6. GENERAL CALIBRATION &
MAINTENANCE PROCEDURE
6.1 Pre-Calibration Input
In case the Oxygen Exchange Cap is re-
placed it is neccessary to set the sensor
constants (grey highlighted boxes) which
can be found on the Final Inspection
Sheet delivered with OEC (FIS, see Fig.
17). For the detailed setup instruction
please refer to the FLXA402 4-Wire Con-
verter (IM12A01F01-02EN).
6.2 DO71 two-point (Re)calibration
In case of requirement for recalibration
of DO71 use Two-point calibration in oxy-
gen free environment (e. g. nitrogen 5.0,
1 % sodium sulfite solution) and a sec
-
ond calibration medium between 1 and
2 % oxygen. Best accuracy is achieved
if the calibration temperature is about
the same as the temperature during the
following measurements.
For calibrating sensor using FLXA402
analyzer please refer to section 4 of
FLXA402 user manual IM 12A01F05-
01EN.
Procedures how to prepare oxygen free
and medium oxygen environment in HU-
MID calibration is described below:
• 1st Calibration Point HUMID: Oxy-
gen-free water
To prepare oxygen-free water dissolve 1
g of sodium sulfite (Na2SO3) in 100 mL
water.
Use a suitable vessel with a tightly fitting
screw top and label it cal 0. Make sure
there is only little headspace in your ves-
sel. Due to a chemical reaction of oxygen
with the Na2SO3 the water becomes
oxygen-free. Additional oxygen, diffusing
from air into the water, is removed by
surplus Na2SO3.
Close the vessel with the screw top and
shake it for approximately one minute to
dissolve Na2SO3 and to ensure that the
water is oxygen-free. To prepare oxy-
gen-free water you also can use sodium
dithionite (Na2S2O4).
Fill the calibration solution cal 0 in the
vessel you have mounted the DO71
in. Make sure the probe tip is covered
completely with the liquid. To minimize
the response time, you can slightly stir
the solution. Then follow the instructions
in analyzer calibration setting chapter
respectively.
After recording the first calibration point
remove the calibration solution cal 0, fill
the vessel with distilled water and stir it
for 1 minute. Repeat this procedure at
least 5 times to clean the sensor mem-
brane from sodium sulfite.
For storing the calibration solution cal 0
keep the vessel closed after calibration
with a screw top to minimize oxygen
contamination. The shelf life of cal 0 is
about 24 hours provided that the vessel
has been closed with the screw top.
• 2nd Calibration Point HUMID: Air-satu-
rated water
Add 100 mL water to a suitable vessel
and label it cal 100. To obtain air-saturat-
ed water, blow air into the water using
an air-pump with a glass-frit (air stone),
creating a multitude of small air bub-
bles, while stirring the solution. After 20
minutes, switch of the air-pump and stir
the solution for another 10 minutes to
ensure that the water is not supersatu-
rated.
Fill the calibration solution cal 100 in
the vessel you have mounted the sen-
sor in. Make sure the sensor surface is
covered completely with the liquid. To
minimize the response time, slightly stir
the solution. Then follow the instructions
in analyzer calibration setting chapter
respectively
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