ENMET ISA-44-RALE-OD - Manual (Page 7)

ISA-44RALE-OD/ISA-44RAHE-OD/ISA-44E-OD ENMET Corporation
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1.2 Operation Principles Explained ISA-44RALE-OD and ISA-44E-OD
Connected to the control unit by conduit, two separate sensor enclosures house two different sensing elements: a metallic
oxide semiconductor (MOS) sensor for carbon monoxide detection, and an electrochemical cell (oxygen cell) for oxygen
detection. Each sensor is connected to its own circuit board, alarm LED's, meter and relay circuitry; together, these
components constitute a channel. The CO and oxygen channels monitor a continuous pressure regulated and humidified
air sample for CO and oxygen content.
The MOS sensor, a nonlinear device, detects carbon monoxide via an oxidation process. Inside the sensor, a ceramic
element is heated electronically to certain temperatures. When carbon monoxide is present in the monitored air, an
oxidation reaction occurs on this ceramic surface, reducing its electronic resistance. Therefore, an electronic signal is
transmitted through the conduit to the control unit, where CO channel circuitry analyses the signal, and the meter displays
an approximate CO concentration.
Over time, the MOS sensor becomes contaminated, making its response to CO gas inaccurate. To clear the sensor, a
Purge/Operate/Horn off switch has been installed on the outside of the control unit. When switched to the PURGE
position, this switch heats the sensor to very high temperatures, clearing off contaminants in the process.
The electrochemical sensor (oxygen cell), a linear device, monitors the partial pressure of oxygen in the air. As the air
sample passes through the sample head assembly and over the cell, oxygen in this air diffuses through a thin, selective
membrane on the surface of the oxygen cell. This generates an electrochemical reaction inside the cell, producing a small
electrical current proportional to the partial pressure of oxygen. The current signal is transmitted to the control unit,
where oxygen channel circuitry analyses the signal and the meter indicates the oxygen content of the sampled air.
The ISA-44RALE-OD operates continuously on single phase 110 V AC . Because the instrument should be on at all times,
it does not have an on/off switch.
The ISA-44E-OD uses the same principles as the ISA-44RALE-OD. However, by changing the type of MOS sensor, the
type of gas monitored changes. Also, depending on the type of gas monitoring required, a purge switch may or may not
be required.