EVERYBODY wants to blame the probe for their BOD problems, especially for problems with dilution water blanks.
The following procedure will allow you to check your own probe without sending it out to anybody. Run the following tests exactly as stated. If the probe fails any of the tests, it needs to be serviced or replaced. If it passes all of the tests, I give you at least a 95% probability that it is OK.
A. Stability Check:
- Check for air bubbles. Change membrane is necessary.
- Allow AT LEAST two hours after changing a membrane for the probe to stabilize. Overnight is better.
- Have the probe situated in the air-calibration bottle, connected to the meter, and the meter turned on. Shake or blot any water droplets off of the probe membrane.
- Allow the system to warm up for at least 30 minutes.
- Shake the probe to remove any droplets of water that have formed on the bottom of the membrane.
- Air-calibrate your DO probe at 20°C. Click here to learn how to do this.
- Set a timer for two minutes and observe the reading while the probe is still in the calibration bottle. Be sure the temperature is constant. Watch the readings carefully. DO NOT just record the initial reading and come back two minutes later to record that reading. You need to actually see what happens over the time period.
Following are the possibilities:
If the reading drifts slowly downward, a longer warm up time is required.
If the reading jumps around, there is something wrong with the probe.
If the reading is stable in the air-calibration bottle, the sensor is probably good. (If the readings are stable in the air-calibration bottle but not in the solution, the membrane is probably defective.)
B. Zero Oxygen Check: (Response Check)
- Dissolve 3-5 grams of Sodium Sulfite in 300 ml of water. Do this by stirring slowly so as to avoid cavitation. Slowly pour this solution into a BOD bottle.
- Place the probe into the “Zero Oxygen” solution bottle and observe the results.
- Meter should read less than 0.10 mg/l within two minutes.
C. Temperature Thermistor:
- Record temperature reading from meter.
- Using a known good thermometer, measure the temperature and compare the results. Observe that the temperature reading is stable.