Photoionisation technique (PID)
Photoionisation sensors use UV light to ionise gas molecules and are used to detect volatile organic compounds.
A special UV lamp generates UV radiation energy, which ionises the gas molecules. The measuring head converts the UV radiation energy measured at this point into a gas concentration. This UV energy is measured in electron volts. Standard UV sources are 8.4 eV, 9.6 eV, 10.6 eV and 11.7 eV, with 10.6 eV being the most common because it is a stronger source. 11.7 eV is a lithium fluoride source, which is softer and more fragile. The photoionisation technique detects those gases whose ionisation potential is below the energy level of the radiation from the UV source. For example, benzene has a photoionisation potential of 9.24 eV, so light sources of 9.6 eV, 10.6 eV and 11.7 eV are available.
The advantages of PID sensors are good sensitivity and fast response. This measuring head can respond quickly to many low concentrations of gases. Since PID sensors are not harmed by highly concentrated gases, they are often used to decide which PPE to use.
The disadvantage of PID sensors is selectivity.PID can only detect those gases where the gas photoionisation potential is below the level of radiation from the light source. As the light source needs to be cleaned frequently, the meter needs to be calibrated frequently to ensure accuracy.
How Sensors Work
Electrochemical gas detection has many advantages and is considered to be the best technology to use where gas detection is required. The vast majority of electrochemical toxic gas sensors are manufactured on the same principle. However, there are significant differences in the sensors produced by different manufacturers. Assuming that a gas detection system is important to your facility, it is important that you understand these differences, as well as the usual limitations of this technology.
Electrochemical sensors typically have three main components: an electrode (one or more electrodes coated with a catalyst), an electrolyte, and a permeable membrane. A gas diffuses through the membrane and reacts at the electrolyte-catalyst junction to produce an electric current.
A measuring head measures the resulting current and converts it into gas concentration. Because the number of electrons released is proportional to the gas concentration, the output of the sensor is linear.
