E C O P H Y S I C S Measurement principle
CLD 780 TR / July 2000 16
3.2 Description of the CLD 780 TR analyzer
3.2.1 Introduction
Great effort has been made during the design phase of the CLD 780 TR to
achieve a modular construction. A further characteristic of the instrument,
essential for airborne installations, is the location of all external connec-
tion points (both electrical and pneumatic) on the front panel. In addition
to this, the depth of the housing has been minimized to reduce the prob-
lem of lack of space which must often be overcome in aircraft. The main
electronic boards (power supply, microprocessor controller and pressure
sensor) are mounted on a supporting plate which can be folded upwards
for ease of access.
The arrangement of the individual modules in the instrument is shown in
Fig. 3.1.
The construction and operation of the most important components, to-
gether with the flow diagram and electronics system, is described below.
3.2.2 Detector unit
(see Fig. 3.1)
Despite the fact that the CLD 780 TR analyzer contains two reaction
chambers (pre-chamber and main chamber), the instrument is in principle
a single-channel analyzer. When combined with a photolytic converter
the analyzer is able to measure NO and NO
x
(NO.c). The detection of
both components in the sample gas is accomplished through automatic
switching of the gas flow in a two step cycle. The measurement of NO is
achieved in a first step, that of NO.c in a second step (see “MODE“-
window (3), paragraph 5.2.2.2). Referring to fig. 3.1, the pre-chamber
[7] serves to determine the interference signal and the chemical zero
point. The actual chemiluminescence reaction that produces the measure-
ment signal takes place in the main reaction chamber [8]. A 3/2-way tef-
lon solenoid valve [6] switches the ozone flow to either the fore-chamber
or main chamber. When a main chamber measurement occurs the sample-
gas and the ozone-flow are concentrically guided into the main chamber
where the two streams are mixed. The main chamber is viewed by the
photomultiplier (PMT) unit [14].
The photomultiplier (PMT) is a “head-on“ type which collects the light di-
rectly, not reflected by a mirror. The photons emitted by the chemilumin-
scent reaction in the main chamber are converted by the PMT into current
pulses and amplified. The cooled PMT is thermally insulated against heat
flow from outside and from the reaction chamber. The inside of the PMT
housing is flushed with dry air in order to prevent condensation. The cool-
ing to below –15 °C greatly reduce fluctuations in the dark current, so
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