Choosing the right PID for your applications

PIDs are necessary in many applications

Fire departments, HazMat teams, refineries and, most recently, homeland security personnel are all deeply concerned with the detection of hazardous compounds. Both gas detection and monitoring are needed at sites of spills, leaks, and other emergencies. Photoionisation detectors (PIDs) are capable of effectively detecting and monitoring a vast number of hazardous gases and have become popular in recent years as sensors in hand-held multigas detection instruments. If well-engineered, a PID can offer an ideal combination of fast response, ease of use and maintenance, convenient size, the ability to detect very low levels of hazardous compounds, and affordability. What follows are basic guidelines to help you make the best choice when selecting a PID instrument.

The basic principle of photoionisation

PIDs rely on a process called ionisation as the basis of gas detection. The energy required for a neutral molecule to be ionised (i.e. to have an electron removed) and thus change to a charged molecule (an ion) is known as its ionisation potential (IP). The IP is expressed on an electron volt (eV) energy scale. These removed electrons generate a small current, which is in proportion to the concentration of ionisable gas that is present. There are several excellent sources available to research the IPs of particular gases, such as the National Institute of Science and Technology (NIST) [http://webbook.nist.gov/chemistry/ie-ser.html] and the National Institute of Occupational Safety and Health (NIOSH) [www.cdc.gov/niosh/npg/npgd0000-all.html].

To learn more about PID sensor principles see MSA’s White Paper 0803-11/January 2004.

PID lamps

The energy for ionisation is provided by a special lamp designed to emit ultraviolet (UV) light. The “heart” and most critical part of the instrument is the PID lamp, which consists of a glass tube filled with low-pressure gas and sealed with a crystal window. The gas within the tube (or “bulb”) is usually an inert gas such as krypton (Kr). There are two basic versions of PID lamps: with or without internal electrodes. The basic operational principle of both versions is the same; they emit UV light by electrical breakdown (discharge) of the gas sealed in the tube. Well-sealed and cleanly processed lamps provide an UV output with a defined and repeatable energy spectrum as shown on the left, compared with that of a poorly processed lamp as shown on the right (Figure 1). Contaminated lamps like this example whose spectrum is shown on the right, will not provide consistent performance from lamp to lamp, especially if the lamp processing is contaminated and results in outgassing of extraneous compounds into the volume of the tube. This will result in erroneous use of correction factors and possibly reduction of expected lifespan of the lamp.

Figure 1: UV spectra of low-energy PID lamps from two different manufacturers.

9.8 eV and 10.6 eV lamps

The most common and easy-to-use PID lamps operate with photon energy of about 9.8 eV or 10.6 eV. A practical feature of the lower-energy lamps is the ability they provide to discriminate against higher-energy compounds that may not be of interest in a particular detection application. For example, a 9.8 eV lamp can detect benzene which has IP = 9.25 eV, but not acetaldehyde, which has IP = 10.21 eV. To make sure that the gas of interest will be ionised, the lamp energy has to be higher than that of the gas. 9.8 eV and 10.6 eV lamps are usually advertised as requiring low maintenance and having a typical lifetime of one year.

Lifetimes of three years for some PID lamps are now being offered, but of course the actual useful life and maintenance will largely depend on the hours of day-to-day use and the cleanliness of the atmosphere to which they are exposed in the field.

11.7 eV lamps

High-energy lamps (11.7 eV) are also available for some PID models, which allow ionisation of gases with higher IPs such as chloromethane and acetyl chloride. However, the theoretical ability of high-energy lamps to ionise larger numbers of compounds is offset by practical shortcomings.

The lithium fluoride (LiF) windows of these lamps absorb water, causing the chemically unstable crystal to degrade more rapidly than those of lower energy lamps. Even in ideal applications with no water vapour present, the higher energy UV photons from these lamps naturally degrade the transmission of the LiF windows through an effect called solarisation. With the presently available technology, the typical lifetime of these lamps varies from one to two months with intermittent use, to about 25 hours of continuous use. The absorption in the windows of water vapour from the air also prevents long-term exposed storage (unless a desiccator is provided), which is a practical consideration. Additionally, the cost of 11.7 eV lamps is generally quite high, despite their considerably shorter lifetimes compared with that of 9.8 eV and 10.6 eV PID lamps. Its many weaknesses have inhibited widespread use of the 11.7 eV lamp.

‘No cleaning’ lamps

A recent PID marketing trend concerns claims of “no cleaning” or “self cleaning” lamps. What does this really mean in useful terms? As a PID lamp is operated, its window is exposed to trace components in the sampled air.

These easily ionised components tend to form a residual film on the window’s surface, which in turn causes the lamp’s UV output intensity to decrease slowly with operating time. This film can be removed simply by cleaning the lamp window with a methanol-dipped cotton swap, if the lamp is able to be removed from the unit.

