4
A detector can be tested with methane bump gas. Such tests merely verify that the gas
sensor(s) are in operating condition. To verify and adjust, if necessary, their sensitivity they must
be calibrated.
A detector can be automatically calibrated in approximately one minute using Bascom-
Turner's calibration gas (2.5% CH
4
and 100 ppm CO in air). Given the ease and speed of automatic
calibration, it pays to calibrate as frequently as possible, and certainly, no less than monthly.
Accuracy.
A properly operating and calibrated detector will respond with the specified
accuracy. If combustible gases other than the gas used for calibration are likely or suspected, the
instrument
cannot
be relied upon to give a proper indication of their concentration and hence of
how close to their combustible limits they may be. For example, the detector responds quite
differently to gasoline, to methane, and to propane. Accordingly, readings of % LEL (or its
equivalent in percent gas) refer only to the calibration gas and can be relied upon only in this
respect in assessing an atmosphere sampled by the detector. Furthermore, concentrations displayed
by the detector refer to a local sample at the tip of the instrument probe. Low gas concentrations
at one spot do not necessarily mean that the gas concentration is low throughout a wider area.
Safety.
These instruments measure the concentration of natural gas in air and give, more
generally, an indication of the presence of other combustible gases. When used by trained
personnel, they constitute one element for assessing the safety, or lack thereof, of a particular
atmosphere.
However, they must not be relied upon by themselves for judging safety; all
other significant factors must be taken into account.
The measured concentration refers only to
the immediate vicinity of the probe – concentrations in a wider area may be significantly different.
A trained technician should consider possible interferences, physical or chemical, and be aware
that calibrated values are only valid in air. If other gases, including inert gases such as carbon
dioxide are present, the measurements may not be reliable. Finally, although there are built-in
safeguards, a specific instrument may malfunction. It is therefore imperative that other indicators,
including, for example, odorant levels, must be taken into account.
WARNING:
Each detector responds to the gases for which it was designed. Other toxic
or dangerous gases may not be detected.
WARNING: Catalytic combustion sensors do not respond in inert or reducing
atmospheres. They should not be used for detecting combustible dusts or
mists.
Summary of Contents for GAS-EXPLORER II EGI-201
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