Fail safe hydrogen sulphide detection using an open path gas detector

Hydrogen sulphide (H2S) has one of the most readily identifiable odours known to the petrochemical and waste water industries. Hydrogen sulphide is both toxic and flammable; research has shown that long term exposure to concentrations above 1 ppm can irreparably damage the sense of smell, whilst higher concentrations can prove fatal.

It is known that gases and vapours are transparent to visible light but block light of specific wavelengths.  Intensive research has lead us to develop a technique using a single tuneable laser diode source coupled with advanced software algorithms to pinpoint specific "wavenumber" where there is little or no interference from other gases. 

Traditional Techniques for H2S Detection

Modern industrial hazardous 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 environment conditions where the unit will be installed.

Electrochemical Cell Technology

Generally speaking 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.

By their very nature electrochemical cells are a consumptive technology.  The units cannot be exposed continuously to the target gas of interest as this greatly 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. 

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 extensively used in the Middle East.

Infrared (IR) light absorption

Gas molecules are composed of different types of atoms (e.g. hydrogen and carbon, in Methane CH4 or hydrogen and sulphur in Hydrogen Sulphide).

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.  Although filtered into the absorption regions of interest for hydrocarbon gases, the light source and optical filter arrangements have a spectral width of around 2 nm, this is too broad to pick out the single absorption lines necessary to monitor for a single gas species, like Methane or Hydrogen Sulphide, typically 0.05 nm.  A new type of IR source is needed, and with the recent telecommunications boom, tuneable laser diodes have become more reasonably priced to allow use in industrial detector products.

Tuneable Laser Diode

The laser diodes used for gas detection have emerged from high capacity fibre optical telecommunication equipment where e.g. multiple 10 Gbit/s (10Gbit/s = one full length movie per second) is packed into a single optical fibre, separated by wavelength. 

This wavelength separation asks for 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 is very narrow banded, 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 potential selectivity when you compare a laser generated detector to a conventional detector using a broadband source narrowed by mechanical filters is in the range of 20 000: A laser band width of approx. 0.0001nm compared to more than 2 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. This is done by sweeping a small area, typically 0.2 -0.3 nm, where you can find both a reference gas and the target gas.

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 units to be employed with high availability, and minimal maintenance into 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.