Detcon IR-700 - Manual (Page 5)

Model IR-700
IR-700 Instruction Manual Rev. 3.1 Page 1 of 40
1. Introduction
1.1 Description
Detcon Model IR-700 combustible gas sensors are non-intrusive “Smart” sensors
designed to detect and monitor combustible hydrocarbon gases in air. The range of
detection is 0-100% LEL or 0-50% LEL. The Model IR-700 CO
2
Sensor is designed
to detect CO
2
in air at ranges from 0-.3% to 0-100% by Volume. The sensor features
an LED display of current reading, fault and calibration status. The unit is equipped
with standard analog 4-20mA and Modbus™ RS-485 outputs. A primary feature of
the sensor is its method of automatic calibration, which guides the user through each
step via fully scripted instructions shown on the LED display.
The microprocessor-supervised electronics are packaged in an encapsulated module
and housed in an explosion proof casting.
The unit includes a 4 character
alpha/numeric LED used to display sensor readings and the sensor’s menu-driven
interface when the hand-held programming magnet is used.
1.1.1 Non-Dispersive Infrared (NDIR) Optical Sensor Technology
The sensor technology is designed as a miniature plug-in replaceable component, which can easily be changed
out in the field. The NDIR sensor consists of an infrared lamp source, two pyro electric detectors, and an
optical gas sample chamber. The lamp source produces infrared radiation, which interacts with the target gas
as it is reflected through the optical gas sample chamber. The infrared radiation contacts each of the two pyro
electric detectors at the completion of the optical path. The “active” pyro electric detector is covered by a
filter specific to the part of the IR spectrum where the target gas absorbs light. The “reference” pyro electric
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 detective decreases accordingly. The
reference detector output remains unchanged. The ratio of the active and reference detector outputs are then
used to compute the target gas concentration.
The technique is referred to as non-selective and may be used to monitor most any combustible hydrocarbon
gas. The technique for CO
2
is similar except that the sensor provides a selective response to CO
2
. Unlike
catalytic bead type sensors, Detcon IR sensors are completely resistant to poisoning from corrosive gases and
they can operate in the absence of an oxygen background.
The sensors are characteristically stable and
capable of providing reliable performance for periods exceeding 5 years in most industrial environments.
Figure 1 Sensor Cell Construction