2.7.5 The Exhaust Gas
Exhaust gas is the sample (or zero/span) gas that has passed through the analyser. For
hazardous gases (high concentrations, flammable or poisonous), exhaust gas must be carefully
routed to a safe venting point.
It is important to minimise ‘back pressure’ at the GAS OUT port, as this will affect readings and
reduce the efficiency of the sample pump. Where long runs of pipe are necessary, larger bore
pipe should be used via a suitable adapter. Where the exhaust pipe is connected to a forced air
extraction duct, make sure that any suction is minimal.
2.7.6 Gas Pump(s)
Gas pumps fitted in the instrument are of the vibrating diaphragm type and are capable of up
to 1.0 I/min
1
under pressures normally encountered.
The sample pump operates at fixed speed and at a flow rate of nominally 500 ml/m1n
1
. If it
should be necessary to adjust the flow, it can be set using the potentiometer VR2 on the power
supply board and / or the throttle (when fitted).
The internal sample pump is operated via the front panel 'PUMP' menu button.
Certain instrument configurations may be fitted with a second internal pump which is used to
circulate purge gas. This pump would be set to run continuously at a factory set rate.
2.8 Use of Chemicals (certain models only)
Certain instruments use chemicals to remove (or strip) certain gases from the air for zero
adjustment, or to purge the optics to prevent ambient air from affecting readings. These
chemicals must be replaced regularly to ensure efficient operation and hence good accuracy.
The chemicals are contained in glass ‘columns’ which are accessible from the front of the
instrument to aid inspection and refilling.
The chemicals that may be used are listed below under the gas application.
2.8.1 Water Vapour
An indicating form of calcium sulphate (CaSO
4
) known as 'DRIERITE' is normally supplied with
new instruments and replacement supplies are available from ADC (Spares & Service) Ltd or
their local authorised agents. In use, drierite changes from blue to pink as it becomes
exhausted, making it obvious when the container should be refilled with fresh chemical.
Drierite can be recovered by drying.
Magnesium Perchlorate (Mg (ClO
4
)
2
0 (also known as Anhydrone) is also suitable, but it is a
strong oxidising agent, which may restrict its use. It is an effective drier, but does not provide
any indication when it is exhausted and is difficult to recover. As more water vapour is
absorbed it takes on a solid appearance, at which time it should be replaced.
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