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CUP vs. PSI
 Moderated by: Timberghozt, Rockydog, fryboy
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 Posted: Sat Feb 25th, 2012 09:15 PM
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Rockworx
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 Hello all, I am confused with the current Hodgdon load data. They have some pressures listed in PSI, and others in CUP.  I don't know the difference, so perhaps the knowledge of the board can enlighten me. Thanks.



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 Posted: Sat Feb 25th, 2012 09:21 PM
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SavageShooter
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Copper units of pressure or CUP, and the related lead units of pressure or LUP, are terms applied to pressure measurements used in the field of internal ballistics for the estimation of chamber pressures in firearms. These terms were adopted by convention to indicate that the pressure values were measured by copper crusher and lead crusher gauges respectively. In recent years, they have been replaced by the adoption of more modern piezoelectric pressure gauges that more accurately measure chamber pressures and generally give significantly higher pressure values. This nomenclature was adopted to avoid confusion and the potentially dangerous interchange of pressure values and standards made by different types of pressure gauges. Pressure is a fundamental thermodynamic parameter that is expressed in units of force divided by area. In the avoirdupois system, the units of pressure are pounds per square inch and in the metric system, the units or pressure are newtons per square meter (pascals). A chamber pressure measured with a copper crusher gauge would be expressed as psi (CUP) in the English system or MPa (CUP) in the metric system

Courtesy of Wikipedia



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 Posted: Sat Feb 25th, 2012 09:24 PM
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Plainsman
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Here are a couple of links that may help...

http://en.wikipedia.org/wiki/Copper_units_of_pressure

http://www.shootingsoftware.com/ftp/psicuparticle2.pdf



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 Posted: Sat Feb 25th, 2012 10:18 PM
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Ozark Ed
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CUP can also be used to measure women's breast sizes whereas PSI cannot.



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 Posted: Sat Feb 25th, 2012 10:36 PM
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TMan51
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Ozark Ed wrote: CUP can also be used to measure women's breast sizes whereas PSI cannot.
Ozark Ed is a genius :lol::lol:(We'll both be banned, I'm sure).

The PSI/CUP methods/data are used to compare pressures from load to load, they are not the same, and they don't convert easily.   40,000CUP on a one cartridge, is not the same as 40,000CUP on another cartridge.  PSI is more of an absolute value.

As Plainsman suggests, google both of them up, then read a few explanations.  Look at SAAMI too while you're looking.

The important thing to consider is whether you are above the limit for pressure with your load.



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 Posted: Sat Feb 25th, 2012 11:01 PM
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Rockworx
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Here's the reasons I've asked. I'm thinking of trying some Varget for .308 win. My Lyman manual says 61,100 PSI (max load @45.7 gr.) and the Hodgdon data says 50,600 CUP (max load @46.0 gr.) Thats roughly a 10,000 point difference. See my confusion.
Thanks for the replies btw.



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 Posted: Sun Feb 26th, 2012 04:54 AM
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Rockydog
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Rockworx, I understand your confusion but, as stated above, there really is no correlation. The 10,000 difference is a meaningless number. Kind of like subtracting 6 apple slices from 12 orange slices and wondering just how much juice will be extracted. RD



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 Posted: Sun Feb 26th, 2012 06:55 AM
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runfiverun
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they don't relate.
the crusher method is done by measuring the crush on a copper pellet.
it gives you the maximum amount of pressure applied to it.
it doesn't tell you when the pressure is applied.
it could go from zero to maximum instantly and then drop straight off. [which is a bad thing]
the piezo transducer gives you instant time readings that are traceable, and a lot more believable.
this has lead to some loads being reduced.
it has also lead to some new powders that are truly awesome.
they can be tailored to keep more gas volumn behind a bullet longer, producing more velocity without the high pressure.
but by a prolonged time under a lesser pressure.
something that a copper pressure unit [pellet] could never do.



 Posted: Mon Feb 27th, 2012 01:40 PM
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Rockworx wrote:  My Lyman manual says 61,100 PSI (max load @45.7 gr.) and the Hodgdon data says 50,600 CUP (max load @46.0 gr.) That's roughly a 10,000 point difference. .
Your confusion is well founded.  You will be even more confused when you find that the PSI/CUP conversions are not the same from round to round, and not all crusher systems work precisely the same.

