Purging/Hold Stage - Ametek 241CE II User Manual

Purging/Hold Stage

The measuring cycle begins with the Purging/Hold Stage. The Purging opera-
tion 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 flowing, 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 contained
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 mir-
ror temperature to its setpoint temperature.
You can change both the mirror High setpoint temperature and the duration 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 temperature 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 set-
point 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 pres-
sure drop at pipeline pressures of 1400 KPAG (200 PSIG) or higher. The HCDP
temperature is, therefore, determined at the process stream pressure.
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 Measuring 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 photo-
transistors for both mirror surfaces (to ensure the mirror 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).
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Installation and Start-Up 2-31
PN 903-8797, Rev A