
16 INM MTL 130-0116 Rev 14
5.2.3 Calibration of instruments fitted with ‘N’ and ‘E’ Type cells
5.2.3.1 Calibration gas level
For optimum accuracy it is best to calibrate with a gas of the same composition and
oxygen concentration as the normal sample. Refer to section 4.6.2 if calibrating an
‘N’ type instrument where the density of the calibration gas and sample gas are
significantly different. If calibrating on ambient air, take into account the dilution effect of
its humidity using the table below.
Temp °C 20% RH 40% RH 60% RH 80% RH 100% RH
0 20.9% O220.9% O220.9% O220.8% O220.8% O2
10 20.9% O220.8% O220.8% O220.7% O220.7 % O2
20 20.9% O220.8% O220.7% O220.6% O220.5% O2
30 20.8% O220.6% O220.4% O220.2% O220.1% O2
40 20.6% O220.3% O220.0% O219.7% O219.4% O2
5.2.3.2 Calibration method
Flow the calibration gas through the analyser and wait for the reading to become
stable. If the reading is the same as the concentration of the calibration gas, no
adjustment is necessary, and the analyser may be put back in normal service.
Otherwise, press and hold the Cal button until the display shows “H xxxx”; where
H indicates that the high calibration point is selected. Use the and arrow buttons
to set the reading to that of the calibration gas, and then press the Edit button
momentarily to confirm the calibration. The display will now show “H xxxx”,
where xxxx is now the correct concentration of the test gas. Press the Cal button
momentarily to return to measurement mode. This completes the calibration and
the analyser is ready for use. Re-enable any disabled control functions.
5.2.4 Calibration of instruments fitted with ‘L’ and ‘H’ Type cells
CAUTION
Typically, these instruments will be measuring parts per million (ppm) concentrations of
oxygen, and care must be taken not to expose ‘H’ and ‘L’ type cells to ambient air for more
than a few minutes while changing pipes etc. This is because the time taken to purge the
cell down to low ppm levels depends on how much oxygen the cell has absorbed. If the cell
has only been exposed briefly to air, the purge time will be quite short. If, however, the cell
has been exposed to air for several hours, the purge time may become far longer.
See Figure 7
Refer to section 4.6.2 if calibrating an ‘L’ or ‘H’ type instrument and the density of
the calibration gas and sample gas are significantly different.
The method used to calibrate ppm analysers depends on the concentration to be
measured during normal service.
5.2.4.1 Calibrating instruments typically measuring more than 50ppm oxygen
Ideally use a standard gas with a concentration of oxygen approximately that of the
typical in-service concentration. However it is possible to use concentrations up to
10% oxygen.
Flow the calibration gas through the analyser and wait for the reading to stabilize.
If the reading is the same as the concentration of the calibration gas, no
adjustment is necessary, and the analyser may be put back in normal service.
Otherwise, press and hold the Cal button until the display shows “H xxxx”; where
H indicates that the high calibration point is selected and xxxx is the measured
concentration. Use the and arrow buttons to set the reading to that of the
calibration gas, and then press the Edit button momentarily to confirm the
calibration. The display will now show “H xxxx”, where xxxx is now the correct
concentration of the test gas. Next, press the Cal button momentarily to return to
measurement mode. This completes the calibration and the analyser is ready for
use. Re-enable any disabled control functions.