Need Inexpensive Method to Check Seals with Vacuum

Interesting. Coming at it from a completely different direction from what I was imagining. Worth exploring as an option. I'm at the very beginning of this aspect of the overall design, but figured I'd start thinking about it as well.

But let me try to be clear on the most basic question that I don't know the answer to. And that is... would pressurizing the inside of my canister, and then submerging it, in order to look for bubbles... would that also be a valid test of my seals, even though I'm reversing the direction of my pressure... would that tell me if my seals are good? I ask because checking for bubbles escaping would be a lot easier than checking for internal leakage. And also I could perform the test on the completely assembled unit, with electronics already inside, instead of inserting them afterward.

Okay, cool. Thanx Doorman.

Oops... i didn't see your answer to the question i just asked doorman. Ok. Thank you.

That's great! I talked to the team and we are going to try to build a test chamber. Problem is, I'm great at certain mechanical prototyping, but not so much experience with this specific field, involving air pressures and plumbing. So, I'm going to try to follow your instructions and not blow anything up. So if you wouldn't mind, it would be great if I could refer to you as I go along.

What do you think about the other design that was mentioned here? I'd rather not have to leave it in the chamber for days to get results. The sight glass will be perfect. I'm already buying heavy walled polycarbonate tube for my end cap. I can get some smaller stuff for the sight glass.

And if I can use a hand air pump, that would great as well.

So if you, and any of the old-timers here who read this would like to collaborate on my project, and offer any suggestions or additional details, so that I can start buying parts... that would be extremely helpful. Meanwhile, I'll do a little more research on my own so I can have a better understanding.

My device has grown to 3 1/2" OD at it's widest part. And OAL is now 18". Nothing sticking out the sides except for a small rubber boot for a connector.

Thanx a lot man. This is very helpful.

And to anyone assisting, know that you will be helping in a lot of conservation fish and coral reef research after this project is completed. I'm building this through a grant from National Oceanic & Atmospheric Administration (NOAA).

I got asked to consult on the design of the enclosure itself, which is coming along fine. I'm getting a flat rate which is almost nothing. But it's a great cause, so I'm glad to help them out.

I think the suggestions to use a pressure chamber are right. Pressure inside your device is not the same as pressure outside.

When you have new appliances connected to a domestic gas supply in the UK, the gas engineer does the connection then pressurises the system. They then close the supply valve to isolate the house system & use a sensitive manometer type reader to monitor pressure for a few minutes, don't know the specific device.

In your situation, you would monitor the pressure in your chamber, any pressure drop could indicate leakage into your assembly.

This is not in my wheelhouse, but I will pass it along to our electrical engineer. We don't have a lot of room left inside for additional circuitry. Your suggestion would require another section of circuit board. He can't change the boards he's got as they are already built.

At this point in our development, I think we just need the ability to check for watertightness at the end of assembly, as opposed to leakage in the field. But a re-design further down the road, after we have working units, might include what you suggest. These devices will be in remote areas so I'm not sure we'd be in range of an alarm that would indicate a leak some time after it is placed. But I'll see what he thinks.

Thank you for the suggestion.

Hello,

I concur with the suggestions to use a pressure system that models the installed situation.

I also caution about using air pressue. If the test rig fails, then the expelled air can also take shards of the test rig with it. The suggestion to utilise water volume alone has significant merit as the incompressible water simply ruptures the containment.

Local plumbers and gasfitters here do the static test described by others for new dwellings, tested at 7bar and left for 24 hours. With system properly primed and no air included, even 1 drop shows as observable pressure change.

If you use the rig multiple times, be aware of pressure cycle fatigue potential.

You think just .7bar would be dangerous as far as a test rig failure? I think I'm leaning toward using a high quality bicycle pump to create the pressure. I'd possibly use SCH80 PVC pipe if that was necessary.

As far as fatigue... at this early stage, we'd only be checking two or three prototypes a couple times. And then in a couple of months, I need 20 operational units after prototype. So I would be testing those as a final QA inspection.

