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CUP vs PSI |
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bkcorris
.416 Rigby aka The Cheesehead Savage Joined: 11 June 2003 Location: United States Status: Offline Points: 2120 |
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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Stupid people are like a slinky, they don't serve much purpose in the world but they sure are fun to watch tumble down the stairs! |
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Orion
.375 Holland & Holland Magnum Joined: 10 August 2003 Location: United States Status: Offline Points: 555 |
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:
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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Orion
.375 Holland & Holland Magnum Joined: 10 August 2003 Location: United States Status: Offline Points: 555 |
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
.243 Winchester Joined: 16 September 2003 Location: Cuero, TX Status: Offline Points: 176 |
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
Administrator aka The Gipper Joined: 10 June 2003 Location: Chinook Montana Status: Offline Points: 14749 |
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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TasunkaWitko - Chinook, Montana
Helfen, Wehren, Heilen Die Wahrheit wird euch frei machen |
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Ranch Dog
.243 Winchester Joined: 16 September 2003 Location: Cuero, TX Status: Offline Points: 176 |
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
.375 Holland & Holland Magnum Joined: 10 August 2003 Location: United States Status: Offline Points: 555 |
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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EDip
.375 Holland & Holland Magnum Joined: 12 June 2003 Location: United States Status: Offline Points: 562 |
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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Ranch Dog
.243 Winchester Joined: 16 September 2003 Location: Cuero, TX Status: Offline Points: 176 |
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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EDip
.375 Holland & Holland Magnum Joined: 12 June 2003 Location: United States Status: Offline Points: 562 |
Posted: 25 January 2004 at 02:23 |
Yep - I agree with ya Ranch Dog.
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Rockydog
Administrator Joined: 13 June 2003 Location: United States Status: Offline Points: 3191 |
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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