Dew strap - how warm?

[Ags]

My dew buster for my nifty fifty lens is complete and undergoing field testing! It runs off four C rechargeables, with parallel resistors giving 8 ohms resistance. It's been going for nearly three hours and is still distinctly warm, so it is doing exactly what I wanted.

It turns out the high capacity rechargeables deliver 16% more voltage than it says on the packet, at least for the first 45 minutes. This makes the dew strip mighty warm, although I am admittedly testing on a warm day... How warm is warm enough? I have no commercial strips to compare with.

[Ags]

In the end it lasted for 6 hours before the voltage dropped to 4.2V, although it was still warm at that point. So one charge should last me a couple of nights :-)

[focaldepth]

 How warm is warm enough? I have no commercial strips to compare with.

Hi, I only have limited experience with commercial and home made strips. I would not look at it as how hot the strip gets but how many watts it takes to raise the lens (etc) above dew point. In use, on a scope you can hardly tell they are on as it only takes a few degrees rise to work.

Here is the data I used when I made a carbon fibre tape heater.  http://www.kendrickastro.com/dew_premierheaters.html

[Ags]

Thanks. My strap is producing 3.8W for the first hour of a session, 3.0W for the bulk of the session, and 2.2W in the last hour. This is applied to a 52mm metal  screw-on lens hood. I expect heat loss to the air is quite high as I have not insulated the outside of the heater yet.

I am thinking about removing one resistor from the circuit, which would raise the resistance to 9.2 ohms. This would change the wattages to 3.2W at start, 2.6W for the bulk of the session, and 1.91W at the end of the run. Which looks a bit low compared to the Kendrick figures. My current setup in fact looks right. One other thing I might do is distribute the resistors more evenly, that would get rid of one particular hot spot.

[focaldepth]

If it helps I just measured my 6" Astro-Zap heater as 13.2 Ohms. I have used it on a 6" Skymax which has a massive thermal inertia so it would be on 100% for an hour then down to about 25%. I am still learning how to best handle dewy nights. I am using the VERY cheap Chinese LED dimmers from Ebay as controllers. They get several mentions elsewhere on SGL.

[Ags]

Thanks. I reread the Kendrick reference, and I now see that I'm producing enough Wattage to fight off a Siberian freezing fog. I will look for the LED dimmers you mention.

Still... it's good to know I can run at 120% required output for 6 hours.

[Ags]

I rewired it today, and took out 2 resistors (1Watt). It feels more reasonable now... I should have a couple of hours extra runtime, maybe 8 hours, more than enough for several sessions- so I won't have to pack the bulky charger.