Tons per square inch - The DoubleGun BBS @ doublegunshop.com

Is there a chart or formula for converting British tons per square nitro proof into US psi.

thanks

One UK Long Ton = 2240 psi

I don't think there is an exact formula as they are measured differently. Roughly 8,500 psi is thought to be around the same as 3 tons per square inch pressure rating and 9,000 for 2 3/4 inch 3 1/4 tons per square inch. That is as far as I am aware unless someone else has a more exact idea. Lagopus.....

According to the 1954 British Rules of Proof,
3 tons = 8,938 psi
3 1/4 tons = 9,682 psi
3 1/2 tons = 10,427 psi
4 tons = 11,917 psi

But if you plug those British long ton numbers into a converter like this, the numbers don't come out the same:

https://www.convert-me.com/en/convert/pressure/longtoninsq.html?u=longtoninsq&v=3.25

The best explanation for the difference that I've seen, when this has been discussed in the past, came from Miller (2-Piper)

Originally Posted By: 2-piper

The Ton used in British proof was 2240lbs. Thus tons x 2240 is a direct conversion from tons to pounds. What is "NOT" a direct conversion is crusher pressures to piezo-electric pressures which are considered to give true PSI. In the early days of working with the piezo-electric pressures bbls were set up with both P-E & crusher gages & simultaneous readings taken at the same 1.0" station. A "Workable" formula for conversion, within the normal pressure range of shotgun bbls was found to be;
P-E (tons) = (Crusher tons x 1.5)-.5 Multiplying this figure by 2240 will then give psi. Note that this formula was not given to be "Exact" but to give reasonable working pressures. By working through the pressures given per the Birmingham proof house & comparing the greatest difference will be found to be on the order of 3% & this on the 4ton pressure, the other 3 being from 2% down to near zero. The reason for the differences & why no exact formula can be given is because of the varible "Lag Time" in crushing the piston. Within this range crusher pressures will always read less than the P-E pressures.

And this is why we need Miller to get well and stay well!

Probably more than you want to know, but scroll down about 1/2 way here for a discussion of Crusher Units and Tarage Tables
https://docs.google.com/document/d/1F2sQuPm05IE4VWYYnCkvuXmYEzQoWd_SQgaAfUOZEFU/preview

thanks all. I purchased a 20 nitro proofed damascus hammer gun with 3 tons per stampings and I did not want to shoot off the shelf high pressure loads. Am I correct in asuming that 3 tons is service load and not definitive pressure, have I got that correct?

John

Note that while the piezo-electric pressures should be interchangeable anywhere as they are true pressure the crusher pressures are not universally interchangeable.
The British & the US did not necessarily use the exact same set up for taking crusher pressures, therefore should only be compared to their individual set up. The same load which measured 3 tons (6720 lbs) my well have not read 6720 on an American crusher gauge due to the different set-up. The formula cited was thus good "ONLY" for British crusher pressures, though the conversion to psi would be good for US guns.

susjwp, yes 3 tons is the service pressure. 'Off the shelf' high pressure loads would not be advisable. Standard British 20 bore 2 1/2" cartridge carries a load of 13/16th. ounce of shot. I would use only CIP approved ammo marked 65mm. or 67mm. Lagopus.....

Our British cousins have done their best to confuse us with proofmarks. Even when they converted to metric proofmarks back in the mid to late 80's (850 bar etc), you could not convert that to American psi via a mathematical formula. They were still expressing crusher values in metric figures while we had converted to piezo-electronic.