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overlapping dew bands

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#1 Nicole Sharp

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Posted 01 July 2022 - 06:24 AM

I have been informed that dew heating bands can't overlap by more than 0.5 inches.  What are everyone's experiences with this?

 

Specifically, I need dew bands for three camera lenses with maximum circumferences of 9.04 inches, 10.1 inches, and 11.4 inches.  Is it possible to use a single 12-inch heating band for all three lenses or do I have to get a different sized band for each lens (much more expensive)?  Only allowing 0.5 inches of overlap would also require custom sizes of 9.5 inches instead of 10 inches and 10.5 inches instead of 11 inches and 11.5 inches instead of 12 inches.

 

I had thought that you could velcro the ends of the band at a right angle to form a looped ribbon shape but apparently this is not recommended.  If the band is 1.25 inches across, even connecting at a right angle will overlap for more than the minimum overlap of 0.5 inches.



#2 MikeTahtib

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Posted 01 July 2022 - 07:30 AM

I have never heard that rule. I use the same heater band for all my eyepieces and it overlaps more than .5" on my smaller eyepieces.  I've never seen anything bad happen as a result.  Perhaps there's something about camera lenses that demands more precision, I don't know.

 

What was the basis of the .5" rule?  If there is no danger fo equipment damage (which seems unlikely with teh minimal temperature rises from dew heaters), maybe get the biggest heater band you know you will need, and see if anything negative happens.  If not, you've saved yourself 2 heater bands.  If the image distorts, then go  buy the other bands.


Edited by MikeTahtib, 01 July 2022 - 07:33 AM.


#3 wrvond

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Posted 01 July 2022 - 07:51 AM

I’d like to know the logic behind that claim as well.

I not only overlap straps regularly, I often use a single long strap to warm both objectives and both eyepieces on binoculars.

It’s not rocket science, it’s just dew prevention.



#4 decep

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Posted 01 July 2022 - 04:21 PM

If you run your dew heater bands at or near 100%, you can run the risk of melting things if you overlap too much.  I have seen straps melt themselves.  If you are running them at 50% or less, it is not much of an issue.

 

It really depends on how much energy your straps use.  It is fairly standard for 12v straps to be 200-300mW/cm and these will melt if overlapping too much.

 

USB straps may be different.



#5 wrvond

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Posted 01 July 2022 - 05:19 PM

If you run your dew heater bands at or near 100%, you can run the risk of melting things if you overlap too much.  I have seen straps melt themselves.  If you are running them at 50% or less, it is not much of an issue.

 

It really depends on how much energy your straps use.  It is fairly standard for 12v straps to be 200-300mW/cm and these will melt if overlapping too much.

 

USB straps may be different.

How do you know they melted because they were overlapping? Straps that are not overlapped melt themselves frequently. When you consider that no properly functioning dew strap generates so much heat that you can't hold it in your bare hand, it doesn't stand to reason that they would overheat just because the ends overlap. Another question would be - exactly what part of the dew strap do you think would "melt" due to the overlap? Were the straps you saw made with resistors or were they made with nichrome? Were they felt on one side and nylon on the other, or were they made using neoprene? There are too many variables and construction methods to be able to make an off-hand assessment that the cause of all the malfunctioning heat straps you've seen were due to overlapping.


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#6 decep

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Posted 01 July 2022 - 05:59 PM

How do you know they melted because they were overlapping? Straps that are not overlapped melt themselves frequently. When you consider that no properly functioning dew strap generates so much heat that you can't hold it in your bare hand, it doesn't stand to reason that they would overheat just because the ends overlap. Another question would be - exactly what part of the dew strap do you think would "melt" due to the overlap? Were the straps you saw made with resistors or were they made with nichrome? Were they felt on one side and nylon on the other, or were they made using neoprene? There are too many variables and construction methods to be able to make an off-hand assessment that the cause of all the malfunctioning heat straps you've seen were due to overlapping.

I know they melted due to the overlap because only the overlapping part melted.  I have also had melting when running them on high when covered with an insulator like a foam dew shield.

 

I generally make my own dew heater bands with nichrome wire in heatshrink tubing.  My specs are usually pretty precise, I originally targeted 300mW/cm, although I try to go to 200-250 these days.

 

This is an anecdote--my experience in the matter.  It is possible for dew heater bands to melt by overlapping.



#7 Nicole Sharp

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Posted 02 July 2022 - 07:57 AM

I have never heard that rule. I use the same heater band for all my eyepieces and it overlaps more than .5" on my smaller eyepieces.  I've never seen anything bad happen as a result.  Perhaps there's something about camera lenses that demands more precision, I don't know.

 

What was the basis of the .5" rule?  If there is no danger fo equipment damage (which seems unlikely with teh minimal temperature rises from dew heaters), maybe get the biggest heater band you know you will need, and see if anything negative happens.  If not, you've saved yourself 2 heater bands.  If the image distorts, then go  buy the other bands.

I was told this by the manufacturer of the Dew-Not heating bands.



#8 Nicole Sharp

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Posted 02 July 2022 - 08:00 AM

If you run your dew heater bands at or near 100%, you can run the risk of melting things if you overlap too much.  I have seen straps melt themselves.  If you are running them at 50% or less, it is not much of an issue.

 

It really depends on how much energy your straps use.  It is fairly standard for 12v straps to be 200-300mW/cm and these will melt if overlapping too much.

 

USB straps may be different.

That makes sense I guess.  If 0.5 inches is the maximum overlap at 100% power, then with an overlap of 1.25 inches, the maximum usable power might be only 40%.  So if you want to save money on heating bands, you would have to compensate by using less heat on the bands if they have to overlap.



#9 KLWalsh

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Posted 02 July 2022 - 01:34 PM

If you run your dew heater bands at or near 100%, you can run the risk of melting things if you overlap too much. I have seen straps melt themselves. If you are running them at 50% or less, it is not much of an issue.

It really depends on how much energy your straps use. It is fairly standard for 12v straps to be 200-300mW/cm and these will melt if overlapping too much.

USB straps may be different.


I can’t imaging running a heater band so hot than anything comes anywhere close to melting. That’d be crazy hot and would risk damaging the scope.
The times I’ve used dew straps, they were just barely warm. They need to be just warm enough to keep the dew away.

#10 decep

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Posted 02 July 2022 - 02:22 PM

I can’t imaging running a heater band so hot than anything comes anywhere close to melting. That’d be crazy hot and would risk damaging the scope.
The times I’ve used dew straps, they were just barely warm. They need to be just warm enough to keep the dew away.

Dew heater straps are very "lossy" systems.  Just making a wild guess, probably way less than 25% of the heat generated actually makes it into the scope.  Most of the energy is lost to the open air.

 

In reality, the heater wire probably gets hotter than you expect.  The strap might be "barely warm to the touch", but that is through a couple of layers of insulation.  The wire might actually be 50C (120F) internally.  You want all that heat carried away from the wire.  If you overlap the wire, it starts applying heat to itself and you can end up with a bit of a thermal runaway.




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