Apparatus and methods for performing acoustical measurements

Details Apparatus and methods for performing acoustical measurements Apparatus and methods for performing acoustical measurements

1999-11-19

2001-03-13

Noori, Max (Department: 2855)

Measuring and testing

Instrument proving or calibrating

Apparatus for measuring by use of vibration or apparatus for...

Reexamination Certificate

active

06199423

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to the field of acoustical measurements performed on gases, liquids, and solids. More particularly, in accordance with certain of its aspects, the invention relates to new and improved methods of, and apparatus for, acoustically testing a gas mixture to measure gas concentrations, volumetric flow ratios, and/or mass flow ratios. In accordance with other aspects, the invention relates to the determination and/or the control of the mass (or volume) flow rate of a gas of interest in a carrier gas, where the carrier gas has a known flow rate.
BACKGROUND OF THE INVENTION
A. Industrial Setting
Numerous industries employ processes which require accurate delivery of a binary gas mixture consisting of a gas of interest and a carrier gas. To achieve accurate delivery, these industries require precise measurements of the concentration of the gas of interest in the flowing gas mixture, where the gas of interest is typically of high purity and may be highly corrosive. Examples of these processes include: chemical vapor deposition, dopant diffusion in, for example, the semiconductor industry, etching, the operation of high efficiency hydrogen cooled generators, and the like.
Current practice in the above and other industries is to use mass flow controllers upstream of a bubbler (vaporizer) to predict (control) the concentration of the binary gas mixture which is generated in the bubbler. This approach suffers from insufficient accuracy due to, among other things: variations in the bubbler temperature; instability of the temperature and pressure of the binary gas mixture; possible leakages in the gas lines upstream and downstream of the bubbler; and concentration time delays between the mass flow controllers and the points of interest, especially at low flow rates.
In addition, the existing equations used to predict a bubbler's pick-up rate are inaccurate. The following is an example of such an equation, where G
A
is the mass pickup rate of gas A (the gas of interest), Q
B
is a flow rate of gas B (the carrier gas), P is the pressure of the binary gas mixture, P
VA
is the vapor pressure of gas A at the bubbler's operating temperature, M is the molecular weight of gas A, T is the temperature of the binary gas mixture, and R is the ideal gas constant:
G
A
Q
B
=
(
P
VA
P
-
P
VA
)
⁢
PM
RT
.
This equation can exhibit an inaccuracy as high as 20% when the bubbler's operating temperature is well below the boiling point of gas A, which is the typical operating condition used in practice.
At present there is a wide variety of concentration sensors on the market which use different measurement approaches, including acoustical (EPISON and MINISON devices sold by Thomas Swan of the United Kingdom), optical (IR-5 device sold by MKS Instruments, Inc., Andover, Mass.), thermal conductivity (Varian Model 3400 Gas Chromatograph, sold by Varian Vacuum Products, Lexington, Mass.), and mass spectroscopy. None of these approaches fulfill all the requirements for a binary gas measuring system, including robustness, maintenance free operation, and the ability to produce highly accurate and repeatable real time concentration measurements of high purity and/or highly corrosive gaseous media.
B. Concentration Sensors Which Employ Acoustical Energy
As discussed above, acoustical measurements have been used in the past in concentration sensors. At their heart, such devices involve a measurement of the speed of sound (or, more accurately, the speed of propagation of acoustical energy) in a medium, with variations in the measured speed being indicative of variations in the concentration of the chemical of interest.
There are two main approaches for measuring the speed of sound in a medium, namely, the phase approach and the pulse approach. The phase approach gives a precise measurement of the phase velocity of acoustical energy in the medium, either by means of a fixed-frequency, variable-path, cylindrical acoustic interferometer or by means of a variable-frequency, fixed-path, spherical acoustic resonator. In accordance with this approach, the speed of sound is assumed to be the same as the phase velocity, which, as discussed below, is not always the case.
The pulse approach provides a direct measurement of wavefront velocity (i.e., speed of sound) and can be implemented either in a shadow format with a separate transmitter and receiver, or an echo format with only one transducer which fulfills both the transmitting and receiving functions (see U.S. Pat. No. 5,325,703). The pulse approach as practiced in the prior art has generally been less accurate than the phase approach.
(1) Phase Approach
Phase methods employ a continuous acoustic wave. The principles of the propagation of sound in finite cylindrical and spherical resonators were worked out by Lord Rayleigh and set forth in his 1877 treatise “The Theory of Sound” (Dover Publ., New York, 1945). From a practical point of view, neither a spherical acoustic resonator nor a cylindrical acoustic interferometer can be used widely for several practical reasons.
For real gases with speeds of sound in the range of 100-1500 meters/second, a spherical resonator requires an efficient broad band acoustic transducer, which does not exist at present. State of the art principles of design of such transducers for liquids are known (see Desilets et al. “The Design of Efficient Broad-Band Piezoelectric Transducers”, IEEE Transactions on Sonics and Ultrasonics, Vol SU-25, No. 3, May 1978, 115-125 and Takeshi Inoue et al. “Design of Ultrasonic Transducers with Multiple Acoustic Matching Layers for Medical Application”, IEEE Transaction on Ultrasonics, Ferroelectrics and Frequency Control, Vol. UFFC-34, No. 1, January 1987, 8-16), but their capabilities are very weak for concentration measurements in gaseous media.
A cylindrical acoustic interferometer requires a precision mechanical system with a known distance between a transducer and a moving piston (see Potzick “On the Accuracy of Low Flow Rate Calibrations at the National Bureau of Standards”, ISA Transactions, Vol. 25, No. 2, 1986, 19-23). Such a device thus requires continuous maintenance.
In addition to these specific drawbacks, it is important to note that both phase methods use phase velocity as a measure of the speed of sound. Phase velocity and the speed of sound can be different at different frequencies for different gases due to relaxation processes (see Gooberman “Ultrasonics”, Hart Publishing Company, Inc., NYC, 1969) and/or shock waves (see Elmore et al. “Physics of Waves”, Dover Publications, Inc., NY, 1969). These effects are capable of producing incorrect values for the speed of sound, and, in turn, incorrect concentration measurements.
(2) Pulse Approach
Pulse acoustical concentration measurement methods and instruments for testing binary gases are known. Publications in this area include:
(a) E. Polturak, S. Garrett and S. Lipson, “Precision acoustic gas analyzer for binary mixtures”, Rev. Sci. Instrum 57 (11), American Institute of Physics, November 1986, 2837-2841.
(b) G. Cadet, J. Valdes and J. Mitchell, “Ultrasonic time-of-flight method for on-line quantitation of semiconductor gases”, Ultrasonics Symposium, Proceedings, New York, Institute of Electrical and Electronic Engineers, 1991, 295-300.
(c) G. Hallewell and L. Lynnworth, “A simplified formula for the analysis of binary gas containing a low concentration of a heavy vapor in a lighter carrier”, Ultrasonics Symposium, Proceedings, New York, Institute of Electrical and Electronic Engineers, 1994, 1311-1316.
(d) J. P. Stagg, “Reagent Concentration Measurements in Metal Organic Vapour Phase Epitaxy (MOVPE) Using an Ultrasonic Cell”, Chemtronics, Vol. 3, March 1988, Harlow, Essex, UK, 44-49.
(e) G. Hallewell, G. Crawford, D. McShurley, G. Oxoby and R. Reif, “A Sonar-Based Technique for the Ratiometric Determination of Binary Gas Mixtures”, Nuclear Instruments and Methods in Physics Research, A264, 1988, North-Holland, Amsterdam, 219-234.
(f) M. Joos, H. Muller and G. Lindner, “An ultr

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