About The Measuring Cycle - Ametek 241CE II Operator's Manual

About the Measuring Cycle

During normal operation, the analyzer continuously executes the mea-
suring cycle, which consists of three stages: Purging/Hold, Cooling, and
Warming. 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 longer it will take to find it. The time required for the
Warming Stage is also variable. 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 rela-
tively 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 problem can occur if the High mirror setpoint tempera-
ture (ApplicCFG : MTHi) is reduced too much. The typical default Slow
Mirror Cooling Rate (ApplicCFG : SRate) is 2 °C/3.6 °F per minute. If
HCDP accuracy is important, maintain the SRate at its original setting. In-
creasing the SRate will decrease the HCDP accuracy. A preferred method
of reducing cycle time is to increase the Fast Mirror Cooling Rate (Applic-
CFG : FRate), but this method tends to reduce the life of the Thermoelec-
tric 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.
3-30 | Model 241CE II Hydrocarbon Dewpoint Analyzer