recently asked the zeitgeist about meplat trimming which lead to a tangent regarding internal shell casing volume measuring. I did a bit of research and would like some help fleshing out a concept for quick reliable measuring of internal case volumes. It occurs to me that using a known quantity and pressure of air would be a useful way to go. Imagine a device that would seal around the case mouth and inject air while measuring the change in original quantity. I am thinking of adapting one of my nail guns to accomplish this task. If there were a way to measure resistance in the piston which uses a known quantity of air every time the gun is fired. Any impute is appreciated.
Found a couple of my people
- Thread starter ADIDAS69
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scwhitetail
New member
On the right track, but powder and water have different specific gravities, meaning a specified volume of water will have a different weight than an identical volume of water. You also have to worry about the compaction of the granular powder versus fluid water.
It would be better to fill the case with water, of course making sure that your fill line is below where the bullet will be seated.
For example, a .308 win holds 3.64 ml of water
Since 1 ml=1cm³ of water, the powder capacity of the case is 3.64 cm³
This weighs 56 grains and has a density of 15.39 gr/cm³
IMR 3031 smokeless powder has a density of 13.123 gr/cm³ so the maximum powder weight would be 47.768 gr.
If you added 56 grains of powder (the weight of the water) you would overfill the round and have some dangerous issues with compressing the powder.
So the formula for finding maximum weight of powder would be:
(Volume of Water in cm³) x (Density/specific gravity of powder to be used)
Densities of common smokeless powders can be found here: http://www.tacticoolproducts.com/powder.pdf
Using air pressure would be rather impractical because there is no conceivable way to account for the volume of the neck of the casing where the bullet will be seated.
If you do decide to use a pressure based system, refer to your high school chemistry and physics. Do you remember the ideal gas laws PV=NrT or re arranged V=(NrT)/P where V is volume, N is moles of substance, r is the ideal gas constant, T is temperature, and P is pressure. There are just too many variables to consider to make this feasible without spending a ton of money on equipment. You would have to monitor temperature, and pressure of the air inside the casing and do some pretty serious math. And the water thing works just fine, as long as you don't mind drying the casings, which a quick shake and about 30 seconds in an oven should take care of that.
Jim Watson
New member
The old Powley PSI Computer figured everything based on water capacity of brass and an IMR powder bulk density of .86. (Grains per cu cm is just cruel.)
moxie:
you're probably right about thread location I wouldn't be offended if one of the admins moved it or some one told me how to move it myself. You're also probably corrected about the futility but i have time to trim meplats and i have time for this.
Jim243:
not a huge fan of the water, if i use a liquid it will be a 99.9% pure alchohal
scwhitetail:
I certainly wasn't going to use the air volumes measured to determine powder charges. Simply to determine similarity. Imagine an apparatus that has two exhausts that discharge the same volume and pressure, one port connected to a vessel of a known volume and the other able to receive 30 caliber case neck and successfully seal around it. A meter on both would indicate the over flow from either side. So one ideally should be reading zero every time and the other would give a reading that would then allow you to separate casings with matching volumes. The air concept came to mind as a way to measure several thousand casings quickly instead of over the course of a month using the liquid method.
Generally:
if i use a solid it will be silicone beads.
you're probably right about thread location I wouldn't be offended if one of the admins moved it or some one told me how to move it myself. You're also probably corrected about the futility but i have time to trim meplats and i have time for this.
Jim243:
not a huge fan of the water, if i use a liquid it will be a 99.9% pure alchohal
scwhitetail:
I certainly wasn't going to use the air volumes measured to determine powder charges. Simply to determine similarity. Imagine an apparatus that has two exhausts that discharge the same volume and pressure, one port connected to a vessel of a known volume and the other able to receive 30 caliber case neck and successfully seal around it. A meter on both would indicate the over flow from either side. So one ideally should be reading zero every time and the other would give a reading that would then allow you to separate casings with matching volumes. The air concept came to mind as a way to measure several thousand casings quickly instead of over the course of a month using the liquid method.
Generally:
if i use a solid it will be silicone beads.
scwhitetail
New member
Oh ok, I didn't understand what you were getting at. That makes a lot more sense then what I was imagining. Its a very good idea. If you have the time and expertise to build it it should work very well. I've been giving it some thought and simply using pressure may work after all. How about a simplified design in which you have one container of known volume and pressure attached to a valve. That container would attach directly to the casing. You could then measure the pressure after the valve has been opened. Theoretically the pressure should be the same if the original container is always charged to exactly the same pressure. This way you wouldn't have to have the first meter.
The only concern with your original two meter design is that your first container will never actually read zero because the gas pressures will equalize. Unless you are using a pump to pump the air into the casing of course. So if you are using 15psi of air, the container may only empty to about 7. With only one pressure meter, it should work because you will be able to determine at what pressure it reaches equilibrium with the first container. Since you are using the same container, with the same volume of gas (determined through a fixed fill pressure) two cases of equal size will reach this equilibrium at the same pressure. A smaller case will read with a higher pressure, and a larger case will read with a lower pressure.
