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want to buy a dew zapper for my C14 Classic!

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#1 scopelover

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Posted 08 December 2012 - 12:25 AM

Ok folks ...Whats the cheapest and best way to get a zapper for my C14 Classic? Do I really need the variable heat controller gizmo?? :question:
Can u recommend a brand??

#2 GlennLeDrew

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Posted 08 December 2012 - 12:56 AM

If cheapest is desired, build a ring of resistors to attach to the corrector's retaining ring, and cover with a layer of foam or other insulating material (to lessen radiated heat going directly to the sky.) For a 14", in conjunction with a (ideally non-metallic) dew cap, about 10 Watts of heat should do the job. And no variable controller is required.

I've made a variant on this, where instead of individual resistors I cut a handy length of the inner heater wire taken from pipe heating cable. The resulting 4.8W works beautifully on my C8, where the heating element is attached and insulated as outlined above, and augmented with a commercial, flexible plastic dew cap.

And I've never bothered with heater controllers, relying on designing to a wattage which will just comfortably do the job while continuously powered.

#3 JJK

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Posted 08 December 2012 - 08:23 AM

If cheapest is desired, build a ring of resistors to attach to the corrector's retaining ring, and cover with a layer of foam or other insulating material (to lessen radiated heat going directly to the sky.) For a 14", in conjunction with a (ideally non-metallic) dew cap, about 10 Watts of heat should do the job. And no variable controller is required.

I've made a variant on this, where instead of individual resistors I cut a handy length of the inner heater wire taken from pipe heating cable. The resulting 4.8W works beautifully on my C8, where the heating element is attached and insulated as outlined above, and augmented with a commercial, flexible plastic dew cap.

And I've never bothered with heater controllers, relying on designing to a wattage which will just comfortably do the job while continuously powered.


Glenn, is your local weather that steady? Here, it pays to tweak the dew heater power.

#4 korborh

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Posted 08 December 2012 - 11:42 AM

Ok folks ...Whats the cheapest and best way to get a zapper for my C14 Classic? Do I really need the variable heat controller gizmo?? :question:
Can u recommend a brand??


I was looking to build the heating strip on my own but found that it would not save much over getting the Dew-not heat strips.
You can directly connect the heating strip to a fixed power supply that can supply the amps, and see how it goes. I am currently using my own made supply made from old laptop charger.

#5 GlennLeDrew

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Posted 09 December 2012 - 12:18 AM

JJK,
It's not about the 'steadiness' of the weather. It's about supplying enough heat to work in the worst conditions.

What the heater needs to do is compensate for that heat radiated directly to the sky. No matter the season or ground temperature, a clear sky presents as a 'heat sink' of about -30C. I've realized experimentally that a one-value power level for a particular application can fill the bill for all eventualities. Moreover, I attempt to reduce wastage of heat by achieving good thermal coupling and using some form of insulating 'blanket.' this allows to use relatively low-power heaters by not losing so much heat to the surroundings. I'm using heaters made nearly 20 years ago, powered continuously when in use (and when deemed necessary to run), and have never once suffered dewing or frosting on my optics. And the total power consumption is probably no higher than 'regulated' systems for similar application.

#6 LouHalikman

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Posted 09 December 2012 - 08:37 AM

How many and what resistance/wattage resistors did you use? I was aiming for 20 watts, as I live in a high dew area. Lou

#7 rflinn68

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Posted 09 December 2012 - 09:07 AM

Dew-Not strips and its not the cheapest but I highly recommend the DewBuster. It is temperture regulated and not time regulated. It puts only enough heat into the tube to keep the dew clear. I'm sure with that C14 you know what heat in the optics will do.

#8 GlennLeDrew

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Posted 10 December 2012 - 12:08 AM

If resistors are used in series, and 20W of heat is desired, each resistor must be able to dissipate its own share of heat without running too hot. For example, 20 resistors would need to be rated at 1W each, and 40 resistors 1/2W apiece.

At 12V, 20W is generated by a resistance of 7.2 Ohms and a current draw of 1.67 Amps. To deliver the heat relatively evenly, a somewhat large number of resistors is required. If 1/4 Ohm resistors can be found, 36 of them rated for 1/2 Watt would serve well. A larger power dissipation rating is fine, but they get physically larger. Actually, I wonder if such a low resistance resistor can be found which is rated for less than a couple of Watts...

A more elegant solution would be to obtain a pipe heating cable of length about 24 feet. The inner, jacketed heating element is about 0.1" in diameter and quite flexible. A suitable length cut from it should nicely encircle the corrector's retaining ring and provide the required heat at 12V.

If we assume a diameter of 16", the length of heater wire would be 50", which for a resistance of 7.2 Ohms is 0.144 Ohms/inch.

If you can find the wattage for a pipe hearing cable of given length, you can calculate the resistance per unit length. Here's an example to show how:

A 24' pipe heating cable available here in Canada is rated 168 Watts. Such a cable has two elements, resulting in a real length of 48'. When run at 110V, the total resistance is V^2 / W, or 110^2 / 168, or 72 Ohms. With a length of 48', or 576", the resistance is 72 / 576 = 0.125 Ohms/inch. But the final word is obtained by measuring with an Ohmmeter.

A 50" length would give 50 * 0.125 = 6.25 Ohms, and at 12V delivers 12^2 / 6.25 = 23W.

One of these pipe heating cables provides 10 such heaters, and so the cost is about $5 each.






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