Operation overview – Det-Tronics OPECL Infrared Hydrocarbon Gas Detector User Manual

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10.1

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 (digital signal for EQP model), 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 (digital signal for

EQP model), 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.

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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.

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.

STaNDaRD OUTpUT
A 4 to 20 mA current loop corresponding to 0 to 5

LFL-meters is provided for connection to analog input

devices such as gas controllers, logic controllers, or

distributed control systems (DCS). To convert the mA

reading to LFL-meters, use the following formula:

mA Reading –4

X 5 = LFL-Meters

The EQP system displays the gas concentration in

LFL-meters.

OpTIONal RElayS
The Model OPECL can be furnished with a factory

installed relay output board that provides two

programmable alarm relay outputs and one fault

relay output. All relays are sealed and provide

form C (NO/NC) contacts. The high and low alarm

relays are programmable and can be set for latching

or non-latching operation. When the high alarm

relay is configured for non-latching operation,
the detector must be connected to an auxiliary
system that provides the latching function

. The low

alarm cannot be set above the high alarm threshold.

Alarm configuration can be done with the HART or

MODBUS interface. The onboard multi-color LED

indicates a LOW alarm condition via a flashing red

color, and a HIGH alarm condition via a steady red

color. The OPECL internal magnetic switch or HART

Field Communicator can be used to reset latched

alarms. A short-duration magnetic switch activation

of 1 second will reset latched alarms. Holding the

magnetic switch closed for 2 seconds will start the

calibration sequence. The external calibration line will

not reset latched alarm relays.

Alarm levels for the EQP model can be set using S

configuration software.

When the optional relay output board is installed, the

OPECL receiver is certified for EEx d operation.

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Refer to “Alarm Relays” in the Specifications section
of this manual for important information regarding
alarm relays.

1M @ 100% LFL

2M @ 50% LFL

10M @ 10% LFL

OPECL OUTPUT EQUALS 1 LFL-M IN ALL THREE SCENARIOS

A230B

Figure 1—Detector Response to Three Gas Clouds of Different Size

and Concentration