Hydrocarbon Gas Detectors
Catalytic Filament Combustible Gas (CFCG) Indicator (Explosimeter)
- In taking a measurement with explosimeters, the manufacturers’ detailed instructions should be followed. After the instrument has been initially set at zero with fresh air in contact with the sensor filament, the sample shall be drawn into the meter by means of a rubber aspirator bulb or a pump. The reading is to be taken when the pointer has ceased to rise on the scale.
- When using a CFCG indicator it should be borne in mind that the instrument does not give a reliable reading in atmospheres deficient in oxygen, such as those in inerted tanks, or in atmospheres with HC vapour above LFL. The meter must not be used in such atmospheres.
- Non hydrocarbon gases, such as hydrogen sulphide or carbon monoxide, or gases from lead compounds, which may be present in the tank atmosphere can affect the explosimeter response, but only if they are present in very high concentrations.
- The instrument is set up to read correctly in the factory using a hydrocarbon gas/air mixture, the composition of which will be indicated on the label fixed to the instrument, all personnel using the instrument shall be familiar with this information.
- The response should be checked at the beginning of every day during which it is intended to use the instrument. Such a check should also be made after replacing a filament. Test kits for use on board are available for this purpose providing a mixture of a hydrocarbon gas in air (such as 50% LFL butane in air). At intervals the instrument should be checked more thoroughly in a laboratory equipped with suitable gas blending facilities.
During operation it is important to check the instrument and sample lines occasionally for leakage, since the ingress of air would dilute the sample, giving false readings.
- Leak testing may be achieved by pinching the sample line and squeezing the aspirator bulb; the bulb should not expand as long as the sampling line is pinched.
- To avoid the effect of gas flow rate, take a reading when there is no flow, i.e. between two squeezes of the rubber aspirator bulb.
- Use of dilution tubes which enable catalytic filament indicators to measure concentrations in over-rich hydrocarbon gas/air mixtures is not recommended.
- Only instruments fitted with flashback arresters in the inlet and outlet of the detector filament chamber should be used. The arresters are essential to prevent the possibility of flame propagation from the combustion chamber; a check should therefore always be made that they are fitted properly in their place.
- Certain designs of CFCG indicators should have their PVC covers fitted around meters with aluminium cases to avoid the risk of incandescent sparking if the case impacts on rusty steel. These meters shall always be used with their covers in place.
- When hydrocarbons are being measured no filters should be used other than the cotton filter inserted in the gas inlet of the detector to remove solid particles or liquid from the gas sample, although a water absorbent material or water trap may be necessary in the sampling line if the gas is very wet.