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Model 4688-IR / 4679-IR
Operation & Maintenance Manual
Manual #087-0007G, 10/2003
Introduction
The GasPlus-IR 24VDC powered INFRA-RED GAS TRANSMITTER uses
infrared absorption technology to monitor the level of combustible
hydrocarbon gases (Model 4688-IR) or CO
2
(Model 4679-IR) in the
surrounding atmosphere and transmits an analog (4-20 mA) or digital
(Modbus
®
) signal that is proportional to the measured concentration. The
output signal is linear for the specific gas being measured. Linearization
curves for numerous gases are built into the instrument and are user
selectable.
The instrument features 2 alarm relays capable of indicating high and high-
high alarm conditions, and a separate relay for indicating faults (relays are
optional). A local display reports concentration readings in units of either
%LEL or %V/V (user selectable), as well as system status. The operator
panel interface uses a Scott Instruments magnetic screwdriver for accessing
the system parameters and may be password protected. Non-intrusive user
setup and calibration permits operation and maintenance from within
classified areas.
Principle of Operation
Infrared radiation propagating from an incandescent emitter passes through a
volume of gas, then is reflected by a mirror onto 2 fixed detectors. One
detector is tuned to a wavelength that is absorbed by hydrocarbons or CO
2
,
while the other detector is tuned to a nearby wavelength that is not absorbed.
The ratio of the output voltages of the 2 detectors is used to compute the
concentration of gas. As gas concentration increases, the incremental output
produced by the detectors decreases. The instrument then uses a gas-
specific algorithm to linearize the signal. Since the amount of infrared
absorption differs with each gas, the instrument's cross-sensitivity will vary.
For example, an instrument setup to detect methane, which absorbs less
infrared radiation than most other hydrocarbons, will produce high readings
when exposed to most other combustible gases.
Other Gas Sensitivities -
Model 4688-IR
ONLY
The Model 4688-IR is specifically tuned to detect certain hydrocarbon gases
and will not detect non-hydrocarbon combustibles such as NH
3
, H
2
, etc. Scott
Instruments' Model 4600 GasPlus Universal Toxic Gas Transmitter provides
an excellent solution for many of these gases. Hydrocarbons not listed as
available (see parameter SR.GN) may or may not be detectable by the
transmitter. Consult the factory for further information.
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