EUTECH INSTRUMENTS DISSOLVED OXYGEN PROBE User manual

Eutech Instruments

Dissolved Oxygen

Probe

USER MANUAL

1DESCRIPTION

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.

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.

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

oC). 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.

2INSTALLATION

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.

3DIRECTIONS 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!

3.2 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!

4SERVICE 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.

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.

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 probe-

especially 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.

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.

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: 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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