specific details of which are
available in the open literature. The circuit uses a very accurate resistance divider string in a D/A converter for measuring the actual signal ratio E.sub.o /E.sub.i with a closed loop servo continuously driving the ratio device to null. The ratio
output is digital by taking advantage of the digital drive in a D/A converter which is used backwards as an A/D readout. This approach to determining the ratio E.sub.o /E.sub.i is more accurate because it is independent of the relative oscillator signal
amplitudes, and is a bridge ratio concept involving the use of R.sub.s and R.sub.l as two bridge arms against the D/A resistor string (i.e. the other two bridge arms).
Briefly explained, a variable potentiometer or resistance string 90 receives input voltage E.sub.i. Potentiometer 90 includes a wiper 92 which traverses the resistor string to define a voltage proportional to input voltage E.sub.i. This
proportional voltage is fed to a differential amplifier 94 which compares the proportional signal to output voltage E.sub.o. The output of amplifier 94 is fed to a combined synchronous demodulator and low pass filter 96 which converts the sine wave
differential input signal to a dc voltage and hence to a comparator 98 which determines whether the resulting voltage signal is positive or negative. The output of comparator 98 is fed to a counter 100 which receives, for example, a 10 MHz clock input
signal for counting. The output of comparator 98 directs the counter to either increase or decrease its pulse count. The output of counter 100 is fed via line 101 back to wiper 92. The feedback circuitry continously operates to drive wiper 92 to null
whereupon the voltage signal taken from the resistance string is equal to output voltage E.sub.o. Once nulled, the resulting ratio E.sub.o /E.sub.i signal 101 is fed to computer 44 (FIG. 2) for use in equations (5) and (6).
Along with the transducer apparatus, the invention comprises the generalized method for determining multiple properties of a test gas having varying pressure and composition as set forth above. In particular, the method includes the steps of:
causing a reference vibrator and a detector vibrator to vibrate at a resonancy frequency, whereby the frequency of the detector vibrator (exposed to the test gas) varies with variations in the gas pressure and composition; deriving frequency signals
corresponding to the frequencies of oscillation of the reference vibrator and the detector vibrator; comparing the corresponding frequency signals and producing an output signal proportional to the difference in their frequencies; determining the series
resistance of the reference vibrator and the series resistance of the detector vibrators; comparing the series resistances of the two oscillators and producing a signal proportional to the difference in their series resistances; and deriving signals
representative of two gas properties based upon the proportional differential frequency signal and the proportional differential series resistance signal produced from the reference and detector vibrators. The two properties determined can comprise
pressure and the property combination (molecular weight.times.viscosity), which are determinable by equations (3) and (4). Preferably, the two comparing steps, i.e., comparing the frequencies and comparing the series resistances occur substantially
simultaneously such that the two properties of the test gas are derived in the same measurement cycle.
While the invention has been described in detail herein in accordance with certain preferred embodiments thereof, many modifications and changes therein may be affected by those skilled in the art. Accordingly, it is intended by the appended
claims to cover all such modifications and changes as fall within the true spirit and scope of the invention.
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