Patent ID: 6192739
Filing Date: 2001-02-27
Classification: G01N

Abstract:
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.sub.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.sub.p ;wherein the transmitter has a near field zone of length L.sub.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.