4. CALIBRATION
Gas readings made using IRGA optical benches are relative rather than absolute. Before
precise gas readings can be obtained, the analyser must be adjusted so that it reads zero when
there is no trace of the measured gas present, then, when a known concentration of the gas is
applied, the reading is adjusted to agree with this concentration. These adjustments are known
as ‘zero’ and ‘span’ respectively.
Routine calibration is important, not only to verify measuring accuracy, but also to spot
changes over time that may indicate problems.
Note: because the instructions will vary slightly from gas to gas, the instructions given here are
fairly general. ADC (Spares & Service) Ltd. or their authorised local agent will be pleased to
answer any questions.
4.1 Initial Checks
It is assumed that the instrument is correctly installed and all gas connections are correctly
made – if not, refer to section 2.
Before operating the Instrument, ensure all the relevant installation checks have
been carried out [refer to Section 2.9], and that if a filter column is fitted, it is
properly charged with the chemical supplied.
As one of the first operations concerns Zeroing and Spanning the Instrument, ensure that
external Zero and Span Gas supplies are available for connection to the Instrument. If gas
cylinders are used, check the Pressure Regulators are set correctly.
Make sure that the Power Settings [on the Rear Panel] are correct for the supply used [refer to
Section 2.3].
4.2 Warm Up
If previously removed for any reason, re-fit and secure the enclosure's front or top covers to
the Instrument. The covers are necessary to ensure that the internal temperature stabilises.
Switch on the Instrument via the Rear Panel mounted Power ON/OFF switch, and allow the
instrument about 30 minutes to warm up.
Until the instrument has reached operating temperature the display will indicate a symbol which
represents a thermometer. This symbol will be replaced by the gas measurement when the
correct temperature has been achieved.
4.3 Zero Adjustment
Before a zero adjustment is made ensure that the gas cylinder is connected and that
the correct pressure is set. See section 2.7.4.
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