Model IR5500
4
2.0 Product Description
2.1 General Description
The Model IR5500 Infrared Open Path System is a hydrocarbon gas detector
(Figure 1). The system consists of a Source and a Receiver. Both units are
calibrated at the factory and need no further alignment. The operation of the Model
IR5500 can be checked by placing a Test Gas Film in front of the Receiver. The
system requires only a periodic cleaning of the windows to assure dependable
performance, and is recommended prior to zero check, test gas, and optical
integrity testing. The Model IR5500 Infrared Open Path System continuously
monitors hydrocarbon gases. The gas detection range of a Receiver calibrated for
methane is 0 to 5,000 ppm•m and 0 to 5 LEL•m while that for propane it is 0 to
2,000 ppm•m and 0 to 1 LEL•m. The Receiver provides two 4 to 20 mA analog
signals, one proportional to ppm•m and the other to LEL•m. Alarm and Fault
relays, split range (A/O-ppm), Modbus, and HART outputs are optional.
The Model IR5500 Infrared Open Path System is easily aligned using the digital
display and adjustable mounting arms, therefore does not require any bulky setup
equipment such as digital volt meters and handheld alignment aids. For setups that
require the distance from the Source to the Receiver to be greater than 100
meters, General Monitors offers an optical alignment scope that makes the initial
alignment easier.
Prior revisions of the product were marked as the Ultima OPIR-5 Detector.
2.1.1 Infrared Detection Principles
Most gases absorb infrared radiation. Hydrocarbon gases absorb infrared radiation
at specific wavelengths but with different degrees of absorption. Absorption of the
radiation follows the Beer-Lambert Law, which states that the transmittance (T) of
radiation through an absorbing medium decreases exponentially by the product of
the absorption coefficient (A), the concentration (C), and the path length (L):
T = T
0
exp(-ACL)
Here T
0
is the transmittance of light containing no absorbing medium.
2.1.2 IR5500 Detection Method
The Model IR5500 uses a single beam, dual wavelength method of infrared
absorption detection. The gas absorbs one wavelength but not the other, which is
the reference wavelength. By comparing the signals from these two wavelengths,
the detector measures gas concentration. The reference wavelength is chosen to
compensate for interferences that can otherwise occur from atmospheric variation,
such as humidity, rain, dust, snow, fog, steam, and temperature. This method of
detection comes under what is commonly known as the non-dispersive infrared
(NDIR) absorption principle.
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