anyone used these ...

[sparkyjon]

not sure if this is classed as diy but i've stumbled across these. and i did search for a thread already going but had no luck...

http://www.dewityourself.co.uk/standard-dew-heater-bands.html

has anyone had any experience with them how easy are they to make ?And are they any good? i'm an electrician and build pc's so not too dumb with wires...

[wookie1965]

They look very interesting and quite easy to make i use a dew shield had no problems up to now (cross fingers) but ive bookmarked that page just in case thanks for the heads up.

[sparkyjon]

if i do buy this celestron c8 tomorrow i think i'll have a pop at one of these..

[The Sailor]

Yes booked marked here, looks interesting, thanks.

Jim

[Islander]

Or make it yourself completely. Some ideas on this site:

http://www.dewbuster.com/heaters-330ohm_resistors.html

[simonmc]

To be fair its a kit of almost the same thing. If you did not have the bits on hand then you would almost spend the same after driving out and getting them. I was going to suggest the same thing. As i will be needing some soon and after giving it some thought, i prefer his method of using braid as the main conductors rather than solid core. I think i would rather buy the kit if it was just for one bit of kit.

If however i was going to do the whole of my kit in one go then i may go the diy route, but that would involve thinking about the resistor values to use to give the required heating, then convert that how many 1/4W resisitor that need to give even heating, what im going to power it with, and thats ignoring the thermal mass of the object to be heated... (Sorry rambling at the moment due to the film thats on and the rum and coke in my hand )

Sorry, my last job was doing somthing similar in the oil industry using peltier TECs to cool things then heat them back up in a controlled fashion and its not always as simple as one thinks.

Simon

[Islander]

For a one off, it may not be cost effective but I bought a large quantity of 390R 0.5W metal film resistors from eBay for a very low price. I've made a dew heater for my Tal 100RS and also for the star diagonal - I'll be building other heaters at some point. I already have a shop bought tape and Telrad heater and to be honest, the home built ones perform every bit as efficiently as the shop bought ones. Braid is an excellent idea but tinned copper wire works just as well. They aren't that expensive to build really - it just depends on how much you want to do yourself.

It's not hard to calculate the values if you're used to Ohm's Law (my first profession was electronics engineer). The power required doesn't have to be ultra precise - I work on a value of 2W per inch diameter for objective lenses and that seems to cover it nicely. 0.5W resistors rather than 0.25W - otherwise they'll overheat. If you want to do it precisely then you'll need to measure the objective temperature and the dew point and have some feedback control loop to keep the objective a couple of degrees about dew point. I'll be doing that once I've built an observatory and have a permanent setup.

[simonmc]

Totally agree. I also was an electronic engineer till recently, and TBH i ment 1/2 watt resistors but been working on SMT stuff to much! But you could use 1/4 watt, just twice as many at double the resitance. Maybe its the rum and coke coming out now, but on a project i started for my fish tank i got a load of LED strips. These were constructed of a plastic ribbon cable with lots of high power smt LEDs in parallel with a series resistor for each one covered in some sort of silicone. If the silicone was stripped off, the LEDs and resistors swapped out for resistors that worked out better for heating they would be ideal. Very thin, very narrow and you could use smt resistors of about 1206 size , very flexable with a substrate that could handle the current. A real pain to make though after cleaning nearly all the bits you paid for. Maybe im thinking to far out the box :lol:

Simon

[IanL]

Nichrome wire is the other option. It seems a bit more daunting to work with, but it is actually pretty easy once you get down to it. It is generally cheaper than using resistors if you are only making a few heaters. Also less soldering involved so quicker to build. You can pick it up of eBay for a few quid for a reasonable length and they have various gauges available (with different resistances per metre).

I have found that somewhere in the region of a third of a watt per centimetre length of the heater band is ideal for typical UK conditions. The only thing to watch out for is that the quoted resistance per metre on the specification for the wire may not be that accurate. If anything I would go for a thinner gauge/higher resistance wire than your initial calculations would suggest.

It is then easy enough to twist together several lengths of the wire in parallel to achieve a lower resistance and less power per cm. That also has the benefit of making a thicker "rope" of wire that is easier to work with, and also it tends to combat the springy curve of the wire when it comes off the reel. (You can also reduce the unwanted twisty curve by stretching the wire and heating it with a heat gun or a flame, but it still tends to want to spring back in to a coil).

Alternatively if your wire turns out to be too low in resistance and you need to increase it, you can use one run of wire and double it back and forth along the strip to increase the total resistance and power output per cm. That is harder to do though since you have to ensure that each run of the wire doesn't touch the others.

The one problem of Nichrome is that you can't actually solder to it; you either have to use crimp terminals (hard with fine gauge wire) or do what I did on the advice of others. Basically you make a noose at the end of the Nichrome wire by doubling it back and twisting the loose end. Then you feed your (braided) copper wire through the noose, double it back on itself and twist in to another noose; twist both nooses until they are tight and then fold them together. You can then solder the copper wire to itself, trapping the nichrome inside the resulting lump of solder. I then added some heatshrink tube to keep the whole thing stable.

You need a short copper wire at one (near) end, and then a long insulated copper wire that you solder at the far end and run back along the strip to the near end. N.B. If you find you have to go for the single long run back and forth, you may be able to arrange it so that both ends of the wire end up at the same end of the strip, meaning you only need two short copper wires. Then solder a connector (usually a phono-type socket ) to the copper wires.

You can feed lengths of heatshrink tube over the nichrome rope to ensure it is insulated from the OTA, but it isn't absolutely necessary. Then stick the nichrome to a strip of Duck Tape which you fold over and stick to itself, followed by a second layer of duck tape which you wrap around the first in a helical pattern. Finally wrap around the OTA, and then add some stick-on velcro to the right places on the ends of the strip so you can fix it in place and remove it as required.

I then used some heavy-gague speaker wire with phono plugs at each end to connect to my home-made dew heater controller. (See the long-running thread about LED dimmer dew heaters for info on that one). You could save a couple of quid by soldering one end of the speaker wires direct to the nichrome instead of using a socket and plug, but I felt it was better not to, since I could easily make up a shorter or longer cable if needed without having to take the heater band apart.

See my thread here on my adventures with Nichrome and some of the calculations I used: