Probably more than you want to know Tim.
In the 1954 British Rules of Proof the service pressure was expressed in TONS and was measured by LUP.
There is no mathematical conversion for LUP as measured by change in dimensions of a lead cylinder in a crusher apparatus and a Tarage Table, to PSI as measured by modern piezo transducers.
Adding 10 - 14% to LUP numbers works pretty well.
The 1954 Rules of Proof “Highest Mean Service Pressure” equivalent transducer values as converted from LUP by Burrard’s formula: estimated Long Tons/ Sq. Inch Lead Crusher Pressure (Cp) conversion to PSI (pound-force per square inch): (Cp x 1.5) - .5 = TSI, TSI X 2240 = PSI.
2 3/4 tons = 8,120 psi = 560 BAR
3 tons = 8,960 psi = 618 BAR
3 1/4 tons = 9,800 psi = 676 BAR
3 1/2 tons = 10,640 psi = 734 BAR
4 tons = 12,320 psi = 850 BAR
Great Britain joined the Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives in 1980 but the new Rules of Proof were not introduced until 1984. Lead Crushers were used however until 1989; the proof marks in BAR (and shell pressures) thereafter are PSI by transducers.
