The words below are from a post by 2-piper on the DoubleGun BBS some time ago. I would prefer to have posted the link but that board has a short longevity before threads are vaporised. This is reproduced here without 2-piper's permission, but he posted it on DoubleGun without reservation, so I trust he would be OK with its reappearance here.

Note that it refers to pressure in the shotgun range.

For the information of all:

"Note that in sir Gerald Burrard's work "The Modern Shotgun" Appendix III was a work on "The Piezoelectric Pressure Gauge and it's Application to the Measurement of Pressure in Shotguns" by A Watson, B.Sc., F.Inst.P. In this work he discusses the mounting of a piezoelectric (P) gauge & a lead crusher (L) gauge on opposite sides of a pressure bbl, both @ exactly 1.0" from breech giving simultaneous readings of the same shot. After a careful study of these readings the following was stated; "Over the range of pressures usually encountered at the one inch position in shotguns the reaings given by the two gauges are related by the equation; P=1.5L(tons)-.5tons"
These pressures are all given in tons & are the British Long Ton @2240lbs. Working this formula gives the following with all converted to equivelent piezoelectric psi;
3 tons (6,720) = 8,960psi
3Ľ tons (7,280) = 9,800psi
3˝ tons (7,840) = 10,640psi
4 tons (8,960) = 12,320psi
Note the similarity of these conversions to the pressures given in the 1954 Rules of Proof per the very excellent previous post.

Per the 1954 Rules of Proof, here are the equivalent service pressure values:
3 tons--8,938 psi
3 1/4 tons--9,682 psi
3 1/2 tons--10,427 psi
4 tons--11,917 psi

Also note this conversion is applicable "Only" to those 1.0" pressures in the normal range of shotgun loads. They are also only applicable to those pressures recorded under the British Crusher system. Other crusher systems would likely be similar, but not necessarily exact. Piezoelectric psi readings are generally considered to be "Exact" & will almost always be higher than crusher pressures. There "May" be a few instances where time lag of the crusher is so low the two give essentially equal readings.
Also "Nota Bene" the difference in the two systems over this limited range varies from 1ton (2240psi) to 1˝ton (3360psi). The use of the oft stated 1,000 lbs would thus seem to be useless & in some cases could prove dangerous."

PS: apparently I failed to save the "very excellent previous post"! Sorry.....