4.4
The oxygen content of the Span gas should be at least 5% O
2
greater than the oxygen content
of the Zero gas.
Instrument quality air may be used as the span calibration gas. Alternatively, dry air from a
cylinder, or clean, dust free atmospheric air may be used, provided it is dried.
For the best accuracy, the span gas should be at least 10% O
2
greater than the oxygen content
of the zero gas. The oxygen content of the calibrating gases may be outside the range of the
analogue output.
If the analyser is to be used to monitor a sample gas with an oxygen content above 21% then a
span calibration gas with an oxygen concentration greater then the sample gas should be used,
typically this will be pure oxygen. Its actual analysis must be known to the accuracy desired and
should be determined by an absolute method.
WARNING
Compressed air from an ordinary shop compressor must not be used for
setting the span. The oil vapour present will contaminate the pipework of
the analyser and sample system. This could be a fire hazard if the
instrument is used to monitor pure oxygen. Refer also to gas cylinder
warning.
WARNING
Gas cylinders when full are pressurised to 15500 kPa (2500psi). They must
be installed as detailed below to ensure maximum safety.
All gas cylinders must be fastened securely by fittings specifically designed
for the purpose. A cylinder and/or valve could be violently propelled by the
sudden expansion of the highly compressed gas contained in a cylinder if it
falls and the valve ruptures.
All gas cylinders must be fitted with a regulator whose output pressure can
be limited to a maximum of 70kPa (10 psi). This prevents serious over
pressuring of the measuring cell. Damage to the cell could result if it is
subjected to serious over pressure.
The reaction between pure high pressure oxygen and residual oil or grease
in the system can reach a temperature sufficiently high to ignite the metal
tubing. The resulting fire, fuelled by pure oxygen, will present a very
serious hazard. Therefore all regulators valves, pressure gauges, tubing
etc. which may come into contact with pure high pressure oxygen must be
carefully degreased before use.
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