Fortunately, nature has provided a counter-balance to this reaction, as UV light emitted by a PID lamp will convert oxygen molecules in the background air into ozone molecules. Ozone, if allowed to build up enough concentration adjacent to the lamp’s window, will loosen or even remove some of the residual film from the window. In diffusion-type PIDs versus pumped-flow PIDs, there exists a trade-off. With a pumped PID unit, response and clear times are usually faster, but the ozone is typically continuously flushed out and the window surface film builds up and hence the eventual need for lamp cleaning as well as recalibration. With standard diffusion-type PIDs, ozone builds up and slows the production of the window surface film, but the response and clear times are noticeably slower. This characteristic is in addition to poor humidity response, which is inherent in diffusion-type PIDs.“No cleaning” or “self cleaning” lamps are misnomers. All PID lamps will build up a window surface film which needs to be cleaned, hence the lamp cleaning instructions included in manufacturers’ operating manuals despite any other claims of “no cleaning necessary”. Lamp cleaning frequency depends upon the specific PID design, the gases sampled, the ambient sampling environment and the sample/filter system.

MSA’s patented PID design

MSA has recently designed and patented the interchangeable and disposable “button-cell” ionisation chamber used in the Sirius® Multigas Detector portable instrument (Figure 2). This innovative chamber takes advantage of the positive aspects of both pumped and diffusion-type PIDs while overcoming their respective disadvantages. Because the central volume of the button is shielded from the direct flow (unlike most competitors’ designs for pumped PIDs), a high enough concentration of ozone is allowed to build up, slowing the formation of window surface film and allowing for very infrequent window cleaning. Fast response and clear times are also offered due to the exchange of gas through the chamber via the pumped flow, as well as insensitivity to noncondensing humidity. When the need to clean the lamp does arise, an automatic signal will display during the PID calibration procedure. Lamp access in the Sirius Multigas Detector is uniquely placed to save precious maintenance time and can be performed within seconds.

Figure 2: Disposable ionisation chamber used in the Sirius Multigas Detector portable instrument. Gas outlets on other side of the chamber are hidden in this view.

Significant advantages of pumped-type PID instruments

• The instrument’s pump draws in a filtered flow of gas that passes through the ionisation chamber’s gas inlets. This steady exchange of sample gas insures the sensor’s fast response time even amidst changes in the ambient hazardous gas concentration. Likewise, the gas impurities will also clear rapidly, allowing for quick rezeroing when the sensor is returned to a fresh-air environment.
• The novel design of the sampled air flow path minimises the effects of humidity, which can degrade PID performance. Peak instrument performance is assured by optimising the gas-flow conditions before the gas sample reaches the ionisation chamber.
• Minute particles and water droplets which potentially could continue past the upstream filtering system will be steered away from the ionisation chamber, so there is no need for additional particle-collecting electrodes inside the chamber.

The importance of customer field trials

Look for basic features such as resolution to assess the instrument’s ability to measure at sub-part-per million (ppm) levels. Some instruments only provide readings at 5 or 1 ppm resolution, while others can display 0.1 ppm or even parts-per-billion (ppb). Calibrating the PID sensor before use and rechecking the calibration at the day’s end will reveal the instrument’s ability to hold its calibration.

This calibration check can be repeated over a few days to a week to determine and establish the instrument’s performance in your environment. Well-designed PIDs will show a stable zero reading while poorly designed PIDs will show instability or “zero drift” when run for at least 60 minutes in changing conditions. When doing field tests, make sure to subject the unit to changes in temperature and humidity, as going from cool/dry to warm/humid conditions can cause condensation in the ionisation chamber or on the lamp window in some designs. If this humidity effect occurs and is not controlled by the PID, it can abruptly shift the zero level of the readout, which could produce a false reading of significant concentration. When evaluating a multigas PID instrument, subject the unit to higher concentrations (>500 ppm) of VOCs instead of the usual 100 ppm of dry isobutylene gas. An easy test method involves exposing a felt marker tip to the instrument’s pump inlet. This very simple test will reveal cross-sensitivity as well as stability of other installed sensors, and will reveal the time needed to re-zero reading all sensors. And of course, the filter water-trap system should be tested.

Choosing the right PID for your applications - a basic checklist

Some issues are crucial in the choice of a PID, especially for real-world applications. Be sure to incorporate the following guidelines when researching your choice of an instrument:

• Durability (Ingress Protection ratings for water and dust, drop testing)
• Repeatability (consistent readings) and stable zero in changing humidity conditions
• Ease of calibration and ability to hold calibration
• Adequate resolution in ppm and ppb
• Cross sensitivity and stability of installed electrochemical sensors (marker test)
• Ability to quickly change response factors in the field
• Rugged housing of the instrument’s design
• Convenience of replacing the water and dust filters
• Accessibility of PID lamp and ease of changing the type of lamp
• Ease and projected frequency of lamp cleaning

Summary

Select the right PID for your application by looking closely at the available technologies. Performance, ease of calibration, maintenance frequency and lamp access are key decision-making factors. All PID lamps will degrade in performance unless they are cleaned, and the appropriate frequency depends upon the application and the design of the overall system. Higher energy lamps will ionise a broader range of gases but have other significant disadvantages. Instrument field trials with high concentration of VOCs in dirty environments along with temperature and humidity changes will reveal a great deal more about PID performance than idealised specifications which may have been generated only under laboratory conditions. Many instruments are available with diverse price tags and real-world performance; be sure to compare wisely before choosing and investing in a PID instrument.

Note: This bulletin contains only a general description of the products shown. While uses and performance capabilities are described, under no circumstances shall the products be used by untrained or unqualified individuals and not until the product instructions including any warnings or cautions provided have been thoroughly read and understood. Only they contain the complete and detailed information concerning proper use and care of these products.