E C O P H Y S I C S Measurement principle
CLD 780 TR / July 2000 15
For this reason the CLD 780 TR employs a pre-chamber where NO com-
pletely reacts with ozone. If the residence time of the sample gas in the
pre-chamber allows the complete deactivation of the excited NO
2
*
, the
signal S
NO
detected by the PMT in the main reaction chamber is zero. Al-
ternating measurements with and without the pre-chamber allow the calcu-
lation of the correct NO concentration in the sample gas:
S
1
=S
NO
+S
In
+S
D
S
2
≈
S
In
+S
D
--------------------------------------------------------------------------------------------------------
S
NO
≈
S
1
-S
2
One measuring cycle consists of two steps:
Step 1 (measuring mode) producing the signal S
1
.
Step 2 (zero mode) producing signal S
2
.
In both modes the sample gas flows first through the pre-chamber and
then through the main chamber. In Step 1 the sample gas is mixed with
high-concentration ozone in the main chamber. During Step 2 (zero
mode) the ozone is added to the sample gas in the pre-chamber. The
PMT, of course, always measures the optical emission from the main
chamber only.
Thus the duration of a complete measurement cycle is twice the integration
time
∆
t plus a few seconds of purging time. In NO measurement mode,
with an integration time
∆
t = 30 s, the instrument displays a new NO con-
centration value in ppb every 62 s. See also section 5.2.2.2 (3).
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