Ametek 241CE II Operator's Manual page 76

After Purging is complete, the Hold portion of this stage begins, lasting
for approximately 10 seconds. At this point, the solenoid valve is closed
to stop (block in) the main flow of sample gas and to allow the Measur-
ing Cell to equilibrate to the process stream pressure. The Microprocessor
then performs a series of diagnostic tests to check the operation of the
LEDs and phototransistors for both mirror surfaces (to ensure the mir-
ror has been cleaned of all condensates). The two phototransistor signals
are compared against the allowable limits. These diagnostic checks take
approximately 2–5 seconds to complete. If the phototransistors are within
the allowable limits, the Measuring Cycle will continue normally (the
Cooling Stage will begin).
If any of these diagnostic checks fail, the Possible-Cell-Contamination
("w Dirty Cell") SCode is generated and the Purging/Hold Stage is repeat-
ed. The Purging/Hold Stage will then be extended by a duration equal to
the purge cycle time (PTime). If not already done, you can also open the
Bypass Valve (on the analyzer backpan, Figure 3-1) to allow a very low
purge flow rate (trickle purge) to flow continuously through the Measur-
ing Cell. This flow will be approximately 10 percent of the Purging/Hold
Stage flow.
Since the analyzer uses only the Measuring Cell, sample gas, and Heat-
sink as heat sources to warm the mirror, it is possible at the end of the
Purging/Hold Stage for the mirror temperature to be less than its High set-
point temperature. This is a frequent occurrence when the purging time is
short and the difference between the ambient temperature and the High
setpoint temperature of the mirror is minimal. When this occurs, the Cool-
ing Stage is initiated and the Stage-Time-Out ("w TimeOut") SCode and
Purging/Hold-Stage-Time-Out ("w Hold TimeOut") XSCode are generated.
3-32 | Model 241CE II Hydrocarbon Dewpoint Analyzer