130-0047 Issue 8
Isolate the analyser from the process gas and pass the calibration
gas through it. A full calibration requires two calibration points
referred to as upper and lower, although the lower point can be
omitted except when replacing a sensor. Safeguards are built into
the instrument to prevent the calibration being set outside of the
sensor’s operational limits.
It should be noted that it will take time for the calibration gas to flush
the previous gas out of any filters etc. upstream of the analyser.
This means that the reading will take time to stabilise. Refer to the
response graphs (Fig. 5) for guidance on this. When the reading is
stable, press and hold the Calibrate button for approximately 8
seconds. The display will go blank for a moment and then show “H
xxxx”; where H indicates that the high calibration point is selected.
Use the up/down arrow buttons to set the reading to that of the
calibration gas, and then press the Edit button momentarily to reset
the calibration. The display will now show “H xxxx”, where xxxx is
the correct concentration of the test gas. Unless a low level
calibration is required, normally done only when a new sensor is
fitted, press the Calibrate button momentarily to return to
measurement mode. To access the low calibration mode, press
and hold the Calibrate button for 8 seconds. The display will now
show “Lxxxx” where “L” indicates that the low
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calibration point is selected and “XXXX” is the measured value of
the calibration gas. Lower point calibration is performed in the
same way as the high calibration, however it may be necessary to
allow a longer time for the reading to stabilise.
5.5 Calibration of a ppm range instrument
Piping
See section 3.5 for advice on pipe work etc. If there are regulators
in the gas stream they should have a low volume and a metal
diaphragm. Most pressure gauges etc. contain cavities that entrain
air/oxygen and can take several hours to purge down. For this
reason they should be avoided.
Calibration gas level for L sensors
Ideally the calibration gas level should be as near to the normal
working concentration as possible. However it is possible to use
concentrations up to 5% oxygen in nitrogen. Beyond 5% the sensor
begins to deviate from its standard output equation. N.B. Air may
be used for convenience if nothing else is available. In this
case, to correct for the deviation from the standard equation,
the reading should be set to read 19.4% - NOT 20.9%.
Calibration method
Isolate the analyser from the process gas and pass the calibration
gas through it. It should be noted that it will take time for the
calibration gas to flush the previous gas out of any filters etc.
upstream of the analyser, which will increase the time taken to
stabilise.
WARNING: Care must be taken not to expose a L type sensor to
ambient air for more than a few minutes while changing pipes etc.
The time taken to purge the sensor down to low ppm levels
depends on how much oxygen the sensor has absorbed. If the
sensor has only been exposed briefly to air, the purge time will be
quite short. If, however, the sensor has been exposed to air for
several hours, the purge time may become far longer. See Fig. 5.
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5.6 Sensor replacement - overview
Before proceeding identify which sensor is to be replaced, then read
and fully understand the following appropriate procedure.
For best accuracy it is necessary to re-calibrate the instrument
following a sensor change. The calibration procedure depends on
the type of sensor fitted to the instrument. See sections 5.3 to 5.5
for additional information on calibration before proceeding.
Each instrument is built to accept a particular type of sensor and for
the most part cannot be used with one of a different type.
For instruments fitted with L type sensors it is necessary to reset
the sensor zero offset before replacing the sensor as detailed in the
following sections. For instruments fitted with N sensors proceed to
section 5.7.
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