The development of a fast-response portable photo-ionization detector: a model of the instrument's response and validation tests in air

A newly developed fast-response portable gas-detector instrument is described. The principle of operation is that of the photo-ionization detector (PID), using radiation in the vacuum ultra-violet range. The instrument described here is referred to as the ultra-violet ion collector (UVIC, registered as a trademark) detector. Several innovative design features provide very substantial performance enhancements compared with other battery-operated portable PID devices and patents have been granted in the UK, the USA and Europe. The most notable advantages are with regard to speed of response and sensitivity; the UVIC detector provides a useful calibratable range of about 0.01-1000 ppm (by volume), with a response time of about 0.02 s. A generalized model of the operation of the detector is presented in the form of a set of equations that describe the ionization, advection and recombination processes taking place in the instrument. The model is generalized in the sense that the user may specify values of a wide range of parameters, so that the model can be used both to compare predictions with the performance of a specific instrument and to predict the performance of the instrument in other possible configurations and conditions in order to investigate design options. For validation of the model, values of the input parameters are measured when that is practicable; otherwise, they are estimated on some suitable basis. A software implementation of the model is tested using data obtained under a wide range of conditions both in the laboratory and in field experiments using atmospheric air as the carrier gas and propylene as the contaminant. These tests show that the model performs very well in predicting the response of the detector.