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Model 4688-IR / Model 4679-IR
Operation & Maintenance Manual
Manual #087-0007G, 10/2003
Lower Explosive Limit -
Model 4688-IR ONLY
Combustible gas mixtures can be ignited only between certain concentration
limits. These limits are known as the Lower Explosive Limit (LEL) and the
Upper Explosive Limit (UEL). When these terms are used, it is assumed that
a normal atmospheric air background (20.9% O
2
) is present. For example,
the LEL for methane is a concentration of 5.00 %V/V (volume per volume) in
air.
Combustible gas sensors are designed to operate at concentrations under
the LEL, and typically the readings are calibrated in %LEL. For methane, 100
%LEL is defined as a concentration of 5.00% in air; 50% LEL is defined as a
concentration of 2.5%, and so on. If the reading is under 100% LEL, the gas
at the sensor cannot ignite. However, even very low readings indicate that
there may be a significant gas leak at some distance from the sensor.
Readings exceeding 100% LEL are possible.
Suitability For Use in Hazardous Locations
The Models 4688-IR and 4679-IR are designed to be acceptable for use in
Class 1, Zones 1 and 2, Groups B, C, and D hazardous (classified) locations
(these locations are approximately equivalent to Class 1, Division 1 and 2,
Groups B, C, and D, as defined in the pre-1996 NEC). The instrument
accomplishes this primarily through the use of the explosion-proof method of
protection (Ex d). The 1 exception is the heater which uses the intrinsically-
safe (Ex ib) method of protection.
Using a Flowcell
When using the GasPlus-IR with a flowcell, it is important to recognize that
output will vary with gas pressure. The Ideal Gas Law establishes a
relationship between pressure and the number of molecules. Where
P=pressure, V=volume, n=number of molecules, R=gas constant, and
T=temperature):
P V = nR
P V = nR
P V = nR
P V = nR
P V = nRTTTTT
If temperature is held at a constant, the number of molecules in a given
volume of gas (gas density) will vary linearly with pressure. Since the amount
of infrared absorption increases with gas density, the higher the pressure in
the flowcell with respect to atmospheric, the higher the instrument's
concentration output will be (and vice-versa). Note that the flowcell pressure
can be either positive (i.e., gas is being pushed into the flowcell) or negative
(i.e., gas is being pulled through the flowcell).
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