Ideas on how to calculate the volume of a telescoping assembly.

[ENGR]

Hey guys,

First time poster. Glad to have this as a resource.

This is a real work related problem so any help would be greatly appreciated.

I have a closed telescoping assembly made of a round upper and round lower tube. They connect with a seal.

I need to find the precise volume of the assembly when the unit is at full extension (about 1000ml) and when it is at full compression, (about 300ml). So, volume 1, (V1) volume 2 (V2) and the delta between V1 and V2.

Obviously, V1P1=V2P2 applies.

Due to the complexity of the assembly, I can't measure the internal volume of it by putting anything but air (gas) into the unit.

My idea was to attach a sub tank that I know the precise volume of. Then connect a gauge to the sub tank and close the sub tank off from the unit I want to test. Then charge the sub tank to a given pressure, then open up the valve (connecting the two) and note the change. Run this test with the unit at full extension then again at full compression.

Does that seem like the best and most accurate way to get the numbers I need?

Thanks in advance. I'll check back often to answer any questions.

VOL SAMPLE.jpg

[JAlberts]

You will also have to measure the air temperature during the test and allow enough time for the air temperature to restabilize back to the original tank temperature after the air injections this procedure should work but its accuracy is fully dependent upon the accuracy of your tank measurement and the pressure gauge installed.
One issue that srikes me regarding the test versus the application of the unit is that, assuming this unit is going to be utilized for gas operation then the pressures in the cylinder at the open and closed positions will not be same due to the change in cylinder loading to extend the unit under load, so your test will not reflect the actual gas volume consumption between the open and closed positions in the cylinder application(s).

[ENGR]

You will also have to measure the air temperature during the test and allow enough time for the air temperature to restabilize back to the original tank temperature after the air injections this procedure should work but its accuracy is fully dependent upon the accuracy of your tank measurement and the pressure gauge installed.

Thanks!!!

Temps are held very constant, but I see your point as that could effect the reading.

One issue that srikes me regarding the test versus the application of the unit is that, assuming this unit is going to be utilized for gas operation then the pressures in the cylinder at the open and closed positions will not be same due to the change in cylinder loading to extend the unit under load, so your test will not reflect the actual gas volume consumption between the open and closed positions in the cylinder application(s).

I think I may not have explained some of the details correctly. The telescoping cylinder remains sealed (closed) during operation and no sub tank is involved.

The only purpose of the sub tank is to take a ready in a disconnected and connected state in order to establish a volume read on the assembly.

The test would look like this.

1. Measure the volume of the sub tank and hose.
2. Connect the sub tank to the telescoping assembly.
3. Close the valve between the two.
4. Charge the sub tank to a given pressure.
5. Make sure the telescoping assembly is locked at full extension and at zero pressure.
6. Open the valve.
7. Note the shift in the pressure drop on the gauge. Use the difference in pressure readings to calculate the volume of the telescoping assembly.

Test 2
1. Bleed off all pressure.
2. Fully compress and lock telescoping cylinder. Bleed off pressure.
2. Close valve.
3. Charge sub tank to test pressure.
4. Open valve.
5. Measure pressure difference to calculate volume.

[JAlberts]

Your test procedure is exactly what I would have expected; but, one thing I failed to include in my response is to remind you to use absolute pressure values in your PV = PV calculations, i.e. the initial pressure in the cylinder is: P cyl = 14.7 psia; and, your initial pressure in the tank is: P tank = P gauge + 14.7.

As to my second comment, I was simply questioning how your test volume measurements might relate to the actual cylinder field operating conditions.

[ENGR]

Your test procedure is exactly what I would have expected; but, one thing I failed to include in my response is to remind you to use absolute pressure values in your PV = PV calculations, i.e. the initial pressure in the cylinder is: P cyl = 14.7 psia; and, your initial pressure in the tank is: P tank = P gauge + 14.7.

As to my second comment, I was simply questioning how your test volume measurements might relate to the actual cylinder field operating conditions.

Ah...you bring up a good point.

If I fill the external/sub tank to a given pressure, (let's say 60PSI) then open the valve to connect the two volumes, then take a read, I'm adding (connecting) 60 PSI to 14.7. Well...so to speak.

In other words, I'm not filling a chamber that is completely void of any air pressure, and that needs to be calculated for.

[JAlberts]

The key issue is that in the PV = PV calculation 14.7 must be added to the tank gauge pressures; or, simply put, the correct formula for your tests is: (P gauge + 14.7)V = (P gauge + 14.7)V.

Also, just to be safe,I remind you that in both tests the total volume calculated after filling the cylinder is the sum of the cylinder volume plus the tank volume so you need to subtract the tank volume to obtain the correct resulting cylinder volume.

[fredapinto164]

Telescopic cylinders are a special design of a pneumatic cylinder that provides an exceptionally long output travel from a very compact retracted length. Typically the collapsed length of a telescopic cylinder is 20 to 40% of the fully extended length depending on the number of stages.