INFICON Composer Gas Concentration Controller User Manual
Page 64
3 - 2
IP
N 07
4-
28
9L
Composer Operating Manual
NOTE: Always wait for the STEADY lamp on the Controller’s front panel to
illuminate before completing the Reference Zero establishment
process; that is, actually pressing the switch. This indicates that the
measurement conditions are stable and a relative equilibrium has been
reached. It sometimes takes many minutes for the light (low molecular
weight) carrier gas to clean the Resonant Chamber of any residual
Precursor or other heavy gasses. Remember that some gasses may
also absorb on the Resonant Chamber’s walls and are slowly released,
further extending the time to reach pure carrier gas as indicated by
stable frequency.
CAUTION
Setting the Reference Zero before the Resonant
Chamber is filled with only pure reference (carrier) gas
will result in an offset and scaling error in the
measured composition (until the next time the
Reference Zero is set). See
3.1.3 How is Performance Affected if I Use the
Factory Set Reference Zero Value?
While it is most accurate to reset the Reference Zero on a daily basis, good
results can be obtained for many processes by using the factory set default
value for the Reference Zero. While the exact loss of reproducibility is hard to
predict, it is reasonable that it will be about 2-4 times worse than the specified
value when monitored over extended periods of time. This may be acceptable
for the process and might also be convenient. Alternating between the factory
default and carefully setting the zero occasionally will not be productive. It
would be better to extend the interval between reestablishing the Reference
Zero than to alternate. An example of long term stability is shown in
.
3.1.4 What To Do when the Specific Heat Ratio
for a Gas is Unknown?
Quantitative accuracy for an acoustic measurement technique partially
depends on accurate knowledge of the Specific Heat Ratio,
γ
, of the individual
gas species. While this ratio is known for many pure common gasses, little
information is available in the literature on many of the complex Precursor
molecules. The information we have at the time of this printing is given in
. Even without exact knowledge of the Specific Heat Ratio, useful
information and reliable operation can still be obtained. The Specific Heat Ratio
of gasses is a parameter that does not have a large range, it is literally confined