CUP vs. PSI

[AtlLaw]

I tell ya, once you get me on a subject I won't shut up... 
Now I'm getting old and senile and sometimes I wonder if my memory is as sharp as it should be.  One of the things that has been bothering me of late is that some folk talk about PSI as a new concept.  Somebody please correct me if I'm wrong, but I'm sure I remember when internal ballistics changed the unit of measure from PSI to CUP.  Maybe about the time there was the push to go from the inch to the decimal system of measurement, who knows...  I also know I could never get my head around the CUP and was never comfortable with it.  Am I completely off base here or are we simply returning to the former unit of measure, that being PSI.   

[LaOtto222]

I ain't no spring chicken myself so I am a little fuzzy on this. I was under the impression that the CUP came first. I think it means Copper Units Pressure. It was well before strain gages and little black boxes were even available. They basically drilled a hole in the side of the chamber and put a copper plug in the hole. They knew what the size of the plug was before firing and then measured it after firing. The crush was interpreted to and called CUP. While I was typing this, just thought I would look it up on the 'net. Here is one explanation.

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 the copper crusher and lead crusher gauges respectively. This was necessitated 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 or 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.

Here is some further explanation

Chamber pressure is generally measured in CUP or LUP (lead units of pressure), using the crusher method. In short, a hole is drilled into the chamber of a pressure barrel, and a crusher assembly is placed over that hole and sealed. Inside the assembly is a piston which, when a cartridge is fired in the chamber, is forced to move when the chamber pressure bleeds through the hole in the chamber. This force causes the piston to crush a slug of copper (or lead when measuring shotshell pressure) of known hardness and length. Pressure is quantified by measuring the length of the slug after it has been crushed and comparing that with recorded data tables.

While looking this up, I noticed that one sage said that you can not rely on these pressure readings because of variances. He went on to talk about reloading and that there are differences in gun to gun (variables) and then went on to say - "Chamber pressure is the pressure exerted within the chamber of a gun or firearm when a cartridge is fired in it. This pressure can be important when handloading ammunition, but at all times one should depend on examining the fired cartridge case for the final answer on whether a given load is acceptable." We all should know that this is not entirely true. It just will not die, I thought it amusing after several of us have had a fairly through discussion on this very subject.

Of course we should all know that there is no correlation between CUP and PSI, they are two completely different ways of measuring pressure.

[Siskiyou]

Darn, you must be old!   Or I must have lost the few brain cells I had.  I checked some of my old manuals and it shows pressure in p.s.i. in some of my early manuals. (Late 1960’s)

I can remember the gun writers of the 1960-1970-era writing about the subject but I have forgotten their pro and con opinions on the subject.  It was like listening to two attorneys in a courtroom.  The debate became more important then the subject.

I found an interesting article on the subject in the 46th Edition of the Lyman Reloading Handbook by W.T. McDonald and H.C. Howarth.  The article dates the copper pressure method back to around 1860 and subject to refinement since that date.  The intent was to obtain maximum chamber pressure.

The article dates the concept of the Piezo system back to WWI, but emerging around 1922 and being refined since that time.  A quote from the article, “The key advantage of the piezoelectric transducer is the ability to generate the complete pressure-versus-time history of the internal ballistic process, whereas the copper crusher gage is capable of measuring only the peak magnitude of the pressure pulse.”

They give the advantage to the Piezo system and did a number of comparisons using both systems for the load.

Unfortunately there is not a cost effective system that I can afford as a reloader.   The best I can do is work with data published by one or more reliable source.  But I also need to compare data because reliable sources are subject to error.  We have witness a number of recalls from the commercial ammunition manufactures, powder manufactures, and publishers of reloading manuals. 

The best I can do is compare data, select proper components, fire test rounds, and now days use my Chrony.  Post firing requires an analysis of firearm, cases, and targets, along with data collected by the Chrony.

Sometimes it is not the ammunition but the firearm.  Years ago one of my wife’s elder uncles offered me a Mossberg in .243 Winchester.  He was honest with me, and advised me that it leaked gas when fired.  The only ammunition he fired was Winchester factory ammunition.  After an evening of examining the rifle, and looking at some cases I passed on the rifle.  When I had the cases on my reloading bench I ran a bent paper clip in each case and found the case body was about to separate from the head.  I crushed over one hundred cases.

While there has been a lot of discussion regarding pressure signs I try and reload safely and that is to look for any sign that indicates that I am experiencing excessive pressure, or case fatigue from too many loadings, or over working the case.  I would be following that practice even if I could afford a current generation Piezo system.

P.S.  I have not had the misfortune to blow-up a firearm, but I have been observed test firing rounds with a piece of carpet over the action, goggles over my glasses, and leather gloves on.  Fortunately they were not needed.

