New Zealand has a rich heritage of mining. For more than a hundred years, Kiwi blokes have extracted minerals from the ground, but this has always been a precarious occupation both then and in very recent times.
Mines by their very nature are a hazardous environment. Metres and sometimes hundreds of metres underground, workers are deprived of light, air and often are exposed to toxic gases.
Methane gas is one of the main hazards. Its produced naturally by bacterial decomposition of organic (carbon based) materials where there is little or no oxygen. This occurs in mines, land-fills and indeed the human (and bovine) digestive tract.
The hazard is two-fold; it can replace oxygen, causing asphyxiation, or if mixed with a certain quantity of oxygen, it can burn very quickly; indeed, explosively.
Before modern replacements excused them, poor old canaries were used as a warning of gas in mines. Canaries were used as ‘sentinel species’. A sentinel species is an animal that can warn humans of a hazard in advance. Canaries have a small lung capacity, fast metabolism and rapid breathing rate and hence succumb to gas before humans. If the bird died, it was an advance warning to the miners to don their breathing equipment or to evacuate.
Gas needs two things to ignite; the right amount of oxygen and sufficient temperature to ignite it; the “ignition temperature”. Humphry Davey conceived the Davey Safety Lamp in 1815.
A common high school experiment is to hold a metal gauze in a Bunsen burner flame. The gas above the gauze will not burn, while that below does. The gauze, being a good conductor of heat, lowers the gas temperature to below the ignition point such that it will not light above the gauze.
This was the principle of Davey’s lamp. A wick burning within the gauze gave light but the flame, contained in a gauze cage, never allowed the gas outside the lamp to reach its ignition point despite being flooded with gas.
The brilliance of this design was two-fold; not only did the gauze prevent explosion and provide a safe source of light, but the flame inside the gauze burned safely in the atmosphere to which it was exposed. Hence one could view the way the flame burned (height and colour) and learn about what gas was present around one.
Catalytic Bead Detectors
Many of today’s gas detectors work on exactly the same principle. Rather than a kerosene or paraffin flame, we have a tiny electric element, but we still have a gauze to prevent ignition. And we add a bed of catalyst beads to lower the temperature at which the gas burns.
And finally we add a temperature sensor to measure the burning rather than viewing the flame.
Definition: Catalyst. A chemical that increases the speed of a reaction without being used up in the reaction.
By comparing the temperature of a catalytic bead exposed to the atmosphere under test, with one sealed against the gas, we can determine when the gases are ‘burning’.
These catalytic bead sensors, sometimes identified by one of their trade names “Pellistor”, were a brilliant leap forward in detection technology and are used today in the vast majority of detectors for flammable gases.
Hydrocarbons often form very long ‘chains’ of molecules, so one might easily imagine that such a giant molecule would, and does take a long time to diffuse through our microscopically small gauze.
Hence we have other technologies such as Infrared Extinction* (IR) or Photo Ionisation* (PID) to quickly detect these molecules when rapid response is needed.
Also some gases don’t burn, so that a catalytic bead is of no use in detecting other gases such as Hydrogen Sulphide (which is not flammable).
Other technologies have come along since that add to our arsenal of weapons in the fight against danger.
*Please call to discuss these if required or your other gas detection inquiries