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Functions
8.5 Automatically executed functions
ULTRAMAT 23
158
Manual, 3/2016, A5E37100388-003
8.5.3 Probe purging function
Hydrogen sulfide measurement: Purging function of 50 vpm probe
Hydrogen sulfide (H
2
S) is a corrosive gas, especially in wet condition and in combination with
other gases. A selectable protection function has been implemented (see section Probe
protection function (Page 156)) since H
2
S concentrations above the permissible continuous
concentration impair the functionality and service life of the H
2
S probes. A purging function
has additionally been integrated for the 50 vpm probe, enabling a discontinuous
measurement above its permissible continuous concentration.
The permissible continuous concentration is 12.5 vpm. Although a measurement above such
a concentration is still correct, the probe is damaged after a certain period. This period can
be set in accordance with the experience gained using the sensor between 10 und
20 minutes using the AUTOCAL purge time (the purging duration corresponds to twice the
AUTOCAL purge time). For this reason measurements above a concentration of 12.5 vpm
must be carried out discontinuously and alternately with purging gas. The probe can be used
for a measurement again following a purging time of equal duration with air.
The function test is set during execution of the purging function in order to signal that the
displayed value is incorrect.
How the purging function works
The purging function is triggered if the continuous measured value of H
2
S is greater than the
permissible continuous concentration (12.5 vpm) in measuring mode for a period equal to
the duration of the zero gas purge time.
Following triggering of the purging function:
● The last measured values of all components are 'frozen' if the 'Analog outputs with
FCTRL' parameter has been set to 'Hold', or the current measured values are still
displayed for all other settings.
● A "V" (H
2
S purging function running) is displayed flashing in the measurement screen at
the right edge where the test letter "!" appears (fault logged, no longer pending).
● The zero gas valve is opened.
● The "Function control" status is set.
As long as the purging function is active, the zero gas valve remains open during the zero
gas purge time. The device then switches to the sample gas path. The status 'Function
control (FCTRL)' remains set and the test letter "V" flashes during the zero gas purge time
and the subsequent pre-purging phase. This signals that the displayed measured values are
incorrect. The status 'Function control' and the test letter are deleted following the pre-
purging phase, and the current measured values displayed again. Monitoring of the H
2
S
threshold for the permissible continuous concentration is already re-activated during the pre-
purging phase.