Introduction to PID Detectors

PID Detectors are frequently employed to monitor VOC’s (Volatile Organic Compounds). Numerous vapors and gases fall into the VOC category and IGD’s PID’s can monitor for more than 300 types of gas. PID is an ideal first responder for VOC leaks, even though the PID cannot directly establish the type of gas detected.

For instance, in a chemical plant where both acetone and propylene are present, leaks of either gas will be identified by a PID detector. The PID will report a ppm level leak detection in both instances. However, the PID cannot establish whether the leak detected is propylene or acetone.

The PID is a first responder for leak detection for that reason, both gases have occupational exposure limits and neither gas should be present in the atmosphere. The PID can identify a leak has occurred, however pinpointing the source would then need to be undertaken by competent persons.

Utilizing PID’s as fixed gas leak detectors can be done as part of a sites risk mitigation strategy. To undertake this, multiple factors must be considered:

1. In humid and dirty applications or where other non-responsive gases are present, PID’s can be affected adversely. Humidity can refract the UV light or introduce leakage currents, leading to lower sensitivity.

To minimize these effects, IGD PID’s utilize fence electrode technology and miniature sample cavities. If other gases are present that do not respond on a PID, these also work to decrease sensitivity (quenching) as the UV light’s energy is absorbed in collision with molecules which are not the target gas.

If UV photons collide with a gas, they can’t ionize, so that photon’s energy is lost which decreases the amount of UV photons available. It is crucial to understand the characteristics and the application of the PID being considered.

2.  PID’s are first responders as they are not specific to one gas but respond to multiple. If a specific gas detector is available for the target gas it would be the best choice.

For instance, PID’s can detect hydrogen sulphide but specific hydrogen sulphide detectors are readily available and, for this application, would be a better choice than installing a PID.

3. In order to ionize the sample gas stream, PID’s utilize ultraviolet light. Some vapors and gases can photochemically react and form compounds which can coat the internal surfaces of the PID, leading to decreased sensitivity, or even device failure.

A good example of this type of compound is Phosphine. Using PID’s, it can be detected at low levels, making it a good first responder for occupational exposure levels.

Yet, it will react photochemically, producing what are thought to be phosphorus oxides that can coat internal surfaces affecting performance. This can be accounted for during calibration at low levels.

The PID should be cleaned where significant exposure may occur. Phosphine detection is a good example where there are alternative detector methods, but handling phosphine for calibration is undesirable making the PID a potentially better alternative. When making a detector choice it is crucial to fully understand the application.

4. PID’s respond to a wide scope of VOC’s which must be considered to ensure false alarms or alarms to VOC’s which exist in the area of application, do not cause detection problems.

Correctly sited, PID detectors in a factory producing cyanoacrylates can produce great results to protect workers from emissions; ensuring processes are well ventilated and secure.

Yet, positioning the same detectors onto a vehicle production line in order to help protect workers using adhesives in the production could cause poor results. That is because the new vehicles coming down the line are emitting many types of VOC’s from new paintwork, oils, waxes, solvents etc.

Each of these VOC’s will be detected together with any VOC emissions from adhesives being employed at workstations. So for this application it would not be possible to isolate just VOC’s from the adhesives because the factory has a high potential VOC background.

Is a PID Detector a Good Choice for Your Leak Detection Application?

PID’s could be the correct choice, but if an electrochemical, infrared, semiconductor, or catalytic based detector provides a better solution then IGD are able to advise accordingly, as they manufacture all of those types of sensor.

If a PID is the correct solution for your application, then there are numerous types available to meet your applications exact requirements. The different options IGD manufacture and are able to supply are detailed below.

Fixed Diffusion based PID detectors

ATEX or safe area are the two formats in which these are available. Diffusion based detectors enable gas to migrate into the sensor for measurement naturally. Detectors can work in either ‘stand-alone’ operation or by utilising IGD’s addressable systems, which permits them to be part of a wider detection network.

Sampling PID Detectors

Samplers are available in either ATEX or safe area versions, the same as the diffusion-based version. Samplers are a good choice for many applications:

• Areas which are difficult to access benefit from samplers, the sampler can be placed somewhere easy to locate, at a convenient point for access, and tubed off to the location needed. That point can be a sump, or high-level point that would be difficult to service and monitor otherwise.

• Clean rooms where a standard fixed detector acts as a dirt trap or ‘bug’. Samplers only require a bulkhead fitting for the sample point, minimising the impact in a clean room.

IGD’s 750 series PID samplers provide more advantages, they can be configured to automatically self-zero at predetermined intervals by drawing a scrubbed ‘clean’ air sample across the detector. This extends service intervals and supplies excellent zero stability. PID samplers can operate addressable as part of a deployed IGD detection system.

Portable PID Detectors

If it is practical to fit fixed gas detection then this should always be performed as part of any gas risk mitigation strategy, but this is not practical in some cases. For example, confined entry for inspection into fuel tanks or down sewer systems are not places where fixed gas detection would be employed.

Portable detectors VOC Detectors are available for personal protection in these instances. In these cases a portable version may also be employed to pinpoint the source of a VOC emission picked up on a fixed detector system of the PID.