2
95-8556
2.1
OPERATION OVERVIEW
THEORY OF OPERATION
The OPECL transmitter module illuminates a direct
linear path ending at the OPECL receiver module. As
flammable hydrocarbon gases intersect the light beam
between the two modules, certain IR wavelengths are
absorbed by the gas, while other IR wavelengths are
not. The amount of IR absorption is determined by the
concentration of the hydrocarbon gas. A pair of optical
detectors and associated electronics located in the
receiver module measure the absorption. The change
in intensity of the absorbed light (active signal) is
measured relative to the intensity of light at a non-
absorbed wavelength (reference signal). The
microprocessor computes the gas concentration and
converts the value into a 4 to 20 milliampere current
output signal, which is then communicated to external
control and annunciation systems. No filter wheel
motors or other moving parts are utilized in either
module.
The output signal is a 4-20 mA signal, which
corresponds to 0-5 LFL-meters. To better understand
the concept of LFL-meters, refer to Figure 1, which
shows how three gas clouds of different size and
concentration would produce the same output of 1 LFL-
meter by the open path gas detection system.
NOTE
The system must be configured for less than 60%
of the full scale measuring range, with
consideration to the accepted gas cloud size and
concentration.
LAMP REDUNDANCY
The OPECL transmitter provides a unique operational
feature. A xenon lamp illuminates the linear detection
path from the transmitter to the receiver. In the event
that the primary lamp fails, the back-up lamp is brought
into service. The receiver recognizes this condition by
the change in flash pulse coding and responds by
initiating a “failed lamp” warning signal to indicate that
the back-up lamp is in use. Total system operation is
maintained, with no loss of sensitivity or detection
performance. In this mode of operation, the following
occur:
1. The indicator LED turns amber.
2. The 4-20 mA signal drops from a normal 4 mA to 2.4
mA. The analog output value is overridden if the gas
level exceeds 0.5 LFL-meters. All gas alarm signals
will occur as when operating with the primary lamp.
3. HART and MODBUS communication warn of a
“failed lamp” condition.
Service should be arranged as soon as possible. A
new transmitter module should be installed.
NOTE
The “failed lamp” condition indicated by the 4-20
mA output can be overridden by applying a
magnet to the transmitter module. The coded
flash sequence will then return to normal, which
causes the 4-20 mA signal at the receiver module
to also return to normal. This condition will
continue until power to the transmitter unit is
cycled.
DETECTABLE GASES
OPECL is capable of detecting most hydrocarbon
gases and vapors including methane, ethane, propane,
butane, and propylene. Gas type and other operational
parameters are selected via digital communications.
The factory calibrated setting is methane.
1M @ 100% LFL
2M @ 50% LFL
10M @ 10% LFL
OPECL OUTPUT EQUALS 1 LFL-M IN ALL THREE SCENARIOS
A2304
Figure 1—Detector Response to Three Gas Clouds of Different Size
and Concentration
