H74.0.05.6C-06
Operating Manual GMH 3651
page 10 of 18
8. Check cathode for air bubbles.
If there are large air bubbles, remove diaphragm head again and repeat process as of point 5. If O-ring
has been damaged, it has to be replaced.
When maintenance has been completed plug on protective flask. Re-connect electrode to measuring device
and wait for at least 3 hours till electrode can be calibrated..
7
Oxygen Measuring - please note
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 [%] and the oxygen partial
pressure [hPa] from the electrode signal and the temperature. According to DIN38408-C22 all
measurements refer to steam saturated air.
7.1 Correction of salinity ('SAL')
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).
7.2 Ambient pressure and measuring depth of the electrode
The pressure at the sensor membrane is important for:
The calculation oft he 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 practised with
more primitive instruments via tables and manual input of pressure or Elevation above sea level values.
7.2.1 Measurement without Water depth measuring assembly: Menü dEPt = OFF
The integrated pressure sensor is permanently measuring the ambient pressure to:
Compensate the pressure for the concentration [mg/l or ppm] or saturation [%O2]
Pressure compensate the calibration
7.2.2 Measurement without Water depth measuring assembly TMV 3600 (dEPt = On)
Necessary accessory: Water depth measuring assembly TMV 3600.
The pressure sensor measures the hydrostatic pressure to evaluate the measuring depth of the sensor with
a resolution of 0.1m
(e.g. for comfortable logging of oxygen profiles in lakes)
Before starting the measuring (during segment test) and during calibration the pressure port has to be at
ambient pressure: The reference ambient pressure is taken (display
P.REF
and pressure) and memorized for
calculation of saturation [%O2], and as reference for the hydrostatic depth measuring.
Info: The hydrostatic pressure does not really play a significant role for the calculation of concentration (mg/l
or ppm) or saturation [%O2] up to 100m depth, but it is an important unit to evaluate oxygen layering in
waters. Combined with the logger function evaluation can be performed very comfortably!
More detailed Info: Please refer to manual of TMV 3600.