To be kind, even metric conversions make way more sense.

I looked at this for a long time early in my handloading obsession.  The internet made it much easier.  Not that it helps a lot, but most manuals state maximum charges based on one or the other, most of the time.  The 61,100PSI and 50,600CUP are roughly the same.  I say roughly, because the manual may have stopped increasing charge due to space, loss of accuracy, or any other reason. ???  If you buy a Lee manual, you find that many loads are listed, and some are rated in CUP, some in PSI.  They are essentially the same.

What ISN'T happening, is the freedom to crank up the CUP load another 10K.



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 Posted: Mon Feb 27th, 2012 03:17 PM
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LOL, I thought I had if figured out till I read everything--now my head hurts, my eyes are watering, and am a bit dizzy--then there is the "cup" size to farther confuse the situation. Sometimes it is hard to keep my mind out of the gutter--I keep getting these visions of "cup sizes".



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 Posted: Mon Feb 27th, 2012 04:10 PM
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Rockworx wrote: Here's the reasons I've asked. I'm thinking of trying some Varget for .308 win. My Lyman manual says 61,100 PSI (max load @45.7 gr.) and the Hodgdon data says 50,600 CUP (max load @46.0 gr.) Thats roughly a 10,000 point difference. See my confusion.
Thanks for the replies btw.

And the reason that they have two different max loads is simple.Different component/lot #'s and pressure guns.



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 Posted: Sun Mar 4th, 2012 11:31 AM
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OldStuffer
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Rockworx wrote:  Hello all, I am confused with the current Hodgdon load data. They have some pressures listed in PSI, and others in CUP.  I don't know the difference, so perhaps the knowledge of the board can enlighten me. Thanks.

The difference between PSI and CUP is "a mess", and "a minefield".

PSI is what it says it is, Pounds Pre Square Inch pressure exerted. This is found using piezoelectric strain gauges in the side of the chamber like SAMI uses in it's test barrels NOW (used to be the older way of course).

Copper Units Of Pressure is the older way of trying to measure the pressure. A hole is drilled in the barrel or chamber/cartridge case, a copper chunk is put in that hole and held in place.

The military still uses this older method, this sparks much heated debate over the supposed "differences" between GI ammunition and it's civilian counterparts (mostly 5.56x45/.223Rem & 7.672x51/.308Win currently). A few posted, quoted, and reposted/requoted typos of PSI instead of CUP and the misunderstanding arround them help fuel the fire/debate/discussion/urination contests.

Cartridge is fired. The pressure shortens the copper slug.

Now, depending on the exact hardness of that exact lot of slugs and exactly how far it deformed compared to un-fired, a pressure value is determined.

Let me be very clear on this next part:

THERE  IS  NO  DIRECT  GOOD CONVERSION  FROM CUP  TO  PSI  THAT  WORKS  AT  ALL  PRESSURES!

Conversion @ 50,000CUP is not the same as conversion @ 30,000CUP, or 20,000CUP.

 

Now, just to add to your confusion, in the world of shotguns much lower pressures (7-12,000psi), it is PSI or LUP, Lead Units Of Pressure.

I have not tried to convert this one, but when you look over decades of load data, they look so very close that I consider them much the same.



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 Posted: Sun Mar 4th, 2012 11:36 AM
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OldStuffer
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Rockworx wrote: Here's the reasons I've asked. I'm thinking of trying some Varget for .308 win. My Lyman manual says 61,100 PSI (max load @45.7 gr.) and the Hodgdon data says 50,600 CUP (max load @46.0 gr.) Thats roughly a 10,000 point difference. See my confusion.
Thanks for the replies btw.


The 2 loads are very close to the same pressure level.

My older load data manuals that show specifications on .308 Winchester state it is a maximum 52,000CUP cartridge.

Check SAAMI and you will find the same cartridge is specified as 62,000PSI.

Same pressure level, 2 differnt methods of putting a number on that pressure.

The cartridge, nor the load data, has not ever changed, the method of describing the pressure has.



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