Detcon CX-IR - Manual (Page 6)

Model CX-IR
Principle of Operation
The target gas diffuses through a stainless steel screen and into the volume of the sample gas optical chamber.
An alternating miniature lamp provides a cyclical IR radiation source, which reflects through the optical gas
sample chamber and terminates at the two pyroelectric detectors. The active and reference pyroelectric
detectors each give an output which measures the intensity of the radiation contacting their surface. The active
detector is covered by an optical filter specific to the part of the IR spectrum where the target gas absorbs light.
The reference detector is covered by a filter specific to the non-absorbing part of the IR spectrum. When
present, the target gas absorbs a fraction of the IR radiation and the signal output from the active detector
decreases accordingly. The signal output of the reference detector remains unchanged in the presence of the
target gas. The ratio of the active/reference signal outputs is then used to compute the target gas concentration.
By using the ratio of the active/reference signal outputs, measurement drift caused by the changes in the
intensity of the IR lamp source or changes in the optical path’s reflectivity is prevented (Figure 2).
Figure 2 Principle of Operation
Performance Characteristics
The IR sensor maintains strong sensitivity to most all combustible hydrocarbon gases within the LEL range.
When compared with the typical catalytic bead LEL sensor, the IR sensor exhibits improved long-term zero
and span stability. Typical zero calibration intervals are quarterly to semi-annual and typical span intervals are
semi-annual to annual.
NOTE Actual field experience is always the best determination of appropriate calibration intervals.
NOTE
The CX-IR sensor will not respond to combustible gases that are not hydrocarbons, such as
H
2
, NH
3
, CO, H
2
S….etc. It can only be used to measure hydrocarbon type gases.
The IR sensor generates different signal sensitivity levels for different combustible hydrocarbon target gases.
Unless otherwise specified the CX-IR sensor will be factory calibrated for methane service. If the target
hydrocarbon gas is other than methane, then the unit will have to be span calibrated and configured in
accordance with this CX-IR sensor instruction manual.
1.2 Modular Design
The CX-IR sensor assembly is completely modular and is composed of four parts (Figure 3).
1. CX-IR Intelligent Transmitter Module (ITM)
2. Plug-in Infrared Sensor
3. CX Series Bottom Housing
4. Rain Shield
CX-IR Instruction Manual Rev. 1.2 Page 2 of 34