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Oxygen Monitors & water quality monitoring Instrumentation

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Oxygen Probe Manual

• No zero point error
• Very accurate usually better than +/-  0.2mg/l
• Calibrate to 100% in air
• Self-temperature compensating from 5 to 40 deg C
• Connect with good quality two core cable,  tested up to 2000m with ordinary cable
• Very stable, usually 3 to 6 months between calibration checks
• Very easy to maintain, can be serviced in the field
• Strong Teflon membrane
• Very heavy duty sub-sea urethane cable, into epoxy sealed internals
• Diameter = 63 mm, length = 73 mm. Cable length = 5 m (standard)
• Weight 1000 g incl. cable 5m
• 6 to 12 mill volt per ppm (mg/l) ( depends on temp)
• pressure to 10 atmospheres
• Water flow should be at least 4 cm/sec
• Response time, to change in DO, approx 15 to 30 seconds
• Standard probe is a 2 wire,  with temp sensor there are 5 wires
• Temperature sensor PT100 built in to probe is available as an option

Calibration of the oxygen probe is very easy, just follow the steps below;

Standard calibration

1. Remove probe from water, dry and clean the probe and membrane with a soft clean cloth.

2. Using a pin, clear the breather hole on top of the probe

3. Suspended the probe in air above the surface of the water, try to keep the probe out of the wind and direct sun-light. The probe should be left for a period of at least 30 minutes,  or until you are sure that the temperature of the probe is the same as the air temperature.  The best time to perform this task is early in the morning or late evening.

4. The probes will now be reading the equivalent of 100% saturation. Take an accurate reading of the air temperature by the probes. Use the table below to establish the concentration in mg/l for the probe, and use this figure with your oxygen monitoring equipment.

5. Example. if the air temp is 20.5 deg C,  and the probe is at the same temperature as the air,  the probe should be reading 8.99mg/l. Set the display to give this reading

Quick calibration

Sometime you do not have time to wait 30 minutes or more to perform a calibration, we therefore have a quick calibration technique that often proves more accurate than the standard technique.

1. Remove the probe (or probes from the water), do not dry the probes, suspended in air out of direct sunlight

2. Clear the breather hole with a pin and wipe the membrane with a soft cloth

3. Measure the temperature of the water  ( if more than one probe is being calibrated,  the water temperature should be know for each probe)

4. Calibrate the probes using the water temperature. The probes must be calibrated within 5 minutes of removing from the water, otherwise the temperature of the probe may have changed too much from the temperature of the water

The oxygen probe requires very little attention,  the degree of attention will depend upon the water type in which the probe is immersed. For most applications we recommend that the probe is removed from the water once a week and the membrane cleaned with a soft clean cloth. The breather hole on top of the probe should also be cleaned using a pin. This is all that you need do with the probe on a regular basis.

In the event that the membrane is damaged,  the readings will become very erratic. Under these conditions the membrane should be replaced.

Membrane Replacement

The following procedure describes membrane replacement.

1. Remove the oxygen probe from the water and clean with a cloth or paper towel

2. Unscrew the bottom end cap, please note that the electrolyte containing a white deposit of colloidal solution of zinc oxide may be captained in the cap.   Discard this solution.

3. Using a coin, unscrew the threaded retaining ring in the membrane cap, remove and discard the membrane and small `o` ring located below the membrane

4. Clean the inside of the membrane cap thoroughly with a damp cloth, and finish off with a clean dry cloth.

5. Insert a new `o` ring,  and then the membrane on top of the `o` ring. Screw down the threaded retaining ring until you feel tension, then give is a further 1/4 turn. If  the membrane wrinkles,  you will need to replace the `o` ring and membrane and try again.

6. Clean the inside of the probe top, you may clean the silver cathode with 1500 grade paper,  take care as damage to the silver cathode can affect probe readings.  You can clean the angular zinc anode with rough abrasive paper in order to remove any oxidation layer. Clean the probe in freshwater to remove any of the fines.

7. Fill the membrane cap with electrolyte, and holding the probe vertically, screw the membrane cap on to the top of the probe, making sure that the large `o` ring is in place. Slowly screw up the cap,  the excess electrolyte will escape though the breather hole.  Take care that you do not screw the cap on too quickly since this will stretch the membrane. Screw up the cap until it seals on the large `o` ring a then give it a further 1/4 turn.

8. Immerse the probe into the water, after 24 hours perform a calibration as per the instructions above.

Components supplied with the probe

The standard  probe is supplied with he following components;

1. Oxygen probe fitted with 5 meters of heavy duty cable

2. 50ml bottle of electrolyte

3. 5 spare membranes and membrane o rings

Note.

The probe is supplied dry without any electrolyte. On receipt of the probe, remove the end cap, fill end cap with electrolyte and slowly screw the cap back up onto the probe.  Screw the end cap up slowly to allow the excess electrolyte to squirt out of the top breather hole. You should then leave the probe for 30 minutes suspended in air before you perform a calibration. Once the probe is calibrated immerse the probe into the water for a period of at least 24 hours and then perform another calibration. Thereafter a calibration will be required at approximately 1 to 2 week intervals. After a period of approx. 4 weeks,  the calibration frequency can be reduced to once every month to several months.

Low Temperature applications

If the oxygen probe is subjected to sub-zero temperatures,  the electrolyte may freeze and cause damage to the probe.  In order to avoid freezing down to temperatures of -25 deg C glycerine is added to the electrolyte at a level of 55% glycerine to 45% water by volume. The glycerine will reduce the milli volt out-put of the probe down to approximately 30mv.