INFICON Composer Gas Concentration Controller - Manual (Page 46)

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Composer Operating Manual
Figure 2-1 Mole Fraction of Trimethylindium in H
2
vs. Instrument’s Frequency
2.2 Measuring the Speed of Sound
A simple explanation of the functioning of this Instrument System is that it
measures concentration by first determining the resonant frequency of the gas
flowing through it and then comparing the measured resonant frequency to that
of the pure carrier gas. Utilizing this frequency ratio, , and the physical
parameters of the gasses,
and , the concentration is derived. The
instrument determines the basic resonant frequency by varying the frequency
across the operating range and then operating at the frequency where the
largest amplitude of sound is transmitted. Because the composition is
controlled, or varies slowly, and the Resonant Chamber has good acoustic
properties, the frequency may be measured with great precision. From
equation
[6] on page 2-2 , the speed of sound may be derived from knowledge of the
Resonant Chamber’s length, L. In operation, the exact length is unimportant as
only the ratio of frequencies, , is used to determine concentration.
One critical issue confronted in the measurement of the speed of sound is the
ability to correctly measure or control the temperature of the gas. The
Transducer is designed to provide a user-set isothermal environment for the
gas in the Resonant Chamber and to precondition the gas as it enters the
Resonant Chamber. To aid in this preconditioning, the Transducer’s inlet tubing
is intentionally longer than is strictly necessary and is contained within the
insulation of the Transducer’s enclosure. This helps smooth the gasses’
temperature transition from the temperature of the Reactor’s supply tubing to
the Resonant Chamber’s carefully controlled temperature. There is a feedback
loop between the temperature (Controlled Variable) as measured by a PRT,
Platinum Resistance Thermometer, and the power level applied to the heaters
λ
m γ
λ