91.Tankscope

The sensing element of this instrument is usually a non-catalytic hot filament. The composition of the surrounding gas determines the rate of loss of heat from the filament, and hence its temperature and resistance.
The sensor filament forms one arm of a Wheatstone Bridge The Initial zeroing operation balances the bridge and establishes the correct voltage across the filament, thus ensuring the correct operating temperature During zeroing, the sensor filament is purged with air or inert gas that is free from hydrocarbons

The presence of hydrocarbon changes the resistance of the sensor filament and this is shown by a deflection on the bridge meter. The rate of heat loss from the filament is a non-linear function of the hydrocarbon concentration and the meter scale reflects this non-linearity. The meter gives a direct reading of % volume hydrocarbons. When using the instrument, the manufacturer’s detailed instructions should always be followed. After the instrument has been initially set at zero with fresh air in contact with the sensor filament, a sample is drawn into the meter by means of a rubber aspirator bulb. The bulb should be operated until the meter pointer comes to rest on the scale (usually within 15-20 squeezes) then aspirating should be slopped and the final reading was taken. It is Important that the reading should be taken with no flow through the instrument and with the gas at a normal atmospheric pressure.

The non-calatytlc filament is not affected by gas concentrations In excess of its working scale. The instrument reading goes off the scale and remains in this position as long as the filament is exposed to the rich gas mixture.