User guide

9 - 4
PN 074-566-P1B
Composer Elite Operating Manual
9.2 Measuring the Speed of Sound
A simple explanation of the functioning of Composer Elite 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
gases, and , the concentration is derived. Composer Elite 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 9-2, 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 Acoustic
Sensor is designed to provide a user-set isothermal environment for gas in the
Resonant Chamber and to precondition the gas as it enters the Resonant
Chamber. To aid in this preconditioning, the Acoustic Sensor’s inlet tubing is
intentionally longer than is strictly necessary and is contained within the insulation
of the Acoustic Sensor’s enclosure. This helps smooth the gases’ temperature
transition from the temperature of the process tubing to the Resonant Chamber’s
carefully controlled temperature. There is a feedback loop between the
temperature (Controlled Variable) as measured by a RTD, Platinum Resistance
Thermometer, and the power level applied to the heaters (Manipulated Variable). It
is normal for this temperature to be controlled to +/- 0.05°C. It is also normal for an
offset to exist between the value of the temperature parameter and the actual
temperature.
Measurement of the speed of sound is also dependent on the instrument’s ability
to precisely measure the amplitude of sound transmission through the Resonant
Chamber. This is especially difficult as the pressure in the Resonant Chamber is
lowered. Because of the need to operate some Delivery Systems at low pressures,
a method was needed to couple energy more efficiently from the Excitation
Microphone through the target media and into the Detecting Microphone. The use
of a Helmholtz Resonator was chosen because its careful shaping allows relatively
efficient energy coupling into the Resonant Chamber by providing a better
impedance match. The Helmholtz design also provides a means of building a
compact & low volume structure so that its Fundamental Resonance is at
frequencies below the Self Resonance of both the Diaphragms and the Drive and
Detecting Microphones.
A potential difficulty of operating at low frequencies is an apparent loss of frequency
resolution (at least on a relative basis), when compared to operating at the
frequencies produced by a high harmonic. But this is only an illusion. This apparent
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