Applications of Stationary Photo Ionisation Detectors
Bernd Rist, Compur Monitors GmbH & Co. KG Tel: ++49 89 620 38 0 • E-mail:
compur@compur.de • Web
www.compur.de
PID (Photo - Ionisation - Detectors) are used to detect substances which are not detectable with other methods, like in solvents or fuels. These so – called VOCs (Volatile Organic Compounds)
are not only present in all chemical industries, but also in areas where one would not expect them. The food industry is using fast ink jet printers to print the best before date on food packages, for example; these printers use relatively little ink. One gallon of ink is enough to print 2 – 3 million characters, but these printers work extremely fast: they can mark 10,000 packages per hour.
Typical ink solvents are; MEK (0,2 ppm), Ethylene cellosolve (10 ppm), N – Methyl pyrrolidone (20 ppm), Tetraethylene cellosolve (toxic) and Pentan dione (toxic). If we assume each package is marked with only 10 characters, this adds up to 1 million characters printed per workday. This means, 1/3 gallon solvent evaporates per shift. In a badly ventilated workplace, during 3 shift operation, or if several printers operate in the same location, the allowable concentration may be exceeded. Gas detection to protect the workers is imperative.
Operation
A PID uses a high energy ultra violet light beam to break the gas molecules into radicals. These radicals discharge on the electrodes of a condenser. This discharge current is proportional to the number of molecules, i.e. the gas concentration. The Statox 501 PID operates with a 10,6 eV lamp, consequently detecting all substances which are ionised below 10,6 eV. As the molecules recombine after leaving the measuring cell, the PID can be regarded as a physical measuring method, as there is no material transformation taking place.
Physical methods of gas detection are fast and robust; PIDs are capable detecting gases which cannot be measured with other sensors. Therefore one could expect that this technology is a worldwide standard in gas detection. In reality, this technology has just started to expand. The first PID instruments were brought to the market in the United States of America. In 1970s they were first used to detect Vinyl Chloride monomer. Incrementally, scientists discovered how versatile this technology is and then started measuring reference factors for other gases. PID sensor technology has been used throughout history, primarily in portable instruments for leak detection and industrial hygiene applications.
Positive and Negative Aspects of PID
In comparison to FID which is also capable of detecting VOCs, the PID does not need hydrogen as fuel. FID, Flame Ionisation Detection, uses a hydrogen flame to break the gas molecules into radicals. Applications with hydrogen involved can prove unpopular because they tend to be labour intensive. Apart of the safety risk caused by this highly explosive gas, its transportation and storage is subject to strict regulations.
PID and FID are not specific to a certain substance. While FID will detect anything that has a C-H bond, the PID will detect all substances with an ionisation potential below 10.6 eV. Both
sensor technologies are very sensitive: reliable measurements in the lower ppm or the upper ppb range are attainable.
As a PID is not specific to certain gases, it will monitor all gases that are present; therefore the user must know exactly which gases might be present in the area.
Not all gases will be detected with the same sensitivity, response factors must be observed when calibrating the instrument. For this reason, any sensor interface can be made to specification by Compur Monitors ex works. The user only needs to specify his application to avoid nasty surprises such as false alarms.
For field calibration, Isobutene, is always used; this gas is easy to manage, as it is non-toxic, non-corrosive and therefore stable and, in the concentration used for calibration, non-combustible (LEL = 1,6 %). Isobutene has therefore become a standard for PID calibration. The correct reading of the Statox 501 PID is achieved by correcting the isobutene reading with the relevant response factor.
As the sensor is using light as an energy source, gases which are light – absorbing will have an affect. When gases occur in very high concentrations they might absorb so much energy, that there is not enough energy left for the ionisation of the target gas; as a consequence the reading will be too low.
The user needs not only knowledge of interfering gases, but also about gases that might cause this so - called quench effect.
Marketing Myths Extreme sensitivity
Some manufacturers praise the nearly unlimited sensitivity of the PID. Indeed is it possible to detect some gases in concentrations as low as 2 ppb. This may be a nice feature as long as the target gas is measured in a binary gas mixture, i.e. air plus one well known substance. As soon as a PID is operatational in an industrial atmosphere, super sensitivity is no benefit. It is common knowledge that in a plant there are always some ppb of gas around; there is no need to spend money to determine this. Frequent false alarms will be a too high price paid for super sensitivity. Compur Monitors recommends not selecting an overly sensitive measuring range for fixed PID systems.
Maintenance free
A physical method of gas detection is maintenance – free. No! To an even greater degree than a normal light bulb, a high energy PID lamp is subject to increased wear and tear. A PID
IET November / December 2011
www.envirotech-online.com
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