Eutech Instruments
Dissolved Oxygen
Probe
USER MANUAL
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1 DESCRIPTION
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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.
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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!
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3.2 Maintenance
The probe’s membrane must be kept free from deposits. All surfaces in a
biologically active system will be covered by a film composed mostly of
bacteria. This bacteria film acts as a diffusion barrier for the oxygen that must
diffuse through the membrane. The membrane must, therefore, be cleaned at
regular intervals, the frequency depending on the actual condition. Cleaning can
be performed with a cloth or soft paper. The membrane is strong and not easily
damaged, but do not try to scratch it clean with a fingernail!
The probe should not be taken apart unless it needs renovation. There is no
need to exchange the electrolyte regularly as in conventional probes!
Other Points Worth Considering
Even though the Eutech probe is one of the most robust available, it should be
treated carefully. It measures at few thousands of a gram of oxygen, which it
must “drag” out of the water around it. So, if, in your opinion, it performs
mysteriously, ask your dealer or us about it. You are certain to get the help you
need!
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4 SERVICE PROCEDURE
4.1 DO Probe membrane replacement and probe renovation
All probe types need membrane replacement if the membrane is damaged.
Probes used in aquaculture (type 1) may need renovation if, after some years use,
it is not possible to calibrate up to the correct value. Other probes should NOT
need renovation unless subject to extreme conditions.
Eutech probes do NOT need regular renovation.
1) Start by cleaning the outer of the probe, then unscrew the cap.
2) TYPE 1 PROBES ONLY: inspect the anode. Clean off any loose white
deposits with anon-metallic brush. If very corroded this type of anode can be
replaced. Check that the nut under the anode is tight before fitting a new
anode.
3) (All probes types) Rinse the probe inside.
4) Check the cathode and remove any deposits using a little wet or dry emery
paper, grade 600. The cathode MUST NOT BE POBLISHED.
5) You can at this stage perform an easy check on the probe. Dry the probeespecially the cathode and are around it – completely, then observe the
output signal – the probe should have zero output (less than 0.01 mV when
measured at the probe). Contact your distributor if this is not the case.
6) The small vent hole in the top of the probe must be open, it can be cleaned
with a needle. Insert the needle 12mm to clean the hole.
7) Fill a new (or renovated) cap to the brim with the electrolyte, and fit it to the
upper part by holding the upper part, moving the cap up to the upper part
and slowly screwing the cap into place without turning backs at any time.
Excess electrolyte should dribble from the thread.
After renovation the probe can be regarded as new. It should be hung up in air to
stabilize for at least an hour before calibration. If possible re-calibrate after a day
or two.
A new membrane can be easily be fitted to the cap – see diagram. The membrane
must be flat – if it wrinkles removes it and dry again with a new one. It is
important at all parts are clean and dry. Neither O-ring nor membrane can be
used more than once.
This is normally only necessary after several years’ use, or if the membrane is
damaged. Start by cleaning the outer of the probe. Then unscrew the cap and
inspect the anode. Clean off any loose white deposits with a non-metallic bruch,
e.g. a hard toothbrush. If the anode is very corroded replace it. Check that the nut
under the anode is tight before fitting a new anode. Rinse the probe inside. Check
the cathode and remove any deposits using a little wet or dry emery paper, grade
600. THE CATHODE MUST NOT BE POLISHED. The small vent hole in the
top of the probe must be open, it can be cleaned with a needle and rinsed.
Replace the large o-ring with a new one.
Fill new (or renovated) cap to the brim with electrolyte, and fit it to the upper
part by holding the upper part with the vent hole away from you, moving the cap
up to the upper part and slowly screwing the cap into place without turning
backwards at any time. Ensure that the o-ring seats properly. Excess electrolyte
should dribble out of the vent hole.
After renovation the probe can be regarded as new. It should be hung up in air to
stabilize for at least an hour before calibration. If possible, re-calibrate after a
day or two.
Tighten the cap
firmly, but remember
that you may have to
loosen it yourself the
next time the probe
needs renovation!
We do not
recommend that a
cap be re-used
without replacing the
membrane, as the
membrane stretches
to fit the cathode, and
will not fit perfectly
a second time.
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A new membrane can easily be fitted to the cap – see the drawing overleaf. The
membrane must be flat – if it wrinkles remove it and try again with a new one. It
is important that all parts are clean and dry. Neither O-ring nor membrane can be
used more than once.
4.2 Spare Parts
The Eutech probe is manufactured from tried and tested parts of the highest
quality, so the need for spare parts is minimal.
A kit can be purchased containing: A cap mounted with a membrane, an anode,
nut and 50 ml of electrolyte. Loose membranes and the tool to remove the ring
are also available.
Users with many points of measurements are recommended to stock one or more
probes. The shelf-life of a ready-to-use probe is very long – several years, and it
can be stored dry if the ambient temperature is not too high. It should be stored
upright. It takes two minutes to assemble a probe.
Do not remove the tape that protects the membrane until the probe is taken
into use.
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4.3 Probe Maintenance - Drawing
5. SPECIFICATIONS
Dimensions: 58 mm dia x 59mm. 3 m cable standard.
Weight: 350 g with standard cable.
Measurement Principle: Self polarizing, self temperature compensating
galvanic cell.
Output: approximately 25 mV in normal air
Operating Conditions:
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0 to 40° C, pressure to 10 bar or more.
Connection of Eutech DO probe with built-in Pt 100
BROWN - DO probe +
BLUE - DO probe -
YELLOW
BLACK Pt 100 sensor
BEIGE
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