HAFFMANS c-DGM/i-DGM
c-DGM/i-DGM Page 47 of 78
7.4 *Calibration O
2
sensor
Because the oxygen sensor only undergoes a very slight amount of wear as a result of
exposure, regular calibration is not required. It has been experimentally determined that after
1 million exposures (i.e. 1 million O
2
measurements) the O
2
deviation from the zero point is
merely 5 ppb and at higher O
2
values no deviation is detectable.
A light-pulse counter, integrated in the software, counts the number of exposures. On reaching
1 million exposures the warning “Calibrate or replace O
2
sensor” appears and the sensor must
be re-calibrated or replaced. As a reminder, a warning symbol is displayed continuously on the
display.
After a successful calibration, the light-pulse counter is reset and the warning symbol
disappears.
In the following two principle methods are described to calibrate the oxygen sensor:
1. One point calibration
This method is recommended if a high accuracy for higher oxygen
concentration when measuring with the LHO-sensor or for low
oxygen concentration when measuring with the WLO-sensor is not
required.
2. Calibration with two gases
This method is recommended to ensure the specified accuracy
over the whole oxygen measuring range.
7.4.1 *Calibration frequency
At a rate of 500 measurements per day, 7 days per week, 52 weeks per year, the calibration
would have to be recalibrated once every five years.
A monthly check of the zero point is recommended (also by ISO procedures) and can easily be
performed by
- flushing the c-DGM with a class 5 CO
2
or N
2
calibration gas.
- filling the o-DGM with a zero-point liquid without O
2,
, such as a sodium sulphite solution
(Na
2
SO
3
; 2%).
The temperature of the c-DGM and sodium sulphite solution or calibration gas may only differ
by a maximum of ±1,5°C.
Attention!
A sodium sulphite solution needs to be stored in a closed
bottle due to the consumption of O
2
.
If an O
2
deviation of > 1 ppb is observed, then a zero-point calibration is recommended.
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