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# How to calculate vaccum pressure ?

## How to calculate vaccum pressure ?

(OP)
Dear all,

I am having a 12 mm diameter piston and cylinder arrangement,Using a handle i am moving the piston upward for 35 mm for inflow of fluid from reservoir against a check valve.
My question is how much partial vacuum pressure is created in the cylinder for the rise of 35 mm movement of 12 diameter piston?
Please provide any formula or sample calculation.

Regards,
Dinesh

### RE: How to calculate vaccum pressure ?

Assuming that the system is leak free, you can use Boyle's law to calculate the air pressure in the bigger volume...

### RE: How to calculate vaccum pressure ?

With a liquid you should get close to a perfect vacuum (or the vapor pressure of the liquid) minus the cracking pressure of the check valve. For a gas it all depends on the "head space" between the piston and the check valve. The gas in this space will be expanded as the piston moves. If you double the volume, the pressure will be half an atmosphere.

### RE: How to calculate vaccum pressure ?

What do you mean by fluid? a liquid or a gas? If a liquid, some of that will turn into a gas during a vacuum; if the fluid is a gas then boyle's law could be an adequate approximation.

### RE: How to calculate vaccum pressure ?

I'll rephrase "... If a liquid, some of that will turn into a gas or vapor during a vacuum"

### RE: How to calculate vaccum pressure ?

Does't the density of the fluid matter in this?

### RE: How to calculate vaccum pressure ?

Boyle's law does not include the density of the fluid.

Boyle's law does include the volume and any change in volume is dependent on the bulk modulus. If the cylinder contains a gas, then it is straight forward. If the fluid in the cylinder is a liquid, then much more force would be required to change the fluid volume.

If adequate force is applied such that the fluid reaches its vapour point, then Boyle's law can be applied as above.

As the original post refers to a handle, it is unlikely that there would be enough force to change the volume by enough to change the pressure.

So, in summary, it's Boyle' law if the cylinder contains air. If the cylinder contains a liquid, then it would not be possible to change the volume to cause a change in the pressure.

### RE: How to calculate vaccum pressure ?

Since the question requires the suction intake of a liquid it very much involves the density of that liquid.

"i am moving the piston upward for 35 mm for inflow of fluid"

### RE: How to calculate vaccum pressure ?

Ignoring the fluid intake for now, the absolute pressure can be calculated from the displacement of the cylinder.

The lower pressure will then cause the liquid to rush in.

I guess that the OP wants to know what spring is required in the check valve, so needs to know the pressure that will be needed to open it.

Again, it’s just Boyle’s law...

Density is only needed if the flow rate and pressure drop at the port need to be known.

### RE: How to calculate vaccum pressure ?

So lifting mercury will cause the same pressure as if the fluid is air?

### RE: How to calculate vaccum pressure ?

We are not calculating the head, just the pressure to open the check valve.

One also has to be realistic and take the safe assumption that the medium is not mercury or molten lead...

### RE: How to calculate vaccum pressure ?

The check valve already has a pressure drop. And if it's already liquid filled, Boyle's law won't apply.

At the best, since there's been no OP reply, this was probably a question on a quiz to see if he could figure out what other information was needed to solve a real problem.

### RE: How to calculate vaccum pressure ?

It’s just for fun...

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