# Thread: How to calculate with vacuum

1. I have several sizes of tubes, 1.5 cm diameter, 2.5 cm diameter, 3.5 cm diameter (give or take). All are 10 centimeter long, made of PE with PP cap. They have a conical bottom which adds another 2 centimeters.

I want to know the total pressure on the walls when it is at vacuum, at any given height. Where would the force be the highest, and where would it be the lowest?

To take the entire force into the equation, i know i needed a height, but all i can do is calculate the entire force of the total distance, not if the properties of the material are exceeded at any time in any direction, which would make the entire tube fail. Irregular shapes like the cap and the bottom make calculating this even more difficult, as a different force distribution is applied to this. So i took 1/4th of pi as a distance on either side to calculate. 1,5 cm would turn into 1.1781 cm long as a maximum force applied to every bit. And the 2.5 cm would be a 1.9635 cm long bit of maximum shared force. Etc

Is this a correct way to estimate the total force applied to the walls at vacuum? And what is a better way to do this?

I know about P = F/A = (m*g)/A, but this doesn't apply for imperfect materials. As force is also applied inside the material, to counter the force of the pressure.

2.

3. Without knowing what the end-game is here, I can tell you the smallest diameter tube will withstand the most vacuum without collapsing due to it's smaller diameter. Outside air pressure is approximately 14.7 PSI (or 101.353 kPa, or 1 Bar) at sea level. The most likely place where it will buckle is in the middle of the straightest length of tube. Sorry, I'm not sure what formula to use for this. This is purely base on experience. I hope it points you in the right direction.

4. Originally Posted by Zwolver
I want to know the total pressure on the walls when it is at vacuum
When it's a vacuum inside the tube or when the (pressurised) tube is in a vacuum?

at any given height.
I don't understand this. If it's a vacuum in the tube then the variation of external atmospheric pressure over a 10cm height is going to be all but negligible1. Likewise if it's a pressurised tube in a vacuum then the internal pressure is going to be same throughout the length.

i know i needed a height
Unless it's some phenomenally precise and sensitive system I don't think you need to bother with the height.

but all i can do is calculate the entire force of the total distance, not if the properties of the material are exceeded at any time in any direction, which would make the entire tube fail. Irregular shapes like the cap and the bottom make calculating this even more difficult, as a different force distribution is applied to this. So i took 1/4th of pi as a distance on either side to calculate. 1,5 cm would turn into 1.1781 cm long as a maximum force applied to every bit. And the 2.5 cm would be a 1.9635 cm long bit of maximum shared force. Etc

Is this a correct way to estimate the total force applied to the walls at vacuum? And what is a better way to do this?

I know about P = F/A = (m*g)/A, but this doesn't apply for imperfect materials. As force is also applied inside the material, to counter the force of the pressure.
Without you supplying the answers requested above (and, if it's tubes in vacuum the we'd also need to know what pressure is applied internally) I can't help.

1 This calculator says that atmospheric pressure is the same at sea level as it as 10 cm above SL. A 1 metre height variation gives a mere 0.000118% reduction. That's not going to affect PE in the real world.

5. Originally Posted by Dywyddyr
When it's a vacuum inside the tube or when the (pressurised) tube is in a vacuum?
The tube is itself under vacuum. With regular air pressure above.

I don't understand this. If it's a vacuum in the tube then the variation of external atmospheric pressure over a 10cm height is going to be all but negligible1. Likewise if it's a pressurised tube in a vacuum then the internal pressure is going to be same throughout the length.
Yes, the outside pressure is the same everywhere, and the inside pressure is also the same everywhere. And still the force distribution isn't the same everywhere. You have weak points, and strong points. And i want to know if there is a benchmark way to gauge the weakest point. I want to make a vacuum tube, and i want to know if the force anywhere ever exceeds the limit of what PE can take at the maximum temperature (probably 32 celcius in the sun) it will reach, at the weakest point.

Unless it's some phenomenally precise and sensitive system I don't think you need to bother with the height.
true, height is not important, as long as the height isn't the limiting factor. If the height was so low the bottom and top were reenforcing the structure of the tube to an extend.

Without you supplying the answers requested above (and, if it's tubes in vacuum the we'd also need to know what pressure is applied internally) I can't help.
vacuum tubes, so the tubes are under vacuum, regular atmosphere above, and PE will turn liquid when the walls have to much stress. Diameter matters for the structural integrity.

1 This calculator says that atmospheric pressure is the same at sea level as it as 10 cm above SL. A 1 metre height variation gives a mere 0.000118% reduction. That's not going to affect PE in the real world.
Well, yeah. i get that.. I will assume pressure is the same over the length..

6. Originally Posted by Zwolver
Yes, the outside pressure is the same everywhere, and the inside pressure is also the same everywhere. And still the force distribution isn't the same everywhere. You have weak points, and strong points. And i want to know if there is a benchmark way to gauge the weakest point.
The wording on this is slightly weird. If the pressure is the same everywhere then, by definition, the force is the same everywhere. The ONLY way to gauge weak/ strong points is to test it.
Weak points arise from faults in the material or faults in the construction (i.e. joints/ bends etc) - these aren't calculable.

I want to make a vacuum tube, and i want to know if the force anywhere ever exceeds the limit of what PE can take at the maximum temperature (probably 32 celcius in the sun) it will reach, at the weakest point.
There's not only a list of mechanical properties of (HD)PE on this pdf (for temperature data you may have to request - from here for example - a more detailed brochure) but 32C shouldn't be any problem that I can see.

Diameter matters for the structural integrity.
Uh, what? Wall thickness matters.
Page 5 of the linked document gives an equation to determine what that should be for a given pressure (and up to 80C temperature).
If that doesn't help then there's this (as a primer) and these (as calculators).

7. Originally Posted by Dywyddyr
Originally Posted by Zwolver
Yes, the outside pressure is the same everywhere, and the inside pressure is also the same everywhere. And still the force distribution isn't the same everywhere. You have weak points, and strong points. And i want to know if there is a benchmark way to gauge the weakest point.
The wording on this is slightly weird. If the pressure is the same everywhere then, by definition, the force is the same everywhere. The ONLY way to gauge weak/ strong points is to test it.
Weak points arise from faults in the material or faults in the construction (i.e. joints/ bends etc) - these aren't calculable.

I want to make a vacuum tube, and i want to know if the force anywhere ever exceeds the limit of what PE can take at the maximum temperature (probably 32 celcius in the sun) it will reach, at the weakest point.
There's not only a list of mechanical properties of (HD)PE on this pdf (for temperature data you may have to request - from here for example - a more detailed brochure) but 32C shouldn't be any problem that I can see.

Diameter matters for the structural integrity.
Uh, what? Wall thickness matters.
Page 5 of the linked document gives an equation to determine what that should be for a given pressure (and up to 80C temperature).
If that doesn't help then there's this (as a primer) and these (as calculators).
I mean a formula to calculate it.

Something like this combined with this (page 12).

8. The fourth link I gave should give you the method.

9. Thank you , i don't know how i sort of missed it..