How hot should the dew heater strip be?

[Martin-Devon]

After being plagued by dew so much, even with a dew shield fitted, last week I bought the Orion 8" dew zapper heater strips, and a Maplins 12v/5A mains adaptor. When connected the dew strip rapidly got very hot, to the extent that I could hardly hold it in my hand anymore. If connected to the OTA of my CPC800 I must admit I was worried that it may be too much.

Anyhow, since the 12V/5A adaptor made such a noise (has a built in fan & I just could not tolerate the racket) - I exchanged it for a Maplin 5 in 1 power tank, which after some tribulations at first, I now have working properly. When I plug in the dew strips now, they get warm, but not hot and certainly nowhere near as hot as with the 12V/5A adaptor.

So my question - what temperature should the dew strips actually reach when set up and used outside? Will "warm" be good enough, or do I have to get more current running through to get it much hotter instead?

[Beyond_Vision]

What kind of dew strips do you have and does it have a controller?

[brantuk]

That sounds very odd - it's only supposed to supply enough heat to keep the mirror or corrector at ambient temperature so dew can't form. I'll let someone else supply the technical answer but it doesn't sound right. My next question would be "how quickly is the band flatenning the battery?"

[brianb]

The heater band is supposed to be in contact with metal which will conduct the heat away. It should be used in conjuction with a controller and adjusted so that the tube an inch or so either side of the band is dry whilst the remainder is wet with dew (if you don't have a supersophisticated controller with ambient & protected element temperature sensors).

If used in free air, I would expect the heater band to get distinctly warm. It's not good for the element either.

[brantuk]

The Orion Dew Zapper is made to be used without a controller direct from 12v dc power.

http://www.harrisontelescopes.co.uk/acatalog/Orion_Dew_Zapper_8_.html

Edit - woops wrong link corrected now.

[blinky]

It only needs to be luke warm - remember you only want the mirror/corrector whatever at a couple of degrees above the dew point. These bands should be used with a controller, if you dont then they will run at full power.

[Glider]

The band has a cigarette light plug on the end that implies it plugs direct into a power tank. I've seen this type of direct connection on large bands. It does rely on a good thermal contact to the OTA so that the heat is carried away. When you tested it and it got very hot did you have it connected to a good lump of something or just in your hand? In that case I would expect it to feel very hoy.

[Martin-Devon]

I do indeed have the exact Orion dew strip heater that Brantuk posts above. It has no controller unit, just a cigarette adaptor end that plugs into a 12V outlet. On the 12V Maplin 5 in 1 power tank, it is probably OK then, since it is warm, no more. I have no idea the amperage output of this Maplin power tank though - nothing is stated on the info that came with it - just that it is a 12V output. My "A" level physics is very rusty now (nearly 40 years ago!), but is heating output current squared x resistance or something similar? If so then the original Maplin main power unit pumping out 5A may have been too much, which is why that set up got far too hot.

[blinky]

Power (i.e. Heat) = Current X voltage or Current Squared X Resistance

[gedmac]

i suspect when you used the Maplins 12v/5A mains adaptor. it was outputting more than the 12 volts reqd so consequently using power(heat)=ampsxvolts it will have got hotter,another way to look at it is current = volts divided by resistance.resistance being constant an increase in volts equals an increase in current.an increase in current equals in increase in power or heat,hope i havent made that too complicated:icon_scratch: