INFICON Composer Gas Concentration Controller User Manual
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Composer Operating Manual
(Manipulated Variable). It is normal for this temperature to be controlled to +/-
0.05 °C. It is also normal for an offset 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 5 chamber Helmholtz Resonator was chosen
because its careful shaping allowed 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 advantage is negated by modern electronics’ ability to generate
precision sine waves with resolutions of better than one part in 50,000.
Improving the generation precision of the operating frequency to 0.1 Hz has the
same affect on relative precision as operating in the tenth harmonic, without the
viscous energy losses associated with the higher frequency sound waves. The
basic measurement scheme employed is to generate a frequency and measure
its amplitude. By intelligently varying the frequency it is easy to find the
maximum amplitude, which corresponds to the Resonant Frequency. It is
possible to further enhance the measurement system’s resolution by
methodically Curve Fitting the frequency vs. amplitude data around a
Resonance peak to match a Lorentzian shape.
Another important aspect of this instrument’s design is that the composition
measurement does not depend on absolutes. How does this help? Think about
the difficulty of measuring temperature to 0.01 °C in an absolute sense. A quick
review of the equations (
section 2.1, Speed of Sound and Gas Composition, on
) shows that concentration measurement by this technique is
dependent only on maintaining the temperature at the same temperature at
which the reference zero measurement was taken. The PRT is a sensing
element that is stable to fractions of a PPM per day, and all the relative error
due to temperature variation is eliminated every time the instrument is zeroed.
Likewise, the speed of sound depends slightly on the pressure, very slightly on
the flow rate and even slightly on the local barometric pressure changes. Daily
setting of the Reference Zero minimizes the possible influence of these
variables on the concentration measurement.