How to check the vacuum correctly


I am doing the vacuum test of the enclosures, I make the vacuum reach 15 inHG and every 10-15min my pressure drops 0.3/0.4 inHG according to your page I would be doing the right thing. “Monitor the gauge for about 10-15 minutes and make sure the vacuum pressure does not decrease by more than 0.5 inHg.” But I have some questions:
-Is it normal that every hour you lose 1 inHG until you lose the vacuum completely in one night?

I have not done the test of closing the vacuum with a valve and removing the vacuum gun but I understand that the mityvac gun is losing the vacuum and not the enclosure, is this so? So, always doing the 15-minute test (- 0.5inHG) could I already be sure of the immersion?

Another thing regarding the mityvac pistol, it has a very limited use, even making all the recommendations to tighten as close to the lever point, after it broke I was looking and there is the same metal pistol “MV8510”, I have already bought it and It has a much longer life than PVC, after using PVC and getting the metal gun, I don’t understand why you don’t use this model.

Thank you!!!

If you see any pressure drop, even very small pressure drop, even over 24 hours, means you have a leakage.
These leaks may be very small and very hard to detect, but it’s still a leakage and the deeper you go the more apparent that’s going to be.
We experienced the same problem we had a very small pressure drop but we could not detect any leak using the finder and searching for bubbles inside the canister. The leak was so small that for most cases it only led to fog inside the canister and also in front of the camera. This happens normally at shallow dives down to 10 m. When we performed a deeper dive to about 50m we got water inside the canister.
As we could not detect any leak by using League find her and looking for bubble inside the bottle while the bottle was under vacuum, we reversed the process, putting over pressure inside the canister. We applied about 0.2 bars and applied leak finder on the penetrators on the back plates of the canister. Be mediately observed bubbles arising from one of the penetrators and could confirm that leak was between the cable going into the penetrator and the resin used to pot the penetrator. Please be aware that adding over pressure inside the canister made lead to catastrophic failure. We did a test on an empty canister and the canister dome popped when we reached a pressure of 0,24 bar creating a bang and a crack in the canister. Use a jig to secure the end lids.
We have experienced leaks several times, all of them via the penetrators.
Unfortunately for us this leak was very hard to detects because the small size of the leak. We believe that using hard marine resin to pot penetrators is a poor solution, so avoid any penetrators of such kind. use a proper potting solution or mechanical seal.


Hi @jua123

Are you doing the vacuum test with the tether connected?

Through the gaps between the twisted pairs of the tether, air can enter from the surface very slowly.

To be sure and know if the pressure is lost through the tether, you can try to replace the tether with a blank penetrator and do the test again.



I had an enclosure with all its connections, I did a vacuum test with its WetLink cables and other types of connectors, every 15 minutes it loses 0.5inHG or less, until I lose the vacuum, but, I have done a test with the enclosure all clean, all greased to perfection with Molikote, I have put a single cover with a single hole where I make the vacuum and it continues to lose vacuum, with a new enclosure without using it, that’s why my question, to you with the machine vacuum Mityvac maintains the vacuum for a whole day at the same pressure? Because I think it is impossible for an unused enclosure, all clean, recently lubricated, to continue losing vacuum.

As for the connectors, it is clear to me that it is not the error, I use WetLink and when I vacuum, I vacuum the two interconnected enclosures, that is very clear, but if I take a new enclosure with its joints recently greased, only With a well-tightened and lubricated hole, I don’t understand how it continues to lose vacuum at the same rate as if I had all my connectors on. I have used two different guns and the same thing keeps happening, that’s why I wonder if it’s the gun that loses vacuum or I have a problem, but I don’t know what problem is happening.

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Hi @jua123,

Did you test the setup before trying to test the enclosure(s)? If the setup has a leak then that will show up even if the enclosures are fine.

The vacuum pump with the seal placed directly as you show in the configuration does not lose anything in the 30 seconds to 1 minute that you recommend, but if I leave it longer if it loses vacuum, at the same rate that it loses if I connect it to the enclosure with a single hole the same as the one in the photo, This occurs in two different pumps, one that you sell and another that I bought made of metal like the one in the photo, This means that the pump does not maintain a vacuum for a long time but According to you and the other people who comment, this should not be happening.

At this point of knowing that the pump you sell loses its vacuum, following your step-by-step manual, I don’t understand how I can get to know if my enclosure is totally safe, (well, actually yes, I could cut it off with a valve and a manometer and isolate the pump, but I want to follow your steps) now I want to return to the initial question, according to you
“4. Monitor the gauge for about 10-15 minutes and make sure the vacuum pressure does not decrease by more than 0.5 inHg”

Perfect! I meet this requirement but when 50 minutes pass I have increased the loss from 0.5 inHG to 1 inHG, the error increases over time (the pump loses vacuum) but this does not appear in your guide and according to what I am reading this should not be happening, I am using the same tools as you, I just want to find the logic to be able to be sure that my enclosure is not going to have any problem, if I am using the same tools as you, if I am using the same enclosures as you but the result of the empty is not the same following the indications that you give, the magic question is, What am I doing wrong to not have the same result as you?

Thank you!!!

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I’ve brought up your questions internally and been told the following:

  1. The cables going into an enclosure (from accessories and tether) will generally allow some amount of air into the system during a vacuum test, which is why we have a 10-15 minute time recommendation for the test
    • This should only be noticeable if there are actually cables going in, so shouldn’t be the case for a sealed enclosure with a single entry point
  2. If you’re concerned about a particular part of the system you can try to test it in isolation, and add components from there until you can replicate the leak (if there is one)
    • For example, to determine whether the pump is leaking you can stopper it directly and see whether it maintains the pressure