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Technical Article: Tuneable Laser Diode for Toxic Gas Detection

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Hydrogen sulphide (H2S) has one of the most readily identifiable odours known to the petrochemical and waste water industries. The familiar “rotten eggs” smell is detectable to the human olfactory system at concentrations below one part per million (ppm). Hydrogen sulphide (H2S) is both a toxic and flammable gas; research has shown that long term exposure to concentrations above 1 ppm can irreparably damage the sense of smell. Concentrations in excess of 100 ppm can lead to an immediate loss of smell; those over 300 ppm are potentially fatal.

It is known that gases and vapours are transparent to visible light but block light of specific wavelengths. The interaction between absorption and wavelength generates a profile or “fingerprint” for each gas molecule; the Beer-Lambert law defining the absorption physics from which a gas concentration can be quantified.

Methane gas detection has been achieved due to the strong absorption of its hydrocarbon (H-C) component bond and the commercial availability of IR sources and filters. It should be noted these devices are really hydrocarbon, not methane detectors. Traditional IR sources provide a wide band of light that in essence “wash out” some absorption lines of a given gas species; a new approach was therefore needed to improve specificity. Intensive research has lead Simtronics to develop a technique using a single tuneable laser diode source coupled with advanced software algorithms to pinpoint specific “wave number” where there is little or no interference from other gases.

Modern industrial toxic gas detection systems tend to use one of two main technologies, these being Electrochemical Cell (EC) or Metal Oxide Semiconductor (MOS). Each technology has their own merits with the selection mainly depending on the local environmental conditions where the unit will be installed.

The laser diodes used for gas detection have emerged from high capacity fibre optic telecommunication equipment where e.g. multiple 10 Gbit/s channels are packed into a single optical fibre, separated by wavelength. As reference, please consider 10 Gbit/s is the equivalent of one full length movie per second.

This wavelength separation demands accurate tuning of the laser diodes, and it is this tuneability that is used to achieve gas detection by scanning gas absorption lines. As the light source bandwidth is very narrow, almost down to a single wavelength, it is possible to pick up single absorption lines and thus achieve gas detection with virtually no cross sensitivity to other gases.

The difference in bandwidth, and hence the potential selectivity when one compares a laser based detector to a conventional detector using a broadband source narrowed by mechanical filters, is in the range of 1,000,000: A laser band width of approximately 0.0001 nm compared to typically 100 nm for conventional IR gas detectors. Where a conventional IR gas detector is tuned to a major absorption band for the target gas, a laser based detector looks for single absorption lines.

Once the absorption line of interest has been identified a best match laser diode is selected which must be capable of scanning an active and reference wave number. The reference is selected to provide a datum point using a commonly available atmospheric gas, namely carbon dioxide.
The laser is finely tuned to the wave number of interest, by temperature, ±5 mK, a ramp current is then employed to sweep the region of interest, typically 0.2 - 0.3 nm. The sweeps this region at 1 kHz and continuously monitors the active and reference wave number.

The selectivity of laser based gas detection, and the fact that one laser may be used to cover both target and a reference gas like CO2, gives a very low “probability of failure on demand.” This allows the detectors to be employed with high availability and minimal maintenance in Safety Integrity Level (SIL) rated systems.

Moreover, it must be remembered that the target gas offers a primary risk to health and reducing the duration and frequency of exposure should be a key component of modern safety system designs.

For more information on Simtronics GD1 Open Path for H2S or CO2 Toxic Gas Detection click image




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