Calibrating the dissolved oxygen sensor

The direct measurement signal for the Aanderaa 4835 Dissolve Oxygen Sensor is the partial pressure of O2 (pO2), which can be expressed in percent saturation relative to the air saturated value. The oxygen reading in percent saturation can then be converted to concentration (µmol/L or µmol/kg), based on the Henry’s law of solubility: [O2] = K × pO2. The solubility constant (K) is a function of salinity and temperature. The salinity value is either read from the SBE 45 salinity sensor or entered manually.

If the SBE 45 is connected to the pCO2 system, you can let the software read the salinity from the sensor by clicking the Manual button to select From SBE-45. Alternatively, you can set the salinity manually and enter the salinity value in the text box. This setting applies only to manual data acquisition. When measuring automatically, the system always takes the salinity readings from the SBE 45.

Material preparation

Prepare a suitable container with freshwater. Aerate the water using an ordinary aquarium pump together with an air stone, and let the temperature stabilize, which might take hours. Prepare a zero-oxygen solution by dissolving 5 grams of sodium sulfite (Na2SO3) in 500 mL of water. Other substances that remove oxygen can also be used.

Note: To obtain the highest accuracy the sensor(s) to be calibrated should be submerged into water at least 24 to 48 hours prior to the calibration. If a sensor is allowed to dry out this could lead to a bias in the readings of up to 2%. This effect disappears when the sensor is submerged into the water.

Note: Stripping of the oxygen with N2 gas is also possible but not recommended, since it is uncertain when an absolute zero oxygen level is reached or if ever it can be reached using this method.

Communication parameters

Connect the sensor to a computer with the Sensor Cable 3855 (RS-232 serial communication). Start a terminal program such as Tera Term with the following configuration:

9600 Baud, 8 Data bits, 1 Stop bit, No Parity, and Xon/Xoff Flow Control

Note: Select one of the options Sent line ends with line feeds or Echo line ends with line feeds in the terminal program. If using Tera Term Pro, after setting up the com port according to settings above select Terminal in the Set up menu and click Local echo also select CR+LF for both Receive and Transmit under New line.

If there is a communication failure, follow these steps:

  1. Identify the COM port and open it using Tera Term with the following set-up:
  2. Select Terminal in the Set up menu and click Local echo also select CR+LF for both Receive and Transmit under New line.
  3. Check data output format:
    1. Do sample
    2. If the results turn out to be only numbers like shown below, then type:
    3. Set enable text(yes)
    4. Save
  4. Close the terminal.

    Return to the main user interface and try to connect again.

Calibration procedure

  1. Submerge the Optode into the aerated water.
  2. Set the Interval property to 30 seconds, enter the save command, and wait until both the temperature and the phase measurements have stabilized:
    1. Set Passkey(1000)
    2. Set Interval(30)
    3. Save
  3. Store calibration values by typing:
    1. Set Passkey(1000)
    2. Do CollectCalDataSat
    3. The save command is performed when you type Do CollectCalDataSat.
  4. Set the CalDataAPress property to the actual air pressure in hPa at the site.
    1. Set Passkey(1000)
    2. Set CalDataAPress (..)
    3. Save
  5. For maximum accuracy, do not compensate the air pressure for height above sea level.
  6. Submerge the Optode in the zero solution. Make sure that the sensing foil is free from air bubbles. Wait until both the temperature and the phase measurements have stabilized.
  7. Enter the Do CollectCalDataZero command to store calibration values. The save command is automatically performed.
    1. Set Passkey(1000)
    2. Do CollectCalDataZero
  8. Enter the Do Calibrate command to effectuate the new calibration and store the new coefficients in the sensor memory. The save command is automatically performed.
    1. Set Passkey(1000)
    2. Do Calibrate
  9. Check
  10. Check that the sensor is working properly by taking it up into the air and rinse off, flush the sensor well to remove all Zero Oxygen water to not have cross contamination of your saturated sample. In dry air, the sensor should show close to 100% oxygen saturation at sea level. Put the sensor back into the anoxic water; the reading should drop to zero.

Maintenance

The oxygen optode requires little maintenance. Clean it if you observe algal growth or calcareous deposits on the sensor. If you are measuring water that is rich with algae, the sensor may need to be cleaned regularly. A thorough cleaning may be required if there are calcium deposits.

Complete maintenance information provided by the manufacturer is available at www.aanderaa.com/oxygen-sensors.