4.3
4.2 End Boss Purge Connection
Depending on application, fittings for 1/8" o.d. tube may be provided to purge the end
bosses on either side of the sample cell and these should now be connected. Refer to
the following application notes and Figure 4.2 where necessary.
4.2.1 Introduction
The 2500 measuring cell is mounted on two short tubes or "end bosses" which
preserve the integrity of the optical system, while providing thermal isolation. The
measuring cell and the analyser enclosure are both independently sealed using
windows or lenses and elastomeric seals. The end bosses form the interface between
the two, and the implications of a cell window leak must be considered when
flammable, corrosive or toxic gases are measured.
Under ideal circumstances, the gas in the end bosses should be totally transparent to
the wavelengths of infrared energy used for the measurement. However, simply filling
the bosses with nitrogen and sealing them is not always satisfactory, because the
elastomeric seals are permeable and atmospheric gases such as carbon dioxide or
water vapour can leak in. Specifically, a problem will arise when the gas being
measured is also carbon dioxide or water vapour, or when the measurement is cross
sensitive to either of those gases: the effect of a gradual ingress of the atmospheric
gases will be seen as drift in the measurement. A further problem arises with low-level
measurements of carbon monoxide. In this case it is out-gassing of carbon monoxide
from the seals themselves which can cause drift.
4.2.2 End Boss Scrubbers
Problems, relating to cross interference within the end bosses, can usually be resolved
by fitting chemical "scrubbers" to the end bosses. These are small metal housings
packed with a chemically active material, which absorbs or reacts with the problem gas.
A number of materials are available which absorb water vapour (molecular sieve),
carbon dioxide (soda lime) and carbon monoxide (Hopcalite).
When a 2500 is specified, a suitable scrubber material is stipulated for the end bosses,
where necessary, and the analyser will be delivered with these scrubbers fitted.
NOTE
If the analyser is dismantled for any reason, the scrubbers must not be left exposed
to normal atmosphere for more than a few minutes or they will be exhausted.
Similarly, after long-term use they may need to be replaced, and this must always
be with the correct type.
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