Detcon IR-522 - Manual (Page 3)

3.0 D ESCRIPTION
Detcon MicroSafe™ Model IR-522, combustible hydrocarbon gas sensors are non-intrusive “Smart” sensors
designed to detect and monitor combustible hydrocarbon gas in air over the range of 0-100% lower explosive limit
(LEL). One of the primary features of the sensor is its method of automatic calibration which guides the user
through each step via instructions displayed on the backlit LCD. The sensor output is a standard 4-20 mA signal.
The microprocessor supervised electronics are packaged as a plug-in module that mates to a standard connector
board. Both are housed in an explosion proof condulet that includes a glass lens. A 16 character alpha/numeric
indicator is used to display sensor readings as well as the sensor’s menu driven features via a hand-held program-
ming magnet.
The sensor technology is a f ield proven “plug-in replaceable” non-dispersive infrared (NDIR) optical type. NDIR
optical sensors show an excellent response to a long list of combustible hydrocarbon gases. This technique is non-
selective and may be used for the detection and monitoring of many target combustible hydrocarbon gases. As
compared to catalytic bead sensors, with NDIR there is no risk of sensor poisoning, no risk of high concentration
saturation, and no need for O2 to be present. The NDIR type sensor is characteristically stable for both span and
zero and is capable of providing reliable performance with low maintenance requirements for periods approaching
5 years in most industrial environments.
3.0.1 Non-dispersive Infrared (NDIR) Optical Sensor
The Detcon NDIR sensor is designed as a miniature single piece “plug-in replaceable” component, which can easily be
changed out in the field. The NDIR sensor consists of an infrared lamp source, two pyroelectric detectors, and an
optical gas sample cavity. The lamp source produces infrared radiation which interacts with the target gas as it travels
through the optical gas sample cavity. The infrared radiation contacts each of two pyroelectric detectors at the comple-
tion of the optical path. The “active” pyroelectric detector is covered by a filter specific to the part of the IR spectrum
where the target gas absorbs light. The “reference” pyroelectric detector is covered by a filter specific to the non-
absorbing part of the IR spectrum. When the target gas is present, it absorbs IR radiation and the signal output from
the “active” pyroelectric detector decreases accordingly while the “reference” detector output remains unchanged. The
ratio of the “active” and “reference” detector outputs is then used to compute the target gas concentration.
Model IR-522 Combustible Hydrocarbon Sensor PG.3