ENMET ISA-44-2OD - Manual (Page 6)

ENMET Corporation ISA-44-2-OD
2
2.0 Features and Controls of the 2.0 Features and Controls of the ISA-44-2-OD
How the ISA-44-2-OD Works
The ISA-44-2-OD uses gas-sensitive Metallic Oxide Semiconductor (MOS) sensing elements that react to toxic and
combustible gas molecules. An oxidation reaction occurs on the sensor surface when contaminants are present. The
oxidation reaction changes the gas sensor's electrical resistance, which then triggers the alarm circuitry when the
concentration of gas goes beyond a preset level. Also, an oxygen micro-fuel cell and associated circuitry maintain a
steady current indicating the presence of a satisfactory oxygen level. Should there be a lack of oxygen, the current
emitted by the micro-fuel cell will decrease accordingly, triggering the alarm in the same manner.
The ISA-44-2-OD has two similar circuits (printed circuit assemblies PC1 and PC2, see Figure 2) with related
sensors. Each of these circuit-sensor combinations makes up a separate channel of gas detection. The combustible gas
detection circuit is on PC2; the toxic gas detection circuit is on PC1. A third circuit-sensor combination on the right-
hand side of PC1 comprises the oxygen deficiency channel.
2.1 Standard Calibration
The majority of ISA-44-2-OD instruments are furnished with a standard calibration, which is as follows:
Combustible Gas Channel:
Alarm Level Alarm Threshold
Low Alarm: 10% LEL methane
High Alarm: 20% LEL methane
Meter Full Scale: 50% LEL methane
Toxic Gas Channel:
Alarm Level Alarm Threshold
Low Alarm: 10 ppm hydrogen sulfide
High Alarm: 20 ppm hydrogen sulfide
Meter Full Scale: 50 ppm hydrogen sulfide
Oxygen Deficiency Channel:
Alarm Level Alarm Threshold
Alarm: 19.5% oxygen
Meter Scale: 16% to 26% oxygen
This calibration corresponds to one of the major applications of the instrument. Because the instrument may be
purchased with many different calibrations, the manual has been written in a general fashion.