High pressures?

[Arkansas Paul]

I'm new to reloading and am spending a lot of time pouring over loads in manuals. Why do IMR powders seem to give so much higher pressures than Hodgdon powders. With the same velocities, IMR seems to be consistently higher. Am I right to be concerned about this? Also, would someone explain the difference between psi and cup. I know psi is pounds per square inch, but what's the difference? Help a rookie out please.

 

[faire bien de...!]

No need for concern, just use the powder that gives the least pressure. Wikipedia explains PSI, CUP, and other measurements of pressure quite thoroughly.

 

[Steve C]

Why do IMR powders seem to give so much higher pressures than Hodgdon powders

This question really doesn't make sense. Are you asking about similar numbered powders like IMR 4831 vrs H 4831? Or are you asking a general question regarding IMR vrs Hodgdon? If you are looking at data using the different scales CUP vrs PSI the comparison is like feet vrs meters.

Also, would someone explain the difference between psi and cup.

CUP (Copper Units of Pressure) is the older method of detrmining chamber pressure in cartridges. A calibrated copper slug was set in the chamber and how much it was compressed was used to determine a "number" called CUP. CUP is a non linier scale and there is no direct or exacting calculation to convert CUP to PSI though some approximations can be made and at certain levels CUP is close to PSI while at others there's a wide disparity.

PSI is a more scientific measure of Pounds force per Square Inch. More exacting measurements are now made using electronic sensors that show how much pressure is generated by how much the chamber is expanded and contracted in real time during the firing of the sample load. This piezo electronic pressure method is capable of showing pressure spikes and peak pressures while the copper crush method would only give a rough average pressure level.

 

[Shoney]

The biggest difference in CUP and PSI is that CUP is an average of the pressure that develops from a load, while PSI readings are reading of pressure during the entire length of the burn.

CUP
Before the powder ignition the pressure is zero in the copper crusher chamber. At ignition the pressure rises to it's maximum and then decrease to zero as the burn is completed. CUP measures sort of an average pressure in the cylinde and does not tell you the highest pressure or when, during the burn, that the pressure was highest. You don't know if the pressure spikes rapidly at some point or rises and decreases evenly.

PSI
The use of peizoelectric transducers measures the pressure during the entire powder burn. It tells you not only the highest pressure, but at what time in the burn that high pressure came.

There is no way of translating one into the other.

 

[fguffey]

http://www.frfrogspad.com/intballi.htm



Arkansas Paul, a neighbor of yours from the north of Little Rock called and ask if I would have an interest in 100,000+ test fired cases complete with CUP imprint, I told him I would have no problem with loading the cases after examination but as to determining useful information from the cases the answer was 'no'.

Then I qualified my answer, if I was offered the opportunity to examine the cases (dimension, case head diameter and case head thickness) before firing and again after firing and then given the results of the CUP test for each case, the answer would be yes.

Even when copper billets are used for testing chamber pressure every batch is tested for yield, every lead billet batch is tested for yield, brass will support more weight therefore the amount of yield will be less, to measure the increase in case head diameter and case head crush would require an accurate gage that could measure .0001, better yet .00001.


So we are left with firing first with factory ammo to determine the expansion of the case head for a standard? for normal expansion, some say .0005, then someone will say they they fire cases 50 times with maximum loads (higher than normal loads) without maintenance to the case, .00025 expansion carried out to 50 firings = .0115 case head expansion, .0005 expansion carried out to 50 firings=.025 thousands, then there comes a times when case head expansion goes to .005 for one firing, that is the one that will be remembered, the primer pocket expands, the flash hole expands, the case head shortens and at the same time increases in diameter and the case requires extreme measures when removed from the chamber.


Then there is the standard answer, (it can't be done because of 'spring back' and if you did the results would not be accurate).

then there is 'absurd'

"the idea a few cling to, that uncalibrated brass can do better, is just absurd"


F. Guffey

 

[rcmodel]

IMR seems to be consistently higher. Am I right to be concerned about this?

No.
But I think you already got your answer, in that Hodgdon shows a lot of it's inherited IMR data in CUP, and almost all it's Hodgdon powder data in PSI.

Comparing any other data, such as Speer's IMR data against Hornady's Hodgdon data is comparing apples to oranges, because the data was shot in completely different test conditions with different bullets.

If you look at only one source of data, such as the Lyman #49, you can directly compare IMR against Hodgdon using the same test condition, bullet, and pressure standard (All CUP or all PSI for a specific bullet).
If you do that, I think you will see very little if any difference when comparing similar burn rate powders from the two companies.

The fact remains that IMR powder was one of the first, and still one of the best for a lot of uses.

Don't rule it out based on apples & oranges.

rc

 

[Arkansas Paul]

I appreciate the help. Like I said, I'm just starting out and want to make sure I do things the safest way possible. I'm not taking any chances with a controlled explosion six inches from my nugget. Hope you guys don't mind simple questions, because I'm sure I'll have a lot more of them.