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LOTS of dew in all-sky-camera dome :(


Gonzo

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Well it would appear that my all-sky-camera dome does dew up quite a lot....

I don't have a dew heater built in, and was thinking about adding one now.

I don't have +12v to this box, only +5 as the box is powered by a Raspberry PI.

What are my options please?

I saw a post somewhere here or another forum about drilling a few small holes at the bottom of the dome to get an air flow, good / bad idea ?

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You definitely need heat inside to keep dew at bay, how much 5v current can you supply? Voltage doesn't matter - just current. You can use a string of suitable resistors inside to convert the 5v current to heat. Trouble with drilling holes in the case to allow airflow is that you can end up with condensation inside, I keep mine sealed with silica gel inside so it's perfectly dry - no chance of misting.

ChrisH

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I do have quite a beefy +12v PSU powering up the observatory, I guess I could route a +12v cable to the all-sky-camera along side the +5v cable.

Issue is that I need a simple dew heater, automated of course.

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Well it would appear that my all-sky-camera dome does dew up quite a lot....

I don't have a dew heater built in, and was thinking about adding one now.

I don't have +12v to this box, only +5 as the box is powered by a Raspberry PI.

What are my options please?

I saw a post somewhere here or another forum about drilling a few small holes at the bottom of the dome to get an air flow, good / bad idea ?

That may have been me, I did try that - very bad move as just caused the dome to mist up inside. I now use a sealed chamber (with some desiccant bags ) and use a dew heater band around the inside, this works ok but things do get mighty hot if you forget to to turn off the  strap on during the day, to that end I now have a simple arduino controller that regulates things based on dew point.  

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Well it would appear that my all-sky-camera dome does dew up quite a lot....

...

I saw a post somewhere here or another forum about drilling a few small holes at the bottom of the dome to get an air flow, good / bad idea ?

The basic problem is that the air inside your dome contains too much water vapour.

That could have happened because the dome is not sealed, letting in warm and humid air during the day which cools down at night and the water condenses onto the first cold surface that falls below the dew point.

I would suggest that your best option would be to fill the dome with cold, dry air e.g.from the inside of a fridge or freezer and then to seal it - maybe with a desiccant bag inside. You could also try getting an aerosol of dry air / gas. Here's an example, though I've never tried it myself. Or maybe if you're really confident in the airtight seal around your dome, suck some/lots of the air out :lipsrsealed:

If you drill holes in the bottom all that will happen is that during the day your dome will get filled with warm wet air. Although the daytime relative humidity might appear low that's because the air temperature is high. Once the air temperature falls, the relative humidity will increase (although the absolute humidity will remain the same).

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I use a ring of 6 turns of nichrome wire insulated with small heatshrink sleeving just inside the dome.  This is switched on and off using a power MOSFET type IRLZ44N (available cheaply on ebay or other places) fed from the Arduino Nano.  I use 12v but wouldn't be a problem with 5v with lower resistance ring of nichrome wire.

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I use a ring of 6 turns of nichrome wire insulated with small heatshrink sleeving just inside the dome.  This is switched on and off using a power MOSFET type IRLZ44N (available cheaply on ebay or other places) fed from the Arduino Nano.  I use 12v but wouldn't be a problem with 5v with lower resistance ring of nichrome wire.

I think I've read your post earlier with that nichrome wire ring, I need to do more research on it.

All I need/want is the simplest basic heater for the dome if I really have to have one...

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If you are happy going the DIY route, I can recommend Robert Brown's design for a fully automated two channel dew controller. Does everything including humidity and temperature measurement, data logging. Based on an Arduino Nano, so fairly low cost. The details are on Robert's SourceForge pages, just search for dew controller. The only drawback - from your perspective I think - is that the set-up program is a Windows one. Still, it is fully stand alone once set up.

