When encountering a problem with your values, most operators blame the instrument or probe first. After all, it’s what gives you the values! There can be many factors leading to the problem from dirty bottles, DI water, bad seed, bubbles, non-linearity and so on.
Calibration: Regardless of technology, it is recommended to only calibrate the instrument once a day before readings are taken. Make sure there are no water droplets on the probe tip before calibrating and that the probe is in a 100% water-saturated air environment. A BOD bottle with a little water in the bottom is all that’s needed. Place the probe back in this bottle when not in use. Make sure this bottle is kept clean as shown in the bottle on the right of the image. The bottle on the left is not a clean environment and could affect the calibration.
Probe Care: Polarographic sensors must have fresh electrolyte and membranes along with a maintained anode and cathode for optimal performance. Optical sensors will need an occasional visual check of the paint layer on the sensor cap.
Warm-Up Time: Polarographic probes require sufficient warm-up time of 5-15 minutes before calibrating and using the probe. Improper warm-up time can easily lead to data drifting due to an inaccurate calibration. Optical probes do not require a warm-up period.
A variety of strategies are employed to deal with specific sample types. These include varying dilutions and diluent seeding. It is often desirable to distinguish between carbonaceous and nitrogenous demand, in which case a nitrification inhibitor is used. Toxic and chlorinated samples also need special handling. The operator must be familiar with standard methods and with the technical literautre on the subject.
When properly used, the test provides a reliable characterization of wastewater. It can be expected to be a standard for regulatory agencies for many years even though its use as a control tool is limited by the 3 or 5 day wait required for the test (and sometimes 20 days!). Various methods (based on short-term monitoring and extrapolation) of quickly estimating the probable results of the test on a sample have been devised and the interested reader is advised to consult appropriate literature but a 'true' test requires time and incubation.
View our optical ProOBOD probe and learn more about this technology or read our tech note: Accurate BOD Measurements with Electrochemical and Optical Probes.
