3.
Sample gas connection
Since pressure differences between sample gas- inlet and -outlet in-
fluence the measurement result and if exceeding differential pressure
of 50 mbar (0.7 psi) they might even damage the sensing detector, it
is essential that gas samples tapped from the plant are returned to
the plant through a port very close to the suction point!
The sample-gas feed hoses must be provided with a dust filter version 2
(see also
chapters 7.3
).The filters must correspond to the requirements
as per
chapter 3.1
and be installed at the beginning of each gas hose. By
means of the filters the measuring instrument and the measurement
system hoses are protected against pollution. Suitable filters are of such
quality that they will neither adsorb measurable gas components in the
filter element nor in the filter housing. As an additional feature the filters
must permit the filtration of particles size 5 micrometers.
In order to avoid pollution of valves and of measuring chamber, the
instrument may only be operated with filters protecting all measur-
ing channels. The filters are to be placed right behind the valves at
the plant respectively at the sample gas port.
Polluted gas hoses will adsorb molecules of the gas components to be
measured and will lead to incorrect measuring results, i.e. the sample gas
arriving at the measuring instrument is picking up additional molecules
of the gas under test from the hose walls and is therefore reaching a
higher concentration than at the sample gas port. Although this 'enriched'
concentration will decrease continuously, it will, nevertheless, in
correlation to the degree of hoses pollution delay the indication of the
true concentration value.
a2
