Instructions

DO-400 Bases for measurement | 6
B-H86.0.21.DB214-1.0 15 / 29
– Mechanical stress of the sensor membrane
– Storage in dry air
– Continuous use in elevated carbon dioxide concentrations
6.1.4 Operating position
The oxygen sensor should be arranged vertically upwards with the connecting cable.
A slight angle of inclination does not impair the measurement.
6.1.5 Measurement accuracy
The measurement accuracy can be impaired by:
– An inadequate flow below the necessary value of approx. 30cm/sec.
– The water temperature and sensor temperature must be the same. The most ac-
curate measurements are provided when the measuring temperature is calibrated.
6.1.6 Residue
Visible residue collecting as a reaction product in the interior of the membrane cap on
the lead-oxide lead anode is red or brown from reaction with oxygen and lead carbon-
ate and is white from reaction with carbon dioxide. These substances can collect on
the membrane, but do not normally impair the measuring function and can be mostly
removed in the course of maintenance of the sensor. Prior to screwing on the mem-
brane cap, they should be removed as far as possible in order to prevent particles
from being trapped between the membrane and the platinum cup. A rapid or excess-
ive formation of lead carbonate after commissioning is an indication of air in the
sensor. This is usually due to incomplete filling or a leak due to improper fitting of the
cap / fill screw or membrane leak.
6.2 Instructions for oxygen measurement
The following must be observed when measuring dissolved oxygen:
– The storage bottle must be removed before the measurement.
– The sensor must have been calibrated.
– The sensor and the liquid to be measured must have the same temperature. The
temperatures of the two must match.
– The sensor must be immersed at least 3 cm into the liquid to be measured.
– A flow speed of at least approx. 30 cm/sec is necessary for exact measurements.
Either stir continuously or use an appropriate stirring device
– The measurement is sensitive to jarring! Therefore, make sure that the container
is not struck with the sensor while stirring, because this can significantly impair the
measured value.
– The oxygen partial pressure, oxygen concentration in mg/l and oxygen saturation
in % are calculated from the sensor signal and temperature. In accordance with
DIN38408-C22, the measurement is relative to water-vapour-saturated air.
6.2.1 Salinity correction
With increasing salinity SAL, which is the value for the salt content in the water, the sol-
ubility of oxygen in water decreases, i.e. with the same oxygen partial pressure, fewer
mg of oxygen are dissolved per litre of water. To determine this oxygen concentration,
therefore, the salinity of the medium must be entered first; see Configuring parameters
of the configuration menu [}p.22]. A salinity correction is not necessary in freshwa-