Watertightness - spray testing: The physics of leaks
Watertightness - spray testing: The physics of leaks
(OP)
A common method of demonstrating that a thing is "watertight" is to spray water on it.
Typically, there is a (seemingly arbitrary) set of requirements for water pressure, water flow rate, water velocity (usually not specified where the velocity is to be determined), and sometimes nozzle distance from the thing under test. For example, MIL-STD-810.
But what determines the necessary values for these parameters?
Or, the actual point of my question - if a certain test facility can't simultaniously meet all of the required parameters, what compensation might be made to allow for an "equivalent" test.
Is a 20 minute test with a nozzle velocity of 100 ft/min and a flow fate of 100 gpm "equivalent" to a 10 minute test at 200 ft/min and 100 gpm?
What is it that actually challenges a seal and causes a leak?
My thoughts are that you need to consider the kinetic energy impinging on the surface under test, but I'm not certain that's sufficient.
It's easy to imagine a pressure washer causing a leak that a garden hose wouldn't. But it's also easy to imagine something that would never leak when subjected to a garden hose - no matter how long you sprayed it.
Your thoughts on the subject would be welcomed, and actual references even more so.
Typically, there is a (seemingly arbitrary) set of requirements for water pressure, water flow rate, water velocity (usually not specified where the velocity is to be determined), and sometimes nozzle distance from the thing under test. For example, MIL-STD-810.
But what determines the necessary values for these parameters?
Or, the actual point of my question - if a certain test facility can't simultaniously meet all of the required parameters, what compensation might be made to allow for an "equivalent" test.
Is a 20 minute test with a nozzle velocity of 100 ft/min and a flow fate of 100 gpm "equivalent" to a 10 minute test at 200 ft/min and 100 gpm?
What is it that actually challenges a seal and causes a leak?
My thoughts are that you need to consider the kinetic energy impinging on the surface under test, but I'm not certain that's sufficient.
It's easy to imagine a pressure washer causing a leak that a garden hose wouldn't. But it's also easy to imagine something that would never leak when subjected to a garden hose - no matter how long you sprayed it.
Your thoughts on the subject would be welcomed, and actual references even more so.





RE: Watertightness - spray testing: The physics of leaks
RE: Watertightness - spray testing: The physics of leaks
By the way your phrased your question, I'm assuming keeping water out of an area. In that case, I agree it has to do with the kinetic energy imparted on the surface. I'd also imagine that there is some normalizing factor, so that "equivalent" tests may be performed.
V
RE: Watertightness - spray testing: The physics of leaks
Having said that, what is the core of your problem? A watertight tank/pipe should have different tests than a water tight pump volute, a coating or a cloth. On a tank you'd want the pressure of the fluid to act for quite some time to detect leaks.
But on a strictly spray scenario, I'd have to think that velocity is the determining factor unless there is a wetability component to the material being sprayed. If with time and as TheTick mentions, capillarity could play a role, then you have time to consider as well.
<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying " Damn that was fun!" - Unknown>>
RE: Watertightness - spray testing: The physics of leaks
RE: Watertightness - spray testing: The physics of leaks
So, for a rigorous test, you need a pressure spray test as well as a static temperature cycle test.
TTFN
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RE: Watertightness - spray testing: The physics of leaks
Bigger issue is what the customer would say if you propose to take exception to the spec.
RE: Watertightness - spray testing: The physics of leaks
RE: Watertightness - spray testing: The physics of leaks
RE: Watertightness - spray testing: The physics of leaks
As such I would be very wary about trading off between the variables in a spray test without some form of comparison test.
Cheers
Greg Locock
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RE: Watertightness - spray testing: The physics of leaks
Dynamic pressure X mass flow rate returns a unit of power.
Perhaps two test setups with equivalent power would be equivalent for an equal duration.
RE: Watertightness - spray testing: The physics of leaks
Mike Halloran
Pembroke Pines, FL, USA
RE: Watertightness - spray testing: The physics of leaks
Dynamic Pressure X Volumetric Flow Rate => Power
Does trading velocity and flow with equal power being considered an "equivalent" test make sense?
RE: Watertightness - spray testing: The physics of leaks
Although, thinking about it practically--I don't know if they're equivalent. Something feels different to me between water at a higher velocity with a lower volumetric flow rate versus a lower velocity with a higher volumetric flow rate. Seems to me the higher velocity would do more damage. That's just my feeling though.
V
RE: Watertightness - spray testing: The physics of leaks
<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying " Damn that was fun!" - Unknown>>
RE: Watertightness - spray testing: The physics of leaks
Hmmm... Interesting problem.
V
RE: Watertightness - spray testing: The physics of leaks
A lip seal would seem vulnerable to high kinetic energy - causing deflection of the seal and opening a gap.
