A sample of gas is drawn into the sample chamber using the hand pump. Any combustibles land on the surface of the hot wire and burn increaseing the temperature of the wire thereby increaseing the resistance. This causes an imbalance in the wheatstone bridge arrangment indicated on the display. This display is scaled in percent lower explosive limit and is calibrated for hydrocarbons. Note that the unit will operate against any combustible gas although the readout would be erroneous.
Calibration gas is used to check the accuracy of the meter. In addition the unit must be landed on a regular basis for formal calibration.
Chemical

The teflon is semipermeable letting in gasses and not moisture.

The instrument functions by the oxidation of the electrolyte which increases or decreases the conductivity of the cell depending on the oxygen content.

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