3-16 Chapter 3: Module components SHB_U14
Infrared detector
Location in module
The infrared detector is placed at the end of the sample cell.
Depending on the application, there can be from 1 to 4 detectors installed.
A maximum of 2 detectors are possible per beam path.
Structure
The infrared detector consists of the following components:
• An optopneumatic portion consisting of two chambers, one located behind the
other, separated by an infrared-transparent window.
The chambers are filled with a gas that corresponds to the sample component
involved.
In the part not affected by the beam there is a membrane capacitor consisting of
a metal membrane sealed in a gas-tight manner between the two chamber
portions and a fixed counter electrode. Both components are linked to the
preamplifier via gas-tight openings.
• The preamplifier is a fixed component of the detector.
The preamplifier has a temperature probe for optional temperature
compensation.
Function
The emission entering the detector is absorbed in the region of the fill gas
absorption lines and by molecular collision is momentarily converted to heat energy
corresponding to pressure. This pressure change is received by the membrane
capacitor.
The 150-V voltage applied now generates a corresponding mV signal via a high-
resistance circuit.
The primary emission absorption occurs in the forward chamber (positive
deflection). In the rear chamber there is a flank-driven emitted beam portion of the
rotation lines which in the case of overlap with interference gas components leads
to cross-sensitivity. The greater depth of the rear chamber causes the absorption of
most of this interference emission and there is also a pressure increase here to
counter the pressure in the front chamber. This suppresses a certain amount of the
cross-sensitivity effect.
The preamplifier has 3 jumpers to allow signal matching to x1, x3 or x10. These
jumpers are set during the initial calibration.
The temperature probe is used to determine the ambient temperature of the optical
components and is therefore part of the optional measurement signal temperature
compensation circuit. Only the first infrared detector’s probe is used for
temperature measurements.
Types
All infrared detectors are mechanically identical. Detector fill is determined by the
application. The detectors are suitable for all measurement ranges shown on the
data sheet.
Sample components:
CO, NO, SO
2
, N
2
O, Frigen/R12, H
2
O, SF
6
, CS
2
, NH
3
CH
4
, C
2
H
2
, C
2
H
4
, C
2
H
6
, C
3
H
6
, C
3
H
8
, C
4
H
10
, C
6
H
6
, C
6
H
14
CO
2
with 10% fill for small measurement ranges
CO
2
with 100% fill for large measurement ranges
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