Tech Note 206 Detail

DO Probe Troubleshooting

Affected Products:

CI-6542 Dissolved Oxygen Sensor
PS-2108 PASPORT Dissolved Oxygen Sensor

Problem/Symptom:
Dissolve Oxygen Sensor Troubleshooter

PASCO Solution:

Symptom: The values are inaccurate.
Problem: The sensor is unresponsive, has not been calibrated under the proper conditions, the sensor amplifier is not functioning properly, or there is electrical interference.

Remove other electrically conductive sensors from the solution. See Tech Note 611 for details.

Check that the sensor is responsive to changing oxygen conditions by alternately removing and then submerging the probe in the water. If not see, the troubleshooting section on an unresponsive sensor.

Check that the sensor has been properly calibrated per Tech Note 497 - Dissolved Oxygen Probe Care.

Symptom: The sensor fails to be detected by a compatible interface with a port that is known to be working.
Problem: Either a poor connection between the PASPort interface and dissolved oxygen sensor box or a defective sensor box or interface port.
Solution:
  1. Unplug and replug the sensor box from the PASPort interface.
  2. If the unit remains undetected, try swapping probes and sensor boxes to determine the probable point of failure.
Symptom: The signal is very low or jumps between values.
Problem: Air bubbles are trapped near the surface of the DO sensors.
Solution: Gently tap the sensor while it is submerged to remove air bubbles trapped near the diaphragm.
Symptom: The DO value decreases over time in a new electrode.
Problem:

Either the solution is not stirred adequately to account for oxygen consumption in the vicinity of the probe, the probe is polarizing, or the water is naturally de-oxygenating: When water is poured into a container, air bubbles mix with the solution. As water stands it slowly de-oxygenates to an equilibrium level. If the water temperature increases, then the water will typically become super-saturated and very slowly come back to saturation.

Solution:
  1. Constantly stir the solution in which the probe is placed at speed that does not create turbulent mixing. A magnetic stirring bar is strongly recommended.
  2. If you are looking for oxygen production from photosynthesis, de-oxygenate two water samples (one of which will serve as a control) by heating both to boiling, allow the water samples to cool to ambient temperature, add a photosynthetic organism to one water sample, apply light to both samples, and, while stirring, measure the oxygen concentration of both the samples over time.
  3. Connect the probe to a powered interface for 10-50 minutes before use to repolarize the electrode.
  4. If you are running the DO sensor connected to a GLX, please check that either the sampling rate is greater than once per minute or that the GLX is connected into external power. (This prevents depolarization of the electrode.)
  5. Immerse the dissolved oxygen sensor below the chrome band.
  6. Tap off any air bubbles that are adhered to the electrode.
  7. Begin recording and wait until the value begins to stabilize.
Symptom: The signal drifts and is too high.
Problem: The membrane may be perforated or the chloride layer may be contaminated.
Solution:
  1. Clean and replace the membrane.
  2. Connect the probe to a powered interface for 10-50 minutes before use to repolarize the electrode.
  3. If this doesn't solve the problem you may have a salt bridge in the cable or connector. To check this, remove the electrode from the chamber, remove the membrane, and dry the tip of the electrode. If the electrode does not read zero %O2, you have a salt bridge will have to replace the electrode and cable.
Symptom: The signal wanders at typical levels.
Problem: Since the electrode signal varies 4%/degree C, the temperature of the solution should be fairly stable.
Solution:
  1. Make sure the temperature of your system is stable and that the solution is stirred sufficiently. (A magnetic stir bar is recommended.)
  2. Remove any air bubbles on or under the membrane.
  3. If you are still having problems, polish the silver electrode and refill with fresh filling solution.
Symptom: The value decreases over time in an old electrode.
Problem: The sensor slowly consumes oxygen to produce a voltage; however, rapid oxygen consumption indicates that the probe may contain aerobic bacteria. It is also possible that the value is decreasing because the probe is depolarizing.
Solution:
  1. Remove the membrane cap.
  2. Discard the filling solution.
  3. Bleach the probe components to clean them andkill any microorganism on the electrode surfaces.
  4. Rinse the probe components with deionized water.
  5. Polish the silver electrode tip to remove accumulated silver sulfide and silver chloride deposits.
  6. Replace the electrode membrane using the CI-6541 Electrode Maintenance Kit.
  7. Fill the electrode cap with fresh polarographic filling solution and replace the cap.
  8. Connect the probe to a powered interface for 10-50 minutes before use to repolarize the electrode.
  9. If you are running the DO sensor connected to a GLX, please check that either the sampling rate is greater than once per minute or that the GLX is connected into external power. (This prevents depolarization of the electrode.)
Symptom: The sensor is unresponsive.
Problem: The electrolyte has either dried up, the pores on the membrane have clogged, and/or the probe has a broken electrical connection.
Solution:
  1. Check that the dissolved oxygen probe is fully and correctly inserted into the sensor box.
  2. Remove the membrane cap, discard the filling solution, and polish the silver electrode tip to remove accumulated silver sulfide and silver chloride deposits.
  3. Replace the electrode membrane using the CI-6541 Electrode Maintenance Kit.
  4. Fill the electrode cap with fresh polarographic filling solution and replace the cap.
  5. If you are running the DO sensor connected to a GLX, please check that either the sampling rate is greater than once per minute or that the GLX is externally powered. (This prevents depolarization of the electrode from power cycling.)
  6. Connect the probe to a powered interface for 10-50 minutes before use to repolarize the electrode.
  7. If you still experience problems, check for a broken electrode or cable, by removing the membrane and dipping the probe into deionized water with the sensor turned on. If the amplifier does not register a high reading, replace the electrode.
To reduce the probability of future problems, please store the electrode properly.


Creation Date: 01/1/2000
Last Modified: 05/14/2012
Mod Summary: