ELMETRON CX-401, Руководство пользователя

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20. BASIC INFORMATION ABOUT OXYGEN MEASUREMENT
Measurement of dissolved oxygen in water solutions is performed with use of
the oxygen sensor. The basic element of the sensor is a teflon semi-
permeable membrane, which enables penetration of oxygen contained in the
measured solution, into the electrolyte – inside of the sensor. The sensor
generates a cell, which voltage depends on the oxygen content in the
electrolyte.
The meter enables measurement in % of oxygen saturation and in mg/l.
Calculation of the mg/l value is based on the saturation measurement in %
and the temperature measurement. During mg/l measurements, the values of
salinity and atmospheric pressure should be additionally introduced. The
saturation measurement in % does not depend on these factors.
The quality of the oxygen sensor has a major effect on the
measurement accuracy. Complications arising during measurements
are caused mainly (98%) by the sensor, not the device. In many cases
problems result from negligence of basic maintaining activities of the
sensor from the user’s side. It is worth remembering that during
measurement the sensor absorbs oxygen from the environment of the
membrane.
The sensor’s manufacturers recommend in their instructions the
minimal flow-rate of the tested water, assuring a stable result. When
this requirement is not complied, the result will regularly decrease.
During measurements in stagnant solutions the flow can be partly simulated
by keeping the sensor in motion with a suitable speed. In laboratory
conditions, i.e. performing measurements in a vessel, the flow can be forced
with a magnetic stirrer. However, when measuring low O
2
saturation,
intensive stirring can cause increase of oxygen content in the tested solution.
Transferring water samples to the laboratory can alter their O
2
concentration.
The best results can be achieved only in conditions recommended by
the manufacturer of the sensor in the operation manual.
Long-lasting storage of the sensor without performing any measurements
(more than 2 months) requires removing of the electrolyte. After this period
the container must be filled with a fresh electrolyte and the sensor stored in
distilled water for about 24 hours.