A compressed gasket may be more sensitive to capillary action and the wetting properties of the fluid on the mating surfaces.
A labyrinth might be sensitive to both kinetic energy and individual droplet size.
Since my TUT has all of these seal types I can't ask for different tests for different seals - well I could, but I'd get flack for it, and at the moment couldn't justify it.
Also, my original test parameters are "historical" - which means nobody knows how or why they were selected. Most likely they were what some test facility that no longer exists was capable of doing.
It is an interesting problem however - might be a paper topic for some obscure conference...
RE: Watertightness - spray testing: The physics of leaks
I also just reciently sat through a Design News web seminar on sealing/venting electronics. Was pretty informative (although was 70% product pitching). The jist is that without vents a completely sealed product can go through a rigorous even if not intended change in atmosphere. So much that the pressure differences in the enclosure and outside of it will cause an almost vacuum like effect and actually suck in moisture past even the best seals.
The idea of a vent that would allow a vapor/moisture guard, but still be breathable to atmosphere would take almost all of the stress off of the seals. Allowing them to be much more efficient since they don't have to seal against differentials in pressure.
James Spisich
Design Engineer, CSWP
RE: Watertightness - spray testing: The physics of leaks
As an example. There is a test for packaging called the ISTA test. One of the tests is to drop a weight from a certain height onto the center of a long box supported on both ends. A little bit of physics says you have a shorter drop with a bigger mass and have an equivalent test. This should give the same energy and the same results.
However, a box that will pass the original test will fail the equivalent because it can not support the larger mass that is resting on it after the drop.
In a similar way the water my drain off at a certain rate so that a different flow rate would create different levels of water a each location. A seal that is only wet in one test cloud be submerged by another.
(However this is still a very interesting physics problem)
RE: Watertightness - spray testing: The physics of leaks
RE: Watertightness - spray testing: The physics of leaks
V
RE: Watertightness - spray testing: The physics of leaks
You don't need a 'vent' to eliminate pressure/vacuum if you use a diaphragm, therby maintaining the 'atmosphere' within the unit. Obviouisly designed large enough to cope with temperature changes involved.
B
RE: Watertightness - spray testing: The physics of leaks
James Spisich
Design Engineer, CSWP
RE: Watertightness - spray testing: The physics of leaks
The box example second test failed because the Supports weren't strong enough, not the box (it survived the falling weight).
Anyhow, getting back to what I was saying. The test should test the seal effectiveness of the seal compound or material, as you have mentioned. So, in your case, a baseline test would have to be designed based on the sealing compound or sealing material's typical or actual use and presumed light, normal and heavy water conditions.
RE: Watertightness - spray testing: The physics of leaks
The pressure spray environment would occur as part of cleaning or decontamination, which may not be part of your specific UUT. The dunk test would apply to UUTs that might get submerged, like a military vehicle that must ford a river or shipboard equipment that might get overwashed with waves. These environments are all "soft" and uncontrolled, so many of the tests might well be "legacy", based on some historical or even mythical conditions.
Interestingly, the one test that isn't performed, is the "moisture getting sucked in by a pressure differential trick." We had a camera system that did that so well there was water halfway up the camera aperture.
TTFN
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RE: Watertightness - spray testing: The physics of leaks
I guess it didn't occur to the military that something that is actually hot inside when operating might get cooled when it starts raining.
At any rate, my test is not MIL-STD-810 per say, simply a similar generic spray test of unknown and likely arbitrary origin.
RE: Watertightness - spray testing: The physics of leaks
RE: Watertightness - spray testing: The physics of leaks
Will you be varying the volume and rate of the water water by the size opening of nozzle or at the pump producing the water's force?
RE: Watertightness - spray testing: The physics of leaks
i am a little bit off topic here , but in case people want to look at leak testing for tanks , vessels , boxes etc
we often use compressed air on one side of the test object , and spray soapy water on the other side , it sometimes allows testing where hydrostatic or chemical penetrants like dyecheck cant be used.
either pressurise the vessel with 2 - 3 PSI , or run a blowgun along the internal seams while spraying soapy water outside.
hope its useful to someone :)
RE: Watertightness - spray testing: The physics of leaks
The test arrangements I encountered when working with automotive door seals made me believe that in order to assure leakproof joint one needed to have jets from multiple angles, volumes, and pressures. One of the more interesting stations resembled a car wash with the nozzles moving the streams in cone shaped paths to cover all angles from which water could hit the vehicle. Looked a lot like a carwash with the nozzles mounted on the top and each side of the carriage that usually carries the brushes. IIRC, the techs said that the velocities had to simulate 120 mph. That seemed unreasonably high until looked at in the perspective of 70 mph auto into a 50 mph wind.
Griffy