User guide

9 - 5
PN 074-566-P1B
Composer Elite Operating Manual
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 effect 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 Composer Elite design is that the concentration
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 9.1, Speed of Sound and Gas Composition, on page 9-1)
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 RTD 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 calibration, reestablishing Reference
Zero, minimizes the possible influence of these variables on the concentration
measurement.