Dew bands how hot is enough.

[Doc]

Never seen or used dew bands before so how hot does one get.

I'm doing a little testing and have made up one that uses nicrome resistance wire.

It states 30mm = 1 Ohm.

I have 300mm so in theory 10 ohms

Attached to a 12v source should give me 1.2 amps.

Is this going to be hot to touch or just warm. Remember it needs to go through the dew band.

[Reggie]

sounds a bit less than the draw from each of my heater bands, mine are always warm to the touch on the outside and a bit warmer inside, neither are overly hot, although it is usually wrapped around a cold metal tube and outside:D

[barkis]

That adds up to over 14 Watts Mick. I suppose it depends on what you are wrapping it around. Is there a controller driving it, or is it a constant operation? Over a long period, that would warm up quite a bit I would think.

Ron.

[Merlin66]

I use NiCr wire for my home made dew heaters..

1.2A @ 12V is only 14.5 W, this will struggle to feel hot, but may be warm enough to keep the objective above the dew point.

Base on some research by the guys in Oz

P = 0.0606 x D +1.18

Where P = power in Watts

D = nominal diameter of the lens in mm.

So, for example, an 80mm lens requires 0.0606 x 80 + 1.18 = 6 Watts.

Hope this helps

[Doc]

It's a wrap around stalk heater for my secondary. And yes I have a controller driving it.

I also have some 10 ohms per metre wire.

So 300mm of this should produce 3 ohms.

12v divided by 3 ohms = 4 amps.

Will this be better or overkill.

[adamsp123]

Doc your last post....

Thats 48 watts!! wow that should get WARM! still if you have a controller you should be OK.

I reckon a nice steady low heat right from the off would be the best policy.

Pete

[Doc]

God am I a idiot.

I had the controller on Low

Turned it to max and it's glowing red.

[themos]

When I looked around the internet for this, I came up with values like 1/4 or 1/2 Watt for heating a secondary. Remember, you only want to keep the secondary about 2 degrees above ambient.

[brianb]

you only want to keep the secondary about 2 degrees above ambient.

No, you want to keep it at ambient - despite the tendency to radiate heat away until it's below ambient because of exposure to the "cold of space" (yeah I know that's nonsense physically but it's a good way of explaining things).

If it's above ambient there will be warm air currents rising from it & ruining definition at high power.

[Merlin66]

Actually you only have to keep the temperature above the Dew point, which varies with humidity and temperature. The secondary and any other objectives etc will tend to ambient anyway.

[Rusty Strings]

Is there a way of sensing ambient temperature and comparing it with the scope temp/humidity and automatically controling the dew heater to keep it optimal?

John

[johninderby]

There's a very good article that goes into real detail on how to make a homemade dew heater on Dewbuster's website.

http://www.dewbuster.com/heaters/heaters.html

http://www.dewbuster.com/

They do make the best quality dew heaters avaiable, real industrial quality compnents used throughout.

John

[Reggie]

Rusty, I am part of a project that is doing just that for a dew heater, (I'm writing the code).

Check the link:

sgl_observatory_automation : SGL Observatory Automation

We measure ambient temp, RH and scope temps from this we can get the dew point and set our 'heater on' levels accordingly, I currently have 3 scope sensors working but can add more. we have accuracy to around 0.4-0.5DegC with these sensors.

The hardware is pretty cheap, about £15 for the main circuit (which is adaptable so it can do other things, like focuser control), £8 for the ambient/humidity sensor and £2.50 each for the scope/kit sensors. For each dew tape a transistor is needed to switch the higher voltages needed, these are around £1.20 each. To add another heater tape to the unit would cost you under £4 + the cost of the tape

We are developing a number of 'units' based around the core controller board, Focuser, Dew heater, Cloud detector, fully featured Intervalometer + I am open to suggestions for other ideas. we are providing all of the circuit designs (that we make) and all of the code that we write for free, all you have to do is get the kit and hook it all together. Some of the units will work together, so again this will help keep costs down should you want to expand.

One of the cracking features of the base boards is that you can 'build' your own with minimal components, the main chip, a timing crystal a couple of caps and resistors, about £5 worth of components is all it takes, this makes for an incredibly cheap way to develop a whole raft of astronomy devices.

All units will run standalone, each unit will have ascom drivers where appropriate (Focuser will have ascom support, whereas the Dew heater won't), where possible we will provide a windows standalone interface for our units for the purposes of logging data or just to be able to keep any eye on/control things in your warm room.

For the dew heater I would like to expand on the automatic side of things and was wondering whether it would be worth logging the scope data along with PWM duty cycles to watch the response of the heater tapes to try and make the heaters as efficient as possible, possibly adding some kind of hysterisis to allow you to keep the scope at a particular temperature rather than allowing it to wander up and down.