Condensed from an OPSIS AB article by Bengt Löfstedt, first published 2017-11-03.
Any measurement starts by finding a property of the target that is easy to measure. For gases that property is often optical: a light source, a detector, and a characteristic that separates the target from everything else in the mixture. DOAS is one such method — xenon lamp, spectrometer, a selected wavelength window, and mathematics against stored spectra of known concentration. The same instrument can therefore report several gases. Other optical methods add a forced chemical reaction. In a chemiluminescence NOx analyser, NO2 is converted to NO, which then reacts with ozone from an internal generator; the emitted light is proportional to the NO or NOx concentration. In a fluorescence SO2 analyser, narrow-band ultraviolet light excites SO2 molecules, which emit at another wavelength in proportion to concentration. The same principles serve ambient air (µg/m³, ppb) and process gas (mg/m³, ppm). DOAS covers the difference with path lengths of very different size; traditional analysers dilute the sample instead. Traditional analysers cost less per unit, but one is needed per molecule and maintenance is heavier. A DOAS analyser costs more to buy and considerably less to own, with fewer interruptions to the data.
The comparison in the original is fair, and it matches what we see in the field: the deciding cost is rarely the purchase price, it is the number of instruments, sample lines and interventions per year. A multi-component site that needs four or five gases ends up with four or five extractive analysers, each with its own probe, heated line, pump and filters — and each an independent point of failure. On the other hand, a single-gas duty with a simple, clean sample is often better served by a conventional analyser. Two practical points we add. First, unit conversion: µg/m³ to ppb and mg/m³ to ppm depend on temperature, pressure and molecular weight, so the same ppb value gives a different µg/m³ for each gas; conversion factors are not interchangeable between components. Second, when comparing instruments, compare the averaging volume as well as the accuracy — a point sample and a path average are not the same measurement, and the difference matters when results are checked against a reference method.
How Does a Gas Analyser Work — read the original at opsis.se ↗
Written by Bengt Löfstedt, OPSIS AB, first published 2017-11-03. The summary and the A.Johnson note above are our own text; the OPSIS article remains the authoritative version. Chinese reader? See the full Chinese article.
Send the gases, ranges, duct or path dimensions and required certification — we will respond with a technical proposal.