(meteorobs) Heat required to prevent dew in a box for allsky camera

Sat Nov 22 12:54:37 EST 2014

Ok guys, engineer here...

Jim is correct on the sandia all sky system.  The heaters are simple and work very well.

You don't want to use low voltage for heating.   It is a waste of energy for conversion and transport, in addition to adding more stuff that can break.  With standard insulation techniques and standard wire, it will be safe.

The sandia NMSU design uses a simple DIN mount thermal controller with a settable turn-off temperature, feeding two metal cased power resistors that deliver about 36 watts of heat.  They are mounted on a small metal plate.  Together with the little 2 inch fan to circulate the air, they do pretty well.

With that said, I found the heat output on mine to be too high.  When it was on, it cooked the surrounding plastics with enough heat to fog the dome with what looked to be a fine oily silt. Probably the plasticizer from the wires, mount, and PVC.

Tired of constantly cleaning the lenses, I reconnected the two resistors in series, which reduced the heat down to 16 watts and it worked fine.  I've had no issues with fogging, condensation, or ice since then.  I eventually fed the unit with a COTS light sensor (modular unit for Christmas lights) that turns the heat on after dark.  Since the software is disabled in the day, there was no reason to cook the unit needlessly.

Jim, one thing on these, don't forget the .636 factor when using AC.  They are published as 50 watts, but that's the DC value.  

Sent from my iPhone

On Nov 21, 2014, at 6:22 AM, Jim Wooddell <jimwooddell at gmail.com> wrote:

Jay,

With all due respect, there are hundreds of sky cams out there that use 110vac line voltage to power a heater circuit.  There are millions upon millions of 110vac products out there in outdoor service.  They are safe.  Additionally, you seem to assume everyone has 12v available at their camera capable of powering a Walmart heater/defogger.
So, I think you are talking out your arse.

Regards,

Jim Wooddell

> On Thu, Nov 20, 2014 at 9:46 PM, HEADLIGHTS Studio <studio at studioofbluelight.com> wrote:
> OK, it is just plain dangerous, fraught with the possibility of disaster, to have live Line Voltage inside a small outdoor enclosure in a wet environment. Exposed NiChrome Wire is better known under another name, “Death Wire”. While it may have been prudent in 1994 to use line voltage for de-fogging a limited number of pedestal units, it is 2014; much more attractive, safer, and inexpensive alternatives are available off the shelf at Wal Mart.
> 
>  
> 
> My Meteor Observatory just went through driving rain, 29 degree freezing all night, heavy fog, and has been exposed this way for years, without requiring attention or maintenance; providing uninterrupted observation.
> 
>  
> 
> I drew a quick and dirty schematic…
> 
> http://www.salsburg.com/NAVSPASUR/CameraPedestal.jpg
> 
>  
> 
>  
> 
> Good Luck.
> 
>  
> 
> Jay
> 
>  
> 
> From: meteorobs-bounces at meteorobs.org [mailto:meteorobs-bounces at meteorobs.org] On Behalf Of Howard
> Sent: Wednesday, November 19, 2014 10:19 AM
> To: Meteor science and meteor observing
> Subject: Re: (meteorobs) Heat required to prevent dew in a box for allsky camera
> 
>  
> 
> Its very common to use resistors as heating elements for dew heaters in astronomy applications. Or nichrome wire if you want to work with that (its sometimes a bitch to solder). Its fairly simple to size the resistor network to get the desired wattage.  Google it, there are plenty of examples on the web.  You only want enough heat to raise the window temperature a degree or two above the air temp. 
> 
> Typically I use 33 ohm 1/2w resistors in parallel to achieve whatever wattage I think or determine is needed. Running off a 12v DC source it works fine but of course runs down a battery eventually. You switch the voltage on and off using either some kind of relay or use a pulse width circuit. The later is the exact principle used by Kendrick type dew heater controllers.
> 
> Easy stuff, not rocket science. I've built a boat load of these over the years: highly effective.
> 
> 
> 
> Using Line Voltage and Resistors like those in the Sandia rig is expensive
> and dangerous, it is not advisable; besides the resistors are difficult to
> obtain. Relate to the fact that those Sandia rigs were designed in the 1990s
> by highly experienced Engineers with large budgets. Running Line voltage to
> something on your roof or outside your observatory is fraught with problems
> and dangers (Lightning, Sunlight, Animals).Inside my pedestal, I use a 12VDC
> Windshield Heater (has a fan) from Wal-Mart, switched by a Car Horn Relay
> from an auto parts store, and a cheap electronic thermostat from Lowes.
> Total cost, about $35. I run it on 2, 5 Volt USB Chargers connected with
> Malibu Lighting Wire (Lowes) which is Sunlight resistant. The problem with
> internal Moisture is caused by an inadequately sealed Pedestal. It is
> advisable to use a large number of desiccant bags inside the Pedestal.
> Moisture will eventually destroy the Lens and Camera. I use 6 inch PVC Sewer
> Stand Pipe and Joiners with modified screw-on end caps; on the bottom for
> connections and mounting and on the top, the dome. This makes it large and
> heavy, but is very durable, inexpensive, easy to service and maintain.
>  
> -----Original Message-----
> From: meteorobs-bounces at meteorobs.org
> [mailto:meteorobs-bounces at meteorobs.org] On Behalf Of Jim Wooddell
> Sent: Friday, November 07, 2014 7:38 AM
> To: meteorobs at meteorobs.org
> Subject: Re: (meteorobs) Heat required to prevent dew in a box for allsky
> camera
>  
>  
>  
> Hi Mark,
>  
> The Sandia Allsky cams use two resistors on a metal plate, a thermostat and
> a fan to keep everything toasty in the winter.  Works fine.
>  
> Jim Wooddell
>  
>  
>  
> On 11/6/2014 11:59 PM, Mark Bowling wrote:
>   
> What do you guys do in the summer?  I get a lot of moisture build up 
> inside. I've been using desiccants, but that gets to be a pain.
>  
> Mark Bowling
> Vail, AZ
>     
>  
> _______________________________________________
> meteorobs mailing list
> meteorobs at meteorobs.org
> http://lists.meteorobs.org/mailman/listinfo/meteorobs
>  
>  
> _______________________________________________
> meteorobs mailing list
> meteorobs at meteorobs.org
> http://lists.meteorobs.org/mailman/listinfo/meteorobs
>  
>  
>   
>  
> 
> 
> _______________________________________________
> meteorobs mailing list
> meteorobs at meteorobs.org
> http://lists.meteorobs.org/mailman/listinfo/meteorobs

-- 
Jim Wooddell
jimwooddell at gmail.com
928-247-2675
_______________________________________________
meteorobs mailing list
meteorobs at meteorobs.org
http://lists.meteorobs.org/mailman/listinfo/meteorobs
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.meteorobs.org/pipermail/meteorobs/attachments/20141122/d6c3b4a9/attachment.html 