E C O P H Y S I C S Measurement principle
CLD 780 TR / July 2000 20
PMT flush: (see Fig. 3.2)
Dry air (dew point below -40 °C, corresponding to < 127 ppmv H
2
O) is
aspirated through two external drier cartridges in into the dry air inlet
with filter [10], and flows through the PMT housing. The PMT housing flush
outlet is fitted with a flow restrictor [11]. A filter containing a drying agent
[12], located just downstream of the flow restrictor ensures that even
when the vacuum pump [B] is switched off no damp air can diffuse back
into the PMT housing where it might condense, freeze and eventually de-
stroy the PMT by causing a flashover in the hv supply.
The flushing air is vented into to the main vacuum line between the pres-
sure sensor [13] and the main-chamber vacuum connection points. This ar-
rangement protects the pressure sensor from damage by ozone.
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