General > General Technical Chat
Not electronics but who knows where to ask: anti-fog, anti-frost clear material?
mortrek:
I have an outdoor camera that does not have access to AC power and is enclosed in a box with a viewport. It's a project I've worked on for a couple years and it runs entirely off of a limited battery supply. I need to have a polycarbonate or glass window viewport on the front, and it needs to last for a year without fogging or frosting up in varying environments. I know this is asking a lot...
I used an "anti-fog" treated piece of polycarbonate but the problem this time of year is that the hydrophilic surface attracts water, which then freezes and becomes largely opaque. Untreated polycarbonate would have a similar issue. Would hydrophobic treatment work better in low temperatures? I can apply different coatings for different times of year, but preferably I'd get 1+ years without any significant fog or frost, and without any intervention.
Any ideas I'm missing, other than hydrophobic coatings? I don't think I have the battery capacity to blow warm air on it. The viewport will be mostly in shade, to avoid glare. It's possible to redesign the setup a bit to add a little battery capacity, but I'm unsure of how much energy it would really take to warm a thin piece of plastic covered in a layer of frost enough to melt the frost. It's probably more energy than I can justify.
The plastic window is about 3mm thick, 6x6" in size (although the actual used viewport area is much smaller) and the battery bank is meant to last for 1 year, but I've built in an extra 50-100% or so battery overhead, so I may have about 2700mah at 9V to spare, or 24WH. It's likely to be below freezing for 15-20 days a year. That leaves at most 1WH to warm ice per day or ~3600J . Ice heat of fusion is ~334 J/g. I am unsure of the thickness of the water on the hydrophilic material, but a small window of melted ice would be maybe 2x2inches and .5mm water thickness would make just over 1g, so only ~400J in theory, but much more in practice... and again, I could add batteries.
So if no treatment works, should I look in to some sort of targeted heating system that points at the focal area of the window for the camera and heats it for X seconds before taking photos? And then only have it trigger when temperatures are below 0C? it takes probably 8-10 photos per day during the winter... but temps often go over 0C during mid-day. Ugh, I've worked on this project for a while and this was a major concern that was never fully resolved. I'd be fairly happy with it getting a single good photo during freezing periods, rather than multiple blurry iced-over photos, so maybe when the temperature gets low it should heat the viewport and take fewer photos?
Of course, ideally there would be a passive solution that allows all the photos to be crystal clear...
If anyone has any suggestions I'd love to hear them. I'm so close to done with this project, except for the frost issue...
Domagoj T:
Before contact lenses, when I wore glasses I had this anti fog substance that worked miracles. It was decades ago, so I don't remember what it was, but it may be a start for your search.
Can you purge the atmosphere inside the enclosure with nitrogen? Throw in a pack of desiccant for a good measure.
mortrek:
I've done this. The inside is fine, it's the fog/frost on the exterior that is the problem unfortunately.
RoGeorge:
There are some anti-fog/anti-frost gels, some are used for car windshields, too.
Use hermetic cases and add silicagel granules inside the enclosure to absorb any moisture. If it's very exposed to ice/snow, use a defrosting/heating band.
mortrek:
It's an IP67 case with an anti-fog coated polycarbonate viewport. I have lots of silica gel packets in the case. The problem is the exterior.
Does anyone know of any extremely small low-draw (probably <2A) 5-12V heat guns that I could direct onto the viewport area? I have been looking for hours now and cannot find anything even close to what I'd need.
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