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

[Ed Majden]

Chris:
    Maybe where you live but on the WET Coast of Vancouver Island more heat 
is required.  Even the Sandia All-sky can dew up here.
Ed

----- Original Message ----- 
From: "Chris Peterson" <clp at alumni.caltech.edu>
To: <meteorobs at meteorobs.org>
Sent: Saturday, November 22, 2014 10:01 AM
Subject: Re: (meteorobs) Heat required to prevent dew in a box for allsky 
camera


> The Sandia design isn't the best. In fact, for a video system (as
> opposed to the DSLR case that started this discussion) there is no need
> for heaters at all. The electronics in the enclosure generates enough
> waste heat to keep the dome dew and frost free under the worst of
> conditions. All that is required is a simple internal circulation system.
>
> Chris
>
> *******************************
> Chris L Peterson
> Cloudbait Observatory
> http://www.cloudbait.com
>
> On 11/22/2014 10:54 AM, James Beauchamp wrote:
>> 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.
>
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