Shooters Forum banner

21 - 40 of 72 Posts

·
Registered
Joined
·
549 Posts
CUP to PSI:
=-17902+(1.516*(CUP))

PSI to CUP
=((PSI)+17902)/1.516
This is not quite right. If you know the CUP first multiply the CUP by 1.516, then subtract 17902, which you have right, but to get CUP from PSI you divide PSI by 1.516, then add 17902.

If you add the 17902 first and then divide by 1.516, you get the wrong CUP.

So, PSI to CUP = PSI/1.516 + 17902
 

·
Registered
Joined
·
3,986 Posts
Discussion Starter #22 (Edited)
Charshooter...

I will stand by my calculations above. You agree that the first calculation is correct so 40.0K CUP delivers 42,738 PSI.

With my calculation quoted above, 42,738 PSI will return 40.0K CUP. With your presentation of the calculation, 42738 PSI would return 46,093 CUP.

The sum of the first equation should check against the sum of the other for both equations to be correct. At least that is my take on how the math should work?
 

·
Registered
Joined
·
549 Posts
Yes, you are right, I did not try it with any numbers, just thought I saw it, but you are right, but I don’t know why you are right when the 17902 should act as an independent constant and not vary with a ratio? There is something funny about this that I am just not seeing, you have a*b+c where c is any real number and not part of a ratio then you have a+b/c? There is something funny about this; what am I missing?
 

·
Registered
Joined
·
549 Posts
Oh,
You need to subtract C then divide, but still here C is independent, so PSI to CUP is PSI – 17903 equals the quantity that divided by 1.516 is CUP

I just reversed the procedure but heck I think what is throwing me is you have ((PSI) – 17903)/1.518 and I am thinking (PSI – 17903)/1.516
 

·
Registered
Joined
·
549 Posts
And it doesn’t matter because for every single equation PSI is constant, so (( ) ) or ( ) will be meaningless.

Forgive the lack of profundity of an old man, its been too long since I thought this out, but I don’t feel bad. I was talking to a fellow engineer about double line tank pressure and he said he did not remember anything about it and he is not yet 65!

You might know that all the cartridge pressure is like tank pressure and those formulas should work with cartridges with the same general volume and powder. That is why I would say one could load a 35 Whelen as hot as a 30-06 because the elements are the same if the case is the same and the rifle is also the same
 

·
Registered
Joined
·
3,986 Posts
Discussion Starter #26
Charshooter... same here, I read this stuff in a paper and I wouldn't want to say how long it took me to get it worked out right. I had to seek the assistance of a 16 year old! I was too busy making eyeballs at sweet little Elaine to pay much attention in math class when they covered the PSI-CUP conversion. At least I don't remember it so I figure that's what I was doing... You would think I would have become alert when that old bag of a teacher mentioned reloading but I think she camouflaged it with that X & Y stuff...
 

·
Registered
Joined
·
385 Posts
Now, here is another fun question:

Assuming Powder as a constant, Is Velocity a linear relationship to slug weight, and if it is, is Pressure a constant?

Assume for a moment you have a 219gr slug with a MV of 1200 from 12.2gr Powder. If it is a linear value, that parses out to .0000464762 gr powder per fps per gr.

Running that to a 140gr slug w/12.2 gr gives 1875fps - which is about what I've seen.

(A) does that equasion hold true or have I been fooled by a random correlation and
(B) Is the pressure constant between the two loads?

Are there any books that give thee equasions?
 

·
Beartooth Regular
Joined
·
7,777 Posts
The increase in velocity tends to be linear with the increase in powder charge, when you're in the powder's optimum pressure range. Once you're outside that range, pressure and velocity get erratic. You don't want to go out the top end.

Pressure increases at approximately the square of the velocity. That's why trying for that extra 100 fps isn't a good idea.

Check out this thread for the math. The link to the Powley math is still good.
http://www.shootersforum.com/showthread.htm?t=34384

Bye
Jack
 

·
Registered
Joined
·
385 Posts
Thanks - I figured somthing like that for change in powder, but what about change in bullet weight (and just in bullet weight)?
 

