E C O P H Y S I C S Measurement principle
CLD 780 TR / July 2000 19
3.2.4 Gas flow
(see Fig. 3.2 “CLD 780 TR Gas Flow Diagram“)
A powerful, external vacuum pump (B) generates a reaction chamber
vacuum of approx. 14 mbar. It also serves to draw sample gas and
ozone generator supply oxygen into the analyzer. A thermal ozone
scrubber (A) which converts ozone into oxygen, should be fitted to the
pump intake. This prevents ozone from being discharged into the atmos-
phere since when measuring in the ppt range it is prudent to avoid the re-
lease of extraneous gases in the vicinity of the sampling point. For the
same reason a filter packed with activated charcoal should be mounted
on the pump exhaust to remove oil mist.
Sample gas flow:
a) Determining the chemical and electronic zero point S
2
(see chap. 3.1):
Sample gas and ozone-containing oxygen from the ozone generators
are mixed together just before the flow enters the pre-chamber [7]
where practically all the NO reacts with O
3
. From there the mixture
passes through to the main chamber [8] where the PMT measures the
emitted light. This emission is solely due to interfering compounds, the
must faster chemiluminescent reaction of NO and O
3
having been
completed in the pre-chamber.
b) Determining the total signal S
1
: In this case the ozone flow is brought
directly to the main chamber (as in Fig. 3.2), where the signal meas-
ured by the PMT is due to the combined effect of the NO and the inter-
ferences in the sample.
The gas is then drawn through the vacuum port [9] (see Fig. 3.2) to the
ozone scrubber/pump unit [A/B]. The chamber pressure is monitored [13]
by a sensor located at the outlet of the main reaction chamber.
Ozone flow: (see Fig. 3.2)
a) Ozone generator supplied with O
2
:
O
2
is aspirated through the O
2
inlet and inlet filter [3] into the ozone
generators [5]. Ozone-containing oxygen flows out of the ozone gen-
erator and passes through the flow restrictor [4] to the ozone solenoid
valve [6]. At that point it is either mixed with the sample gas before it
reaches the pre-chamber, or it is routed into the main chamber sepa-
rately from the sample gas.
b) Ozone generator supplied with dry air:
Ambient air is aspirated through an external drying cartridge into the
O
2
inlet and filter [3]. Otherwise as in a) above.
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