General Calibration & Maintenance Procedure; Pre-Calibration Input; Do72 Two-Point (Re)Calibration - YOKOGAWA DO72 Instruction Manual

6. GENERAL CALIBRATION &
MAINTENANCE PROCEDURE

6.1 Pre-Calibration Input

In case the Oxygen Exchange Cap
screwed to the DO72 is connected for
the first time you can use calibration
values of the factory pre-calibration.
You can find these values and sensor
constants on the Final Inspection Proto-
col (FIP, see Fig. 15) delivered with your
OEC. Use the values in the grey high-
lighted boxes of the FIP. For the detailed
setup instruction please refer to the
FLXA402 4-Wire Converter (IM12A01F01-
02EN).

6.2 DO72 two-point (Re)calibration

In case of requirement for recalibration
of DO72 use Two-point calibration in
oxygen free environment (e. g. nitrogen
5.0, 1 % sodium sulfite solution) and a
second 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 environment in HUMID and
DRY 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 DO72 in.
Make sure the probe tip is covered com-
pletely 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 OIM 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.
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