Measure the volume of a cavity with compressed air?

[mugged]

Hello, so I have a pneumatic device with a fixed volume (strange 3D shape) that I'd like measure the volume of. I think traditionally this sort of experiment is done with incompressible fluids like water but I dont want to get it wet.

If I were to do an experiment with a flow meter and charge the volume cavity with say 4 times atmospheric pressure (like 70 psi) and obtain a flow rate over time curve, like a liters/sec vs sec. Would this be enough information to back out the volume of the cavity, the area under the curve represents the total liters or volume? For the setup it would be pressurized air source with pressure sensor -> tube -> flow meter -> tube -> flow restrictor to control velocity -> tube -> valve to open/shut -> tube -> device in question.

Of course it's compressed air so I have to adjust back down to atmosphere to get geometric volume and account for any volume of connection tubes, etc. But am I missing anything here? Having a lot of trouble finding anything online for this sort of thing.

 

[LittleInch]

Don't forget temperature.

But depending how accurate you want to be I would go for 1 bar / 2bara. Then I think the volume of air you inject will be your internal volume. This us based on sea level.

Will your shape deform under pressure?



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[mugged]

No it wont deform, its full metal. How does temperature play in to it, or what formulas? I was assuming I'd ignore temperature since I cant expect it to change much as the flow velocity will be low and the volume itself I'd predict is on the order of 100 cm³

 

[Compositepro]

Study the design and operation of helium pyncnometers. They measure true volume of solid,porous materials. If you pressurize your unknown chamber volume to a known pressure and then vent that pressure into a chamber of known volume, then you can easily calculate the unknown volume from the remaining pressure.
Low pressure is used to avoid temperature changes due to gas expansion. It is all in getting the details right to achieve accuracy.

Here is a video that explains the principles

https://youtu.be/zeErAuwyamI?si=aI-5HyxSg2y4letD

 

[Heaviside1925]

OP,
Unless you are drying your compressed air sufficiently, you very well may get it wet. Even though I have never done it with a gas and I'm sure the other posters will kill this if it's impractical, have you considered inferring the volume by weight? Weigh it empty, then fill it up to a certain psi, let it rest so temperature match ambient. With the temp, pressure, gas weight, gas specific volume and weight gain of the device, you should be able to infer the volume.

As far as getting it wet, I assume you mean water. Have you considered other incompressible fluids? Even something granular like small plastic beads? Unless you need very high accuracy, then I think Compositepro advice would be best.

 

[TugboatEng]

One suggestion is to pick a pressure band to take your measurement within. Fill it to 70 psi, vent through your flow meter, when pressure reaches 65 psi, start the measurement, then stop the measurement at 60 psi. This will help keep conditions more consistent and keep the flow rate inside your meter's most accurate range.

 

[Compositepro]

Flow is not something that can be measured easily or accurately. It is calculated by measuring velocity, volume, temperature, pressure, and other, variables. Each with some uncertainty and requiring calibration.

 

[LittleInch]

How accurate do you need this to be?

You're probably looking at 10% for this method.

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Also: If you get a response it's polite to respond to it.

 

[PNachtwey]

What is the vessel filled with before starting?
What is the volume? At sea level air weighs 1.225 kg/m^3
One could measure by the weight change using the density and pressure. This assumes there is only one type of gas inside.

https://en.wikipedia.org/wiki/Density_of_air

Peter Nachtwey
Delta Motion

http://www.deltamotion.com

http://forum.deltamotion.com/

IFPS Hall of Fame Member

 

[mugged]

Thank you all for suggestions, some answers here:

The method of venting a pressurized chamber into another chamber of known volume I am familiar with. Of course logistically that would involve acquiring a tank of some sort. One thing has bothered me about this idea for some time is how does one know what the volume is of the 'known volume' tank? Practically speaking you'd have to rely on / trust the source you purchase it from no?

Regarding the dryness of air, yes my air source goes through a mist separator, something like 0.03 micron filter so it's pretty dry air. I really cant do water since technically the object doesn't belong to me; plus I dont have water measuring equipment.

The idea of weight is interesting. Suppose the volume is 100 cm^3, that would give me a weight differential of about 0.5 grams when I pressurize to 5x atmospheric. I like this idea but you'd need a super accurate scale in my case, would probably work well for bigger volumes.

 

[TugboatEng]

Mist isn't measured in microns...

 

[mugged]

Thats just what the product is called, its like this:

https://www.smcusa.com/products/awm-d-mist-separator-with-regulator-modular~169597

 

[LittleInch]

Hang on. This is 100cc / ml.

Just buy a 100ml syringe and measure pressure.

When you get to 1 barg, measure the air you've injected. You won't be far out.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[zeusfaber]

How about measuring volume of a more volatile liquid? Put everything in the freezer for a while, then use butane?