Sensing volatile organic gas compounds
September 2005
Sensors & Transducers
Various sensor technologies are currently used for the detection of toxic and combustible gases. The most common ones are solid state, catalytic bead, electrochemical, infrared and photo-ionisation detectors. The characteristics of each sensor type may be summarised as follows:
Electrochemical (EL)
Detects 20-30 toxic gases in ppb to low ppm ranges. Provides good selectivity for some gases, but has a limited lifetime, typically six months to two years. An important application is for those gases (such as ozone) used in the semiconductor industry.
Catalytic bead (CB)
Detects combustible gases in higher concentrations, from 1 000 ppm to %LEL levels. Not prone to interference from low levels of toxic gas. These sensors have relatively short lifetimes (1-2 years) but good selectivity when used for hydrocarbons.
Solid state (SS)
Detects over 150 gases, both toxic and combustible, in ranges from low ppm to %LEL. The technology offers long life, typically in excess of 10 years, with many lasting 20 years or more. They are typically more prone to interference than other sensor types. They are good for general use and recommended for ammonia and hydrogen applications, where interference is not the problem.
Photo-ionisation detector (PID)
Detects most volatile organic compounds (VOCs) in low ppm ranges including benzene, vinyl chloride, phosphene, and hexane, to name a few. PID's UV lamp requires periodic maintenance.
Infra-red (IR)
Detects CO2 and hydrocarbons. Offers good selectivity and good lifetime (typically three years or more). Ranges for CO2 available from ppm to 20% by volume, with the lower ranges offering better resolution. Hydrocarbon ranges are available from %LEL up to 100% by volume, depending on the gas.
A range of portable and permanently-mounted instruments using the above sensors is available from PREI Instrumentation. The company can provide the optimum solution for any specific gas detection application.
For more information contact Steve Herbst, PREI Instrumentation, 011 448 2172, prei@hixnet.co.za, www.eprei.co.za
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