·
Beartooth Regular
Joined
·
7,777 Posts
Vihtavuori had a handy chart in their 1998 manual, but I don't see it in their current online manual.

For a .308 Winchester with 170 grain bullets and N140 powder.

Bye
Jack
 

Attachments

·
Registered
Joined
·
385 Posts
Maybe I'm doing it wrong, but that doesn't seem to work for IMR 4227.

Spec puts 218.75 gr out at 1150fps over 17.5gr
Testing puts 140gr out at 1850 fps over 22gr from the same shell/gun/primer.

Of course, this is an odd testbed too... since I'm using 1/2oz shot .410 data to calculate for a 140 & 175gr cast slug...
 

·
Registered
Joined
·
1,368 Posts
.38 Spl +P Pressure standard.

The .38Spl. +P SAAMI MAP standard was changed in 1994 from 18,500 psi to 20,000 psi when ammunition manufacturers could not provide the performance levels demanded by consumers. This change was not reflected in many loading manuals because it occured after the publication of the ANSI standards manual in 1993.

The current SAAMI MAP pressure for the .38 Spl. +P is 20,000 psi or 20,000 cup. That while the two systems often give different values in this case they read the same. The current +P standard while higher, is still short of the pre 1993 SAAMI standard of 22,400 C.U.P.
 

·
Registered
Joined
·
2,717 Posts
The .38Spl. +P SAAMI MAP standard was changed in 1994 from 18,500 psi to 20,000 psi when ammunition manufacturers could not provide the performance levels demanded by consumers. This change was not reflected in many loading manuals because it occured after the publication of the ANSI standards manual in 1993.

The current SAAMI MAP pressure for the .38 Spl. +P is 20,000 psi or 20,000 cup. That while the two systems often give different values in this case they read the same. The current +P standard while higher, is still short of the pre 1993 SAAMI standard of 22,400 C.U.P.


What about +P+ ammo. I have some Federal +P+ 38 Special Ammo
 

·
Super Moderator
Joined
·
13,164 Posts
Piezo transducers tend to read higher than copper crushers above about 25-30,000 units, but can read lower below that range, and that may account for the discrepancies. The one chart I know of that lists both unit sets still shows 18,500 psi as +P for .38 Special, but also shows 20,000 cup as +P at the same time. It is here. Scroll down to learn where he got his information for the chart. The same chart also shows CIP pressures for both copper crushers and Piezo transducers.

+P+ is not a SAAMI standard, so you don't find it on SAAMI charts. The industry appears to have an unwritten agreement for .38 Special only, to keep it within 22,000 psi, which is 130% of the 17,000 psi standard pressure SAAMI MAP for the cartridge, rounded down to the nearest 500 psi (which is what SAAMI does with all its standards). With 9 mm it appears to be 120% (42,000 psi) is the agreed upon number. So +P+ is unlike +P where the same percentage of standard pressure MAP is used for all +P cartridges (+10% rounded down to the nearest 500 psi).

Father Frog's site has a list of pressures if you care to look. Because it is not a formal standard, the 130% number for .38 Special is not binding on anybody. On another forum one fellow said he measures Federal 9 mm 124 grain Hydrashok +P+ ammo giving him lower muzzle velocity than Winchester 124 grain +P ammo. Different bullet shape can affect that, but it appears the +P+ label is used to generate excitement or to help keep a round in the LE only category.

The interesting thing about .38 Special, in particular, is how the European CIP limits its pressure. They use 150 megapascals, which turns out to be 21,756 psi for all .38 Special loads. No +P or +P+ catagories; just that one number which is considered safe for all. Unlike the U.S., where SAAMI is an industry organization with no regulatory power beyond the voluntary, the CIP proof standards are enforced by law in Europe which requires every gun manufactured to be individually proofed. We don't do that here. It gives Europeans more certainty that their standards are safe in all new guns. Nonetheless, a lot of U.S. made guns are over there and not having problems with the CIP standard. That suggests all but the most frail .38 Special ought to be able to deal with +P+, despite all the liability concern, and provided it really does hold to the 22,000 psi limit, making it basically a Euro-load + a little over one percent. Not a significant difference.
 

