What is a PID gas sensor?

The air we breathe every day seems transparent and pure, but it may hide an invisible threat - volatile organic compounds. They are called VOCs and exist in paints, adhesives, cleaners, fuels, and even new furniture and decoration materials. Some VOCs have a pungent smell, while others are colorless and odorless. However, long-term or high-concentration exposure may pose a threat to health and the environment. How can we quickly and accurately identify these "invisible killers"? The photoionization detector is an efficient "air detective"!

How does a PID sensor work?

The PID sensor measures volatile organic compounds (VOCs) in air by photoionization detection (PID), which is shown schematically below. Test gas (1) is presented to the membrane filter at the top of the photoionization cell and freely diffuses into and out of the underlying chamber formed by the filter, housing walls, and a UV lamp window. The lamp emits photons (shown by arrows) of high-energy UV light, transmitted through the window. Photoionization occurs in the chamber when a photon is absorbed by the molecule, generating two electrically charged ions, one positively charged, X+, and one negatively charged.

Y- (2a). An electric field, generated between the cathode and anode electrodes, attracts ions (2b). The resulting current. which is proportional to the concentration of the VOC, is measured and used to determine the gas concentration. The PID includes a third fence electrode (patented) to ensure that the amplified current does not include significant contributions due to other current sources, such as water condensation on the chamber walls.

What volatile organic compounds (VOCs) are sensed by PID?

Most VOCs can be detected by PID. Notable exceptions are low molecular weight hydrocarbons. Each VOC has a characteristic threshold energy of light (photon energy) which, when directed at the VOC, causes it to fragment into ions. This is called the ionization Potential, or IP. VOCs are ionized (and hence detected) if light of photon energy greater than the IP interacts with the gas sample. The peak photon energy generated in a detector depends on the PID lamp used: Xenon = 9.6 eV, Deuterium = 10.2 eV, Krypton = 10.6 eV and Argon = 11.7 eV. Hence, the use of an argon lamp leads to detection of the largest range of volatile compounds, while using a Xenon lamp can increase selectivity. The instability of the window means that the 11.7 eV lamp is not recommended and is not provided. Lamps of a particular type do not typically vary in spectral fingerprint, so relative responses to a particular gas, e.g., benzene, to a particular lamp, e.g., krypton, do not vary from lamp to lamp. However, the intensity of lamps does vary to some extent, leading to a difference in absolute response to the calibration gas.

Detection range：

A large number of organic compounds containing carbon can be detected by PID. Including:

    (1)  Aromatic compounds: A series of compounds containing benzene rings, such as benzene, toluene, naphthalene, etc.

    (2)  Ketones and aldehydes: Compounds containing C=O bonds. For example: acetone, etc

    (3)  Ammonia and amines: Hydrocarbons containing N. For example, dimethylamine, etc.

    (4)  Halogenated hydrocarbons: Sulfurated hydrocarbons:

    (5)  Unsaturated hydrocarbons: alkenes, etc

    (6)  Alcohols

    (7)  Inorganic gases without carbon: ammonia, arsenic, selenium, etc., bromine and iodine, etc.

Characteristics of PID sensors:

    (1)  Extremely fast response: It can usually provide readings within a few seconds and is an ideal tool for quickly detecting leaks or assessing risks.

    (2)  Wide detection range: It can detect hundreds or even thousands of VOCs, making it a true "broad-spectrum" detector.

    (3)  High sensitivity: It can detect extremely low concentrations (ppb level) of VOCs, far surpassing the perception ability of a human nose.

    (4)  Non-destructive testing: Gas samples are only ionized for measurement and can then be used for other analyses (such as laboratory confirmation).

    (5)  Portable and easy to use: Widely applied in portable detectors, facilitating on-site inspection.

When does PID need maintenance:

• If the baseline rises after you zero the PID - Replace the electrode stack
• If the PID becomes sensitive to humidity - Replace the electrode stack
• If the baseline is unstable or shifts occur when moving the PID - Replace the electrode stack
• If the sensitivity drops too much (pay attention to checking the changes required for calibration) - Clean the lamp.

When do I replace the PID electrode stack?

The PID electrode stack can last the lifetime of the PID if used in clean environments, or may only last a month if used in heavily contaminated sites. The electrode stack is a disposable item, so always hold a spare electrode stack if you are working in a dirty environment. If the PID cell shows signs of contamination after the lamp window has been cleaned or is known to have been subjected to severe contamination, it should be replaced. It is recommended that the PID be recalibrated after the stack is replaced.

When do I replace the PID lamp?

A PID lamp will last a long time - typically five thousand hours and is warranted for six months. The sensitivity of the PID is in direct proportion to the lamp light intensity, so as a lamp ages and loses intensity, the response to a particular, low gas concentration becomes noisier.

The application of PID:

    (1)  Industrial safety: Leakage monitoring and occupational exposure assessment in petrochemical, pharmaceutical, spraying operations and other places.

    (2)  Environmental protection: Pollution source investigation, air quality monitoring, soil and groundwater VOC investigation, emergency accident response.

    (3)  Indoor air quality: Detection of decoration pollution (formaldehyde, benzene series substances, etc.), assessment of furniture/building materials release, environmental monitoring in offices/schools/hospitals.

    (4)  Public Safety and counter-terrorism: Rapid screening of hazardous chemicals (such as some chemical warfare agents and drug precursors).

    (5)  Research and process control: Laboratory analysis, cleanroom monitoring in semiconductor manufacturing.

Due to the limitations of the sensor manufacturing process, the larger the measurement range, the lower the accuracy (i.e., resolution), and it is difficult to balance the measurement range and resolution. It is generally divided into ppb level and ppm level.

ppb level, low range and high precision. If the VOC gases contained in the environment being measured have characteristics that can be harmful to the human body at very low concentrations, then low-range sensors, such as 20 ppm and 50 ppm, should be selected. It is usually applied in leak detection environments, with the main purpose of measuring whether there are VOC organic volatile substances in this environment. Its resolution can reach the ppb level.

ppm level, high range, low accuracy. It is used in environments with high pollutant concentrations, and its main purpose is to detect the concentration value of VOC gases. For example, for online monitoring of TVOCs emissions, it is recommended to choose a sensor with a range of 5000 ppm. Its resolution is basically at the ppm level.

Guarding respiratory safety, PID is everywhere

From monitoring workers' exposure risks in factory workshops to environmental protection departments' investigation of pollution sources; From the rapid identification of hazardous chemical leaks in emergency rescue to the detection of pollutant concentrations such as formaldehyde and benzene after the decoration of new houses; Even in precision industries such as aerospace and semiconductor manufacturing for environmental control, PID sensors play a crucial role. For PID sensors for VOC gas concentration detection, the ION Science OEM gas sensor product portfolio offers market-leading photoionization technology, capable of detecting extremely low levels (1ppb) of volatile organic compound gases. The measurement range is optional from 0 to 4000 ppm and can be used independently or successfully integrated into the product.