Lighthouses of Australia Inc. Bulletin

AGA acetylene/propane lantern LBGA-200 fitted with AGA Solventil, (top right of picture), commonly used on buoys and beacons.
Photograph: Henning Jensen

If kerosene caused the revolution in lighthouse illumination, acetylene was the kind of fuel that made it possible for lighthouse technology to make a quantum leap towards automation. By 1910, when acetylene started to be widely used in buoys and lighthouses, a few technological challenges remained to be resolved before the final step to total automation could be made, but nothing could stop or even slow down the process.

Acetylene is a simple hydrocarbon containing just two carbon and two hydrogen atoms (C₂ H₂ ). It is unusual because the two carbons in the molecule are bound by triple bond. Strangely enough, the triple carbon-carbon bond is not stronger than a single one, just the opposite, it makes the hydrocarbon unsaturated and therefore more susceptible to addition of other elements or radicals.

Cross-section of the acetylene lamp, showing on the left, minus the sunvalve.
Drawing: Henning Jensen

Acetylene is not naturally found and has to be synthesised. The easiest way to obtain acetylene is the reaction of calcium carbide CaC2 with water. It could also be produced by cracking other hydrocarbons in the high energy field of an electric arch or by partial combustion of methane with air.

The gas such produced is colourless and should have a pleasant odour. More often though it smells foul because of contamination with phosphine, arsine and hydrogen sulphate. Acetylene is highly flammable and can explosively decompose into its elementary components by just being compressed. Also, the mixture of acetylene and air is explosive in ratios from 12.5% to 97.5% acetylene to air. It is obvious that before such a dangerous gas could be used, the method of storing it in large quantities had to be devised. Because it could not safely be compressed into a pressure vessel like other hydrocarbon gases, another way of storage had to be devised. In 1896 it was discovered that acetylene keenly dissolves in acetone, a commonly available and cheap organic solvent. Even at normal pressure, 24 litres of acetylene can be dissolved in 1litre of acetone. As the pressure is applied, the ratio rises even more. At the pressure of 10 atmospheres and 15ºC, the vessel can absorb as much as 100 times its own volume of acetylene. At the same time, the dissolved gas loses its tendency to explode. Still, simply dissolving the gas in acetone was not the solution to the problem. As the gas was used, the level of liquid in the cylinder would drop and the vacated space above would fill with pressurised acetylene, creating the same explosive problem as before.

A concrete-like porous material fills the pressure cylinder destined for storage and transport of acetylene.
Photograph: AGA

Enter Nils Gustav Dalén, an inventor and genius of gas technology and the winner of Nobel Prize for physics in 1912. The safe storage of acetylene was one of the three inventions that had huge implications for lighthouse technology. After many experiments, one of which nearly cost him his life and permanently deprived him of his eyesight, Dalén developed a spongy ceramic material which was to totally fill the gas cylinder. It was then half filled in acetone and then saturated with acetylene under pressure. This way, when the dissolved gas was used, the dangerous cavity did not form above the surface of the liquid.

Acetylene burns in air or oxygen, producing bright white light, similar in colour to sunlight. A lot of heat is also produced, the temperature of the flame can reach 3300ºC. The flame can be very sooty if not enough oxygen is available for the combustion but combined with the mantle and sufficient air intake, the light output increases immensely.

The advantage of this arrangement would not be great if the acetylene light had to burn uninterruptedly. Several methods of producing an intermittent light were already known. The flame could be surrounded by moving screens, or the light itself could be made to rotate. Such arrangements needed frequent inspections and involved considerable expense.

Dalén constructed an apparatus, based on an entirely new principle, which by instantaneous opening and closing of the gas pipe and at the same time using the small pilot flame for ignition produced very rapid but distinct flashes lasting down to 0.3 second.

In 1907 Dalén crowned his achievement by designing a solar valve, called the Solventil, which extinguished the light at sunrise and relighted it when night fell. This valve was controlled by four metal rods enclosed in a glass tube. The lower one was blackened, while the others were gilded and highly polished. Daylight was absorbed better by the blackened rod which was heated and consequently expanded, closing the gas valve. As the daylight decreased, the black rod reached the temperature of the other three rods, contracted and allowed the gas valve to re-open. The solar valve combined with an intermittent light produced a saving of gas of up to 93%.