Apparatus and methods for performing acoustical measurements

• US 6,192,739 B1
• Filed: 11/19/1999
• Issued: 02/27/2001
• Est. Priority Date: 04/17/1998
• Status: Expired due to Term

• Current Assignee: Veeco Instruments Incorporated
• Original Assignee: Loren Industries Inc.
• Inventors: Lee, Patrick S., Sirota, Don N., Logue, Raymond C.
• Primary Examiner(s): NOORI, MAX H
• Application Number:
  US09/444,418
• Time in Patent Office: 466 Days
• Field of Search: 73/23.2, 73/23.22, 73/24.01, 73/24.05
• US Class Current: 73/24.01
• CPC Class Codes: G01N 2291/02809 Concentration of a compound, e.g. measured by a surface mass change
• CPC Class Codes: G01N 2291/102 one emitter, one receiver
• CPC Class Codes: G01N 29/024 by measuring propagation velocity or propagation time of acoustic waves
• CPC Class Codes: G01N 29/222 Constructional or flow details for analysing fluids optoacoustic fluid cells G01N29/2425
• CPC Class Codes: G01N 29/449 Statistical methods not provided for in G01N29/4409, e.g. averaging, smoothing and interpolation
• CPC Class Codes: G01N 29/46 by spectral analysis, e.g. Fourier analysis or wavelet analysis spectral signal processing per se G06F17/14
• CPC Class Codes: G01N 29/50 using auto-correlation techniques or cross-correlation techniques

Abstract

Apparatus and methods for performing acoustical measurements are provided having some and preferably all of the following features: (1) the system is operated under near-field conditions; (2) the piezoelement or piezoelements used in the system are (a) mechanically and electrically damped and (b) efficiently electrically coupled to the signal processing components of the system; (3) each piezoelement used in the system includes an acoustical transformer for coupling the element to a gaseous test medium; (4) speed of sound is determined from the time difference between two detections of an acoustical pulse at a receiver; (5) cross-correlation techniques are employed to detect the acoustical pulse at the receiver; (6) fast Fourier transform techniques are used to implement the cross-correlation techniques; and (7) stray path signals through the body of the acoustic sensor are removed from detected signals prior to signal analysis. Techniques are also provided for performing acoustical measurements on gases whose thermodynamic properties have not been measures and on mixtures of compressible gases. Methods and apparatus for performing feedback control of a gas of interest in a mixture of that gas and a carrier gas are provided in which the controlled variable is the flow of the carrier gas.

Claims

1. Apparatus for controlling the composition of a mixture of a gas of interest and a carrier gas, said apparatus comprising:

(a) carrier gas flow control means for controlling the flow of the carrier gas;

(b) mixing means for producing the mixture of the gas of interest and the carrier gas;

(c) determining means for determining a property of the mixture, said property being indicative of the composition of the mixture; and

(d) control means operatively connected to the carrier gas flow control means and the determining means for controlling the carrier gas flow control means based on a difference between the property of the mixture determined by the determining means and a desired value for that property;

wherein the determining means comprises;

(i) a chamber for receiving the mixture;

(ii) a transmitter which, during use of the apparatus, transmits a pulse of acoustical energy through the mixture in the chamber, said transmitter having a characteristic dimension and said pulse of acoustical energy comprising a plurality of oscillations of the acoustical energy, said plurality of oscillations having a center frequency f_c; and

(iii) a receiver which, during use of the apparatus, detects the pulse after the pulse has passed through the mixture in the chamber, said passing of the pulse through the mixture defining an acoustical path length L_p;

wherein the transmitter has a near field zone of length L_n and the characteristic dimension, the center frequency, and the acoustical path length are chosen so that, during use of the determining means, the receiver is within said near field zone.

2. The apparatus of claim 1 wherein the property is the mass flow of the gas of interest provided by the mixture.

3. A method for controlling the composition of a mixture of a gas of interest and a carrier gas, said mixture providing a mass flow of the gas of interest, said method comprising:

(a) determining a property of the mixture, said determination being made on the mixture and said property being the mass flow of the gas of interest provided by the mixture; and

(b) controlling the flow of the carrier gas by determining the difference between said mass flow of the gas of interest determined in step (a) and a desired value for said mass flow;

wherein the property is determined acoustically by;

(i) generating a pulse of acoustical energy, said generation defining a near-field zone;

(ii) passing the pulse through the mixture; and

(iii) detecting the pulse after its passage through the mixture while the pulse is within the near-field zone.