The standard device is the Loos tension gauge. The Standard range is cheaper but slightly less accurate and less convenient to use than the Pro series. Type A or PT1M for 2.5 - 4mm, Type B or PT2M for 5,6,7mm and PT3M for 7,8,9,10mm. Rod rigging gauges are also available. Full details on my website.
Harken does an electronic strain gauge for smaller wire sizes - 2.5mm to 6mm for £200.00.
One of the big spar maker (Sélden I think) publishes a booklet on setting up your rig that includes a very simple but cunning method for measuring rig tension using a 6' folding ruler and a vernier caliper.
I cannot recommend the Selden book too highly. I was really worrying about taking down and re-stepping our mast on our recent trip up the French canals, the book was a huge help and covers all areas and the folding rule tensioner does the job without recourse to expensive kit that you may use very rarely.
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Is there a gauge that measures in the 10-12mm range.
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There is a commercial unit for pylon work and industrial but not cheap even if you could find somewhere to hire one from. I used the selden method of measuring percentage of stretch which seems to work for most people though never had a chance to see how accurate it is with a load cell.