Purging/Hold Stage - Ametek 241CE II Operator's Manual

Purging/Hold Stage

The measuring cycle begins with the Purging/Hold Stage. The Purging
operation occurs first, followed by the Hold portion of this stage.
While Purging, the solenoid valve is opened to allow sample gas to flow
through the Sample Line and Measuring Cell. While sample gas is flow-
ing, the mirror temperature stabilizes at its High setpoint temperature.
Typical start-up values are 20 °C (68 °F) for the High setpoint temperature
and 10 minutes for the default Purge time.
The heat sources for warming the mirror are the Measuring Cell, the con-
tained sample gas, and the Heatsink. If the mirror temperature rises above
the mirror High setpoint, power is applied to the Thermoelectric Cooler to
reduce the mirror temperature to its setpoint temperature.
You can change both the mirror High setpoint temperature and the dura-
tion of the Purging/Hold Stage. The High setpoint temperature, typically
in the range of 15–30 °C (59–86 °F), should be set at about the same tem-
perature as the surrounding ambient temperature and should be at least
20 C° (36 F°) above the highest expected HCDP temperature of the sample
gas. The combination of the purging duration and the sample gas flow
rate should be adjusted to ensure the sample system and Measuring Cell
are flushed completely within the purge duration. A PTime of at least 10
minutes is recommended to ensure all liquids are evaporated from the
surfaces of the mirror. A high upper setpoint temperature also helps clean
the mirror during the Purging/Hold Stage, because any condensed liquids
will evaporate at a high rate. With a higher temperature, the evaporation
rate of liquids is faster.
When the solenoid valve is closed at the end of the Purging/Hold Stage,
flow through the Measuring Cell is restricted except for the small bypass
flow (called a "trickle purge"). Since all the valves between the Measuring
Cell and the process pipe remain open, the sample gas pressure within
the Measuring Cell equilibrates to the process stream pressure almost
immediately after the solenoid valve closes. Even with the Bypass Valve
open, there is minimal pressure drop at pipeline pressures of 1400 KPAG
(200 PSIG) or higher. The HCDP temperature is, therefore, determined at
the process stream pressure.
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Installation and Start-Up 3-31