E C O P H Y S I C S Measurement principle
CLD 780 TR / July 2000 17
that the detector sensitivity remains stable and the signal noise minimized.
All other parameters that influence the reaction rate (chamber pressure,
ozone generator voltage and chamber inlet temperature) are constantly
monitored. To insure optimal NO selectivity a red filter is positioned be-
tween the main chamber and the PMT.
3.2.3 Ozone generator
(see Fig. 3.1)
The integrated ozone generators [5] operate on the “silent electrical dis-
charge“ principle. Oxygen or dry air is passed through an alternating
current electrical field, and ozone is generated through ionization. A sta-
bilized, constant-frequency high voltage is used to create the discharge.
The high ozone yield from the generator assures measurement linearity
over five concentration decades. For maximum sensitivity and accuracy
supply the ozone generators with pure oxygen (O
2
) instead of dry air. The
analyzer is optimized for operating under this condition (see 4.2.3, pt. 3).
The basic version of the CLD 780 TR does not include an internal pressure
regulator for the ozone generator oxygen supply. This option is however
recommended for optimal performance (see section 3.3 in Appendix 4).
It is of course still perfectly feasible to operate the ozone generator with
ambient air. However, the supply air must be dry. For stable ozone pro-
duction the dew point should be -20 °C (corresponding to < 1000 ppmv
H
2
O). Each CLD 780 TR is shipped with three silica gel cartridges as stan-
dard, one of which is for drying the ozone generator supply air (see Fig.
4.2). The cartridge contents must be renewed or regenerated periodically
(see chap. 9).
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