One unit often seen in sewer work, but rarely in mines for some reason, is the Spiralarm. This is a flame lamp of the Davy type, but has a coil of bi-metallic strip above the flame. When the flame burns up to a certain level, it expands the strip which makes a contact and sets of a flashing red lamp and buzzer, set in the base. It also has a small mirror of polished steel on the base, so you can lower it into a manhole on a rope and see the alarm reflected if it goes off; the same mirror can also carry a strip of chemical-treated paper as a test for sulphides.

They require a special fuel, because if the calorific value isn't right the calibration of the flame isn't correct, but they are easier to read than a Davy lamp

some units also have self-igniters, because they are designed to be carried about in a van and lit at the worksite

Spiralarms are more or less obsolete. The point at which the alarm is tripped can vary depending on ambient temperature and pressure.

I'd be curious to know when they were last manufactured.

J Murphy and Sons certainly had a large stock of them when I was working for them in the late 80s, they were usually used in conjunction with a Crowcon as a 'last resort' both for gas detection and light, and I suspect, because some older members of the workforce liked them.

I also had them at Miller Buckley's Bexhill job in 1986, for sewer connections, although since the mining subcontractor was the same they may have been JM ones carried across.

I still have one, plus a can of fuel, which was found in the back of my Land Cruiser when I cleared it out for sale. It carries a sticker showing it was last calibrated in 1990. However I've never used it.

We never used them at Robertsbridge, which was concurrent. The mining subbie was different and had his own way of working.

I haven't seen one in use since, it may be that when JM lost the Thames Water Term Contract in about 1992, they were cleared out and scrapped. JM had been TW Term Contractors for about 25 years at that time, and maintained a yard of plant and materials for that workscope. Certainly that's when I parted company with JM, because there were redundancies across the company in the aftermath of that contract.

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