Dissolved oxygen content in water can be measured by colorimetry, dissolved oxygen detector and meter or titration. Modern technologies include electrochemical or optical sensors. The dissolved oxygen detectors are connected to instrumentation for field sampling and laboratory applications, or to data loggers, process monitors, or transmitters for deployment of measurements and process control.
The most popular method for dissolved oxygen measurements is with a dissolved oxygen meter and sensor. While the general categories of dissolved oxygen detectors 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 detector technologies are available in a smart sensor platforms with a digital output.
A dissolved oxygen detector 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.
What are types of dissolved oxygen detector？
Optical Dissolved Oxygen Sensors
Optical dissolved oxygen sensors measure the interaction between oxygen and certain luminescent dyes. When exposed to blue light, these dyes become excited (electrons gaining energy) and emit light as the electrons return to to their normal energy state 12. When dissolved oxygen is present, the returned wavelengths are limited or altered due to oxygen molecules interacting with the dye. The measured effect is inversely proportional to the partial pressure of oxygen 5. While some of these optical DO sensors are called fluorescent sensors 10, this terminology is technically incorrect. These sensors emit blue light, not ultraviolet light, and are properly known as optical or luminescent DO sensors 11. Optical dissolved oxygen sensors can measure either the intensity or the lifetime of the luminescence, as oxygen affects both 23.
An optical DO sensor consists of a semi-permeable membrane, sensing element, light-emitting diode (LED) and photodetector 3. The sensing element contains a luminescent dye that is immobilized in sol-gel, xerogel or other matrix 23. The dye reacts when exposed to the blue light emitted by the LED 3. Some sensors will also emit a red light as a reference to ensure accuracy 5. This red light will not cause luminescence, but simply be reflected back by the dye 7. The intensity and luminescence lifetime of the dye when exposed to blue light is dependent on the amount of dissolved oxygen in the water sample 23. As oxygen crosses the membrane, it interacts with the dye, limiting the intensity and lifetime of the luminescence 3. The intensity or lifetime of the returned luminescence is measured by a photodetector, and can be used to calculate the dissolved oxygen concentration.
Electrochemical Dissolved Oxygen Sensors
Using an electrochemical dissolved oxygen sensor and meter for measuring dissolved oxygen(photo credit: YSI).
Electrochemical dissolved oxygen sensors can also be called amperometric or Clark-type sensors. There are two types of electrochemical DO sensors: galvanic and polarographic. Polarographic dissolved oxygen sensors can be further broken down into steady-state and rapid-pulsing sensors. Both galvanic and polarographic DO sensors use two polarized electrodes, an anode and a cathode, in an electrolyte solution 7. The electrodes and electrolyte solution are isolated from the sample by a thin, semi-permeable membrane.
Polarographic Dissolved Oxygen Sensors
A polarographic DO sensor is an electrochemical sensor that consists of a silver anode and a noble metal (such as gold, platinum or infrequently, silver) cathode in a potassium chloride (KCl) solution 8. When the instrument is turned on, it requires a 5-60 minute warm-up period to polarize the electrodes before calibrating or measuring. The electrodes are polarized by a constant voltage (between 0.4 V and 1.2 V is required to reduce oxygen) from the cathode to the anode 8). As electrons travel in the opposite direction of a current, the anode becomes positively polarized and the cathode becomes negatively polarized 14. This polarization occurs as electrons travel from the anode to the cathode via an internal wire circuit 19. When oxygen diffuses across the membrane, the molecules are reduced at the cathode, increasing the electrical signal 7. The polarizing potential is held constant while the sensor detects changes in the current caused by the dissolved oxygen reduction 7. The more oxygen that passes the the membrane and is reduced, the greater the electrical current read by the polarographic DO sensor.
Pulsed Polarographic Dissolved Oxygen Sensors
Pulsing polarographic dissolved oxygen sensors remove the need to stir a sample for accuracy when measuring dissolved oxygen. A rapid-pulse DO sensor is similar to a steady-state polarographic DO sensor as both utilize a gold cathode and silver anode. Both steady-state and rapid-pulse sensors also measure dissolved oxygen by producing a constant voltage to polarize the electrodes 7. However, these pulsing polarographic DO sensors turn on and off approximately every four seconds, allowing the dissolved oxygen to replenish at the membrane and cathode surface 7. This replenishing creates a flow dependence of almost zero 7. In order to consistently polarize and de-polarize the electrodes for these short time periods, a pulsing polarographic DO sensor includes a third, silver reference electrode, separate from the silver anode 7. The electrochemical reaction (silver oxidation and oxygen reduction) remains the same.
Galvanic Dissolved Oxygen Sensors
The final electrochemical dissolved oxygen sensor is galvanic. In a galvanic dissolved oxygen sensor, the electrodes are dissimilar metals. Metals have different electropotentials based on their activity series (how readily they give or accept electrons) 17. When placed in an electrolyte solution, the potential between dissimilar metals causes them to self-polarize 16. This self-polarization means that a galvanic DO sensor does not require any warm-up time. In order to reduce oxygen without an external applied potential, the difference in potential between the anode and the cathode should be at least 0.5 volts 16.