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.
WARNING:
Each detector responds to the gases for which it was designed. Other toxic
or dangerous gases may not be detected.
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:
Catalytic combustion sensors do not respond in inert or reducing
atmospheres. They should not be used for detecting combustible dusts or
mists.
3.