Operation
Catalytic pellistor sensors are sensitive to most flammable gases and vapours. It is possible therefore to use one detector to monitor a wide range of flammable gases and vapours.
Pellistor detectors consist of two coils of fine platinum wire each embedded in a bead of alumina, connected electrically in a bridge circuit. One of the beads is impregnated with a special catalyst which promotes oxidation whilst the other is treated to inhibit oxidation. Current is passed through the coils so that they reach a temperature at which oxidation of a gas readily occurs at the catalysed bead (about 500°C). This raises the temperature further which increases the resistance of the platinum coil in the catalysed bead, leading to an imbalance of the bridge. This output change is linear, for most gases, up to and beyond 100% LEL and response time is only a few seconds to detect alarm levels (typically 20% LEL).
Catalytic sensors can be 'poisoned' temporarily (e.g by compounds containing halogens, sulphur or chlorine) or permanently (e.g by substances containing lead or silicones). This has the effect of reducing sensitivity. Crowcon fit poison resisting pellistors to all fixed flammable gas detectors. Further protection against poisoning of fixed flammable gas detectors can be provided by using Crowcon's self adhesive carbon filter which is fitted on the outside of the sintered flame arrestor. It is suitable for hydrogen and methane monitoring but, unfortunately it reduces the sensitivity of the detector to propane (20% reduction), butane and pentane (50% reduction). The filter must always be replaced when a detector is re‑calibrated and at no more than 6 months intervals.
If a pellistor is exposed to gas above the UEL for more that a few minutes, it may suffer reduction in sensitivity caused by "sooting". The degree of sensitivity reduction and whether the effect is permanent will depend on the gas. Aromatic compounds, having a high carbon content cause the worst problems.
In the absence of adverse factors pellistor sensors will last for many years. However they do lose performance continuously, typically by 20% per annum, therefore 6 monthly re‑calibration of instruments employing pellistors is recommended. Pellistor based detectors work satisfactorily in ambient temperatures of ‑40 to +60°C.
Correction Factors
When an estimate of a particular gas concentration is required and where the gas being measured is known to differ from the gas for which the detector is calibrated a "CORRECTION FACTOR" may be applied.
Most equipment is calibrated for either 0 to 100% LEL methane or 0 to 100% LEL n‑pentane. The lists of factors can be used for calculating the concentrations of other gases using either of these calibrations.
Example 1
If a detector is calibrated for n‑pentane and the gas being measured is hydrogen, then from the appropriate list it will be seen that the factor to use is 0.6. Therefore for a meter reading of 65, the corrected reading for hydrogen is:
65 x 0.6 = 39% LEL hydrogen
As the reading is higher with hydrogen it follows that the alarm will operate at a lower level. For an alarm setting for n‑pentane of 20% LEL, the level at which the alarm will operate in hydrogen is 20 x 0.6 = 12% LEL hydrogen. In some instances the correction factor is greater than one. In these cases the detector reads low and the reading has to be increased to bring it up to the proper value.
Example 2
If a methane calibrated detector is used to measure acetone, the correction factor is 1.6, therefore a reading of 65%
LEL would be corrected to 65 x 1.6 = 104% LEL acetone and an alarm setting of 20% LEL would actually operate at 20 x 1.6 = 32% LEL acetone.