H87.0.0X.6C-01
Operating manual G 7500 Series
page 21 of 34
6.9 Basics about oxygen measuring
Please observe the following points when measuring dissolved oxygen:
- For measuring remove the protective flask.
- Do not disconnect electrode from device
.
If electrode has been disconnected, wait 2..3 hours till the final electrode
signal has settled
before carrying out measurements or a calibration.
- Electrode needs to be calibrated
(p.r.t. 'How to calibrate oxygen electrode')
-
The temperatures of the electrode and of the liquid to be measured have
to be identical
(if necessary, wait till temperatures match)
-
The Electrode has to be submerged at least 3 cm into the liquid to being
measured
-
The measured liquid has to stream along the electrode membrane with
at least 30 cm/sec
for measurements to be sufficiently accurate: either stir continuously or
use agitator.
- The electrode measurement is sensitive against shocks!
By stirring of the electrode in the measured liquid be careful that the
electrode does not hit the
container. A vibration of the electrode has a effect to the measured value.
-The
optimum operation position
is: with the sensor inlet pointing downwards
The instrument calculates the oxygen concentration [mg/l], the oxygen saturation [%]
from the electrode signal and the temperature. According to DIN38408-C22 all
measurements refer to steam saturated air.
6.10 Ambient pressure and measuring depth of the electrode
The pressure at the sensor membrane is important for:
The calculation of the oxygen saturation (%sat).
At air water can get 100% saturation. Assumed that there are no oxygen
consuming processes (biological degradation, chemical effects) and that there
are no oxygen enriching processes (e.g. excessive ventilation or
photosynthesis)
The calculation of oxygen concentration (mg/l or ppm)
The electrode evaluation at calibration
Therefore it is necessary to compensate the pressure influence via integrated sensor
or, like practiced with more primitive instruments via tables and manual input of
pressure or Elevation above sea level values.
6.11 Correction of salinity
The higher the salinity (salt content) the lower the solubility of oxygen in water, i.e.
although the partial oxygen pressure is the same, the quantity of oxygen dissolved in
water (mg/l) is lower. Therefore, determination of the oxygen concentration requires
entering the salinity of the medium (p.r.t. 'Configuration'). The correction of salinity is
based on media on a water basis, whose chemical content is similar to sea water.
The corrections are based on the 'International Oceanographic Tables' (IOT).
