Model IR400
32
9.2 Principle of Operation
Most gases absorb infrared radiation in specific wavelengths or bands that are characteristic of
the chemical structure of molecules in the gas. All hydrocarbon gases absorb infrared radiation,
but to differing degrees. Gases, to be infrared active, must have an electric dipole moment. The
Model IR400 is based on measuring absorption of infrared radiation passing through a volume
of gas.
Absorption of the radiation follows the Beer - Lambert Law, which states “the transmittance T of
radiation through an absorbing medium decreases exponentially by the product of the
extinction coefficient A, the concentration C and the path length L”:
T = exp(-ACL)
The Model IR400 uses a dual source, single detector measurement method. One source
operates at a wavelength where absorption of a specific gas (or gases) occurs (the active
wavelength). The reference source operates at a wavelength that is adjacent to the active
wavelength but not absorbed by the gas (or gases). By comparing the signals from these two
sources the concentration of the gas can be measured using the differential absorption
technique.
This method of gas detection comes under what is commonly known as the non-dispersive
infrared (NDIR) absorption principle. The reference wavelength is chosen suitable to
compensate for any interference that can otherwise occur from atmospheric variation (e.g.
humidity, dust, snow, fog, steam, temperature, etc.).
Control Electronics
The Model IR400 operates from an unreg24 VDC (nominal) input, which is fed to an
onboard power-supply that produces all of the necessary voltages within the unit. The
microprocessor constantly monitors the infrared wavelengths and performs mathematical
operations on these values in conjunction with values obtained during the factory set-up
process.
The microprocessor generates output information and feeds it to the digital analog converter to
produce a 4 to 20 milliampere (mA) signal that is proportional to the 0 to 100% LEL (or
methane % by volume) concentration of gas at the sensor. The microprocessor program also
monitors other conditions such as the supply voltage and the optical path integrity.
The Model IR400 provides a two-wire RS-485 addressable communications link conforming to
the Modbus protocol that is used to monitor the IR400’s status and settings in order to simplify
installation and maintenance.
Summary of Contents for IR400
Page 2: ......
Page 15: ...Model IR400 11 Figure 5b IR400 Mounting Dimensions with a European Box ...
Page 20: ...Model IR400 16 Figure 7 Wiring Diagram from IR400 to Control Equipment ...
Page 41: ...Model IR400 37 9 6 Engineering Documentation ...
Page 45: ...Model IR400 HART Field Device Specification for Point IR Gas Detector ...
Page 78: ...Model IR400 Infrared Point Detector for Hydrocarbon Gas Applications Modbus programming guide ...
