There are three methods available for measuring dissolved oxygen concentrations. Modern techniques involve either an electrochemical or optical sensor. The dissolved oxygen sensor is attached to a meter for spot sampling and laboratory applications or to a data logger, process monitor or transmitter for deployed measurements and process control.
Dissolved oxygen can be measured by colorimetric methods, a sensor and meter or by titration.
The colorimetric method offers a basic approximation of dissolved oxygen concentrations in a sample. There are two methods designed for high-range and low-range dissolved oxygen concentrations. These methods are quick and inexpensive for basic projects, but limited in scope and subject to error due to other redoxing agents that may be present in the water.
The traditional method is the Winkler titration. While this method was considered the most accurate and precise for many years, it is also subject to human error and is more difficult to execute than the other methods, particularly in the field 27. The Winkler method now exists in seven modified versions which are still used today.
The most popular method for dissolved oxygen measurements is with a dissolved oxygen meter and sensor. While the general categories of dissolved oxygen sensors are optical and electrochemical, electrochemical sensors can be further broken down into polarographic, pulsed polarographic and galvanic sensors. In addition to the standard analog output, several of these dissolved oxygen sensor technologies are available in a smart sensor platforms with a digital output.
A dissolved oxygen sensor can be used in the lab or in the field. DO sensors can be designed for biochemical oxygen demand (BOD) tests, spot sampling or long-term monitoring applications. A dissolved oxygen meter, water quality sonde or data logging system can be used to record measurement data taken with a DO sensor.
Measuring dissolved oxygen with a sensor and meter (photo credit: Fondriest Environmental; Flickr).
As dissolved oxygen concentrations are affected by temperature, pressure and salinity, these parameters need to be accounted for 7. These compensations can be done manually or automatically with a dissolved oxygen meter or data logging software. Temperature is generally measured by a thermistor within the sensor and is acquired by the meter or data logger without prompting. Many DO meters include an internal barometer, and data logging systems can be set up with an external barometer or water level sensor for pressure measurements. Barometric pressure can also be manually input as altitude, true barometric pressure or corrected barometric pressure. Salinity can be measured with a conductivity/salinity sensor and automatically compensated for, or approximated and manually input as:
|Fresh water||< 0.5 ‰ (PPT or parts per thousand)|
|Brackish water||0.5-30 ‰|
|Saline water||30-50 ‰|
|Brine||> 50 ‰|