Volume of Item being Tested by a Vacuum Tank

[scott3229]

HI All, I am looking for some help on a vacuum testing application.

We have an application where we put sealed flexible plastic pouches into a vacuum tank that is filled with water. When you close the lid, it forces the pouch under the water, and then we slowly evacuate the air from the tank and once we begin to see bubbles forming at the edges of the seals. We record the in-hg that the pouch failed at.

My question is this.

Does the volume of the sealed pouch matter when we test it. Lets say we put the pouch in completely pressed flat and sealed it closed with barely any volume inside the pouch as opposed to giving a light breath of air to "expand" to the pouch then seal closed, not pressurized, but just opened up so that the volume of air inside the pouch is large, then allowing it to equalize to atomospheric pressure before sealing it closed

Assume altittude and temperature are consistant.

My thought is that because the volume of the air inside the pouch will expand as the pressure differential increases, the volume of the pouch DOES make a difference because the volume of air inside the pouch will almost double as the tank pressure drops in half as we evacuate air which would in turn drop the internal pressure of the pouch if there is room for that air to expand which is what is ultimately trying to force the air out of the seals.So as that air expands the pressure drops, which would mean ultimate the vacuum would have to be much greater to have that pouch fail than, if the pouch is already filled to the maximum volume of air before being sealed. Where in that case the tank pressure drops but there is no where for the air to expand because the pouches volume is already near maximum before seal, forcing the pressure to be maintained in the pouch, and exerting a larger force on the seals.

Am I on the right track with this though process?

 

[BigInch]

I think you would be a lot better off if you ditched the water tank, soaped the seals and slowly filled the pouch with air. Read the air pressure inside the pouch from a gauge on the filling tube when you see the soap begin to form bubbles.

 

[scott3229]

BigInch,
Thanks for the feedback, unfortunately, this is a test that is accepted and verified by our customers really as an industry standard. We do some pressure testing, but that testing is geared towards the tensile strength of the seal, and its bursting capacity as opposed to its leak resistance.

 

[BigInch]

I was afraid that you would say something like that. I feel that it makes a large difference how much air you have left in the pouch when starting the test, yet it seems that measuring that initial amount of air is not part of the standard. Therefore it can't be much of a good standard. No doubt you are asking this question because you have realized that of course the amount of air in the pouch makes a difference yet your standard does not require you to standardize that volume. I imagine that you have noticed that, if there is no or little air in the pouch, you can reduse tank pressure a whole lot before the seals pop.

If only a little bit of air is in there, so the pouch is flacid when beginning the test, not prestressed at all, the tank pressure would have be reduced quite a bit more before the pouch began to expand at begin to place stress in the seal than if a lot of air was present in the pouch and it immediately began to expand and place stress on the seal. If the pouch was full enough initially to immediately begin to develop seal stresses when the tank pressure was reduced just a little, the pouch seals would be set to pop at the next little bit of pressure reduction.

For that to be a valid test of sealing, or pouch stress or whatever, you must always be able to discern the difference in pressure between the air pressure inside the pouch and the tank pressure outside. That you know is zero until the pouch (and seal) begins to stress. After that point you no longer know the air pressure inside the pouch, because the pouch film tension and seal strength cause the pressure in the pouch to differ from tank pressure. But (presumably) you don't know the pressure inside the pouch and therefore you are no longer able to determine the pressure in the pouch minus the pressure in the tank and thus lose your reference to judge seal strength based on that difference in pressure.

 

[LittleInch]

"We do some pressure testing, but that testing is geared towards the tensile strength of the seal, and its bursting capacity as opposed to its leak resistance"

Sounds to me like the same thing...

Yes initial volume (mass) of air in the pouch will make a massive difference.

The only plus point of the vacuum test is that the max diff pressure you can get is 1 bar assuming the pouch is simply filled with air at atmospheric pressure, whereas the "pressure test" can clearly go to many more times than 1 bar.

Without a standard volume in the pouch, your "test" is fatally flawed IMHO, as your reading of the reduced pressure is not going to be the same as the differential pressure in the pouch and hence the test is not repeatable.

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

 

[BigInch]

Exactly. That "industry standard", if it does not address the initial volume and initial pressure in the pouch, could not be anything more than rubbish.

 

[scott3229]

"Without a standard volume in the pouch, your "test" is fatally flawed IMHO, as your reading of the reduced pressure is not going to be the same as the differential pressure in the pouch and hence the test is not repeatable."

"Exactly. That "industry standard", if it does not address the initial volume and initial pressure in the pouch, could not be anything more than rubbish."

I would absolutely agree with both of you, this is where I am going with this argument internally, I just needed some validation because the vacuum testing equipment actually comes with the attached chart in their manual, and I think we have technicians that are taking this as the gospel. I thought this chart was completely erroneous based the exact conversation we are having. The chart makes no reference to volume or % of volume, starting pressure etc, and therefore I find it to be flawed as well.

Fundamentally the issue is when our customers validate the pouch using the same test. The volumes need to be the same as when we test it here. Otherwise I have no doubt we could pack the pouch with air and get it to fail, and conversely we could lay the pouch in the tank with less than 1% of it potential volume and get a pouch that would normally fail, to pass.

Thanks for the feedback!

 

[Compositepro]

That chart is pretty useless for you application. It is simply a units conversion chart that assumes that barometric pressure is 760 torr. The pressure inside the bag will equal what is outside the bag only until the bag is fully inflated. At that point the film, and seal, will begin to be stressed and the internal bag pressure will drop below external pressure. It is this pressure difference that is stressing the seal.

Yes, the initial volume of air in the bag is a very important factor in this test, and the procedure you described is almost worthless. If the seal strength is low, then the test simply measures how much air was in the bag. If the seal is really strong you really have to actually measure or control the internal bag pressure during the test, as it would be difficult to calculate it accurately.

The test would be somewhat more meaningful only if the bag were fully inflated with air when sealed or connected to an outside air source. The actual size of the bag is also important. A large bag will burst at a lower pressure than a smaller bag.

 

[BigInch]

CompositePro, No offense, but I think you could just say that you agree with the discussions above, no? Except for the part you mentioned about the internal bag pressure, which has to be higher than the pressure in the tank in order for it to begin stretching, not lesser as I think you said. It is the external pressure that is being reduced. The bag's internal pressure would equal the pressure in the water tank until tank pressure is reduced enough to stretch the bag.

 

[scott3229]

CompositePro,

I agree with the big size. I hadn't even brought that into this discussion, but that is the other factor. The square area of the bag surface that pressure is being applied would certainly affect the force that is being exerted overall on the seals. That's a point I made a while back here. I think of it much like a standard air cylinder, where SqIn x psi = cylinder force in lbs. Thinking of the pouch walls as the cylinder piston. The only caveat to that is that the amount of force that is being exerted on a pouch with a larger surface area will always also have a more linear inches of seals that the force is being distributed upon, so it clouds that up a little bit.

I do also agree with BigInch your reducing external pressure, the bag will never drop below external pressure, it can only be equal to or greater than external pressure given the testing application. And that will only happen once the bag is filled to capacity and the pouch volume can no longer expand. Even at that point the pressure in the bag will essentially begin to plateau, and it will be the pressure differential between inside and outside of the bag that cause the failure as atmospheric pressure is removed around the pouch.​

 

[Compositepro]

Yes, the internal bag pressure will be higher than external, my mistake. But the other points I mentioned are also important; there will be no pressure difference until the bag is fully inflated. This makes the chart completely misleading.

 

[BigInch]

Oh good. We do agree completely I think.