What Is a Dissolved Oxygen Meter?
A Dissolved Oxygen (DO) meter is a device used to measure the concentration of dissolved oxygen in water.
There are two methods of testing: chemical analysis, which uses the behavior of oxygen as an oxidant, and electrochemical analysis, which uses the properties of the current-carrying capacity and fluorescence intensity between electrodes, which vary with the partial pressure of oxygen and the concentration of dissolved oxygen. The electrochemical method is used as the measurement method for most common products.
Uses of Dissolved Oxygen Meter
DO meters are used for water quality and environmental inspections of rivers, lakes, oceans, well water, etc., measurement of dissolved oxygen concentration in biological reaction tanks at sewage treatment facilities, measurement of oxygen concentration in tanks at fish aquaculture facilities, measurement of oxygen concentration in factory effluent, etc. The measurement method, accuracy, and size of the liquid to be treated must be taken into consideration when selecting a DO meter.
If the measurement unit is to be thrown into water, such as a river, it is important to consider the length and strength of the cable. In addition, some products can simultaneously measure not only dissolved oxygen concentration but also pH, etc. Therefore, selection must be made according to the intended use.
Principle of Dissolved Oxygen Meter
DO meters are generally based on electrochemical measurement methods using electrodes, which can be divided into two types: fluorescent and diaphragmatic.
1. Fluorescent Dissolved Oxygen Meter
Fluorescent DO meters consist of a fluorescent substance, an excitation light source, and a light-receiving part, where the fluorescent substance is in contact with the liquid to be measured. To measure dissolved oxygen, the fluorescence emitted by the fluorescent material as it returns from the excited state to the ground state is reduced in intensity by oxygen.
Compared to the diaphragm DO meter, the electrode can be used for a longer period of time.
2. Diaphragm Dissolved Oxygen Meter
The diaphragm DO meter consists of two electrodes and an ammeter. A diaphragm made of Teflon or similar material is attached to one side of the electrodes. The measurement is based on the property of the current-carrying capacity between the electrodes to change with oxygen concentration and partial pressure of oxygen. Diaphragm DO meters are characterized by their ability to make highly reproducible measurements without being affected by pH concentration, impurities, or color in the water.
How to Select a Dissolved Oxygen Meter
1. Measurement Range
Select a DO meter with a measurement range that matches the dissolved oxygen concentration range of the target water body. Typical DO meters cover a range from a few parts per million (ppm) to tens of ppm.
2. Calibration
DO meters need to be calibrated periodically. It is important to make sure that calibration is easy to perform and that it is user-friendly.
3. Durability and Water Resistance
It is important to select a DO meter that is rugged and waterproof for outdoor and underwater use.
4. Battery Life
It is important to select a DO meter with a long battery life if extended monitoring is required.
5. Data Logging Function
A data logging function allows measurement results to be recorded for later review. It is necessary to confirm that the required functionality is available for your application.
Other Information on Dissolved Oxygen Meter
1. Water Quality Evaluation Index Using a DO Meter
Water quality evaluation indicators using DO meters are divided into four levels:
- DO value of 7 mg/L or higher
The water is very clean and in excellent condition for a wild life habitat. - DO value of 5 mg/L or more, but less than 7 mg/L
The water quality is perfect for a wildlife habitat, but a little dirty. - DO value of 3 mg/L or more, but less than 5 mg/L
Not a good habitat for wildlife, but dirty water quality. - DO value is less than 3mg/L
The water quality is not suitable as a habitat for wildlife, and the water is so polluted that it is difficult for organisms to grow and reproduce.
2. Difference Between DO, BOD, and COD
DO (Dissolved Oxygen)
DO measures the amount of oxygen dissolved in water, or dissolved oxygen, to investigate water quality.
BOD (Biochemical Oxygen Demand)
BOD is a method of assessing the amount of oxygen, or biochemical oxygen demand, produced when microorganisms and bacteria break down organic matter in water. If there is an abundance of consumable organic matter in the water, microorganisms will use it as an energy source and reproduce in large numbers.
As the microorganisms multiply, the oxygen in the water is consumed by the microorganisms, and the increase in the number of microorganisms inhibits algae photosynthesis, resulting in an ever decreasing amount of residual oxygen.
COD (Chemical Oxygen Demand)
COD is an assessment method that uses a strong oxidant to oxidize organic matter in water and converts the amount of oxidant required into oxygen equivalents.
The oxidant used varies from case to case, but the general properties of the oxidant are consistent, so basically, the higher the COD value, the more polluted the water is.