Eutech Instruments DISSOLVED OXYGEN PROBE User Manual

Eutech Instruments

Dissolved Oxygen

Probe

USER MANUAL

1

1 DESCRIPTION

N

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.

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.

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.

ote 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.

2

2 INSTALLATION

The Eutech probe is easy to install. It should be placed where there is some
movement in the water-1 cm/sec is enough at 7 mg/1 and 13 C. Ensure that the
probe does not strike against the tank wall, and do not mount it directly above
diffusers etc.

Each probe is connected using ordinary 2-core cable, and it is fitted with 3
meters of PU cable unless otherwise specified. The brown wire is positive, the
blue negative.

3 DIRECTIONS FOR USE

3.1 Calibration

Calibration is carried out by placing the probe in air or air saturated water,
waiting for temperature equalization then adjusting the transmitter or electronics
connected so that the output corresponds to the calibration value. If you are
measuring % saturation on the calibration value is always 100%. If you are
measuring mg/l (ppm) temperature, the barometric pressure and the water
salinity all affect the calibration value.

The probe must have the same temperature as the water or air surrounding it, and
must therefore be allowed to attain this temperature before being calibrated. Any
robust long-life probe such as the Eutech can take up to an hour to respond to a
10-degree temperature change in air, which only takes 10 minutes in water. This
must be taken into account when calibrating.

Take the probe up from the water, wipe the membrane dry, and hang it in free
air, away from direct sunlight. When the probe output is stable adjust the
transmitter or electronics connected to give the correct output.

Calibration value for systems measuring % saturation

The calibration value for systems measuring % saturation is always 100. For
these systems, and only for these systems, temperature, barometric pressure and
salinity have no influence.

Calibration value for systems measuring mg/l (ppm)

For these measurements adjustment until the output corresponds to the
appropriate value from the calibration tables. Temperature, pressure and salinity
affect the value. The first table at the end of this section shows values for various
temperature and air pressures, the other values for various temperatures and
salinity at 760 mbar.

It is essential to correct for temperature-use the air temperature near the probe.
At high altitudes, or for greater accuracy, a correction must be made for pressure.
The salinity should be corrected for in salt-water measurements. For a simple
calibration at low altitudes assume a normal barometric pressure of 760 mmHg.

How often should calibration be performed?

Unfortunately, it is not possible to answer this question. A new probe needs more
frequent calibration than an “old”. Calibration frequently also depends on the
accuracy desired. If an accuracy of +/- 0.5 ppm is sufficient, calibration once a
month is more than enough. It is, however, much better to perform one thorough
calibration than two quick ones. Give the probe time to stabilize, check the
barometer and decide if correction for air pressure is necessary. Check the
salinity if your farm is a salt-water facility!

3