RKI Instruments GP-204 - Manual (Page 5)

Gas Detection For Life
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Certain substances have the property of desensitizing the catalytic surface of the platinum filament.
These substances are termed “catalyst poisons” and can result in reduced sensitivity or in failure to
give a reading on samples containing combustible gas. The most commonly encountered catalyst
poisons are silicone vapors, and samples containing such vapors even in small proportions should be
avoided.
Occasional calibration checks on known gas samples are necessary, especially if the possibility
exists of exposure to silicones. A calibration check on a known methane gas concentration is the
most dependable as an indication of normal sensitivity. A convenient calibration kit is available and
described under “Accessories”.
C. Rich mixtures
When high concentrations of gas are sampled, especially those above the considerable heat is
liberated at the filament. This heat may cause damage to the filament or tend to shorten its life, so
sustained testing of samples beyond the meter range should be avoided. When sampling rich
mixtures, the following instrument action may be expected:
1) Mixtures up to 100% L.E.L. reading on scale.
2) Mixtures between L.E.L. and Upper Explosion Limit (U.E.L.) readings at top of meter.
3) Mixtures above U.E.L. - As sampling continues the meter first goes to top of scale, then
comes back down on scale, depending upon concentration.
4) Oxygen deficient mixtures
Samples which do not have the normal proportion of oxygen may tend to read low, if there is
not enough oxygen to react with all combustible gas present in the sample.
As a general rule, samples containing 10% oxygen or more have enough oxygen to give a full
reading on any combustible gas sample up to the L.E.L.
5) Oxygen - Enrich mixtures
Sample having more than the normal proportion or oxygen will give a normal reading.
However, they should be avoided because the flame arrestor used is not dense enough to
arrest flames from combustible gas in oxygen, which can be much more intense than those in
air. DO NOT ATTEMPT TO USE THE MODEL GP-204 ON SAMPLES OF COMBUSTIBLE
GAS IN OXYGEN.
Instruments for measurement of oxygen concentration are available from RKI Instruments Inc.
6. ACCESSORIES
A. Extension hoses
Additional lengths of hose may be used for sampling from deep tanks and manholes. The
polyurethane hoses are satisfactory for most samples including natural gas, hydrogen, propane, and
gasoline vapors.
B. Water trap (Option)
Where there is danger or water being drawn into the instrument, a water trap should be used. This
glass bodied trap with sintered metal filter couples to indicator inlet and will collect water that is drawn
into or condensed in the sample hose. Inspect trap periodically while in use, and empty or clean