Sealing the aquarium lights - Page 1
Forum section: electronics / projects

author: akis
author_rank: Frequent Contributor
timestamp: December 04, 2015, 01:10:54 pm
content: I have constructed custom aquarium lights made out of a clear acrylic tube inside which I passed a bright white LED strip. I closed both ends with plastic caps that fit perfectly and tightly. I also used some aquarium sealant (like bathroom silicone sealant, only safe for fish). However after 6 months I have noticed water ingress inside the tube. The water seems to be entering from the small hole I made on the cap to pass the power lead - a 3mm hole that *just* allowed the wire to pass through. Even though I had put copious amounts on the inside of the plastic cap to make sure it is sealed.When taking it apart to dry it, I realised that the sealeant has not glued on to the acrylic tube or onto the cap, so it was dead easy to remove the caps.The water comes in from the cable hole, and most importantly probably goes out again and could contaminate and kill my fish (fish are very, very delicate).So how can I seal that cap? I could try using ordinary bathroom silicone sealant but thtat would probably kill the fish. As the wire is quite tight though the hole, there is no space for any adhesive, other than superglue maybe? Should I try superglue maybe? Once set it should be safe for fish I suppose. But superglue is not flexible and everything else is (tube, cap, cable, supports, water wave action etc).

author: Ian.M
author_rank: Super Contributor
timestamp: December 04, 2015, 01:58:27 pm
content: If the light isn't vented, temperature changes will put a lot of pressure on any seals. A push-in bung will leak air out with only moderate internal pressure. Simply switching the light off and letting the air in the tube cool could result in a partial vacuum giving a pressure differential equivalent to several meters of water depth. Obtaining cable with a capillary vent tube in it would go a long way to alleviating your issues - or simply run ordinary insulated wire through a length of aquarium air hose.Silicone sealants don't bond well to surfaces that aren't chemically clean and don't bond at all to any plastic that has a waxy surface feel. Fish-safe epoxy putties like Milliput bond better but still aren't reliable on waxy plastics. Most Cyanoacrylate glues (superglue) don't have long term water resistance and their hydrolysis products include Formaldehyde. A tiny amount of your fish safe sealant on the cable, backed by a 1/2 depth Milliput plug in the end cap which must be freshly sanded on the inside, backed by hotglue might work with mimimal risk of significant water contact with the hotglue, but I believe the only long-term reliable solution would be an IP68 rated cable gland providing a mechanical seal.

author: akis
author_rank: Frequent Contributor
timestamp: December 04, 2015, 02:09:49 pm
content: The wild temperature variations has done it then. The air inside the tubes gets very hot, releasing its heat through the acrylic tube and into the water, and when it cools down when it's lights out, it sucks water in. Very clever. I had thought that the lack of aeration is not healthy for the tubes/lights in general, but there was no other option as they needed to be sealed and immersed.Frighteningly I have an immersible, mains powered, water heater, which shows water droplets inside the glass, that's terrible! I run the heaters off an RCD to be safe but it makes you wonder. A lot of aquarium heaters, pumps and filters, are submersible with the power cord immersed in water.I was thinking why do they not make the easy ones low voltage, like 12V, with an external mains adaptor. I do not get it. But for the 200W heater it makes more sense you'd need 20 amps at 12V, very thick wires and very heavy adaptors.

author: Fungus
author_rank: Super Contributor
timestamp: December 04, 2015, 02:16:02 pm
content: Quote from: akis on December 04, 2015, 02:09:49 pmMost likely, yes.Quote from: akis on December 04, 2015, 02:09:49 pmThe non-isolated electronics will be potted. Hopefully.

author: Ian.M
author_rank: Super Contributor
timestamp: December 04, 2015, 02:27:29 pm
content: As I said, there is an easy option for low voltage lighting - simply run hookup wire through fish-safe tubing. Its vented at the PSU end and the immersed portion can be completely sealed. I would however suggest a brass tube insert where it passes through the compression gland to prevent it collapsing and ensure a good seal..For commercial products, if its cable gland and all other seals can withstand >1 atmosphere pressure differential with no leakage, there is really no issue.If you'd made the tube fully pressure-tight, with solvent welded end caps and a proper gland, and assembled it with a freshly activated silica gel sachet inside, you would also have had no issues. It would be nice to be able to pressurise it to about 0.1-0.2 Bar relative (or freeze it to draw in more air) before tightening the gland, but then you get issues with pressure leakage between the individual wire strands or between the wire insulation and the cable jacket.

author: picandmix
author_rank: Frequent Contributor
timestamp: December 04, 2015, 02:39:11 pm
content: Quote from: akis on December 04, 2015, 02:09:49 pmThats why you want to stick with well known aquarium branded products - cheap and typically far east items are prone to early failure.Lots of underwater led lights from £5 to £25, might be easier than trying to seal your own versions ; plus the old saying you gets what you pays for.http://www.arcadia-aquatic.com/classica-led-spotlight/

author: rdl
author_rank: Super Contributor
timestamp: December 04, 2015, 02:52:33 pm
content: Acrylic fish tanks are glued by solvent welding. Glass fish tanks are glued with silicone adhesive. Trying to do it the other way around won't work. Bear in mind that solvent welding acrylic is permanent. If you need a cap that can be opened and closed, use some kind of threaded cap.

author: Ian.M
author_rank: Super Contributor
timestamp: December 04, 2015, 03:06:18 pm
content: Quote from: rdl on December 04, 2015, 02:52:33 pmA threaded cap is extremely difficult to DIY unless you can use an off-the-shelf plumbing compression fitting for plastic pipe. Probably the easiest option is to solvent weld on a flange then use a flat bolted end cap with a gasket.If you have a lathe, a closely fitted solid acrylic end plug with three O ring grooves machined in it and polished, assembled with O rings and a light smear of pure silicone grease and retained by lateral dowels or screws between the outer end and the first O ring would probably be the best option.

author: akis
author_rank: Frequent Contributor
timestamp: December 04, 2015, 11:20:58 pm
content: I used the glue gun to put a blob of glue on the cable exit, hopefully this will restrict the water somewhat and even more hopefully it will not dissolve in the water leaking noxious chemicals.

author: Ian.M
author_rank: Super Contributor
timestamp: December 04, 2015, 11:30:23 pm
content: I wouldn't do that. Have you read up on the additives in ordinary hotglue?If you put the hotglue on the inside of the endcap with a smear of your safe silicone on the cable where it goes through the cap, it would be a lot safer for your fish.However, I wouldn't trust it unless you can manage to vent it to a dry location to relieve the partial vacuum when its cools.