Regards, Hugh

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An aluminium pipe goes up into mine, being just right ID to hold the QHY5L-II-C camera.  I don't usually have trouble with inside dew on my dome so maybe it prefers the pipe.  The pipe is the mast that holds the camera and goes down into the ground.   The ali pipe/mast also takes heat away from the camera.  Of course the pipe doesn't help with external dew and I need a heater to stop that.

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Mine has this dew heater strap inside, and also an old sock filled with silica gel stuffed in there:

DSC00554_zps13572dfc.jpg

The top plate has a ring of holes around it to let the heat up into the dome area, and also it allows the desiccant in the main part to draw any water vapour out. On the top edge there is a channel filled with silicon sealant which was allowed to dry first, that forms a rubber seal against which the bottom of the dome is pressed:

DSC00555_zps56e6b7aa.jpg

The dome has a lip and this threaded collar presses it again the rubber seal:

DSC00556_zps8db2f852.jpg

The QHY5L-II gets fairly warm in operation and that heat is transmitted via the aluminium plate to the inside - the casing acting like a big heatsink, sadly not enough heat to keep dew at bay but the heatsink effect does keep the camera temperature down and reduces noise a bit:

DSC00558_zps25d1c2e0.jpg

I don't leave mine outside 24/7, I just put it outside during a session. Only takes seconds to set up on a camera tripod.

DSC00557_zpsc7a56ee6.jpg

ChrisH

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  • 3 months later...

bringing this one back from the dead threads... lol...

I need to get this sorted, haven't had the time recently to do anything with it.

I was thinking based on the comments in this post to try the "take the air out method" by drilling a hole in the box cover (where the clear dome seats), install a one way valve and suck the air out with a vacuum pump (I have one in my tool box).

What do you guys think of this?

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bringing this one back from the dead threads... lol...

I need to get this sorted, haven't had the time recently to do anything with it.

I was thinking based on the comments in this post to try the "take the air out method" by drilling a hole in the box cover (where the clear dome seats), install a one way valve and suck the air out with a vacuum pump (I have one in my tool box).

What do you guys think of this?

I don't think it will work, or if it does then not for very long. You need heat on the inside (which keeps the outside of the dome warm as well to stop condensation forming on it) and also dessicant on the inside to keep the humidity low inside. It's up to you how you choose to do that but the combination will keep it functioning in the worst of conditions.

ChrisH

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I would think that if the unit is sealed well enough to hold a vacuum then provided you can dry the air inside with a bit of dessicant it should never dew up on the inside. However, my experience with vacuum equipment is that it is very, very hard to get the required degree of sealing. That said, provided the seal is not too bad, say watertight even if not vacuum tight, then the dessicant should last a reasonable amount of time.

I think without a lot of power available you will have to go down the best seal possible with internal dessicant route.

Regards, Hugh

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Well, I have no experience of these sorts of devices, but as you know, the humidity of the air increases as the temperature lowers, until at some point the water condenses out. It seems to me that the real solution is removing the water from the air, and if you can do that then wasting energy on heating should be unnecessary.

When you consider that double-glazed windows don't dew up because the frame contains desiccant within the channel (OK, with a bit of heat on one side). If the seal fails, of course, then dewing becomes a problem. I wonder if it is at least worth trying a desiccant within the chamber first. I used to deal with air-filled ionization chambers and these need to be kept dry inside, so the chamber is sealed but has a small vent pipe which goes into a desiccant box, so any 'breathing' with temperature fluctuations result in only dried air being sucked into the chamber. Would it be possible to configure you equipment like that? Ideally, use a desiccant with a moisture indicator colourant, and when it indicates that the desiccant is saturated, pop it into the oven to dry and re-use, or just replace.

What about dewing of the outside though?

Ian

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Absolutely...  That was my problem with the ASC.  I found I needed a heater purely to disperse moisture from the outside of the dome.  The QHY5L-2 plus electronics produced enough heat on their own to keep dew off the inside even without desiccant and sealing.  And that was even with an aluminium pipe as a mast conducting heat away from the camera.  I guess having the cooler aluminium surface inside provides a condensation surface in preference to the dome.

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