About The Measuring Cycle - Ametek 241CE II User Manual

About the Measuring Cycle

During normal operation, the analyzer continuously executes the measuring
cycle, which consists of three stages: Purging/Hold, Cooling, and Warm-
ing. The measuring cycle typically takes 15–45 minutes, with the actual time
dependent on the duration required for Mirror Low Temp or HCDP to be
reached.
During the Warming and Purging/Hold Stages, the results obtained from the
Cooling Stage are displayed on the User Interface.
The time required to complete a measuring cycle is variable. The purging time
(ApplicCFG : Purge) is set by the user but it may be doubled if the Possible-
Cell-Contamination (w Dirty Cell) SCode occurs. Since the mirror temperature
is reduced at two different constant rates, the time required for the Cooling
Stage depends on the sample gas HCDP temperature; the lower it is, the lon-
ger it will take to find it. The time required for the Warming Stage is also vari-
able. This time depends primarily upon the temperature difference between
the lowest temperature reached by the mirror and the HCDP determined in
the previous Cooling Stage.
For a gas stream in which the HCDP is highly variable, the measuring cycle
time can vary widely. For a gas stream in which the HCDP is relatively constant,
the variation in the measuring cycle time will be minimal.
For certain applications, it may be desirable to shorten the cycle time as much
as possible. There can be trade-offs in adjusting some parameters to reduce
the cycle time. If the PTime is reduced too much, there may be a problem with
cleaning the liquids from the Measuring Cell between cycles. A similar prob-
lem can occur if the High mirror setpoint temperature (ApplicCFG : MTHi)
is reduced too much. The typical default Slow Mirror Cooling Rate (Applic-
CFG : SRate) is 2 °C/3.6 °F per minute. If HCDP accuracy is important, maintain
the SRate at its original setting. Increasing the SRate will decrease the HCDP
accuracy. A preferred method of reducing cycle time is to increase the Fast Mir-
ror Cooling Rate (ApplicCFG : FRate), but this method tends to reduce the life
of the Thermoelectric Cooler.
The temperature above the last recorded HCDP can also be adjusted. The last
recorded dewpoint is the switch-point of the FRate and SRate. This dewpoint
is also the switch-point between the Warming and Purging/Hold Stages. If
you change the temperature parameter, carefully consider the stability of the
HCDP in the process gas line.
2-30 | 241CE II Hydrocarbon Dewpoint Analyzer PN 903-8797, Rev A