Next year is when I'd need a rig for high volume testing, after field tests are complete on the first batch of 20. Perhaps at that point I'd need a more advanced and heavy duty test rig.

sch 40 should be more than enough...12" has a working pressure of 130 psi...

https://pvcfittingsdirect.com/temperature-and-pressure-ratings-of-pvc/

Yeah, that's what I suspected. Okay. Thanx.

Yeah, nice design though...You want cheap, we can do cheap....

Probably just need an aluminum plate in the end to support the pressure....

The leakage will depend on the differential pressure across the seal and may vary with distortion of the body/sealing faces. If you want to check upto 20' water (0.7Bar) you could apply a vacuum to your test port and leave the outside of the assembly at atmospheric pressure.

You then need to monitor the pressure inside the assembly. A sensitive pressure gauge would do this but you could use a u-tube manometer. You would need to connect both limbs of the manometer to the vacuum connection test point on the assembly, evacuate the assembly down to the pressure you want, isolate the vacuum connection and then seal off one side of the manometer with a ball valve or similar to provide the reference pressure. Any leakage into the assembly will then increase the pressure on one side of the manometer and change the levels. The sensitivity will be determined by the volumes on either side of the manometer so a larger volume on the reference pressure side will make the manometer more sensitive as the movement of fluid in the manometer will change the reference pressure slightly. Be aware of temperature changes which might cause the pressure to change in different parts of the test setup.

Fit a catch-pot or similar on the connection to the assembly as you don't want a catastrophic failure of the seal sucking the manometer fluid into the assembly. Preferably use an oil in the manometer rather than water.

This method only works up to about 30' as the maximum vacuum is determined by the atmospheric pressure. Above this you would need to put the assembly in a pressure chamber as well to get sufficient differential pressure.

Thanx Chris. Yes, your suggested method was along the line of what I was originally thinking... using vacuum instead of pressure. I'm getting some good suggestions here from the guys. Hopefully, there will be a consensus on what fits my needs best. I just need to check our few prototypes for now, then 20 initial test units that will go out into the field for use during our initial trial run.

Once full production commences next year sometime. I would likely need a better test rig that could handle a thousand or so. But just need enough to test the seals designs at this point, in the least expensive, but still reliable and safe way possible.

Yes, in my opinion, applying a vacuum to the inside of the assembly will have the same effect on the seal as pressurising the outside of the assembly. You will still have a positive differential across the seal in the same direction in both scenarios.

A cheap boat type inflator could be used as they have an inlet connection for the air and connecting the hose to this will suck the air out. None of them have any technical data but this one from Decathlon in the UK is a bit more expensive than the cheap plastic units and has screw connections for the hoses which suggests a better seal than the plug in types. I would guess you could probably get 0.7 bar vacuum from this but no guarantee.

https://www.decathlon.co.uk/p/stand-up-paddle-and-kayak-double-action-high-pressure-easy-pump-20-psi

Thanx Chris, I'll take a look.

The pulling or pushing effect of an applied negative or positive test pressure may differ, not show the same test results when subjected to your preferred way of testing? The negative or positive behaviors of the rubber seal to be used..

You may have to consider a particular type of gasket /O-ring, together with its density and how the rubber will react to the push/pull action of the selected test pressures.. just sayin...

Yes, I agree. I'll be using an O-ring, I believe. I'm figuring the relatively low pressures we are talking about shouldn't cause all too much trouble with the O-rings with properly designed gland.

I would build a water tank out of pvc to fit the apparatus and fill with water, then you could use a tire inflator to pressurize the vessel to 30 psi, that should be good for about 2 atmospheres....place moisture detecting paper inside the device for watertight status...

There are several ways to design the tank, which to me would be how often and how long you would need it for testing...

Lots of good solutions here.

As for how often, well I need to test our prototype a few times as we build it next week, then I need a total of 20 working units by end of December, after kinks are ironed out after Alpha and Beta testing. So those need checking.

Then, after some number of months next year sometime, we'll be having them produced by ourselves or someone else and hopefully will make about 1000 sometime next year.

But right now, just the prototypes. And a couple dozen on a few months.

