PID detector principle
Structure of PID detector
The PID detector consists of a UV lamp, a sensor housing, a light window, a filter membrane, electrodes, a circuit, etc. The vacuum glass cavity is filled with high-purity rare gases such as inert gases. The vacuum glass cavity is filled with high-purity rare gases such as inert gases. The glass cavity is sealed with a window that is permeable to ultraviolet light, and an electromagnetic field is applied for excitation to generate ultraviolet light. The ultraviolet light passes through the window and comes into contact with the gas that enters the detector through the filter. The gas is ionized by the light and generates a current signal that is detected by the circuit.
PID detector principle
PID detector is a kind of detector that uses plasma generated by a UV lamp to ionize the molecular components entering into the photoionization gas chamber and generates a current signal, also known as a photoionization detector (Microwave Photo-ionization detector), which is made of quartz or hard glass tube material.
When the molecular components of the sample are in the photoionization detector ionization gas chamber, the molecular components of high-energy plasma excitation for positive ions and free electrons, under the action of a strong electric field for the formation of directional movement of the ion stream and the output signal; current amplified to analog or digital signal output, the size of the current signal and the concentration of the sample is proportional to the photoionization. After being detected, the ions recombine to become the original gas and vapor, so the PID detector is a non-destructive detector for samples, and the detected samples can be collected and reused.
How do PID detectors measure the concentration of different gases?
Different UV lamps have different discharge gases and produce plasma (photons) of different energies, the current commercial PID detector UV lamp specifications are 9.8 eV, 10.6 eV, and 11.7 eV. While different compounds have different ionization energies, most gases have their own specific ionization energies (IPs), which are measured in units of eV, and the energies emitted from UV lamps are also measured in units of eV. The energy emitted by a UV lamp is also measured in eV. If the IP of the gas to be tested is lower than the output energy of the lamp, then this gas can be ionized. The ionization energy determines whether the gas can be detected by the PID sensor. When the ionization energy of a molecule is higher than the photon energy of the detector plasma the sample will not be ionized. When confirming the measurement of a compound, it is necessary to specify the ionization energy of the gas and select a UV lamp with the appropriate energy. Compounds with ionization energies higher than 11.7 eV cannot be detected by commercial PID for the time being. The use of 11.7ev high-energy lamps and lithium fluoride (LiF) light window, the photoionization detector can be used as a general-purpose detector; when the use of low-energy lamps, the range of components to be detected becomes narrower, and at this time, the photoionization detector is a selective detector.
PID detector to measure the characteristics of the gas
1. PID detector without quantitative ability
PID detectors for hundreds of compounds are responsive when a certain energy specification is selected after the PID detection (10.6eV), assuming that there are a variety of gases in the environment at the same time, such as benzene, vinyl chloride, ethane, ethylene, acetic acid. The ionization energy of acetic acid is 11.7eV, which is higher than 10.6eV, so acetic acid cannot be ionized. And benzene, vinyl chloride, ethane, and ethylene can be ionized, this time the current signal is a superposition value, and can not represent the concentration of a gas in the sample, so the PID detector does not have the ability to identify a gas in the gas mixture, and can not accurately measure the exact concentration of a gas. However, if the PID detector is used in tandem with a gas chromatography column, the column is used to separate the benzene, vinyl chloride, ethane, and ethylene in the mixed sample gas one by one, and then enter the PID detector, then the detector will be able to measure the concentration of each gas separately.
2.A certain ability to characterize
Selected ionization energy specifications PID detector although hundreds of compounds are responsive, but when the environment can be a single presence of its ionized gas, PID detector is able to detect it qualitatively. When the PID detector needs to be calibrated for detecting different gases, it is of course best to calibrate with whatever gas is being measured. Because the PID detector can measure a wide range of toxic and hazardous gases, the user site does not have the conditions to provide the corresponding standard gas. So the calibration gas used by the manufacturer is isobutylene, for different gases, the introduction of the calibration factor (CF), that is, different gases measured with the same detector, the signal derived from the detection of the same concentration of isobutylene derived from the signal of a conversion relationship between the CF coefficient is based on their own sensors to do countless experiments, summed up, so different brands of PID detectors, CF coefficient has a certain degree. How to use the CF coefficient, you can consult the PID detector manufacturers too. Different manufacturers of handheld photoionization gas detectors, generally also have built-in CF coefficients, the instrument shows the value is already converted after the concentration value and does not need the user to convert.
According to the definition of VOC gas, VOC is a volatile organic compound, it is not specifically a gas.VOCs are usually divided into non-methane hydrocarbons (referred to as NMHCs), oxygen-containing organic compounds, halogenated hydrocarbons, nitrogen-containing organic compounds, sulfur-containing organic compounds, and other major categories. Toxic gases, it is also a type of gas, many toxic gases are also volatile organic compounds. For the PID detector, as long as its selection of ultraviolet lamp ionization energy is high enough, there is basically no gas that can not be ionized. So PID detectors can measure a very wide range of gases. But there are also many defects, such as high ionization energy detector light source short life, such as mixed gases can not identify a single gas. When selecting a detector, it is necessary to combine the advantages and disadvantages of the detector as well as the customer’s requirements to make a comprehensive consideration.