 

[goodconcretecolor]

It all relates to how the pressure was measured and when. The two methods involved are copper crushers versus piezoelectric tranducers. The copper crushers have been around since the 1800's while reliable piezoelectric transducers date from the mid 20th century. The two methods give different numbers and there is no simple way to relate the two. Relating the two involves the application of integral calculus and differential equations(which most engineers have studied) to the piezo data, that is all of the data, not the peak pressure number that is what you see in the reloading manuals. The older data you see reported in psi is actually CUP as it was measured with copper crushers and calling that psi is not wrong. They just started using psi to denote piezoelectric data and CUP to denote data taken with copper crushers.
I am an engineer and I believe both types of data have their strengths and weaknesses. The crusher method is directly measuring the pressure's ability to deform metal and that is exactly what is most crucial to the reloader from a safety stand point. Piezoelectric can tell you a great deal more about how the powder charge is burning. I believe the peaks reported for piezoelectric data are somewhat misleading and that crusher data is more useful to most reloaders. One way to think about it is that the crusher method does the calculus for you.
In summary, crusher data used to be reported as psi where as now crusher=CUP and piezoelectric=PSI for more modern manuals

[EsoxLucius]

"The Lyman reloading manual is one of my favorites. It’s clearly written, a pleasure to
read, and it sheds some interesting light on the history of terminology in the measurement
of chamber pressure. Before about the 1960's the only measurement system we had for
chamber pressure was the copper crusher method. Up until that time, what we now call
CUP was commonly known by two different names: CUP and PSI. The two were used
practically interchangeably. Of course, this use of PSI was incorrect. It wasn't much of a
problem until piezoelectric and strain gauge systems became commonly available. These
systems, of course really do measure in PSI. When they arrived on the scene, it caused a
lot of concern and confusion. “For years, 52,000 PSI (crusher method with erroneous
designation) had been published as maximum for the 270 Win. Suddenly, there were new
publications showing 65,000 PSI …as maximum.” Lyman 47th Reloading Handbook, p92
If you look at any publications before about 1965, and they say that PSI and CUP are not
the same, and that you should not attempt to convert one to the other, they are talking
about the old, incorrect use of the term PSI, not the modern, correct use of PSI from
strain gauges and piezoelectric pressure meters." Denton Bramwell from Correlating PSI and CUP.

The two methods give different numbers and there is no simple way to relate the two.

For the following ANSI CUP and PSI values a correlation with an R^2=.927 yields an approximation formula of (CUP*1.51586)-17902≈PSI.

Cartridge ANSI CUP ANSI PSI
222 rem 46000 50000
22-250 rem 53000 65000
243 win 52000 60000
25-06 rem 53000 63000
257 roberts 45000 54000
264 win mag 54000 64000
270 win 52000 65000
280 rem 50000 60000
284 win 54000 56000
30 carbine 40000 40000
300 savage 46000 47000
300 win mag 54000 64000
30-06 springfield 50000 60000
303 british 45000 49000
30-30 win 38000 42000
308 win 52000 60000
32 win special 38000 42000
338 win mag 54000 64000
35 rem 35000 33500
375 h&h mag 53000 62000
444 marlin 44000 42000
45-70 government 28000 28000
6.5 rem mag 53000 65000
6mm rem 52000 65000
7mm express Rem 40000 45000
7mm rem mag 46000 51000
7mm SE vH 52000 61000
7x50 R 52000 61000
8mm mauser 37000 35000
8x50R 54000 65000

[AtlLaw]

Okay, maybe I wasn't imagining things after all.  Now, let me see if I got this straight...
way back when I started this hobby there was no PSI only CUP, but they called the CUP PSI even if there wasn't any direct correlation between the two which there now is.  That is because we now we have PSI which isn't CUP but it isn't the old PSI either but it actually is PSI which the old PSI wasn't which was why there was no correlation but now we can correlate the new PSI with the CUP but not the old PSI because... because...        
I need a drink... 

[LaOtto222]

I would like to join in that drink

If you look at the above data, there is no correlation. 35 Remington and 444 Marlin actually have less PSI than CUP for instance. While some are the same and some have much higher PSI values than CUP values - no correlation. Two separate ways of measuring pressure.

[EsoxLucius]

If you look at the above data, there is no correlation. 35 Remington and 444 Marlin actually have less PSI than CUP for instance. While some are the same and some have much higher PSI values than CUP values - no correlation. Two separate ways of measuring pressure.

LaOtto222, you need to learn what correlation means.  There is indeed a strong correlation between the CUP and PSI values listed.  Just because some individual cartridges don't fit as well doesn't mean there is not correlation between all values in the set.  For most cartridges the formula will get one within the error of the testing systems.  Please read the following carefully.

