TM 10-6665-297-13&P
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GAS DETECTOR UNITS (continued)
With the sensing element surrounded by a double flame arrestor, the detector is suitable for location in Class I,
Divisions 1 and 2, Groups A, B, C, and D hazardous locations as defined by National Electrical Codes.
NOTE: Standard detectors should not be used in oxygen-enriched atmospheres. Special detectors
should be chosen for such atmospheres in consultation with the factory. Oxygen-deficient and inert
atmospheres may be monitored by standard detectors if sampled gases are diluted by air to
support an oxidation reaction on the catalytic surface of the sensing element.
1-2.1
DETECTOR SENSING DEVICE. (Figure 1-1) The sensing element of the combustible gas detector is
a platinum-covered bead protected by a porous metal, cylindrical cap that freely passes gas and air while
excluding airborne, nongaseous materials from the area surrounding the element. Individual elements for use
in CD800/830 Series detectors normally sense the presence of combustible gases of any kind. Elements are
also available with special sensivities to particular gases such as methane, hydrogen, or ethylene oxide.
The sensitivity of all catalytic materials may be inhibited by certain compounds, usually called “poisons.”
Tetraethyl lead and silicone compounds are the most critical. J-W sensing elements utilize a unique design
and are resistant to catalytic poisoning. But aerosol sprays, polishes, waxes, and lubricants containing silicones
should not be used in the vicinity of sensing elements. Where poisons cannot be eliminated, frequent gas
testing is essential to ensure safe and accurate operation of the gas detection system.
Figure 1-1. Detector Sensing Element
1-2.2
DETECTOR ELECTRICAL CIRCUITRY. (Figure 1-2) Regulated 6-volt (nominal) DC power from the
control module is applied to balanced pairs of resistances in the detector. In normal atmospheres free of
combustible gases, the paired resistances are in electrical balance, so that no signal current is sensed by the
measuring circuit of the control unit. Oxidation of combustible gas on the catalytic element acts to increase its
resistance,
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