To prevent the possibility of condensation in the Instrument if the Instrument is not in
continuous use, it is recommended that it is left Switched on with the Sample Pump Switched
off.
Samples may contain other gases which ‘interfere’ with the gas being measured. The extent to
which known interferents can affect the measurement are defined in the Instrument
Specification.
Sample gas pipes shall be inert to and shall not contaminate the gas.
The response time for a change in concentration will be affected by the sample flow rate.
Instrument response times are normally quoted for the recommended 500ml min
-1
flow rate,
ignoring the effect of sample pipe volume. Lower flow rates will increase the response time, as
will the volume of the sample piping. Higher flow rates will reduce the response time, but
excessive flow will pressurise the instrument which may cause damage or measurement errors.
If it is necessary to draw higher flows than recommended (e.g. due to long sample gas lines), it
will be necessary to provide some means of bypassing the excess flow to exhaust.
2.7.3 The Span Gas
The Span Gas is used for calibration and is a known concentration of the measured gas -
usually between 50% and 80% of the full-scale range of the Instrument. The gas is usually
obtained specially mixed in a pressurised cylinder and this must be fitted with a 2-stage
pressure regulator, with an output pressure indication of 0 - 0.3 Bar (0-5psig).
The Regulator should be set to give a nominal output pressure of 0.2 Bar (3psig). This will
usually give a suitable gas flow during a span operation (i.e. flow indicator approximately mid-
position). If necessary, a small adjustment to the pressure will provide correct flow.
WARNING – Application of gas pressures above 0.3 Bar / 5 psig will damage the
instrument.
2.7.4 The Zero Gas
The zero gas is used to check/set the reading from the optical benches in the zero
concentration condition. Dry nitrogen is the preferred zero gas as it is cheap and readily
available.
The zero gas must not contain any significant traces of the gas measured or interferents. For all
types of instruments and gases measured, nitrogen (from a cylinder) is usually specified. For
some gases however, fresh air may be used (for example on high concentration CO and CO
2
Instruments). When fresh air is used, it shall be drawn from 'outside’ air and away from any
possible contamination from such as Exhaust Ducts, Chimneys, etc.
In most respects fresh air should be treated in the same way as the sample gas with regard to
filtering, moisture content etc. See span gas, section 2.7.3.
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