Optical dissolved oxygen sensor JUMO digiLine O-DO S10

Dissolved oxygen sensors in a nutshell

All industries however related to water, e.g. heating or aquaculture, measure the oxygen dissolved in water. In the following article, we answer questions related to this process - you will find out, among other things, what types of oxygen sensors there are, what their advantages and disadvantages are and how the measurement takes place.

Where is dissolved oxygen measured in water?

In all water-related industries, the dissolved oxygen content of the water is measured. This parameter is important for the functioning of aquatic ecosystems - it reflects their 'health', i.e., among other things, the balance between oxygen supply from the atmosphere and photosynthesis. It also provides information on water quality.

Oxygen control is therefore necessary, among other things, in heating systems, where the presence of oxygen is a sign of corrosion, as well as in fish farms, where it is essential for the survival of fish and plants.

What is an acceptable level of dissolved oxygen?

Healthy water should have a general dissolved oxygen concentration above 6.5-8 mg/L and between about 80-120%.

Oxygen meters – applications

Typical applications where oxygen sensors are used:

  • Drinking water monitoring

  • Fish farming (fresh and salt water)

  • In municipal and industrial wastewater treatment plants

  • General water and wastewater technology

  • Universities and educational institutions

Dissolved oxygen sensors are used, among other things, in fish farming

Basis for measuring dissolved oxygen concentration in water

Unit of measurement – dissolved oxygen value

Oxygen is measured, for example, in mg/l, ppm, ppb or %/volume.

Influence on measurement values

It should be taken into account that the measurement result is influenced by values such as:

  • temperature of the measuring solution - this can be compensated for by an integrated temperature sensor as in the JUMO digiLine O-DO S10 oxygen probe

  • ambient air pressure

  • salinity

Electrometric versus optical sensors ro measure dissolved oxygen in water

There are two types of sensors for measuring dissolved oxygen in water: electrometric and optical.

Electrometric dissolved oxygen sensors

Electrometric sensors, measure the concentration of dissolved oxygen in water based on the electrical current generated.

Using the electrometric method of measuring oxygen, decomposition of the anode and wear and tear of the electrolyte during operation cannot be avoided.

Optical dissolved oxygen sensors

Optical sensors, also known as luminescence sensors, measure the concentration of dissolved oxygen in water based on the extinction of luminescence in the presence of oxygen by oxygen permeable membrane.

Luminescence – the principle of optical oxygen sensors

Luminescence is a highly sensitive test method that helps to detect various substances. In optical oxygen sensors, measurement using it proceeds as follows:

1. The sensor has a sensor cap with an internal luminophore coating.

2. A light source inside the sensor cyclically illuminates the luminophore.

3. The phosphor, by absorbing energy, changes its ground state to an excited state. After some time, it returns to its ground state, losing heat and emits the remaining energy in the form of red light (called fluorescent radiation), which is detected by a photodiode in the sensor body.

4. In the presence of oxygen, the phosphor collides with it and the emission of luminescent radiation is not stopped.

5. No oxygen is consumed in the process and no minimum flow rate is required. This measurement method allows high measurement accuracy even at low oxygen concentrations.

Optical oxygen sensor – operating principle

Optical dissolved oxygen sensor – advantages

The advantages of luminescence-based optical dissolved oxygen measuring instrument, compared to electrometric sensor, are:

  • stability and precision of measurement over a longer period of time - calibration is carried out infrequently

  • less maintenance work

  • no change of electrolytes

  • no polarisation voltage required

  • no minimum inflow

  • low drift