Detcon 1000_CO2 - Manual (Page 7)

Model 1000 CO2
Model 1000 CO2 Instruction Manual Rev. 2.2 Page 3 of 28
The control circuit is microprocessor based and packaged as a plug-in field replaceable module, facilitating
easy replacement and minimum down time. Circuit functions include a basic sensor pre-amplifier, sensor
temperature measurement, on-board power supplies, microprocessor, back lit alpha numeric display, alarm
status LED indicators, magnetic programming switches, an RS-485 communication port, and a linear 4-20mA
DC output.
1.1.3 Base Connector Board
The base connector board is mounted in the explosion proof enclosure and includes: the mating connector for
the control circuit, reverse input and secondary transient suppression, input filter, alarm relays, lug less
terminals for all field wiring, and a terminal strip for storing unused programming jumper tabs. The alarm
relays are contact rated 5Amps @ 250VAC, 5Amps @ 30VDC and coil rated at 24VDC. Gold plated program
jumpers are used to select either the normally open or normally closed relay contacts.
Figure 4 Connector PCA
1.1.4 IR-640/IR-642 CO
2
Sensor Principal of Operation
CO
2
in the sample stream diffuses through a sintered stainless steel flame 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 filter specific to the part of the IR
spectrum where CO
2
absorbs light. The “reference” pyroelectric detector is covered by a filter specific to the
non-absorbing part of the IR spectrum. When present, CO
2
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 CO
2
. The ratio of the “active” and “reference”
detector signal outputs is then used to compute CO
2
concentration. By using the ratio of the active/reference
signal outputs, measurement drift caused by changes in the intensity of the lamp source and changes in the
optical path are negated.