What Is a Water Quality Sensor?
A water quality sensor is a sensor used to measure water quality.
Major measurement items include pH (hydrogen ion concentration), ORP (oxygen reduction potential), DO (dissolved oxygen), and EC (electrical conductivity), etc. Not only single-item measurement sensors but also combined sensors are available. In addition to these items, there are also a variety of products that can measure water temperature, water level, water pressure, salinity, ammonia, turbidity, and other parameters.
Uses of Water Quality Sensors
Water quality sensors are mainly used by research institutes to measure water quality in rivers, lakes, marshes, and oceans; in agriculture to manage hydroponics, liquid fertilizer, and agricultural wastewater; and in factories to control wastewater.
In addition to surface water and groundwater measurements, the system also plays an important role in water quality inspections at aquaculture farms, where accurate water quality control is required. Water quality sensors are also used in the quality control of grey water, which is sometimes referred to as recycled water after it has been used for drinking water treatment.
Principle of Water Quality Sensors
Water quality sensors mainly measure pH (hydrogen ion concentration), ORP (oxygen reduction potential), DO (dissolved oxygen), and EC (electrical conductivity). Each of these items can be measured separately or as a combined sensor.
1. PH
pH is the concentration of hydrogen ions. The glass electrode method is mainly used as the measurement method. The glass electrode method uses a glass electrode and a reference electrode to determine pH by measuring the potential difference (voltage) generated between the two electrodes.
River water generally has a pH of around 7, but since it varies depending on the location and conditions, it is considered a major parameter in water quality surveys.
2. ORP
ORP is the oxygen reduction potential. It is an indicator of the oxidizing or reducing potential of water and represents the potential determined by the balance between dissolved oxidized and reduced substances.
The measurement method is similar to that of pH measurement, using a pH meter body with an mV measurement function, a precious metal electrode (platinum or gold electrode), and a comparison electrode.
3. DO
DO refers to dissolved oxygen. Dissolved oxygen indicates the amount of oxygen dissolved in water quality, a value that varies with temperature, salinity, and atmospheric pressure. Since living organisms in water consume the oxygen dissolved in the water to survive, it is used as one indicator of the ease of survival of living organisms.
Specific measurement methods include the titration method, diaphragm electrode method, and fluorescence method. In the fluorescence sensor, fluorescence is irradiated onto the sample, and the concentration of dissolved oxygen is calculated from the attenuation of transmitted light using the phenomenon of quenching by dissolved oxygen. In other words, the higher the concentration of dissolved oxygen, the stronger the quenching phenomenon and the lower the fluorescence detected by the detector.
4. EC
EC is the electrical conductivity. Electrical conductivity is an indicator of the ease of conducting electricity in water. It is measured by passing electricity between electrodes immersed in a solution.
Pure water does not conduct electricity, but solutions with high levels of electrolytes in the water conduct electricity easily. An example of an electrolyte in water is nitrogen dissolved in wastewater.
Types of Water Quality Sensors
There are two types of water quality sensors: those that are installed according to the application and portable types that can be carried around. The portable type consists of a stick-shaped sensor and an operating unit, such as an instrument monitor, connected by a cable.
The electrode is placed in the water and the values are read from the panel on the main unit. Because of the risk of dropping the device when used outdoors or in the field, many products are designed to be robust and easy to operate even with one hand. Also available are models that are highly adaptable to heat and have been developed to enable measurement in places with extreme temperature fluctuations.