FT - S8 Dissolved Oxygen Sensor: Unraveling and Measuring Dissolved Oxygen

What is dissolved oxygen?

Dissolved oxygen refers to molecular oxygen dissolved in water. There are two sources of dissolved oxygen in water. One is the oxygen dissolved in water under the equilibrium state between the water body and the atmosphere, and the other is the oxygen formed by chemical and biochemical reactions in the water body.

Dissolved oxygen is a necessary condition for the survival of animals, plants and microorganisms in water. By measuring dissolved oxygen, the pollution situation of water can be known, providing necessary data support and theoretical guidance for sewage treatment and protection.

Most of the biological treatments in current sewage treatment adopt the treatment process combining anaerobic and aerobic methods. Dissolved oxygen plays a crucial role in the actual biological treatment of wastewater. An inappropriate value of this indicator or excessive fluctuations can quickly cause the activated sludge system to be impacted, thus affecting the treatment efficiency. Therefore, in the actual biochemical treatment process, the content of dissolved oxygen needs to be strictly controlled.

The FT - S8 Dissolved oxygen sensor is a water quality sensor launched by Fengtu based on the fluorescence method. Its core working principle is as follows: when an excitation light of a specific wavelength irradiates the fluorescent substance on the surface of the sensor's fluorescence membrane head, the fluorescent substance will be excited and release fluorescence. The decay time of the fluorescence is closely related to the concentration of oxygen molecules on the surface of the membrane head. By accurately measuring the lifetime of the fluorescence, the sensor can calculate the concentration of dissolved oxygen in the water body.

Its measurement range is 0 - 20mg/L, the resolution is 0.01mg/L, and the accuracy is ±2%, ±0.3℃. The output methods are RS - 485 (Modbus/RTU) and 4 - 20mA.

According to Henry's law, the solubility of a gas in a liquid is inversely proportional to the temperature. For dissolved oxygen, when the water temperature rises, the solubility of oxygen in water will decrease; when the water temperature drops, the solubility will increase. For example, when the water temperature rises from 10℃ to 20℃, the saturated concentration of dissolved oxygen in water will decrease significantly. If temperature compensation is not carried out, the measurement results will have a large deviation. Therefore, this sensor is equipped with an automatic temperature compensation function, which will correct the relationship between the fluorescence lifetime of the fluorescent substance and the dissolved oxygen concentration according to the temperature.

It adopts an immersion installation method. Connect the sensor to the cable and directly place it in the water body for measurement. The joint uses a 3/4NPT installation thread, and the cable length is up to 5 meters. Its protection level is IP68, with good waterproof performance. Therefore,

it can directly contact the measured water body to measure the dissolved oxygen content in the water in real - time and accurately.

The Dissolved oxygen sensor FT - S8 adopts a two - point calibration method. The sensor is calibrated by using two standard points with known dissolved oxygen concentrations, one is a concentration point close to zero oxygen, and the other is a concentration point close to saturated oxygen or a known higher - concentration oxygen. Through these two points, the linear or non - linear relationship between the sensor output signal and the dissolved oxygen concentration is determined, so as to accurately calibrate the sensor.