·
Registered
Joined
·
10 Posts
My understanding of some of the discrepancies in the conversion was, the CUP done by the copper crusher was also dependent on the amount of time the pressure was at its peak. If a pressure peaked quick and dropped off fast the crusher would not read as high (the crusher would not flatten as much) as if the pressure peaked and stayed at peak for a longer period of time. That is why for an example a load that was originally checked by the copper crusher may have read 20,000 CUP when the Piezo transducer is used that maps the entire rise and fall of the pressure may show a peak of 25,000 psi. The change from CUP is not always an exact equation, but in most cases it is.
 

·
Super Moderator
Joined
·
13,164 Posts
Most of the non-conformal piezo transducers are read using a peak reading meter, and don't show or integrate the whole curve. Doing so would result in a reading proportional to average pressure during the bullet's time in the bore, which is lower than the peak, not higher. The problems with the copper crushers are inertia in the larger piston, work-hardening of the copper slug that changes its response as it crushes further (so it's not linear) and differing ability to apply a micrometer to a crushed slug among the ballistic technicians. At least the peak meter takes the technician's measuring skills out of the picture.
 

·
Registered
Joined
·
10 Posts
There is a good article in the shooting times on the differance between the to methods.
http://www.shootingtimes.com/ammunition/ST_cuppsireloaddata_200905/index1.html
Quote from article with the info
"The Big Difference
Being strictly mechanical, the crusher system records only peak pressure--nothing else. It's like a black and white snapshot taken with an old box camera. By comparison, transducer testing is a high-definition, full-color video. The new system has a time basis and records from the point the fired case contacts the transducer to the point where the bullet exists the test barrel."
Hope this helps
 

·
Super Moderator
Joined
·
13,164 Posts
Bowfishn,

That article left some things rather fuzzy. The transducer he used is of the current SAAMI standard type (conformal type, as opposed to the older non-conformal or flush diaphragm type that sampled actual gas from the chamber through a hole in the case). It is required to make SAAMI compliant measurements. The graphical readout is an option he enjoyed, but is not required. The list of SAAMI compliant standard transducer measurement equipment is on page 126 (132 by .pdf method of counting that includes non-numbered pages) of their voluntary standards document. It includes a PCB Piezotronics type 451A07 peak detector, but no graphical display.

Bear in mind the standards have not been re-issued since 1993. SAAMI Technical Director Ken Green told me, in a phone conversation a couple of years ago, that they are going to be issuing updated publications over the next few years, so you can look out for any changes that might occur there. In general, though, they tend to be rearward compatible so as not to obsolete existing test equipment investment. That's why you still see Hodgdon and Lyman, for example, giving a lot of c.u.p. data and some p.s.i. data. They don't want to have to capitalize new gear before the old gear wears out.

If you get a chance sometime, take a look at pages 116 through 131 of Lyman's #46 manual (1982) if you can get your hands on a copy. It's an article with some history (the first Piezo gun pressure measuring device was made in 1922; the copper crusher was invented in 1860) and it presents data on a copper crusher vs. a non-conformal type Piezo transducer (gas sampling) that is co-installed on the same .30-06 barrel. The tests are done by H.P. White Laboratories. It's result graphs show c.u.p. readings that are lower then the Piezo transducer p.s.i. readings for slow powders at high pressures using jacketed bullets, but the opposite (crusher reading higher) for fast powders at modest pressure firing lead bullets. Every system seems to have its foibles.
 

·
The Troll Whisperer (Moderator)
Joined
·
23,799 Posts
Owww - my head hurts! :eek:
 

·
Registered
Joined
·
10 Posts
Thanks for the info Uncle, looks like you have given me a bit more reading. That's what I like about these forums, more info to digest. I've certainly got time my SRH 44 mag is at Ruger right now getting the dovetails re-cut or the frame changed out. When it comes back I can spend my time with loads.
 
21 - 40 of 72 Posts
Top