eutech-instruments dissolved oxygen probe User Manual
1 DESCRIPTION
1.1 The Eutech Probe
1.1 The Eutech Probe
The probe is a galvanic measuring element, which produces a millivolt output
proportional to the oxygen present in the medium it is placed in. It consists of an
upper part with cathode, anode and cable, and a cap with membrane and
electrolyte.
proportional to the oxygen present in the medium it is placed in. It consists of an
upper part with cathode, anode and cable, and a cap with membrane and
electrolyte.
Note that the probe
is available in two
different versions.
Type 1 is
particularly suited
for long term
measurements at
relatively high DO
levels – 2 to 50
mg/l. This type is
also used for % sat
measurements
Type 2 is specially
designed for low
DO applications -
0.0 to 10mg/1 and
equivalent % sat
measurements.
Membranes for
mg/l and % sat
measurements
differ. The mg/l
membrane sinks in
water, the % sat
membrane floats.
is available in two
different versions.
Type 1 is
particularly suited
for long term
measurements at
relatively high DO
levels – 2 to 50
mg/l. This type is
also used for % sat
measurements
Type 2 is specially
designed for low
DO applications -
0.0 to 10mg/1 and
equivalent % sat
measurements.
Membranes for
mg/l and % sat
measurements
differ. The mg/l
membrane sinks in
water, the % sat
membrane floats.
Oxygen diffuses through the membrane onto the cathode, where it reacts
chemically and then combines with the anode. This chemical process develops
an electrical current, which flows through a built in resistor. The resistor converts
the current (microamps) into millivolts. This millivolt signal is led to the
transmitter via a two-core cable.
The probe is virtually maintenance free, requiring only that its membrane be kept
reasonably clean. The anode remains fully active throughout its entire lifetime,
which may be several years. The membrane is very robust and can be wiped
clean with a cloth or paper. It is recommended to clean the membrane from time
to time to ensure maximum reliability.
The probe uses very little oxygen for its measurement. This enables it to function
correctly with liquid movement as low as 2-cm / sec (measured at 7 ppm and 13
o
C). The membrane material was specially chosen to ensure a negligible
influence of deposits on its surface, when compared with traditional systems.
Remember that in any biologically active system, deposits will be found on any
surface, including the membrane of an oxygen probe. The deposit acts as barrier
to the oxygen diffusing through the membrane. It must, therefore, be cleaned at
regular intervals, the frequency depending on factors such as the biological
loading of the system, the oxygen consumption of the probe and of course the
accuracy expected.
In a sewage treatment works the percentage of solids and the wastewater
composition are important factors. In a fish farm the species, kind of feed,
population and water exchange frequency are important. This is valid for all
types of oxygen meters.
A small hole in the upper part permits pressure equalizing and CO2 release. The
hole also enables the probe to be filled completely with electrolyte. The probe
can be serviced IN SITU without the use of tools by exchanging the cap with a
new (spare). The membrane in the cap can also be exchanged, giving a very
considerable reduction in maintenance cost compared with conventional systems.
Remember that in any biologically active system, deposits will be found on any
surface, including the membrane of an oxygen probe. The deposit acts as barrier
to the oxygen diffusing through the membrane. It must, therefore, be cleaned at
regular intervals, the frequency depending on factors such as the biological
loading of the system, the oxygen consumption of the probe and of course the
accuracy expected.
In a sewage treatment works the percentage of solids and the wastewater
composition are important factors. In a fish farm the species, kind of feed,
population and water exchange frequency are important. This is valid for all
types of oxygen meters.
A small hole in the upper part permits pressure equalizing and CO2 release. The
hole also enables the probe to be filled completely with electrolyte. The probe
can be serviced IN SITU without the use of tools by exchanging the cap with a
new (spare). The membrane in the cap can also be exchanged, giving a very
considerable reduction in maintenance cost compared with conventional systems.
1.2 Which kind of transmitter can be used with the Eutech
Probe
As indicated above, the probe can be considered a low impedance millivolt
generator. Its output of about 30mV with the probe in air is temperature
compensated and linearly proportional to the oxygen concentration, either in
mg/l or % saturation depending on the choice of membrane.
The output impedance is very low (<1 Kohm) so noise problems normally do not
exist. However, it is important that the input impedance of the following
transmitter is at least 2 Mohms in order not to disturb the temperature
compensation. It is also extremely important that inputs from probes are
galvanically isolated from each other if more than one probe is connected to the
same electronics.
generator. Its output of about 30mV with the probe in air is temperature
compensated and linearly proportional to the oxygen concentration, either in
mg/l or % saturation depending on the choice of membrane.
The output impedance is very low (<1 Kohm) so noise problems normally do not
exist. However, it is important that the input impedance of the following
transmitter is at least 2 Mohms in order not to disturb the temperature
compensation. It is also extremely important that inputs from probes are
galvanically isolated from each other if more than one probe is connected to the
same electronics.
2