Incorrect DO Measurement
The leading cause for this is incorrect standardization of the Winkler reagents, or incorrect calibration of the DO Probe. The Winkler titration is particularly prone to error and can lead to poor blanks. The DO probe tends to drift (especially Clark based systems) which might lead to incorrect measurements. The way to be absolutely certain that the standardization of the Winkler is correct or that the the calibration on a probe is correct is to check the value by making DO water of known concentration. If we know the atmospheric pressure and temperature, we can know the expected DO in water exactly.
The following program will calculate the dissolved oxygen exactly for you. Simply enter the termperature and pressure and this expression will return the dissolved oxygen at 100% saturation Dissolved Oxygen Calculation
One method of DO measurement method that is very accurate and easy to use is the luminescent dissolved oxygen method (LDO). The only problem with using luminescent technology is that usually the probes are to big to attempt to use with BOD. There is one probe that will fit in standard BOD bottles and it is worthwhile to try and use it for BODs.
- Do not calibrate your probe with the Winkler Titration, instead use the Water Saturated Air Procedure for calibration.
- Switch to the LDO BOD probe. They are more accurate than either Winkler or Clark Cells.
- Calibrate your probe as close to 20 degrees as possible to eliminate potential error in temperature correction.
Luminescent Dissolved Oxygen Information