Dew Heaters - How much power to send to them?

[mrflib]

I'm looking to develop some rules of thumb which I can stick to. I am pretty much exclusively an imager.

I have an ASIAIR Pro which can control the dew heater straps % power through the app. I don't think it's in any way automated though, so unless I'm up I need to set and forget. Obvs if I'm awake I can adjust.

Because I image on battery a lot, the dew straps work out as one of the biggest drainers, so I'd like to make this as efficient as possible.

So on my 115mm refractor, how high in % terms should I be running the Lynx Astro dew strap? The colder it is, the more power I run through it? The more degrees below dew point, the more power I run through it? Just whack it on 75% and go to the pub?

I've seen plenty of arguements on various forums about where to position the strap, but not a lot on how much power to give it.

 

Cheers

 

Edited October 6, 2021 by mrflib

[CedricTheBrave]

I made an controller following Robert Browns design.

it has the ability to target dew point so keeps the temp of any surfaces you want dew free above this temp which makes it more efficient.

info is on source forge here https://sourceforge.net/projects/arduinonanodewcontrollerpro/

[fifeskies]

Difficult to have a rule of thumb that is so dependant on ambient temp and humidity level.

A cold damp autumn night and you will need to throw in plenty power or risk losing the struggle to keep clear views.

A warm summer evening and you may get away without it turned on.

Also what works at 10pm may not manage at 4am as mist forms when a weather front approaches.

 

I tend to be generous to make sure I avoid any misting when on mains, but you don't want to overheat and create thermal eddies.

 

If running from battery start lower and check after a while and judge if it is enough.

 

There are some complex control systems that adjust level as temp and humidity sensors send data back , it might seem that is going beyond what is needed most of the time but it will ultimately give the best performance.

Edited October 6, 2021 by fifeskies

[skybadger]

I would try to model it as the balance between power injection and thermal radiation. The target being to keep the lump of glass in positive power budget to maintain its temperature above the dew point.

For example, I put 5 watts in and assume it's all heating the OG.

The OG is at 5 Celsius  and has say an area of pi*100#2/4 ( ie 100mm diameter).

We want to keep the OG above a level which represents the local dew point, say 5 Celsius or 280K

The glass is radiating as a blackbody with temperature 280K into a void of temperature 245K (typical night sky apparent temp is about -30). The rate of radiation of energy is......

Which I make as 0.123 mW assuming an emissivity of 0.8.

(http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/stefan.html#c3)

So, the rate of injection of heat required feels quite low. But losses into the tube probably means the amount of heat we can inject into the glass is a small fraction of the heat we provide. I'm not sure it's as low as 1/5000th though.

Feel free to poke holes.this is a thought experiment on the train home. 

Mike

[M40]

Looks like Pegasus astro do an auto dew heater controller but this is food for thought. I made a simple arduino dew point alarm with the idea being that it just gave a traffic light green - amber - red warning indication that dew was about to form. I wonder if this can be used somehow as you already know temperature, humidity and dew point all you need to add is an output and a few adjustments. Time to blow the cobwebs off.

@skybadger - too heavy for me that

[skybadger]

I also have built an Alpaca ASCOM controlled dew heater setup that uses the temperature of the scope and the domes central measurement of humidity/dewpoint to determine the amount of power to put into the heater band. Servos the value automatically. 

I won't claim its wonderful until more testing is complete but I hope so. 

M