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MD Oxygen monitor

Mobile or stationary display

Buy or view price. A one channel dissolved oxygen monitor, for use with our stationary dissolved oxygen probe code 8.4.37. The instrument provides a LCD display of the oxygen levels in mg/l. The unit is powered by a 9v PP3 battery, and housed in an IP65 polycarbonate enclosure ( 100mm x 60mm x 50mm). The enclosure is suitable for mounting out-doors, on sea cages or tanks. The unit is portable so it can also be used as a low cost, very simple yet robust dissolved oxygen meter. No more costly repairs and servicing, since you can do it all yourself. The unit is designed to work as a stand along instrument, or as a local display of oxygen levels in tanks or sea cages that are monitored by our AquaTraul or PLC based systems. When used as a local display the monitor is wired in parallel with the existing probe. The unit is a simple low cost functional monitor, with an on/off switch and a calibration dial. Probes are connected via a cable gland onto a terminal strip.

Oxygen Probe The MD unit is supplied with a standard dissolved oxygen probe with 5m of cable. The probe is calibrated according to the instructions below and then immersed into the water. The first reading may take around two minutes to stabilise for temperature.  However once the probe is at the same temperature as the water,  subsequent reading only take around 20 seconds. When the unit is not in use,  you may actually leave the probe in a fish tank or cage permanently.  The unit can then be turned on to take an instant reading of the tank,  or moved to take readings in the other tanks. The probes can be order with any length of cable up to 1000m.  The probe is epoxy filled and pressure equalised, and as such it can take tremendous pressures.  The probe has been tested down to 100m,  so it makes a perfect instrument for hydrological surveys.

MD Calibration

Calibration

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

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, or at any time in-doors. The MD unit does not need to be on while you wait for the 30 min.  The probe is self powered.  To take a reading or check calibration, just turn the MD unit on, take the reading and turn if off. The internal 9 v pp3 battery will last for approximately 24 hours

4. The probe 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.

Calibration example.

The probe has been out of the water for at least 30 minutes,  and the air temperature next to the probe is 19.4 degrees centigrade.  From the table below a temperature of 19.4 Deg C should give a concentration of 9.19 mg/l (ppm) of dissolved oxygen, set the MD unit to display 9.19mg/l.  The probe is now ready to be used in freshwater.   If the MD unit is being used in sea water,  then a correction factor needs to be applied.  If the salinity of the water is 30 ppt,  then from the table below the correction factor is 0.83.  Multiply 9.19 by 0.83 to give 7.63mg/l,  this is the figure that you set the MD unit to display.

Using the probe to measure the oxygen content of Air

The Dryden Aqua probe and MD unit can also be used for monitoring the concentration of oxygen in air, the calibration procedure is as follows;

1. 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 in a location where there is a normal atmospheric oxygen concentration. 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.  To take a reading or check calibration, just turn the MD unit on, take the reading and turn if off. The internal 9 v pp3 battery will last for approximately 24 hours

4. After around 30 minutes, use the calibration pot on the side of the unit and adjust the reading to display 20.9.  This value represents the percentage of oxygen in the air. If you move to a new environment the reading will change to reflect the oxygen content. However if the air temperature is substantially different, allow time for the temperature of the probe to stabilise.  The Dryden aqua probe is designed for continuous monitoring of oxygen levels. The response time is approx 2 minutes to a change in oxygen level, however the reading may take up to 30 minutes to stabilise if the temperature difference is more than 5 deg C for the calibration temp

5. The Dryden Aqua probe has a mv out-put, you may use the Dryden Aqua MD unit,  or you could use your own galvanically isolated display unit.  You can even use a multimeter and just read the raw mv out-put from the probe. For example is the probe reads 82mv in normal atmosphere, this reading then equates with 20.95%, the relationship is linear down to zero, and there is no zero point error.

Probe maintenance

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.

If the probe is not immersed in water all of the time,  then it should be stored vertically,  otherwise some of the electrolyte may escape the breather hole. It is best to keep this probe immersed in the water that you want to measure,  this will give you almost instant accurate readings.

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 7 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.

The solubility of Oxygen in water

The solubility of oxygen in water in equilibrium with air at 760mm Hg pressure and 100% relative humidity Units:mg/1

CORRECTION FACTORS FOR CALIBRATION OF A DISSOLVED OXYGEN

METER TO READ IN MG/1 IN SALINE WATER

VARIATION OF MEAN ATMOSPHERIC PRESSURE WITH ALTITUDE

Copyright © 1997
Last modified: July 09, 2007