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

[Chris Potts]

If this is the same article that you refernced in the other topic, even the author admits that the conversion will only get you within 3000 psi two thirds of the time for the data that he tested.  And that it may not work that well for anything except the 30 cartridges that he tested.  The three sentence warning gets lost in the rest of the seven page article where he acts likes it is no different than converting celsius to fahrenheit.

I still stand by the fact that just because there is a correlation doesn't prove anything.  The conversion is an approximation and can not be counted on to give accurate results.  The author knows this hence the three sentence warning where he essentially tells you that the conversion is useless.

Chris

[LaOtto222]

EsoxLucius - I am not attacking your character.

It is obvious that we have a different idea what correlate means.

Webster's - Correlate...to connect in a systematic way

To me; if one cartridge is 10% higher in PSI than CUP, then all of them should be 10% higher (at least within a little bit), then you have correlation. I see no resemblance of a pattern. Some are higher, some the same, and some lower, I just do not see how that correlates.

Show me how I am wrong, please, so we can talk the same language.

[Chris Potts]

If the two correlated perfectly it would be just like you say.  A 10% increase in one would be the same in the other.  Or at least you could calculate one exactly from the other.  Statistically speaking two things can correlate when this is not true.  When the correlation is less than perfect, it is more like "when one increases the other one tends to increase."

Chris

[ricciardelli]

When you start remembering LUP, then you been reloading for a long time!

[quickdtoo]

For those that are gonna ask, LUP = Lead Units of Pressure, pressure value determined by means of lead crusher cylinders.

Tim

[LONGTOM]

Wow! All of this has and is interesting reading with a world of knowledge or lack of from both sides.
The chart is a great help in understanding CUP vs PSI of those stated cartridges.. I do understand the old and new ways of obtaining these figures.
When I first posted the question of CUP vs PSI, I was trying to figure which calibers could be done within the safe operating pressures of a SB2 frame. At this time I can not even remember which caliber I was contemplating.
I do know that what I was trying to see was if a caliber that is not offered in a HANDI could safely be done in one.
Example: if xxx caliber is safe than yyy caliber should also be safe.
The problem arouse when the said two calibers were not listed in the load manuals with the same unites of measure, xxx being in CUP and yyy being in PSI.
With that said, how is a person to know weather a certain round is safe to make into a HANDI or not.
I hope that everyone understands what I was trying to find out when I posted the question.
In no way was I trying to start a heated debate about this subject.
GBO is a great place, and I am very happy that I was allowed to join.



LONGTOM

[stimpylu32]

LONGTOM

 Here is a place to start looking for some answers .

http://www.gboreloaded.com/forums/index.php/topic,129618.0.html

stimpy

[AtlLaw]

Wow! All of this has and is interesting reading with a world of knowledge or lack of from both sides. <snip>
LONGTOM

It really is interesting isn't it LT.   

It's also interesting how threads "morph" as they go on.  When I was reading your thread I thought of my question so I started this one.  Now it seems to have morphed back into your correlation question!    Just shows to go you that there is a lot of knowledge, opinions and feelings on the subject.  I swear, these guys know more about this pressure business then I got years enough left to learn!

Be that as it may, because of these threads I've had long forgotten understandings refreshed, affirmed or brought into question; I've learned a bunch of new information and had the fact that term PSI was used 40 years ago confirmed, so I feel better about my mental faculties.   

I really appreciate it also.

I still would like my own PSI o'meter though!   

Long Tom... hummmm,  You any relation to the deceased movie star "Long John?"   

[ricciardelli]

Maybe this has already been answered...

There is no conversion factor to convert CUP to PSI or the other way around.  They measure pressures at different points and in different manners.

I have a chart at http://stevespages.com/crusher.html that lists both CUP and PSI ratings of the more common calibers.

[MZ5]

In addition to measuring at different points and in different manners, both measure pressure indirectly, making the correlation less perfect.  But more to the point, copper crushers have a range of pressure in which the deformation of the copper appears reasonably linear.  However, the copper crusher method is most definitely not linear down to zero, nor when pressures get too high.  The lead crusher method referenced above works well in a lower pressure range than copper, and is older than the copper method, but works best in a different pressure range.  The piezoelectric transducer method will reveal things that any of the crusher methods do not, which complicates trying to equate them.  The best method of pressure measurement in terms of safety right now is the strain gauge.  It, too, measures pressure in a different way and only indirectly, but it is measuring the strain on the chamber/barrel steel.  That's a better way to keep you safe than trying to see how hard a piece of brass is being pushed into the chamber steel, because it tells you if and when the barrel steel is approaching its 'max' pressure tolerance.