And a bicycle pump was exactly what I figured I'd use. Can't do a whole lot of damage at the low pressures I'll be working with I reckon.

Any additional details would be helpful.

I'm liking the version with the clear sight glass, I would think. Just for these prototype tests, anyway.

Also google "MityVac" vacuum pump. It might provide a simple way to draw vacuum inside the component under test. With small control volumes vacuum leak down may be a quicker, easier test than external water penetration. And small enough to take it out on the boat...

Can the assembled device be dropped into an appropriately sized length of plastic pipe with a solvent welded cap one end, threaded cap on the other? An outer watertight sleeve, if you will. That technology is pretty mature and accessible.

Keep the water from reaching any of these varied seals at the intersections of tubes of dissimilar composition. This scheme may reduce significantly the machining for O rings implied as being used.

Are there sensors, antenna, something on the outside of these? What, in general, do they do?

What happens if/when a field guy crossthreads (installs incorrectly in one fashion or another) the cap, regardless of system used? Restated, are you making your removable cap Idjit resistant? An idjit will drop things (like pipe caps) over the side of a sea vessel.

https://fielddata.tech/about/

Here in the group I'm working with. You'll see on one of the pages, a beige cylindrical PVC assemble, which was the original concept prototype. They gave that to me to make a working, water-tight version. As you see, this has no seals at all.

Yes, cross-threading is an issue. To help prevent that, I've made the window cup clear on the sides so the user can at least see what he or she is threading. The window cup will be replaceable in case it gets scratched, which would effect the optics.

It will be for forest/jungle use as well as submersed use.

The case is now made from machined ASA solid rod and also 63mm extruded ASA tubing. And the window cup will be either polycarb or acrylic.

There is only one penetration of the case, on the side, for a rubber boot covering a port of some type. The widow cup has a seal, and then an additional seal will be for the battery chamber that will be attached to the other end. I'll probably use some kind of draw lathes or over-center cam latches on that seal.

There is not yet a vacuum port, unless I add one.

The leak detector circuit will also work for testing.

Be aware that water contains entrained air,and air and water will expand under pressure which can affect readings.

Water will boil at room temperature if in sufficient vacuum.

A temperature/pressure correction must be allowed if you are using a sealed container for testing.

If you want precision,these factors must be considered.

If you wish to use an old fashioned but very accurate analog method of pressure differential,use an inclined manometer that can measure in the 1/100 inch of water column pressure.They can be ordered with a more extended range if necessary.

If you want a near perfect seal,then make the assembly longer than required,and clean and prime and glue both ends closed.If it malfunctions,cut off the extra portion and replace the casing,which is very inexpensive.

Mark a cut line on the case to indicate where to cut without damaging the internals.

Specify mil spec components throughout the circuit board,they are rated from

0degrees C to 100degrees C.(Freeze or boil).

If using DIP chips in sockets,use gold plated bifurcated sockets to avoid connections and corrosion problems.

I have made these sensors for the bottom of digester tanks,which is a very hostile environment with no problems.

For adjustments after sealing,use a magnetically coupled driver and tools for the adjustments,and this will allow for tuning of the circuits without breaking the seals.

These methods make for a very durable and secure sensor array.

Good luck!

Thanx RedNek I'll pass that on to the team. Basically what we need is a GO/NO GO reading.

As I mentioned,the interior moisture/leak monitor could be incorporated into the original circuit design,then the entire assembly can be hermetically sealed with nitrogen gas under pressure, with charge pressure suitable for the designed external pressure.This would give a zero differential pressure between outside and inside of the tube.

With both ends solvent cement sealed,no worry about leaks except where the signal/power cable enters the tube.

A very simple leak detector could be incorporated in original circuitry.

An example circuit below:

The Buzzer output could be used to signal leak to a remote location.

Thanx HTRN. I'll pass this on to our electronics guy.

Just a simple idea which could be used in conjunction with several other suggestions here.

Instead of messing with pressure pumps etc. just put the header tank in a higher room/on the roof so that the head is 20 foot or whatever is required.

Obviously the tap on the flexible hose has to be closed whenever the test tank lid is open.

Thanx Randall