Apparatus and methods for performing acoustical measurements

• US 6,199,423 B1
• Filed: 11/19/1999
• Issued: 03/13/2001
• 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:
  US09/443,520
• Time in Patent Office: 480 Days
• Field of Search: 73/10.1, 73/10.2, 73/18.2, 73/18.6, 73/10.6
• US Class Current: 73/1.82
• 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. A method for calibrating an acoustical sensor which comprises a test chamber, said method comprising:

(a) introducing a gas, solid, or liquid into the test chamber, said gas, solid, or liquid having a known speed of sound C_N;

(b) producing a pulse of acoustical energy which moves in alternating directions between first and second spaced apart points in the gas, solid, or liquid;

(c) detecting the pulse after it has made n passages between the first and second spaced apart points (the n-passage pulse) and after it has made m passages between those points (the m-passage pulse), where a passage constitutes movement of the pulse between the first and second spaced apart points in either direction, m and n are integers, and

2. The method of claim 1 wherein

(i) the acoustical sensor has a transmitter at the first spaced apart point and a receiver at the second spaced apart point;

(ii) m=3;

(iii) n=1; and

(iv) the method includes the additional step of determining the sum τ

of the time delays in the transmitter and the receiver from the equation;

$\mathrm{\tau <br><br>}=\frac{3<br><br>t_1-t_2}{2}.$

3. A method for calibrating an acoustical sensor which comprises a transmitter, a receiver, and a test chamber, said method comprising:

(a) evacuating the test chamber;

(b) activating the transmitter to produce a pulse of acoustical energy and recording the resulting signal at the receiver;

(c) repeating step (b) to produce a plurality of recorded signals; and

(d) averaging the plurality of recorded signals to produce a stray path signal;

wherein in step (a), the chamber is evacuated so that the amplitude of a direct path signal between the transmitter and the receiver is essentially zero.