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Acousto-microwave system for determining mass or leak of gas in a vessel and process for same

The invention is a method of determining the mass of a gas (such as argon or natural gas) contained within a large pressure vessel (“tank”) with a relative uncertainty of less than 0.2 % by combining measurements of the gas pressure, the frequencies of the microwave and acoustic resonances within the pressure vessel, and an accurate equation of state for the gas.  Part of the invention is accurately determining the volume of the tank using the frequencies of the microwave resonances within the tank.  This can be done even when the tank is filled with gas, provided that the dielectric constant of the gas is known.  Part of the invention uses measurements of the acoustic resonance frequencies of gas within the tank to determine the speed of sound and therefore the average temperature of the gas within the tank.  Because the acoustic resonance frequencies depend on known averages, the acoustic frequencies can be used to accurately calibrate large gas flow meters even when large temperature gradients are formed in the collection tank by flow work heating the gas.  If the speed of sound in the test gas is not known accurately enough from the literature, it can be measured using well-documented procedures at the time and place that it is needed using a small sample in a small thermostatic resonator.

The invention is also a method of determining the average temperature of a gas in a plenum upstream of a critical flow venturi that is used as a flow standard.  One or more of the resonance frequencies of the gas-filled plenum can be measured when there is no flow and the plenum is isothermal, that is the plenum is at a single, well-defined temperature.  Later, when flow that generates temperature gradients passes through the plenum, the changes of the resonance frequencies will, after correction for pressure changes, be determined by an average temperature of the gas in the plenum.  The frequencies of the different resonance modes will be determined by different average temperatures; therefore, a comparison of the changes for the different modes will provide information about the errors of the averaging process.

Current practice uses one of two possible methods for calibrating large gas flow meters: (1) calibrate many small meters, one at a time using a small standard, and then use the small meters in parallel to calibrate a larger gas flow meter or (2) use a large gas-collection tank to calibrate a large gas flow meter and use many temperature sensors within the tank collection tank to estimate the average temperature of the collected gas in the presence of temperature gradients induced by flow work and the tank’s environment.  Method (1) has two disadvantages: (1.1) it is time-consuming (and therefore expensive) to calibrate many small meters, and (1.2) uncertainties grow when many small meters are calibrated and then used in parallel.  Method (2) has at least three disadvantages: (2.1) many temperature sensors have to be periodically recalibrated, a time-consuming practice, (2.2), the temperature field is mapped at only discrete points; thus, it is hard to estimate its uncertainty and the uncertainty of the resulting calibration, and (2.3) the calibrations are time-consuming because the quantity of gas collected cannot be determined until the largest, time-dependent temperature gradients have been greatly reduced, either by natural or forced convection.  The invention can use a large collection tank to calibrate large flow meters without calibrating smaller meters; thus, it avoids disadvantages (1.1) and (1.2).  The invention uses the averaging property of acoustic resonance frequencies to determine the average temperature in a large collection tank, even in the presence of flow-work induced temperature gradients.  Thus, it avoids disadvantages (2.1), (2.2), and (2.3). 

Conventional practice for using critical flow venturis as flow standards is measure the pressure and temperature of the flowing gas in a plenum just upstream of the venture.  The temperature measurement uses one or more temperature sensors to determine the temperature at specific locations in or near the plenum.  The invention uses the change in acoustic resonance frequencies to determine the change of the average temperature.  This is advantageous when the average temperature is temperature of the gas entering the critical flow venturi.  In such situations, the invention can lead to more accurate calibrations. 

The figure below shows how this invention can be used to quickly detect leakages in 3 meter-diameter air locks in nuclear power plants.


The present invention provides a new way to achieve optical frequency division and stable operation of a microwave signal provided by an electrical oscillator having a frequency control input (sometimes called a VCO for voltage controlled oscillator). In the approach two optical signals, provided by lasers, are frequency stabilized so that the relative frequency of the lasers is as stable as possible. The laser signals are then phase modulated using a cascade of phase modulators that are driven by the VCO. In the optical spectrum, this creates a spectrum of sidebands on the initial laser frequencies. The phase-modulated signal can be spectrally broadened to further increase the side-band frequency spectrum. It is important that the frequency separation of the lasers be as large as possible to provide maximum stabilization of the VCO. However, this separation cannot exceed the range of sidebands generated by the phase modulation cascade and nonlinear broadening. This is necessary so that the two innermost sidebands can be close enough in frequency so as to be detected on a photodiode. The photodiode signal contains phase information on the VCO and is used to stabilize the VCO.

Michael Moldover, Keith Gillis, James Mehl
Patent Number: 
Technology Type(s): 
Precision Measurement
Internal Laboratory Ref #: 
Patent Issue Date: 
July 31, 2018
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