Factors Affecting Response
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6. FACTORS AFFECTING RESPONSE
6.1 Combustible Gas
The performance of the combustible gas sensor requires sufficient oxygen
levels (at least 10%) to function properly. When monitoring combustible
gases, always simultaneously monitor for oxygen, particularly in any
application where a lean oxygen atmosphere could be present, as in a
confined space.
6.1.1 Rich Atmospheres:
In a rich atmosphere, the combustible gas concentration is greater than the
Upper Explosive Limit. Several dangers exist in an atmosphere rich in
combustible gas. The introduction of fresh air, or simply the introduction of
some method of air circulation, could dilute the concentration enough to
bring it down into the combustible range. In addition, as the volume of
combustible gas rises, its natural cooling effect, which varies from one gas
to the next, reduces the temperature of the sensor filament and a lower
reading is usually the result.
6.1.2 Oxygen Dependence:
The presence of oxygen in the sampled atmosphere is necessary for the
proper measurement of the combustibles. If sufficient oxygen is not present,
the measurement of combustible gases could be thrown off so drastically
that hazardous levels could be present and they would not be detected.
6.1.3 Pocketing:
Lean mixtures may collect or be trapped in an area of the confined space
such as corners, support beams or equipment and become flammable,
therefore it is critical to test all areas and levels of a confined space for the
presence of combustible gases.
Factors Affecting Response
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6.1.4 Combustible (%LEL) Relative Sensitivity:
The following table shows the sensitivity of the LEL sensor to a number of
other combustible gases. The values listed are relative to a calibration for
100% LEL for Methane.
Combustible Gas
(LEL)
Relative
Sensitivity
Combustible Gas
(LEL)
Relative
Sensitivity
Methane
100
Acetone
45
Propane
53
Methyl Ethyl Ketone
38
n - Butane
49
Toluene
40
n - Pentane
45
Ethyl Acetate
39
n - Hexane
43
Hydrogen
81
n - Heptane
37
Ammonia
126
n - Octane
35
Cyclohexane
40
Methanol
69
Carbon Monoxide
80
Ethanol
59
Isopropyl Alcohol
39
6.2 Oxygen:
The oxygen sensor can underestimate the amount of oxygen present if water
vapor in the area has a higher temperature than the ambient temperature. To
adjust for this situation, use the QTP-2000 Sample-Draw Pump to cool the
air being drawn to the sensor, allowing the water vapor to condense and
permitting sampling at a relatively constant temperature and humidity.
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