H74.0.05.6C-06
Operating Manual GMH 3651
page 11 of 18
8 Calibration of the Sensor
In order to compensate for ageing of the sensor, the sensor has to be calibrated at regular intervals.
The device is equipped with easy-to-use calibration functions: Standard 1-point calibration, sufficient for the
most applications and special 2 or 3-point calibration, if high accuracy is needed in extreme measuring.
We recommend to calibrate the sensor at least all 7 days, or to get maximum precision, before each
measuring series.
If the electrode was dry for one or more days it has to be 'watered' for at least 30 minutes before carrying out
a new calibration.
Remove protective flask prior to calibration and wipe diaphragm with a soft pieceof cloth
8.1 One Point Calibration ('
(AL 1-PT
')
The 1-point calibration adjusts the sensor to the oxygen content of the atmosphere
(20.95%). You can choose between three modes of calibration for which the electrode has to
be prepared accordingly.
Air calibration without accessories ('Cal Air')
The electrode will be exposed to
ambient air.
To protect it
from draughts, wrap electrode in a paper towel. (before
calibration, expose sensor at least 15 minutes to the
ambient air, to let the temperature adjust and to dry
membrane)
Depending on the rel. atmospheric humidity [%] and the
temperature [°C] a small calibration error cannot be
avoided and will have to be accepted. The cooler the air is,
the smaller the deviation.
Recommended temperature: < 25°C.
For error compensation please refer to the opposite table.
Note: A correctly calibrated electrode shows 106 - 108% at air and at room temperature.
Air calibration for highly accurate measurements ('Cal Air')
The electrode is exposed to
air with a relative atmospheric humidity of 100%
.
Proceed as follows: Put some distilled water in a bottle. Close bottle and generate a water steam saturation
(100% rel. atmospheric humidity) in the remaining air by shaking it vigorously for approx. 3 minutes. Both
water and air temperatures should be identical. Open bottle and insert electrode so that the diaphragm is in
the air chamber.
Attention
: By no means must the diaphragm get wet or be immersed in water. The bottle neck opening
should only be slightly larger than the electrode diameter; make sure to avoid over pressure in the container.
Note: A correctly calibrated electrode shows 106 - 108% at air and at room temperature.
Calibration in air saturated water (configuration: 'Cal AqUA')
This mode of calibration is more difficult than the ones already described. As water is easily oversaturated it
quickly results in calibration errors. In order to generate air saturated water, pure water (25-30°C) is poured
into another container from a height of 50 cm. This process is repeated 20 times. In order to let an
oversaturation evaporate, wait approx. 5 min. before carrying out the calibration (do not forget to stir).
Start calibration: press
-key for 2 seconds
The display will show '
A,R
, and as soon the values for oxygen and temperature are stable, the
calibration will be finished.
Then the electrode state resulting of the successful calibration will be shown for a short time (evaluation in
10% steps: xx%
ELE[
).).
20%
40%
60%
80%
100%
5 °C
1.007 1.005
1.003
1.002
1.00
10 °C
1.01
1.007
1.005
1.002
1.00
15 °C
1.014
1.01
1.007
1.003
1.00
20 °C
1.019
1.014
1.009
1.005
1.00
25 °C
1.026
1.019
1.013
1.006
1.00
30 °C
1.035
1.026
1.017
1.009
1.00
35 °C
1.047 1.035
1.023
1.012
1.00
40 °C
1.063 1.047
1.031
1.016
1.00
Deviations when carrying out an air calibration
without accessories,
O2-saturation= display value*corr. factor
