BA 4510
5.3 Calibration
Since the measuring system is linear, two calibration points will suffice for the check:
-Zero point
The zero point corresponds with the measurement if there is no oxygen in the measuring cell and there is neutral gas such as ni-
trogen in the unit.
-Measuring range (sensitivity)
The sensitivity for the measuring range is set with span gas or ambient air (~20.9 % O
2
).
NOTICE
The device must be in operational state for at least 1 hour before calibrating.
5.3.1 Zero Gas Calibration
Checking this is particularly important when measuring near 20 Vol.%. Minor mechanical instabilities or ageing can change, res-
ulting in minor changes in the temperature difference between the electrodes, thus the cell voltage. Zero point calibration com-
pensates this false voltage. Zero point calibration requires ambient air to flow through the cell at the speed sample gas will later
flow. This condition is either implemented by the internal pump (ambient air suction) or external pump (e.g. aquarium pump).
First use the keyboard to open "Calibration", then "Zero gas calibration". After selecting "Calibr. zero:", press the
Enter
key to start
the calibration. The calibration takes approx. 5 s. Now press "Back" and press the Enter key to close the menu. Click "yes" to save.
5.3.2 Span Gas Calibration
For this, certified test gas will be passed through the device (preferably with the same concentration which will later be meas-
ured). The process is determined by the menu.
Use the keyboard to first open "Calibration", then "Span gas calibration". The O
2
concentration of the test gas must be entered
via keyboard. After selecting "Cal. span:", press
Enter
to start calibration.
The stability of the measured value (O
2
concentration) is checked during calibration. The actual calibration does not start until
the signal generated by the test gas is stable. The amount of time required for calibration can therefore vary (the fluctuation
range must be less than 1 % within 4 s).
If it does not stabilise, calibration is cancelled after 60 s.
This process further evaluates the deviation of the measured value from the set point. Zero gas calibration allows ±20 mV (cell
voltage), span gas calibration allows ±20 % from the measured value (cell voltage).
The following formula is used for correction:
U
cell
(corr)=( U
cell
+A)*B
With the values
U
cell
= measured cell voltage
A= cell voltage at zero point
B= value for end value correction
After calibration has completed, select "Back" with the keyboard and press Enter to close the menu. Click "yes" to save.
Calibration status message:
OK< (1.5)
OK (1.5)
last calibration OK (calibration value)
WARTEN! 5
WAIT! 5
Calibrating
ABBRUCH
BREAK
Press key to cancel
FEHLER STABIL.
TIME OUT
Not stable after 60 s
FEHLER BEREICH
OUT OF RANGE
Out of range
FEHLER SENSOR
FAILED
Device error
START<
START
Start calibration
17
Bühler Technologies GmbH
BE550013 ◦ 09/2018
