The standard formula for converting CUP to PSI is to multiply the CUP by 1.516, then subtract 17902.
40,000 X 1.516 = 60,640
60,640 - 17902 = 42,738
....am I missing something or just corn-fused?
That formula is simply a linear fit to a bunch of SAAMI MAP (Maximum Allowable Pressures) CUP and MAP Trans pressure data and though an accurate representation of the general trend on which the fit was applied it can lead to some very dangerous results when we look at specific cartridges.
There is no conversion from LUP/CUP to transduce PSI that works across all cartridges. For any one specific cartridge we can, in theory create a good reliable conversion but that will not be the same mathematical formula for a different cartridge. This is due to how the two measurement systems works making a general one for one conversion not possible.***
Quoting SAAMI (
https://saami.org/faqs/#lup-cup-psi-preassures):
This is important, since the pressure values determined by one method cannot be mathematically converted to values for another, despite claims to the contrary. Likewise, the limiting pressure values for the different systems are not interchangeable.
Try this yourself download the SAAMI spec (
https://saami.org/technical-information/ansi-saami-standards/) and use that formula to convert MAP CUP to MAP PSI on a cartridge at random. Some of the time (especially for rifle cartridges) the formula work acceptably well. Some of the time the results are down right dangerous and there is no way to predict which cartridge the formula works on without already knowing both values.
7mm Rem Mag has a .1% error using the formula.
Try it with 32 S&W and the 12,000 MAP CUP is converted to only 290 PSI for the transducer. Actual transducer it 17,000 psi A safe, but 98% error.
Try that with 357 Magnum and the 45,000 MAP CUP is converted to 50,300 PSI for the transducer. Actual transducer is 35,000 psi A very dangerous 44% overpressure.
Even with our topic at hand of 44 Magnum. the 40,000 MAP CUP is converted to 42,700 PSI for the transducer. Actual transducer is 36,000 psi A 19% overpressure, or nearly shooting proof loads.
***If you're curious on the difference in the method I would point you to this post I did awhile ago that goes into the four major testing methods used (SAAMI CUP/LUP, SAAMI Transducer, CIP (Europe's equivalent to SAAMI), NATO EPVAT) and I think that will give you some insight into why there cannon be a conversion between CUP/LUP and Transducer. (box below will take you to my post in an old thread.)
I have read many times that there are different SAAMI standards for .223 and for. 5.56 NATO concerning pressure. I do find SAAMI data on behalf of .223, unfortunately only in CUP. I dont find any SAAMI data for the 5.56. Could someone please help?
www.thehighroad.org
Remember the whole pressure event happens on the order of .002 seconds or less (2 milliseconds) and the peak pressure is experienced for a very small fraction of a millisecond. Even with modern computers (they sample the pressure at ~1,000,000 times a second during a test), high tech piezo sensors, and well trained technicians this is a fairly difficult measurement to make accurately despite it being done daily by dozens of ammunition manufacturers. Any mistake in the setup leads to serous errors and very small factors can have big effects on the measurements.
️ in my eyes
