INFICON Composer Gas Concentration Controller - Manual (Page 68)

3 - 6
IP
N 07
4-
28
9L
Composer Operating Manual
3.1.8 What is the Effect of Flow Variation?
There is virtually no flow induced effect on concentration measurement in this
Instrument System except at the highest flow rates, see
Figure 3-3 , the
pressure drop across the sensor is about 10 torr (1.33 kPa) per 500 sccm.
Figure 3-3 Effect of flow on apparent concentration
3.1.9 How Does This Instrumental Method Differ from
Measuring the Speed Of Sound by Time Of Flight
The use of a low frequency Resonant Chamber has significant advantages over
Time Of Flight (TOF) methods. First, it allows the constructive build up of
energy within the Resonant Chamber. Secondly, sound transmission in
hydrogen at low frequency is more efficient than at high frequency. These
enhance the instrument’s ability to measure concentration in Hydrogen
mixtures at lower pressures. Finally, the careful intelligent manipulation of the
applied frequency around the resonant amplitude peak allows even greater
precision in the determination of the speed of sound. The resonant method
does not loose accuracy because of uncertainty of guessing when the center of
a low energy acoustic pulse packet precisely leaves the Sender and when the
same center of energy reaches the Receiver. See
Figure 3-4 . For a TOF
instrument to achieve the equivalent resolution as the resonant acoustic
technique’s 1 part in 50,000, it must achieve time measurement precision of 3
nanoseconds. This resolution is achievable for sophisticated instruments
measuring photons or particles, but is approximately 40 times better than the
resolution possible on the poorly defined 15 kHz ultrasonic wave pulse in
hydrogen. One wavelength of a 15 kHz tone is 0.0856 meter, almost one half
the length of a 0.2 meter path length instrument.