ATI Technologies A12-17, Operation And Maintenance Manual

Page: 27 / 32

A 12-17 Combustible Gas Transmitter
O & M Manual
Rev-H, 7/15
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CALIBRATION FOR OTHER COMBUSTIBLE GASES
As previously mentioned, a combustible gas sensor has a slightly different response to each
combustible gas or vapor. In addition, the LEL (Lower Explosive Limit) represents different percent
concentrations for different gases. For instance, the LEL for methane is 5% by volume while the LEL for
butane is 1.9% by volume. Because of these factors, a combustible transmitter must be adjusted
differently if the system is intended to detect a gas or vapor other than methane.
A 1% methane gas standard may still be used for calibration of combustible transmitters when used
for other gases. However, the %LEL value set on the LCD will be different for each gas. Table 1
provides the LEL setting for various gases. The setting assumes the use of 1% methane as the span
gas. As an example, to calibrate the transmitter for hexane detection using 1% methane standards, apply
the 1% Methane gas in accordance with the span adjustment instructions but set the LCD to 50% in step
4.
TABLE 1
GAS % LEL
Methane 20
Propane 35
n-Butane 35
n-Pentane 40
n-Hexane 50
Hydrogen 30
Methanol 30
Ethanol 40
Isopropyl Alcohol 55
Acetone 50
Methyl Ethyl Ketone 50
Benzene 55
Toluene 60
Di-ethyl Ether 10
Ammonia 15
Methyl T-Butyl Ether 46
SENSOR RESPONSE TEST
While zero and span adjustments are required only periodically, gas sensors should be checked
regularly for proper response. The response check can be done quickly by simply aiming the outlet tube
from the span gas cylinder at the face of the sensor and turning on the gas flow for 10-20 seconds. The
sensor should begin to respond within 5 seconds. If the sensor does not respond, it should be replaced.
This type of manual testing is normally not required for systems supplied with the Auto-Test generator.
However, it should be used to test the sensor in the event of an Auto-Test failure.
CAUTION: This response test will cause the output to increase and may cause receiving devices
to activate alarms and/or control devices. Never run a sensor response test without
inhibiting alarm receivers or notifying operating personnel that a system test is in
progress.