Heater Straps

[alistairsam]

Hi,

I bought a 3" Kendrick Heater strap from Bintel and I tried it outdoors on my ST80 last night and, it was barely warm. I plugged it into a 12v 2A supply direct as well.

I know heat is dependant on the resistance and however good the controller may be, it can only heat up to its rating.
resistance of the strap is 29ohms, but if its barely warm how would it keep dew off? I know too much heat can induce thermal currents.

reason I ask is I thought these straps were designed for cold outdoors and resistance would be optimized to what's needed, that is not too high and not too low.

is it just the limitation of the strap and the resistive element? They have more expensive "premium" straps. not sure what the difference is.
when indoors, it warms up really well. last night was round 6C ambient.

Current draw at 12v would be 410ma, and wattage 4.9
I could increase the voltage but I'd rather stick to 12v.

[mikerr]

Alistair, it is probably working fine. I have a Dew Not Strap on my 8 inch SCT and it is just barely warm to the touch and keeps the corrector free of Dew.

Did you still have dew problems with the strap running?

Michael.

[alistairsam]

The strange thing is when the sky was clear, till around 11pm, it was dripping wet everywhere. I didn't have the heater on at that time as I was setting up and sorting out cables.
after 12 when I finally powered up, it was clouded over completely as expected, but there was no condensation even without the heater. I even removed the shield of the ST80, left it open for an hour, stayed clear. so did my finder.
I normally take it off and use the back of my wrist to gauge heat but it was barely warm.

I guess you're right, it may be working well.
I'll have to give it another try.

[erick]

Alistair, my experience is that they only have to be a few degrees above ambient to work for most applications. I test that they are on by pressing my lip against them. You will feel that they are warmer than the surrounds.

But the best way to use them is to have them on from the beginning, rather than turning them on after everything is dewed-up. Start off on low and crank them up as the dew starts to show. So get a big battery or run off a mains-powered supply.

Eyepieces and finderscopes you can crank up and down no problem. I'm guessing the front lens of a small refractor shouldn't be a great problem. But be a bit more cautious of the corrector plate on a bigger SCT. I find corrector on my 8" SCT doesn't like rapid changes in temperature. Before I had a heater strap, I hit it with a hair-dryer one evening while someone was observing - nauseous results for the observer!

[mikerr]

Alistair, Erick is spot on about turnining the dew strap on early. I turn mine on when I set up and leave the mirror to cool down for a while. It probably takes a while for the heat to get through all the insulating surfaces and reach the corrector plate.

Michael.

[alistairsam]

Thanks Mike, Erick,
I'll do just that, turn it on early.
Tomorrow night's forecast looks clear, so will give it a shot.
Cheers

[Nico13]

Alistair I have also had the same results as Erik and Mike with the turn it on early process as it is such a small difference in temp that it does indeed take a little while to reach a dew free temp on the corrector plate, so yeh get it on as early as you can in your setup. :thumbsup: