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What is Gas?
Flammable Risk
Toxic Risk
Oxygen Risk
Gas Detection Applications
Fixed Systems Installation
Sampling Systems
Pellistor Sensors
Electrochemical Sensors
Infrared Absorption
Semi Conductor Sensors
Thermal Conductivity Sensors
The Sulphistor Sensors
Other Gas Sensing Technologies
Detector Calibration
Calibration Ranges & Alarm
Level
PELLISTOR SENSORS
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.
CORRECTION FACTORS FOR PELLISTORS
To obtain an approximate LEL value for the gases/vapours below, multiply the meter reading on a pentane or,
methane calibrated detector by the value shown.
(P) = Pentane
(M) = Methane
Detector Calibration
P
M
Detector Calibration
P
M
Acetaldehyde
0.8
1.6
Ethylene
0.7
1.2
Acetic Acid
0.8
1.6
Ethylene Dichloride
0.7
1.4
Acetic Anhydride
1.0
2.0
Ethylene Oxide
0.9
1.8
Acetone
0.9
1.6
N Heptane
1.3
2.3
Acetylene
0.8
1.7
N Hexane
1.3
2.0
Acrylonitrile
1.0
2.0
Hydrogen
0.6
1.2
Alkyl Alcohol
0.9
1.9
Kerosene
1.4
2.8
Ammonia
0.35 0.6
LPG
1.1
2.1
N Amyl Alcohol
1.4
2.8
Methane (LNG)
0.5
1.0
Aniline
1.2
2.5
Methanol
0.5
1.2
Benzene
1.1
1.9
Methyl Chloride
4.0
8.0
1.3 Butadiene
0.8
1.3
Methyl Cyclohexane
1.0
2.0
N Butane
0.8
1.6
Methylene Dichloride
0.5
1.0
Iso Butane
0.9
1.8
Dimethyl Ether
0.7
1.4
Butene 1
1.0
2.0
Methyl Ethyl Ether
1.0
2.0
N Butanol
1.4
2.9
Methyl Ethyl Ketone
1.1
2.2
I Butanol
0.9
1.9
Methyl N Propyl Ketone 1.6
3.2
Tert Butanol
0.6
1.3
Naphthalene
1.3
2.8
Butyl Acetate
1.5
3.0
N Nonane
1.4
2.8
N Butyl Benzene
1.4
3.0
N Octane
1.3
2.6
Iso Butyl Benzene
1.4
3.0
N Pentane
1.0
1.9
Carbon Monoxide
0.6
1.2
Iso Pentane
1.0
1.9
Carbon Disulphide
4.0
8.0
Petrol
1.0
1.9
Carbon Oxysulphide 0.5
1.0
Propane
0.65 1.3
Cyclohexane
0.9
2.0
N Propanol
1.0
2.0
Cyclopropane
0.8
1.6
I Propanol
0.9
1.8
N Decane
1.4
2.8
Propylene
0.9
1.8
Diethylamine
0.9
1.8
Propylene Oxide
1.0
2.0
Dimethylamine
0.8
1.6
Iso Propyl Ether
1.0
2.0
2.3 Dimethylpentane 1.1
2.2
Propyne
1.1
2.2
2.2 Dimethylpropane 1.1
2.2
Styrene Monomer
4.0
8.0
Dimethyl Sulphide
1.1
2.2
Tetra Hydra Furan
1.0
2.1
Dioxane
1.0
2.0
Toluene
1.1
1.9
Ethane
0.7
1.4
Trimethylbenzene
1.5
3.0
Ethyl Acetate
0.9
1.8
White Spirit
1.5
3.0
Ethanol
0.6
1.85 o Xylene
1.2
2.8
Ethyl Benzene
1.3
2.6
m Xylene
1.1
2.2
Ethyl Bromide
0.5
1.0
p Xylene
1.1
2.2
Ethyl Chloride
0.8
1.6
Ethyl Cyclopentane
1.1
2.2
Ethyl Ether
1.0
2.1
Crowcon – Gas Detection, Gas Monitoring, Gas Sampling