Formula for estimating pressure? (psi)

This is a discussion on Formula for estimating pressure? (psi) within the Reloading forums, part of the Defensive Ammunition & Ballistics category; Is there such a thing, and what data is needed to figure it? I'm sure my daughter could help me with the mathematics if I ...

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Thread: Formula for estimating pressure? (psi)

  1. #1
    Senior Member Array Dadsnugun's Avatar
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    Formula for estimating pressure? (psi)

    Is there such a thing, and what data is needed to figure it? I'm sure my daughter could help me with the mathematics if I knew the formula and what to plug into it.

    Thanks!
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    Depends. What kind of pressure are you trying to estimate? Tire pressure? Pressure of water at a certain depth, or barometric pressure at a certain altitude? Cartridge case pressure?
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    VIP Member Array high pockets's Avatar
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    If the case bulges, or the primer looks flattened, the pressure is too high! That's the only formula I know, for sure.
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    The best formula I know of is to look at the reloading manuals and see what they say a loads pressure is supposed to be. Work from the manual and inspect the cases like hp said.

    It takes a lot of equipment to measure CUP pressure on rounds, something that is left generally to cartridge manufacturers and reloading manufacturers, not the average reloader.

    One question though, why are you wanting to be able to measure the CUP pressure in the first place, are you manufacturing firearm parts, or simply reloading?
    Just remember that shot placement is much more important with what you carry than how big a bang you get with each trigger pull.
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    Senior Member Array Dadsnugun's Avatar
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    Quote Originally Posted by gasmitty View Post
    Depends. What kind of pressure are you trying to estimate? Tire pressure? Pressure of water at a certain depth, or barometric pressure at a certain altitude? Cartridge case pressure?
    Cartridge case pressure.

    Quote Originally Posted by farronwolf
    One question though, why are you wanting to be able to measure the CUP pressure in the first place, are you manufacturing firearm parts, or simply reloading?
    Just curiousity, for reloading. I have one load that I worked up to (4.6g HP38 under a 158g Lasercast RNFP). In the Hodgdon Data it's the max +P load but it's for a 158g XTP. It makes sense to me that the pressure should be slightly lower with the Lasercast bullet, given that it's not jacketed and I just wanted some way to confirm that this is true. I've no intentions to exceed that load, as it shoots well, there is no difficulty with ejection, and I've not noticed any bulging or anything. It's just another way to achieve peace of mind and back up my "real world" testing.

    ETA: oh yeah...and because I still enjoy learning new things.
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    The Powley computer has been computerized and will work for estimating pressure.

    Powley Computer
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    Senior Member Array Dadsnugun's Avatar
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    Quote Originally Posted by msgt/ret View Post
    The Powley computer has been computerized and will work for estimating pressure.

    Powley Computer
    Thanks msgt/ret!...this is going to take some figuring out...
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    Ex Member Array Yankeejib's Avatar
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    Quote Originally Posted by msgt/ret View Post
    The Powley computer has been computerized and will work for estimating pressure.

    Powley Computer
    That's really cool! Thanks. What a great tool.
    I just discovered that if I got into reloading, I'd have to change my handle to either "Kb Master", or "Seven Digits Left"

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    Somewhere around the house I still have an original Powleys Computer like the one in this article. It was purchased back in the day when calculators would only add/subtract/multiply and divide, and the only home computer was the Timex/Sinclair 1000.

    The Powley Computer & PSI Calculator

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    Distinguished Member Array Fitch's Avatar
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    Quote Originally Posted by Dadsnugun View Post
    Is there such a thing, and what data is needed to figure it? I'm sure my daughter could help me with the mathematics if I knew the formula and what to plug into it.

    Thanks!
    The best way to get a feel for pressure is to use the QuickLoad software program which was developed for that exact purpose. It does internal ballistics simulations to predict what happens from powder ignition till the bullet leaves the muzzle. The SW isn't for everybody, but if you want a way to figure it out, that is the way.

