Detcon IR-642 - Manual (Page 5)

The sensors are packaged in a cast metal explosion proof enclosure. The enclosure is f itted with a threaded cover
that has a glass lens window. Magnetic program switches located behind the transmitter module face plate are acti-
vated through the lens window via a hand-held magnetic programming tool allowing non-intrusive operator inter-
face with the sensor. All calibration and alarm level adjustments can be accomplished without removing the cover
or declassifying the area. Electrical classif ication is Class I; Groups B, C, D; Div. 1.
3.1 P
RINCIPLE OF
O
PERATION
The CO2 diffuses through a sintered stainless steel f lame arrestor and into the volume of the sample gas optical
cavity. A lamp source provides a cyclical IR radiation source which travels through the optical gas sample cavity and
terminates at 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” pyroelectric detector is covered
by an optical f ilter specif ic to the part of the IR spectrum where CO2 absorbs light. The “reference” pyroelectric
detector is covered by a f ilter specif ic to the non-absorbing part of the IR spectrum. When present, CO2 absorbs a
fraction of the IR radiation and the signal output from the “active” pyroelectric detector decreases accordingly. The
signal output of the “reference” detector remains unchanged in the presence of CO2. The ratio of the “active” and
“reference” detector signal outputs is then used to compute CO2 concentration. By using the ratio of the active/ref-
erence signal outputs, measurement drift caused by changes in the intensity of the lamp source and changes in the
optical path are negated.
3.1.2 Characteristics
The NDIR optical sensor element maintains strong sensitivity to CO2, as shown in the illustration below. Typical zero cali-
bration intervals would be quarterly and typical span calibration intervals would be semi-annual to annual. However, actual
field experience is always the best determination of appropriate calibration intervals.
The MicroSafe™ IR-640/IR-642 is factory calibrated for a specific CO2 range (see section 3.8 to determine range). The detec-
tor will be factory calibrated for the range specified at time of order. If a CO2 range other than that specified at time of order
is desired, contact Detcon for the appropriate field-replaceable electronics/software upgrade.
The NDIR optical sensor readings can be adversely affected by dust, dirt and oil mist accumulation as well as severe corro-
sion. These deposits may reduce the optical reflectivity inside the sensor, and although accurate readings are continually main-
tained, excessive loss in useable signal eventually gives way to noise and unstable readings. The optical sensor may, over long
periods of time (3-7 years), lose its IR lamp source filament, and in this case an optical sensor modular replacement is
required. The IR-640/IR-642 has an extensive list of Fault Diagnostics to alert and pin-point operational problems. See section
3.10
Detcon Model IR-640/IR-642 Carbon Dioxide Sensor PG.5
IR Lamp Source
Reference Detector Output
Active Detector Output
CO2 Not Present
CO2 Present
Principle of Operation
Sample
Gas