Condensed from an OPSIS AB article by Bengt Löfstedt, first published 2021-04-08.
OPSIS analysers began with broadband DOAS: a white-light source and a spectrometer or interferometer that separates wavelengths close to the detector. There is another route — emit monochromatic light directly and scan its wavelength, with no dispersive element at all. LEDs are close but not close enough: the spectrum of a red LED is a mixture of reds roughly 10 nm wide, while spectroscopy needs a fraction of a nanometre. A laser diode is an LED built with a very precise micro-cavity so that light oscillates and one wavelength is amplified, giving an effectively monochromatic beam, usually in the infrared. The emitted wavelength depends on the temperature of the diode substrate. Stabilise the temperature, then ramp it or the drive current periodically, and the light sweeps a window a few nanometres wide across a chosen absorption line. A synchronised detector accumulates thousands of scans, and comparison with a reference spectrum of known concentration gives the concentration. Two limits follow: the temperature must be held to fractions of a degree, and because the window is narrow, one laser generally serves one or two gases. Carbon monoxide and carbon dioxide happen to have lines close enough to share a window; water vapour, with lines across wide regions, often appears uninvited as a spectral interferent. The OPSIS LD500 carries up to four laser modules, one or two gases each, with light sent by fibre to a transmitter, across the duct to a receiver, and the digitised signal returned over a standard communication fibre.
In China TDL analysers are used most often where a single demanding component matters: ammonia slip at an SCR outlet, hydrogen chloride or hydrogen fluoride in incineration and other corrosive duties, and carbon monoxide. The measurement is a path average, which suits continuous control of a process as well as emission reporting, and in a CEMS the data handling and quality control still follow the HJ 75 and HJ 76 series for calibration, span checks and validity. One design-stage point deserves more attention than it usually gets: because the scan window is only a few nanometres wide, confirm that the target component has a usable absorption line under the actual process conditions, and assess interference from water vapour and CO2 before the analyser is specified. The second point is thermal. A TDL module holds its diode to a fraction of a degree, so the enclosure must not be exposed to fast ambient swings; a shelter that bakes in the afternoon and cools at night will show it in the data even though the module itself is working correctly.
Inside the Gas Analyser: the Tuneable Diode Laser — read the original at opsis.se ↗
Written by Bengt Löfstedt, OPSIS AB, first published 2021-04-08. 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.