Detcon CXT-IR - Manual (Page 28)

Model CXT-IR
3.4.4 Set Gas Factor
Because of the CXT-
IR sensorꞌs almost universal response to combustible hydrocarbon gases, the CXT-IR
sensor can be configured and calibrated to detect a variety of combustible gases. The detected gas is referred to
as the "target gas" and the span calibration gas is referred to as the "cal gas". In cases where the cal gas is
different from the target gas, the Set Gas Factor menu function is used to maintain accuracy. This feature
allows for a significant degree of flexibility in the detection and span calibration process.
NOTE
The default value for Gas Factor is 1.0. This would be used when the target gas is the same
as the cal gas.
Set Gas Factor is used to make the appropriate signal sensitivity adjustment when the target gas is different
from the cal gas. This is necessary because the IR sensor has different signal strengths for each combustible
hydrocarbon gas. The Gas Factor value is adjustable from 0.2 to 5.0. It represents the translation between the
target gas and the cal gas when they are different.
The menu item appears as: Set Gas Factor.
To calculate the correct Gas Factor (Table 1), take the Gas Factor of the target gas and divide by the Gas
Factor of the cal gas. The calculated value is the correct number to enter into the menu as the Gas Factor.
For example, if calibrating with methane when propane is the target gas, the correct Gas Factor to enter would
be 0.63/1.0 = 0.63.
For example, if calibrating with butane when ethane is the target gas, the correct Gas Factor to enter would be
0.38/0.72=0.53.
Table 1 shows the Gas Factors of most combustible hydrocarbon gases that will be measured. Find the gas of
interest for the cal gas and the target gas and follow the above instruction. If there is a mixture of target gases,
use a weighted approach to determine the correct Gas Factor.
For example, if the target gas was 50% butane and 50% pentane and the cal gas was methane, the correct Gas
Factor would be calculated and entered as ((0.5 x 0.77) + (0.5 x 0.77)) / 1.0 = 0.77.
Table 1 Gas Factors
Gas Factor Gas Factor Gas Factor
Acetic Acid 2.00 Decane 1.53 Naphthalene
Acetone 1.21 Ethyl Alcohol 0.35 n-Nonane 1.53
Benzene 1.00 Ethane 0.38 n-Octane 1.34
1,3-Butadiene 1.80 Ethyl Benzene 1.07 n-Pentane 0.77
Butane 0.77 Ethylene 2.39 Propane 0.63
Iso-Butane 0.72 n-Heptane 0.98 iso-Propyl Alcohol 0.54
Butene-1 0.67 n-Hexane 1.00 Propylene 0.80
n-Butyl Alcohol 0.63 Dimethyl Ether 0.40 Toluene 1.00
iso-Butyl Alcohol 0.63 Methane 1.00 Vinyl Acetate 1.43
Cyclohexane 0.89 Methanol 0.41 Vinyl Chloride
Cyclopropane 0.45 Methyl Ethyl Ketone 0.77 Xylene 1.00
From the Set Gas Factor text scroll, hold the magnet over PGM1 or PGM2 until the arrow prompt appears and
continue to hold the magnet in place for an additional 3 to 4 seconds (until the display starts to scroll Set
Factor). The display will then switch to X.XX (where X.XX is the current gas factor). Swipe the magnet
momentarily over PGM2 to increase or PGM1 to decrease the gas factor level until the correct value is
CXT-IR Instruction Manual Rev. 1.6 Page 24 of 44