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Author: Subject: Leaking desiccator
Fyndium
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[*] posted on 18-8-2020 at 13:19
Leaking desiccator


My Nanshin desiccator appears to be leaking somewhere, but I'm not able to determine where. I greased the surfaces and the stopcock joint and the valve, but when evacuating, the pump just keeps pumping, and with control (close the valve) the pump quickly reaches full vacuum by sound(stops rattling and just hums quietly) so the pump or the connection is not faulty. So the leak is either in the stopcock joint or the perimeter surface, which does not seem to be fully flat when mating the pieces. I don't see any visible streams of gas on the greased surfaces through the glass, but it leaches still. I even put a couple of layers of tape around the joint, but it still leaks. The device keeps the limited vacuum only for a very short time, max few minutes after closing the valve.

Any ideas? If I spray some soap water on the joints when evacuating, it should permeate through and form bubbles, as this is a good method to find leaks from positive pressure containers?
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Dr.Bob
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[*] posted on 18-8-2020 at 15:53


Is this a glass desiccator? I've never seen one leak, and good ones will hold a vacuum for quite a while if pulled down well, greased, and has a good dessicant in it to absorb water. That assumes no volatiles, as any trace of solvent, low boiling materials, or water will create a substantial vapor pressure.
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Mateo_swe
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[*] posted on 19-8-2020 at 22:28


I would use soap or any bubbling substance and put on the joint then applying pressurized air.
Or put the soap on the inside joints and apply vacuum.
If you see bubbles forming, that is the leaking area.

I also have a china glass desiccator that doesn't seem to hold vacuum very good.
But i haven't tested it with any good vacuum yet so i really cant tell for sure if its leaking yet.
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wg48temp9
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[*] posted on 20-8-2020 at 01:11


You can check how well ground glass joints mate by lightly smearing one of the surfaces with a brightly coloured lipstick or engineers blue then mate the joints with rotation. Any slight gap in the mating surfaces at any position can be seen by the more intense and opaque colour.

Any rocking on a tapered joint is also indicative of a problem. That is an all to frequent problem with glass I have purchased from China. Possible because I purchased cheap unbranded items that are actually rejects sold by unscrupulous sellers.

[Edited on 8/20/2020 by wg48temp9]




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Fyndium
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[*] posted on 21-8-2020 at 00:34


I should see into it. If it is leaking from there, I consider just putting some silicone caulk into the joint. The joint definitely has some little play in it.
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Heptylene
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[*] posted on 21-8-2020 at 11:32


Try spraying some coloured water on the dessicator while it's under vacuum. The water will get sucked in the leaks and should make them visible.
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Mateo_swe
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[*] posted on 21-8-2020 at 13:38


I think the lab glassware i buy from china have been of worse quality lately, say than what i got a year or 2 ago.
Now many of the pieces actually look like quality rejects from the glassblowing factory.
I got glassware with some small blobs of glass fused to outside or bottom of flasks.
One erlenmeyer flask had one sharp glass piece fused to the bottom that i actually cut myself on dripping blood everywhere.
And those autoclavable reagent bottles with blue caps, latest buy these had uneven glass thickness making them look visually really bad, although nothing wrong with the function.
These reagent bottles usually is flawless, but not in my last order.
In my latest orders i had at least 1 to 3 items on every order where i had to ask them send new because of damage or quality issues.
2 years ago it was very rare items were damaged or bad quality.

If a vacuum desiccator has little play in the ground joint that makes it leak you should ask them to send you a new working desiccator.
If it cant be used for the intended purpose its made for you should ask for a replacement.
Glass desiccators are quite pricey, especially the bigger ones and they should be working OK.
There should not be a unit that works bad, even if they are cheaper than at home and made in china.
My desiccator dont have any detectable play in the joints, it just dont hold the vacuum for long time.
But as i havent tested it with a decently strong vacuum yet it might be good.
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mackolol
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[*] posted on 18-12-2020 at 05:40


Hello, I have a problem with my leaking desiccator too. It's the first time I'm using one and apparently I'm too retarted to use such a simple "device".
My problem is, that I have used petroleum jelly "vaseline" and now I'm using some silicon grease from some construction shop and I just can't get the two layers evenly greased. I take the grease, spread it evenly on each part, but when I link them, there always is some space between where the pressure just leaks. I can't get them to be completely evenly greased when I connect the two parts. Do I use too much or too little grease? Or maybe my grease is just bad?
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Dr.Bob
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[*] posted on 18-12-2020 at 12:34


I would clean the entire surface and wipe with hexanes or DCm to clean the grease, then go back with the best grease you can find, Not sure if silicone grease for construction is OK for vacuum, may have a lot of volatiles. For a desicattor that has a non-flat surface, you may be able to grind down the surface flatter by either using lapping compound on the joint and rotating it like making a lens. Or you could use 400 or 600 grit SIC sand paper and put it on a very flat surface and try to grind the joint flatter. A cast iron table saw top is good for that. But if they are that bad, I would ask for refund and buy a used Corning one. They are great, well made, and last for years if you don't break them, which takes some work.
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