Calculating and Customizing Telescope Dew Heater Bands for Optimal Performance

[ecjoyner]

I want to make my own scope dew heaters, I have 8 meters of nichrome band, with 1,82 ohm/meter resistance. How do I calculate the length for a specific temperature (around 8 Celsius over ambient) and how to wrap it i.e. in a coil or in a series of parallel 320° circles? The circumference of the initial tube is 80 cm and I can vary the power input from 0-12 V and 1-10 amps. Ideally, I would like to set up a chart to use the same power input but with a dimmer to vary the voltage so as to make different sizes (circumference) of the heating bands.

 

[Vanadium 50]

I read the title and thought "correct horse battery staple".

 

[jim hardy]

How do I calculate the length for a specific temperature (around 8 Celsius over ambient)

You'll have to figure out how much power (Watts) you need to achieve that 8° temperature rise. Perhaps the astronomy thread would be a good place to get that advice.

Then you'll figure how to make that many watts appear in your nichrome wire band.

Power in Watts equals Volts^² / Resistance(Ohms)

so to produce 1 watt in one meter of your wire you would cipher thus

1 Watt = Volts^² / 1.82

Volts = √(1 X 1.82) = 1.35 volts

1.35 volts per meter of wire will produce 1 watt per meter of wire.

Current is Volts / Ohms so will be 1.35/1.82 = 0.741 amp to make 1 watt per meter of wire.

So, starting with how many watts you want and how many meters of wire it'll take to encircle your objective lens,

you can now make your table i think...

Resistance of your wire will increase somewhat as its temperature rises , which with constant voltage control will make it somewhat self regulating.

Good luck ! Maybe you'll post some pictures of the project ?

old jim

 

[sophiecentaur]

The commercially available dew heaters are just a very few watts - just enough to keep the optical tube temperature slightly above ambient. They are wrapped around the tube where the objective lens sits.
I was prodigal enough to actually buy one and have never had dew problems since. They are brilliant things. One daft feature of mine is that it has two heat settings (no fancy controller* - just connected straight to the 12V supply) and the small torpedo switch (push off -push low- push off - push high) has a two coloured LED indicator which dangles right in front of your eyes, spoiling your night vision. Another trivial problem to solve.
The wires are zig-zag stitched into the wrap around strip. There doesn't appear to be any 'dropper' resistor or transistor so the resistance of the wires must be somewhat higher than the OP's sample. (High and low settings must use two or one element, I think)
*Commercial "controllers" are very expensive. Much too expensive for a DIYer to bring himself to buy one. Can't think what use they can be, unless they incorporate a temperature sensor in the band and keep the temperature a set amount above the temperature in the controller.

 

[russ_watters]

I read the title and thought "correct horse battery staple".

#amateurastronomerproblems

 

[Tom.G]

You'll have to figure out how much power (Watts) you need to achieve that 8° temperature rise.

A Rule-of-Thumb:
For a thermally conductive material (metal, glass, ceramic, etc) in still air the radiation + convection loss is 1BTUH per sq.ft. per °F. Outdoors with a light breeze blowing figure twice that heat flux. One Watt converts to 3.41BTU.

 

[CWatters]

I can vary the power input from 0-12 V and 1-10 amps

So you have a variable power supply already?

OK so if you wound all 8 meters around key areas the resistance would be 8 * 1.82 = 14.6 Ohms.
On 12V that would draw about 0.8A and generate just under 10W.
Dropping the voltage to 6V it would draw 0.4A and around 5W.

So I reckon it would be reasonable to use all 8m distributed and start with say 2-3V and turn it up if necessary. Monitor the temperature to check it doesn't get too hot. Make sure you can't accidentally turn it up too high etc

 

[sophiecentaur]

On 12V that would draw about 0.8A and generate just under 10W.
Dropping the voltage to 6V it would draw 0.4A and around 5W.

Power = V²/R, remember, so halving the voltage will make the Power 1/4.
On a practical note, the heater element needs to be a "U" shape so it can the removed without taking it over the end of the Telescope Tube.
However, 8m is a lot of wire to be stitching into a fabric collar. An alternative thickness of wire would probably be more convenient to use. (Except, of course, that you actually have the 8m). In the larger scheme of things, a bit of extra cost like resistance wire is a small fraction of the cost of the Astronomy Habit (Sorry - hobby).

 

[CWatters]

Oops good catch I miscalculated.