Detcon CXT-IR - Manual (Page 6)

Model CXT-IR
Figure 1 Sensor Cell Construction
Principle of Operation
The target gas diffuses through a sintered stainless steel flame arrestor 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 CXT-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 CXT-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 CXT-IR sensor instruction manual.
CXT-IR Instruction Manual Rev. 1.6 Page 2 of 44