A caution to anyone translating old plan into metric sizes, especially boilers.

I looked up 18 gauge on the web when looking at an old boiler plan as I thought i may have a tube the right size. I was surprised when 18 gauge came up as 1.0mm as my recollection was that it is slightly thicker than that.

I had foolishly searched for 18g to mm, searching for SWG to millimeters gave me the correct figure or 1.2mm.

The reason is that there are various different wire gauge systems (also used for plate and tube) and 18 AWG (American Wire Gauge) is about 1mm.

Old British plans universally use SWG which is rather thicker for any given number. So be warned, check you are using SWG sizes if converting an old British plan or you may end up with a dangerous boiler that is significantly under strength and won't get passed by a boiler inspector.

Neil

P.S. For beginners – the reason wire gauges get smaller as the number increases is that the originated as the number of times a wire was pulled through a die to make it thinner. 18 gauge wire originally meant it had been pulled through 18 dies, so you can see how the use of different sets of dies led to different numberings (such as Birmingham Wire Gauge). SWG was introduced in the UK and AWG in North America to bring some standardisation, but sadly the opportunity to make both sides of the Atlantic the same was missed.

Thanks to Lambton who has sent me a scan of a table comparing different wire gauges from one of our old Model Engineer Handbooks.

I have created an article where you can download the PDF.

If anyone would be interested in converting this into a spreadsheet with metric equivalents, please get in touch as we could use it as a pull-out in MEW.

Neil

Edited By Neil Wyatt on 02/09/2017 19:22:51