Tunable Diode Laser Absorption

Tunable Diode Laser Absorption


For highly selective gas measurements of one or two gases, Tunable Diode Laser Absorption Spectroscopy (TDLAS) has been introduced by Protea into compact and easy to use industrial gas analyser systems.

TDLAS gas sensor measurement is based around the standard components of absorption spectroscopy – a light source, absorbing medium in cell and a detector. The Beer-Lambert Law still applies, where the absorption of light is proportional to the concentration, given the constants of pathlength and absorptivity.

Rather than the full-spectrum measurement of FTIR, TDLAS uses a laser diode source at a wavelength optimised for measurement of a particular gas species. Modern semi-conductor laser diodes operate at room temperature and emit in the NIR (800 – 2500nm) region. In this region, we see absorption of vibrational overtones of molecules. Compared to the fundamental absorptions we see in the Mid-IR, these overtones are weaker absorbing in the gas phase. For this reason, the measurement technology has to offer increased source power and reduce system noise in order to maximise S/N. This is carried out via the latest in electronic control of the laser and also the detector. TDLAS is ideal for gases such as H2O, CO, O2, CO2, NO, HCl, HF and NH3 which give narrow, distinct and strong absorption peaks that can be measured. For complex mixes of organics, for some gases such as SO2 and for multi-gas measurements, full spectrum Mid-IR such as FTIR is still required.

The laser diode can be tuned over a small (e.g. 10nm) wavelength range by varying its temperature. This tuning allows for the signal to be measured with a very fine resolution and the narrow absorption peaks of the gases required can be identified.

For some tuning ranges, a number of gases can be measured. This commonly involves one gas & water, as water absorption peaks are plentiful. However, given the fine resolution of this method, the interference of water is very low compared to other absorption techniques. We can select absorption peaks free from water interference and use these for measurement. Measurement time is very quick with TDLAS; as low as 1sec can give measurement accuracy required for many process applications.

Increased integration time improves noise and so repeatability, and this is used when measuring for compliance or regulatory monitoring requirements with measurement times of 30-60secs.

Tunable Diode Laser Absorption

TDLAS requires no complex set-up in software or hardware. No regular zero or background measurement is required to make a measurement, so reducing the need for consumable gases. Likewise, the response is inherently linear and repeatable and so regular span corrections of the measurement are not required. Having said that, in applications zero and span gas checks are still commonly required as part of the quality control of running an industrial gas analyser. As a system is built around an optimum laser diode for any given gas, no chemometric modelling or cross-interference compensation is required.

Tunable Diode Laser Absorption

Given the lack of moving parts and compact design, laser absorption systems can be deployed in-situ or on-stack or in extractive analysis systems. In-situ systems are specifically design for mounting in one location, i.e. continuous measurements. Using the pipe, duct or stack as the sample cell, they are confined in their measurement capabilities by what is available on-plant. In some systems, the laser source and detector are coupled to the plant measurement point by expensive fibre optic cable.

Extractive laser systems have more flexibility – they are portable, can be used to measure multiple points for survey measurements and can be located in easily accessible locations in fixed applications. By employing multi-pass gas cells into the portable TDLAS design, analysers can be designed that measure ppb levels of gases. Given the high power and low noise of TDLAS, commonly single pass or single bounce gas cells with pathlengths of 10-50cm are all that are required to give detection limits of 0.1ppm.

TDLAS systems can provide cost effective measurement solutions to many process and emission gas requirements for single gases. Protea offers TDLAS systems combining this new technology with all the benefits of good analyser design and sampling system control.

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