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I need to connect a 3mm inside-diameter tube from a solenoid valve to a plastic box. The box wall thickness is 4.5mm. The purpose is to empty water in the box when needed.

How should I connect it so that it is water-tight (very low pressure - around 70mm water depth)?

Photo:

The photo shows the valve and the plastic box.

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If you don't care it being a little flimsy and just want quick, and the plastic is not too brittle:

Use a barbed fitting with a straight thread on the outside, e.g. #10-32 of M5x.8 (these sizes happen to be interchangable), and a sealant.

SUMMARY:

drill 4.2mm (work up size in small increments to avoid cracking)
tap hole M5x.8 (or #10-32) (with great care, again b/c cracking)
install fitting M5x.8 (or #10-32) x barb
   (type intended for 3mm (or 1/8" = 3.2mm) tube)
add silicone sealant (but make sure gasket is not silicone)
   (may need to abrade and clean surface to get sealant to stick well)

Illustration of fitting type ... you can probably find something for $1 if you try


If the plastic is brittle, as is pretty common for optically clear materials, a glue-in barb fitting or a bulkhead type retained by a nut on the far side, should be used instead.

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    $\begingroup$ Never realized 10-32 and M5-.8 were (nearly) interchangeable. I wonder how many headaches this has caused. $\endgroup$
    – jko
    Apr 6 at 14:40
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    $\begingroup$ @jko It made me learn to always check threads against each other instead of using nuts. Tried to check if threaded rod was metric or imperial, threading a metric nut on each of them felt the same. But then later when it came time to screw the rod into a threaded hole it would only go a few turns, so I wouldn't quite call them "interchangeable" (though it will work well enough in some cases, probably including this one) $\endgroup$
    – llama
    Apr 6 at 20:15
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    $\begingroup$ this one is hard to tell, 25.4/32 = 0.794mm pitch vs 0.800mm pitch ... difference of 0.001" every 4 threads $\endgroup$
    – Pete W
    Apr 6 at 20:36
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If you don't want to drill a hole on the box and mess around with nozzles and sealants, a solution would be to siphon water like

enter image description here

The only difference is that that valve is going to be at the bottom side.

The only "problem" is that you need to fill the hose. A common solution to that is

enter image description here

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    $\begingroup$ Nice layout. Personally I'd drill a hole and install a receiver for the tube (and seal it etc), but the OP can't go wrong either way $\endgroup$ Apr 6 at 13:31
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    $\begingroup$ The obvious choice is drilling. This is just something quick that can be done if you don't have need a long term solution. However, if a long term solution is required I would buy a plastic box with a preinstalled tap. I think just looking for nozzles, sealants and drills is going to cost a lot more. $\endgroup$
    – NMech
    Apr 6 at 13:35
  • $\begingroup$ @NMech - yes I think drilling is a good solution, but I hope it would be long-term. I did try a submersible pump but I got drain-back of water in the tube after pumping (I am weighing the water after emptying it, so don't want errors creeping in). So my question is now - where do I buy a tube to insert into the drilled hole, and how to seal it? Not sure if this is aquarium shop stuff or similar! $\endgroup$
    – keble
    Apr 6 at 13:58
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    $\begingroup$ Following on... Gravity is your friend. In @NMech's first drawing, if you want to "turn off the drain" put a simple rigging on the side of the box to hold the tube above the level of the water. This can be as simple as a small two-ended hook (one end around the edge of the box and the other onto which you rest a portion of the hose (it need not be the end of the hose, as long as the end of the hose is out of the water (so it does start siphoning by accident). If you do it right (by keeping enough water in the hose below the hook), siphoning will start as soon as you drop the end in the bucket $\endgroup$
    – Flydog57
    Apr 6 at 21:57
  • $\begingroup$ @keble I just noticed the weighing part. Why do you need to do that? Is it for keep the box full, or is it for a measurement? If its the first then there are other ways to ensure a water level, if its the latter (measurement) then drilling or replacing with another box with a preinstalled hope/adapter is probably one way. $\endgroup$
    – NMech
    Apr 7 at 5:15

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