    There isn't any simple formulla - just the powder data alone is a mess. QuickLoad contains a powder library, also a bullet library and cartridge library. It has features to screen for powders that might be suitable for the cartridge of interest. It comes with a really good manual which is well worth reading.

    There is a good review of the SW here: QuickLOAD Review & User's Guide

    I've been using it for 5 years now. It's been a really good resource for understanding and predicting cartridge behavior.

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    Distinguished Member Array claude clay's Avatar
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    pressure....re-loading

    look at the spent primer

    is the edge still rounded? is ok
    the case slide out easy from the cylinder, no brass splits
    --blow back on the case can be cause of low pressure; the case did not expand to press against the chamber walls

    is the edge of the primer flat to the brass? you are high pressure...possibly a +p or ++p
    no blow by or some, will be located in one area indicating the cut of the chamber not being perfect or a thin area of the brass. check other spent casings for smiliar marks in the same location.

    primer flat and overflowing the hole, perhaps the firingpin has punctured the face of the primer--
    possibly the case is stuck and when you get it, it shows stress marks/crack
    with a revolver, it may not revolve cause the primer has pushed out

    --you are in dangerous territory. don't shoot any more of those loads..
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    Senior Member Array Dadsnugun's Avatar
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    Quote Originally Posted by Fitch View Post
    The best way to get a feel for pressure is to use the QuickLoad software program which was developed for that exact purpose. It does internal ballistics simulations to predict what happens from powder ignition till the bullet leaves the muzzle. The SW isn't for everybody, but if you want a way to figure it out, that is the way.

    There isn't any simple formulla - just the powder data alone is a mess. QuickLoad contains a powder library, also a bullet library and cartridge library. It has features to screen for powders that might be suitable for the cartridge of interest. It comes with a really good manual which is well worth reading.

    There is a good review of the SW here: QuickLOAD Review & User's Guide

    I've been using it for 5 years now. It's been a really good resource for understanding and predicting cartridge behavior.

    Fitch
    Thanks Fitch, I'll look into this.

    Quote Originally Posted by claude clay
    pressure....re-loading

    look at the spent primer

    is the edge still rounded? is ok
    the case slide out easy fromcylinderender, no brass splits
    --pblow backowback on the case can be cause of low pressure; the case did not expand to press against the chamber walls

    is the edge of the primer flat to the brass? you are high pressure...possibly a +p or ++p
    no blow by or any will be located in one area indicating the cut of the chamber not being perfect

    primer flat and overflowing the hole, pofiring pine firingpin has punctured the face of the primer--
    possibly the case is stuck and when you get it, it shows stress marks/crack
    with a revolver, it may not revolve cause the primer has pushed out

    --you are in dangerous territory. don't shoot any more of those loads..
    Thanks for the very practical information claude . What I am after specifically though is numbers, to use in addition to all of the visual checks that you suggest. It's just a thing with me, I like numbers.
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    Member Array Archie's Avatar
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    Like the Retired Master Sergeant, I have a set of the old, cardboard fabricated Powley Computers - for loads, velocities and pressure prediction. Neat items.

    There are some devices to measure pressure directly from your firearm. Not cheap or as simple as one might like, but available.

    Ballistics Software, Chronographs & Pressure Instruments For Shooters

    Being one such place. I have no connection with them, other than purchasing some of their hard and soft ware.
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    Yes, the bigger the bulge, the higher the pressure.
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    Distinguished Member Array tiwee's Avatar
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    Quote Originally Posted by Dadsnugun View Post
    ...I have one load that I worked up to (4.6g HP38 under a 158g Lasercast RNFP). In the Hodgdon Data it's the max +P load but it's for a 158g XTP....
    I am not a reloader or ballistics guy or chemist. Fired from the same weapon with the same mass(158g), the pressure would seem to be approximately the same for both jacketed and not.

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