Technical Description – Simtronics GD1 H2S Laser Diode Open Path Detector
Fail Safe, Optical H2S Detection using Lasers
Over recent years there has been a marked swing to the use of optical products for the detection of methane leaks in safety related applications. Market research has shown that the motives behind this move are: no unrevealed sources of failure, increased availability and improved likelihood of detection. This presentation discusses recent technological developments in optical toxic gas detection, in particular that of hydrogen sulfide using tunable laser diodes and laser scanning techniques.
Hydrogen sulfide (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 sulfide is both toxic and flammable; 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 vapors 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 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 us to develop a technique using a single tunable laser diode source coupled with advanced software algorithms to pinpoint specific “wave number” where there is little or no interference from other gases.
Traditional Techniques for H2S Detection
Historical methods for detecting gas included the well known use of a singing canary, although this would be mainly for flammable gas detection. 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.
Electrochemical Cell Technology
Electrochemical cells offer good sensitivity and selectivity to a range of inorganic gases . The units provide a direct current output proportional to the gas concentration.
In basic terms, an electrochemical cell is comprised by two or more electrodes surrounded by an electrolyte medium. Each cell has a gas permeable membrane for diffusion purposes. As the target gas diffuses into the cell a reaction occurs at the sensing electrode. Electrons are exchanged generating a change in the potential at the sensing electrode. The counter electrode is employed to balance the reaction. 
The basic reaction for hydrogen sulfide is: H2S + 2O2 → H2SO4
- Sensing Electrode H2S + 4H2O → H2SO4 + 8H+ + 8e
- Electrode 2O2 + 8H+ + 8e- → 4H2O
As the concentration of target gas increases, current flows causing the counter electrode to polarize. If the gas level continues to rise the potential of the sensing electrode eventually exceeds the specific range for a reaction to occur. When a cell reaches this point the output becomes non-linear thus limiting the upper most measurable concentration.
This limitation can be overcome by the use of a third electrode (reference) that allows a cell to work over a much greater concentration range.
By their very nature electrochemical cells are a consumptive technology. The units cannot be exposed continuously to the target gas of interest as this reduces the effective working life of the unit. They also suffer reduced life expectancy in arid environments such as those found in the Middle East.
Metal Oxide Semiconductors
Metal oxide semiconductor (MOS) gas sensors are, generally speaking, good all round devices although they can be affected by relative humidity changes. MOS detectors are able to detect a wide range of gases with selectivity being established through the selection of the metal oxide and catalysts. Additional selectivity can also be gained by using different operating temperatures. 
In recent years, arrays of sensors employing these techniques have also been used to build up a signature profile of the target gas. 
In the most basic terms a MOS sensor is an “air resistor” with the resistance varying logarithmically, at constant temperature, depending on the composition of the atmosphere adsorbing onto the sensor surface. MOS sensors are used extensively in the Middle East.
Infrared (IR) Light Absorption
Gas molecules are composed of different types of atoms (e.g. hydrogen and carbon in methane or hydrogen and sulfur in hydrogen sulfide).
The bonds between these atoms absorb light at specific wavelengths with the amount of absorption defined by the combination of the oscillation and rotation in the bindings between the atoms in the molecules. Resonance occurs at certain frequencies, this is when the gas absorbs most light energy. The Beer-Lambert law defines the absorption physics.
If we look at the infrared spectra we can see similar compounds show similar absorption patterns although each has their unique “fingerprints.”
In basic terms, the interaction of infrared light with the gas molecules leads to a decrease of intensity at the detector. This intensity decrease is directly related to a gas concentration increase.
Most flammable gas point IR detectors are calibrated for methane gas and monitor the region around 3.3 microns. These devices are more correctly termed hydrocarbon detectors as they rely on absorption due to the hydrogen-carbon bonds. Line of sight (LOS) open path detectors typically monitor the region around 2.3 microns. The 3.3 and 2.3 micron regions are known as fundamental and overtone wavelengths.
The infrared light emitted in a point IR or between the transmitter and receiver of a LOS usually originates from a filament lamp, xenon flash lamp or solid state IR source. All these light sources are broadbanded but filtered into the absorption regions of interest for hydrocarbon gases. These detectors use a spectral width in the range of 100– 400nm depending on the target gas This approach is unable to pick out the single absorption lines necessary to monitor for a single gas species, like methane or hydrogen sulfide, typically 0.05 nm. A new type of IR source is needed, and with the recent telecommunications boom, tunable laser diodes have become more reasonably priced to allow use in industrial detector products.
Tunable Laser Diode
The laser diodes used for gas detection have emerged from high capacity fiber optic telecommunication equipment where e.g. multiple 10 Gbit/s channels are packed into a single optical fiber, separated by wavelength. (Examples: www.advaoptical.com). 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 tunability 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 wavenumber. 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 wavenumber 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 wavenumber.
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.
This paper has discussed a novel method for the H2S detection using an open path gas detector for flameproof (explosion proof) industrial applications. At its core is a tunable laser diode, selected to provide little or no cross sensitivity to other gases.
The technology is fail safe in design and complies with the requirements of modern safety systems, including independent SIL certification.
The use of open path detectors for hydrocarbon gases has become widely accepted as they increase the probability of detecting gas accumulations, compared with fixed point detectors.
Moreover, optical detectors generally offer the fastest means of gas detection, their response is not impeded by sinters or diffusion barriers, nor does their detection mechanism rely on a chemical reaction, unlike electrochemical cells and metal oxide semiconductors. This last point is very important when considering the target gas presents a primary threat to health.
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