E C O P H Y S I C S Practical hints
CLD 780 TR / July 2000 95
7.4 Observing the raw signal
The CLD 780 TR allows easy access to the raw signal in counts, making it
possible for the operator to view information about the calibration gas
stability, the condition of the optical system and the gas flow path in the
analyzer.
7.4.1 Stability of the calibration gas
CLD 780 TR analyzers with Option 2 are fitted with a separate calibration
gas inlet. (Other options are not equipped with a solenoid valve to switch
between sample and calibration gas and in these cases, the analyzer must
be calibrated via the sample inlet.)
In order to check the stability of the calibration gas concentration before
performing a calibration on the instrument, the following steps should be
followed:
Access the calibration gas raw signal with the test function (see also
5.2.2.4):
1. Connect a calibration gas supply of known concentration to the cal.
gas inlet
2. Select appropriate range
3. Press the “TEST” key and select the “Counts” window (19).
The raw signals for the NO and possibly NO
2
concentrations in the cali-
bration gas and the dark current signal („Zero“) are displayed in counts
per second . The calibration procedure should only be started after all
signal values have stabilized. It is also possible to log the raw signal
through the serial interface (see 8.3.4 c and d).
7.4.2 Checking the optical system
The formation of solid deposits in the reaction chambers reduces the inten-
sity of the chemiluminescent emission seen by the PMT. This reduces the in-
strument sensitivity. When frequent calibrations are performed this declin-
ing signal is continuously compensated for and is not a problem. Monitor-
ing the raw signal values in counts as described in 7.4.1 reveals the de-
gree of contamination of the optical system and also of the gas transport
lines of the analyzer.
Procedure:
– The raw signal value measured by the analyzer under a given set of
conditions, and with a clean reaction chamber should be noted down.
Select the „Counts“-window (19) or the „Valid“-window (37); use only
one or the other option.
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