A dissolved oxygen meter is an electronic device that converts signals from a probe placed in the water into units of DO in mg/L.
Most meters and probes also measure temperature. The probe is filled with a salt solution and has a selectively permeable membrane that allows DO to pass from the stream water into the salt solution.
The DO that has diffused into the salt solution changes its electric potential. This change is sent by electric cable to the meter, which converts the signal to mg/L on a scale that anyone can read.
If DO is a critical analytical parameter, it is recommended to calibrate at 100 % saturated air, or use a known dissolved oxygen concentration (determined by the iodometric method) for the upper limit, and use a zero DO solution (even if it is not explicitly stated in a particular manufacturer’s manual) for the lower limit.
If DO meter does not allow for a second calibration point, the zero DO solution can be used as a check standard when DO meter is set to the measurement mode.
The DO meter should be able to read less than 0.5 mg/L. If DO meter does not read less than 0.5 mg/L, then there may be a problem with the DO membrane.
If it is determined that the DO membrane needs to be replaced, consult the manufacturer’s manual on conditioning the new membrane before use. It is also possible that other maintenance will need to be performed on the DO meter or the zero DO solution may need to be replaced.
Other factors that affect the accuracy of DO measurements include–improper calibration, not verifying calibration after use and not correcting for ambient barometric pressure/altitude and instrument drift.
Dissolved Oxygen Meter
A typical hand held DO meter with probe for field use is shown in Figure.
Source: M/S YSI.
O&M issues to be cared for are as follows:
• Regular cleaning of diaphragms
• Zero calibration and span calibration
• Regular replacement of internal electrode solution
• Regular cleaning of an electrode; and replacement if broken.