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NOTE: In addition to the sensor long-life filters, the Model 500 emission
analyzer uses mathematical compensation techniques to minimize any
residual cross-sensitivities that its toxic sensors may have to any gases other
than those they are intended to measure. For this reason, never use span
gases that are blends of two or more toxic gases.
f) COMBUSTIBLES SENSOR. This is a two element catalytic type sensor.
Any flammable gas in the vicinity of the active element will be combusted
with oxygen present and cause a rise in the temperature of the detector
element, which is essentially proportional to the heating value of the gas.
The rise in temperature causes an increase in the electrical resistance of the
element, which in turn is converted to a signal proportional to the gas
concentration. For proper operation it is necessary to supply a sufficient
amount of oxygen. This is achieved by mixing an equal amount of air to the
sample gas.
The combustibles sensor will, in principle, detect any hydrocarbon or
organic vapor, hydrogen gas and ammonia. The calibration gas used to span
calibrate this sensor is 1% methane balance nitrogen. Of course any other
gas, such as propane, may be used to calibrate the unit.
The minimum amount of oxygen available to the sensor is about 10%
(assuming no oxygen in the sample). Consequently, the maximum
concentration of methane that can be measured is 5%. For propane the
maximum concentration is 2-3%, and for octane 1%. The type of sensor
used is not dependent on oxygen concentration, as long as there is sufficient
oxygen for the reaction.
NOTE: This catalytic-type sensor is intended to be used as a detector of
dangerous concentrations of unburned gases and is of limited accuracy for
the measurements of hydrocarbons. It will also respond to the presence of
hydrogen and carbon monoxide, which are combustible gases. For accurate
measurements of low levels of hydrocarbons, one must use an NDIR
analyzer.
WARNING: Do not use the Model 500's combustible sensor as a safety-type
instrument in potentially hazardous atmospheres. The Model 500 is NOT
“intrinsically safe” and is only intended to measure the presence of small
amounts of gaseous fuel in stacks.
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