Q. What is a PID?

A. A photoIonisation detector or PID is a type of gas detector.

Typical PID Instruments measure Volatile Organic Compound and other gases in concentrations from sub parts per billion (ppb) to 20000 parts per million (ppm). The PID Instruments are the most efficient and inexpensive type of gas detector. They are capable of giving instantaneous readings and monitoring continuously. They are widely used in military, industrial, and confined working facilities for safety.

PID Instruments are ion detectors which uses high-energy photons, typically in the ultraviolet (UV) range, to break molecules into positively charged ions. As Compound elute from the GC's column they are bombarded by high-energy photons and are Compound ionized when molecules absorb high energy UV light. UV light excites the molecules, resulting in temporary loss of electrons in the molecules and the formation of positively charged ions. The gas becomes electrically charged and the ions produce an electric current, which is the signal output of the detector. The greater the concentration of the component, the more ions are produced, and the greater the current.

The current is amplified and displayed on an ammeter. The ions recombine after passing the detector to reform their original molecules.

As a stand alone detector Photo Ionisation Detection Instruments are broad band detectors and not selective at all as these ionize everything with an Ionisation energy less than or equal to the lamp output. A PID Instrument is highly selective when coupled with some type of chromatographic technique or a pre-treatment tube such as a Benzene specific tube. The PID will only detect components which have Ionisation energies similar to the energy of the photons the detector uses. This selectivity can be useful when analyzing mixtures in which only some of the components are of interest.

PIDs are non-destructive detectors. They do not destroy/consume the components they detect. Therefore they can be used before other detectors in multiple-detector configurations.

Q. How Accurate is a PID

If a PID is routinely gas tested it will provide reliable and accurate data. We recommend that the gas test confirms the instrument is reading between, -10 to +20% of the gas can concentration, typically 100ppm of Isobutylene.

Assuming the PID has passed the gas test it will give you accurate ‘Isobutylene Equivalent’ results.

Because most users are not looking to measure isobutylene, there is probably an apparent error depending on the compound or compounds that are being measured. If one compound only is being measured it is simple to correct the reading.

Assuming we are measuring Toluene with an Isobutylene calibrated instrument,

If we have a reading of 100ppm, this is ‘100ppm of Toluene, Isobutylene equivalent’

It is OK to present data this way. This is the way most organisations use their PIDs.

However, this concentration is not 100ppm of toluene and a correction factor must be applied to ensure an accurate numeric value! The correction factor for Toluene relative to an Isobutylene calibrated instrument is 0.5. The accurate reading of toluene is 100 x 0.5 = 50ppm.

All our PIDs now offer a library of compounds. If sampling a specific compound you can select that compound from the library in the instrument. Selecting the compound automatically applies the correction factor for that compound. The compound library in the PID also gives the correction factor. A table of compounds and a wall chart is available for reference purposes.

When several compounds are simultaneously being measured it is very difficult to correct  the PID to provide an accurate absolute reading. For example if 2 compounds are present and one has a correction factor of 0.5 and the second a correction factor of 4.0. You would also need to know the % of each compound in the mixture before calculating the exact concentration.

Users in many situations including soil analysis on land clean-up sites are usually not aware of the VOCs that are being measured. A PID should therefore be considered as an excellent screening tool for front line assessment. But it is unrealistic to expect it to provide highly accurate results unless a known single compound is being sampled.

Other techniques are available to provide more specific and possibly more accurate data. But these alternatives are sampling devices that provide measurement after several minutes. A PID will provide a real time stream of data up-dated every second. This means that the quality of data you get from a PID is frequently better than the alternatives.. These alternative devices are also significantly more expensive.  

Q. How frequently should a PID be calibrated?

Calibration of a PID is essential to ensure reliable data. It is recommended that instruments are returned to Shawcity for an annual overhaul and re-calibration. But if PIDs are in regular use it is important that they are checked and re-calibrated if necessary.

Gas testing or ‘Bump Testing’ a PID is recommended on a monthly basis. To gas test a PID you simply switch on your instrument and with the instrument sampling in the normal way, feed calibration gas onto the instrument. Isobutylene is the standard calibration gas but you can use alternatives. For instruments with ppm resolution we recommend using a 100ppm concentration. If the instrument has ppb resolution we recommend using a 10ppm concentration.

When gas is applied to the instrument it must read within the range of 90-120ppm. If your instrument is within this range it has passed the Gas test and there is no need to re-calibrate!

If outside the range of 90-120 when testing with 100ppm, calibration is needed. To re-calibrate follow the simple instructions in the operating manual. Alternatively you can telephone for assistance and we can talk you through the calibration.

Q. How do I set an Alarm Level on my PID?

You can set alarm levels on all the Shawcity PIDs. 4 alarms are available. 2 instantaneous or high alarms and the time average alarms, STEL (15min short term exposure level) and TWA (8hour time weighted average).

The HSE publication, EH40 provides guidance regards alarm levels for STEL and TWA.But there are no figures in EH40 for instantaneous alarms.

Some users simply adopt the STEL and TWA alarm figures as instantaneous alarms. This is considered a ‘play it safe’ option and is OK if circumstances allow. But adopting these figures for instantaneous alarms results in more frequent and possibly unnecessary alarms.

Other users choose to consider the 8hour TWA and extrapolate back to arrive at a less demanding instantaneous alarm.

If for example the VOC being measured is Toluene the 8hour TWA is 50ppm. This implies that exposure over 4 hours can be 100ppm, exposure over 2 hours can be 200ppm, exposure over 1 hour can be 400ppm and exposure over 30 minutes can average 800ppm? But there is also a 15 min STEL for toluene and that is 150ppm. So an instantaneous alarm at 3 x the TWA (3 x 50) or 150ppm and a higher instantaneous alarm at 5 x TWA (5 x 50) or 250ppm appears to be a reasonable compromise.

Applying the same logic to benzene gives an instantaneous alarm at 3ppm and 5ppm. Many UltraRae users have adopted 1ppm as an instantaneous alarm. This results in delays as workers are required to wear additional protective masks. This is also unpopular with the workers for comfort reasons.  

It is important to recognise that in the absence of legislation any decision regards alarm levels must be the responsibility of the user. The relative levels indicated above are however useful guidelines that meet the requirements of most users.                                                                                    

Q. I need to know how to get the best out of my PID - How can I get some training?

A. Shawcity define ‘Instrument Training’ and ‘Instrument Awareness’ as two separate issues. If you are considering purchasing or hiring instruments or just want an overview of current practices and instruments available you should consider our Awareness Workshops – we are currently running both Photo Ionisation Detector (PID) and Landfill and Soil Gas Analyser Awareness Workshops which give an overview of the applications, instruments used and their operation and can be carried out at your works at a time to suit you.

These Awareness Workshops normally cost £ 250 + VAT per session but are FREE OF CHARGE to potential purchasers and hirers of our equipment.

Shawcity also offer Certificate of Competence Training in the use of all the instruments we sell which can include both Operator (basic use of the instrument) and Supervisor (Operator level plus basic fault finding and data downloading) training with testing of the trainees knowledge and culminates in the issue of a Certificate of Competence in the use of the instrument.

With this training you can be sure that your operators are fully conversant with the use and operation of their instrument. This can be particularly useful if you are working in partnership with local or municipal authorities or the Environment Agency where this type of training is often mandatory.

Approved Training costs £ 495 + VAT for a ½ day session and

£800 + VAT for a full days session depending on the complexity and number of personnel involved and is carried out at you works by our factory trained personnel.