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DT114+118 Dissolved Oxygen

Type:
Dissolved Oxygen

Range:
0-125% DO2
0-25% O2


Sensor Description:

The oxygen sensor consists of an oxygen sensitive electrode with a processing unit (Oxygen Adapter). The central part of the electrode is a chamber separated from the probe (gas or liquid) by a semi-permeable membrane. In the vicinity of this mem-brane the partial pressure of oxygen is measured. The Oxygen Adapter is used to set the electrode up for measurement in gas or liquid, or to the calibration mode.

How it works:

The oxygen electrode is based on the Clarke cell principle using a polarographic measuring technique. The sensing element consists of a silver anode and a gold cathode. The silver anode is held at a potential of +800mV (polarizing voltage) with respect to the gold cathode. Cathode and anode are immersed in an electrolyte and separated from the sample by a membrane permeable to oxygen.
Oxygen is reduced at the cathode in the following reaction creating a point of zero Oxygen tension (partial pressure) at the cathode:

Provided that diffusion through the membrane is the rate-controlling step in the path of the Oxygen to the cathode, the signal current flowing from the anode to the cathode is proportional to the concentration (partial pressure) of oxygen in the sample.
The adapter converts this current signal into voltage, compensates for the sample temperature, allows the output to be calibrated and scales it appropriately for the MultiLog.
Temperature Compensation A 10 KOhm thermistor is used to compensate the output for changes in the Oxygen permeability of the membrane with temperature. The thermistor is located under the metal disk on the side of the electrode about 35mm above its lower end and should be immersed in the sample medium and at the same temperature as the membrane.

Electrode Assembly

  1. Hold the membrane module in a vertical position and fill it half with electrolyte. Make sure that no bubbles are trapped in the electrolyte as they will falsify the results.
  2. Make sure that the electrode is clean and free from grease.
  3. Hold electrode vertically down and screw membrane module up until the electrolyte is emitted. Tighten the membrane module very slowly until it almost fits. Release access pressure by again slightly unscrewing it. Screw then the membrane fully onto the electrode until it is just tight. Do not overtighten as it must not be pressurized (indicated by bulging away from the cathode).
  4. Plug the Oxygen electrode into the 4 pin mini-din socket on the Adapter.
  5. Calibrate as below:

Define the Sensor

  1. Open Logger << Define Custom Sensor.
  2. Select one of the sensors listed in the listbox.
  3. To define the Oxygen sensor for solutions, type the desired name for your oxygen sensor under Sensor Name (e.g. 'DO2 sol') and '%' under Unit Name.
  4. Value#1: Type '0' under Volts and Measured Unit.
  5. Value#2: Type '4' under volts and '125' under Measured Units (for water and aqueous solutions).
  6. Click OK, the sensor is now available for selection.
  7. To choose DO2, open Logger << Control Panel.
  8. Click the Arrow button on the right side of the desired Input to open a drop down menu, scroll to the desired sensor and double click to choose it.
  9. Define the Oxygen sensor for gas phase by repeating procedure 1-8. To distinguish from the one for solutions, call it 'O2 air'. Enter '25' instead of '250' for Value#2 under Measured Units.
  10. Select DO2 from the drop down menu and click to start recording data.

Calibration

You perform the calibration directly from the Oxygen Adapter. Note that you have to calibrate your sensor always for aqueous solution. Define the sensor as described above and start recording to monitor the procedure of calibration online.

Water/aqueous solution (100% DO2): Select 'DO2 sol' from the Control Panel. Stir a sample of distilled water in an Erlenmeyer flask vigorously for several minutes to make sure that the air above is saturated with water vapor. Position the electrode approximately 1cm above the sample. Start logging and adjust the CAL knob until you see the MultiLog showing 100%. Air (25% O2): Select 'O2 air' from the Control Panel. Start logging and adjust the CAL knob until you see the MultiLog showing 20%.

Note: No compensation is provided for changing atmospheric pressure. Calibration should therefore be carried out at the pressure at which measurements are to be made.

The figure in the right shows a typi-cal graph received from following the oxygen formation in a photosyn-thesis process of a water plant.

Care of the Electrode

  1. Calibration should be carried out regularly. The electrolyte will also need replacing. The interval in both cases depends on the application and the required accuracy.
  2. Ensure that the cathode tip is not scratched or damaged as this will affect the electrode performance.

Treat the membrane carefully as it is fragile. No grease, oil or organics should come into contact with the membrane. Avoid contact with the membrane to prevent punctures.

Specifications:

Range: 0%-125% Notice that the DB-Lab Software will cut everything above 125%, while the MultiLog will still show more. This is done, because the adapter is inaccurate above 125% DO2.

Resolution: +/- 0.25% DO2

Electrode