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Topic ClosedCUP vs PSI

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bkcorris View Drop Down
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aka The Cheesehead Savage

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Direct Link To This Post Topic: CUP vs PSI
    Posted: 22 January 2004 at 16:54
Why are older loads compared with CUP ratings while the newer mags report on PSI ratings. With the new WSM's being SAAMI rated at 65,000 psi, where does that fall in CUP? Or is there no real world way of converting the two numbers?

Edited by bkcorris

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Orion View Drop Down
.375 Holland & Holland Magnum
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Direct Link To This Post Posted: 22 January 2004 at 18:02

BK, there is a formula for conversion and I have it somewhere. But if you look for some of Rick Jamisons' past Shooting Times columns I remember him publishing the formula for that as well as other formulas for recoil, etc.

Basically C.U.P., copper units of pressure and L.U.P, lead units of pressure were commonly used up till the mid-twenties when the piezoelectric transducer was introduced. But due to conflicting manners of measurement and interpretation the pressures measured were never quite right. The copper crusher method is still in use as well however and seems to be the most commonly accepted unit of measurement by SAAMI. There is also the strain gauge method of pressure measurement, but it typically produces lower pressure readings.

Basically, some loads you read about are not tested every time a new manual is published due cost and the fact many older cartridges are lightly loaded due to the many older firearms chambered for said cartridge. Many of the same powders that have been used in the older cartridges are available as well

That said, here is a reference formula:

PRESSURE UNITS: Where density is specified or implied, it is based on the following:

  • Density of water at 60°F = 62.3707 Ib/ft³
  • Density of mercury at O°C = 13.5955 g/cm³

By my best guesstimate and failing memory, use the number 5000 as a reference for converting C.U.P. to P.S.I. IE: 43,000 cup roughly equals 48,000 psi.

That said it is time to hit the sack.......yaaaaaaaaawwwwwnnnnn.

 



Edited by Orion
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Direct Link To This Post Posted: 22 January 2004 at 18:32

Here is one; http://www.reloadbench.com/gloss/cuppsi.html

I'll continue to try and locate the formula for conversion. I'll bet we have another member who has it in arms reach!

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Ranch Dog View Drop Down
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Direct Link To This Post Posted: 24 January 2004 at 03:29
As far as I'm aware, there is no method of converting CUPs to PSI or back the other way.
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TasunkaWitko View Drop Down
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Direct Link To This Post Posted: 24 January 2004 at 03:57
my information is the same as ranchdog's. i forget the reasons, but i could look them up and post them if you want.

Edited by TasunkaWitko
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Direct Link To This Post Posted: 24 January 2004 at 09:20
I will try my best to explain this as I understand it as to why there is not a simple formula to convert existing CUP references to PSI.

When the Copper Units of Pressure was established the reference medium was a standard size plug of copper that was placed in the crusher. The results of the pressure was referenced to the change of this medium.

PSI does not use a reference medium but the pressure developed within the individual cartridge chamber dimensions. That dimension and it's volume is the base or reference for the pressure test. That base or reference is different for every cartridge tested and must be established for the testing to begin. Because each cartridge is different, there is not a constant to convert all the different cartridges that have established CUPs. They all must be developed individually.

PSI is a great source of information if it has been established for the cartridge. Cast bullet users can establish maximum loads for accuracy rather quickly because BHN can be related to a ultimate compressive strength in PSI. Lee publishes such a table.

Edited by Ranch Dog
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Orion View Drop Down
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Direct Link To This Post Posted: 24 January 2004 at 10:22
Ya know, I coild almost swear there is a conversion formula...probably not. Could've sworn I saw one.
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Direct Link To This Post Posted: 24 January 2004 at 17:23

bk - Actual chamber pressures are measured by fastening strain gages on the exterior of the bbl chamber and measures actual strain of the bbl matl. There is no correlation of the two methods. The best you can do, is look at charts for comparisons. C.U.P. readings are usually considerably lower.

 

 Shown below is a C.U.P measuring device used by Hodgdon in an old reloading manual.

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Direct Link To This Post Posted: 25 January 2004 at 00:53
Yep... I did some more checking both via the Internet and with some fellows in the know. No comparison between the two. CUPs are the effect of chamber pressure on an external device and PSI is the effect of chamber pressures internally. It's kind of an apple and organges deal.
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Direct Link To This Post Posted: 25 January 2004 at 02:23
Yep - I agree with ya Ranch Dog.
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Direct Link To This Post Posted: 25 January 2004 at 08:34
Guys, I did a lot of looking a couple of years ago to try to compare LUP to PSI for building trap reloads. No matter where I looked everyone was in agreement. There is no conversion table for all the reasons you just stated. RD
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