Im sure it would be very simple to build once the design is made. Basically it will be one container attached to some tubing and small butterfly valve. tubing from the valve leads to a soft rubber stopper with about a .28" opening. Stoppers like this are used for glass tubing in chemistry all the time and provide a durable air tight seal. A small pressure valve would be attached to a one way valve on your air holding tank so it can be filled with a precise amount of air. Another would be attached between the tank and the casing. It would have to be very precise. attach the casing. open the valve and check the reading!
The only concern with your original two meter design is that your first container will never actually read zero because the gas pressures will equalize. Unless you are using a pump to pump the air into the casing of course. So if you are using 15psi of air, the container may only empty to about 7. With only one pressure meter, it should work because you will be able to determine at what pressure it reaches equilibrium with the first container. Since you are using the same container, with the same volume of gas (determined through a fixed fill pressure) two cases of equal size will reach this equilibrium at the same pressure. A smaller case will read with a higher pressure, and a larger case will read with a lower pressure.
Im sure it would be very simple to build once the design is made. Basically it will be one container attached to some tubing and small butterfly valve. tubing from the valve leads to a soft rubber stopper with about a .28" opening. Stoppers like this are used for glass tubing in chemistry all the time and provide a durable air tight seal. A small pressure valve would be attached to a one way valve on your air holding tank so it can be filled with a precise amount of air. Another would be attached between the tank and the casing. It would have to be very precise. attach the casing. open the valve and check the reading!
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Scwhitetail:
And here my wife says I can't communicate. I do a lot of welding so I think I am going to adapt an argon tank and regulator(s) for this purpose. The static container was in my head as a continuous control, you're probably right that it would present more design problems than it would actually help verify test results. I am also thinking a tire inflation device like those found at petrol stations (a fair bit more sensitive) might also come into play. I'll send pics when the working prototype is sorted.
And here my wife says I can't communicate. I do a lot of welding so I think I am going to adapt an argon tank and regulator(s) for this purpose. The static container was in my head as a continuous control, you're probably right that it would present more design problems than it would actually help verify test results. I am also thinking a tire inflation device like those found at petrol stations (a fair bit more sensitive) might also come into play. I'll send pics when the working prototype is sorted.
scwhitetail
New member
alright man I look forward to seeing it. Best of luck. PM me when you get something working
phil mcwilliam
New member
You still have to linearly measure the critical dimensions being length & diameter. Volume can be the same for different shaped objects - a circle can have the same volume as a square. Just because you checked them for volume doesn't mean they are the same size.
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thedaddycat
New member
Are you starting with new brass or fired brass? If fired brass, will it have all case prep done prior to measuring case volume? Most of what I've seen about case volume is in relation to high, even extreme accuracy shooting. Then again, metplat trimming is right up there, too...
I think I have the mental picture of what you're thinking about building. Here are a couple of thoughts on how it would work:
The gas is supplied from a higher pressure source through a regulator to a fixed volume chamber.
This chamber is then isolated from the regulated supply so you have a fixed volume at a known pressure.
The charged chamber is then allowed to equalize with a case and the pressure drop is proportional to case volume.
Here are some areas of concern:
How will you account for temperature changes?
A gas under pressure will cool as the pressure is reduced. Also, unless your testing environment is held at a constant temperature, changes in ambient temperature will effect your results.
How will you achieve a repeatable seal that will not alter case volume?
If you use something like a single stage press to lower the test mechanism onto the case, won't the edge of the case mouth eventually cut into the end of the stopper? If it does, will the stopper intrude into the case causing a slight but erroneously low case volume reading?
How can you account for or positively determine that there is no leakage?
Will the stopper seal around the case neck without any leakage? Will your valves have tight enough stem seals that they won't leak? Since you are by the very nature of things dealing with small volumes, any leakage at all will introduce a greater error in your readings.
All in all, it is an interesting concept. I know it would be time consuming, but would you use conventional case volume testing to verify your results?
I think I have the mental picture of what you're thinking about building. Here are a couple of thoughts on how it would work:
The gas is supplied from a higher pressure source through a regulator to a fixed volume chamber.
This chamber is then isolated from the regulated supply so you have a fixed volume at a known pressure.
The charged chamber is then allowed to equalize with a case and the pressure drop is proportional to case volume.
Here are some areas of concern:
How will you account for temperature changes?
A gas under pressure will cool as the pressure is reduced. Also, unless your testing environment is held at a constant temperature, changes in ambient temperature will effect your results.
How will you achieve a repeatable seal that will not alter case volume?
If you use something like a single stage press to lower the test mechanism onto the case, won't the edge of the case mouth eventually cut into the end of the stopper? If it does, will the stopper intrude into the case causing a slight but erroneously low case volume reading?
How can you account for or positively determine that there is no leakage?
Will the stopper seal around the case neck without any leakage? Will your valves have tight enough stem seals that they won't leak? Since you are by the very nature of things dealing with small volumes, any leakage at all will introduce a greater error in your readings.
All in all, it is an interesting concept. I know it would be time consuming, but would you use conventional case volume testing to verify your results?
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