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Nichrome Heat
seedlings
Mike...Dan... anyone...

This is based on the charts for nichrome wire on http://www.heater...chrome.htm

OK. I understand the math, as far as determining how many amps I have to spend on heat, then Ohm's Law to determine what length of nichrome wire. I have a question regarding the open coil nichrome. The chart for 10 foot open coil says 10 feet of 24ga is 695 ohms - so each foot is 69.5 ohms??? This doesn't sound close when the same site says 24 ga straight wire is 1.671 ohms per foot. I realize it's coiled, but there's some other math I'm missing.

How do I calculate resistance for the open coil wire?

Thanks in advance
CHAD
Edited by seedlings on 11/16/2007 2:25 PM
Roaster: CoffeeAir II 2# DIY air roaster
Grinder: Vintage Grindmaster 500
Brewers: Vintage Cory DCU DCL, Aeropress, Press, Osaka Titanium pourover
 
Brainiac
CHAD

perhaps the resistance of the coil is for the length of the COIL not the length of the WIRE in the coil? ( the image in Http://www.infraredheaters.com/nicrcoil.htm shows a TIGHTLY-WOUND 10 foot COIL @ 695 ohm) pull this out to straight WIRE and.......:(

Brian
 
Alchemist
They are telling you the total ohms per 10 foot coil. You can run the numbers backwards to confirm.

They say the approximate outer diameter is 0.29". When you coil wire, the outer diameter stretches and the inner diameter compresses, so you want to estimate from the half way point. 24 gauge wire is 0.0201".

If inner diameter is going to be 0.29-0.0201-0.0201, but 1/2 way would be 0.29-0.0201 or 0.2699".

That means each loop of the coil is going to contain a length of straight wire the circumference of that coil or 0.848" ( dia x 3.14... ). Again, 24 gauge steel wire is 0.0201" so there are approximately 5970 coils per 10 feet ( 120" / 0.0201" ).

5970 coil x 0.848"/coil = 5062" of "straight" wire or 421.8 feet.

421.8 feet x 1.671 ohms/foot = 704.9 ohms. A stones throw of 695.

Quote


How do I calculate resistance for the open coil wire?


I would count coil loops for ease.

There are about 50 coils/in. ( 1 / 0.0201 )

Using their total ohms ( it's why they gave it ) 695 / 10 / 12 = 5.791 ohms per inch of coil wire or per 50 coils.

Does that give you what you wanted to know?
 
farmroast
http://cgi.ebay.c...dZViewItem
I'm trying 2 of these in a large roaster. But 1 is 1500w 14amp 115v
Edited by farmroast on 11/17/2007 12:20 PM
Ed B.
DreamRoast 1kg roaster, Levers, Hand Mills http://coffee-roa...gspot.com/
 
Alchemist
Ed,

If you have not purchased these, you may wish to reconsider. That is WAY expensive. You are getting 2 feet for around $44. You can get 5 times that amount of Nichrome for the same price.

It may claim to last 2-3 times as long, but I expect that is at kiln temperatures. At our roasting temps, that is moot point IMO.
Edited by Alchemist on 11/17/2007 12:34 PM
 
seedlings

Quote

Alchemist wrote:
They are telling you the total ohms per 10 foot coil. You can run the numbers backwards to confirm.

They say the approximate outer diameter is 0.29". When you coil wire, the outer diameter stretches and the inner diameter compresses, so you want to estimate from the half way point. 24 gauge wire is 0.0201".

If inner diameter is going to be 0.29-0.0201-0.0201, but 1/2 way would be 0.29-0.0201 or 0.2699".

That means each loop of the coil is going to contain a length of straight wire the circumference of that coil or 0.848" ( dia x 3.14... ). Again, 24 gauge steel wire is 0.0201" so there are approximately 5970 coils per 10 feet ( 120" / 0.0201" ).

5970 coil x 0.848"/coil = 5062" of "straight" wire or 421.8 feet.

421.8 feet x 1.671 ohms/foot = 704.9 ohms. A stones throw of 695.

Quote


How do I calculate resistance for the open coil wire?


I would count coil loops for ease.

There are about 50 coils/in. ( 1 / 0.0201 )

Using their total ohms ( it's why they gave it ) 695 / 10 / 12 = 5.791 ohms per inch of coil wire or per 50 coils.

Does that give you what you wanted to know?


Yes, s:1 John, that sums it up. s:2

This is why I failed physics, then re-took it and got a D. I can figure all the individual math concepts (even calculus bcak then)... but obviously have a problem with the big picture.

It's a little late, but I do have one more question:

A 37 gram stone is tied to a 50cm string. 30cm of that string is the radius from a hole, with the rest of the string hanging down, attached to a 300g weight. The stone is spinning around just fast enough to overcome gravity and stay in a horizontal plane, the hole is it's axis. The weight is pulling the string down the hole at a rate of 10cm per minute. What will the velocity of the stone be in 13 minutes?

CHAD
seedlings attached the following image:
Physics[603].jpg

Roaster: CoffeeAir II 2# DIY air roaster
Grinder: Vintage Grindmaster 500
Brewers: Vintage Cory DCU DCL, Aeropress, Press, Osaka Titanium pourover
 
Alchemist
Zero.
 
Dan
Chad, WHY do you need to know the resistance of straight wire? Surely you aren't going to uncoil that nichrome and use it that way.

Are you confused by the term 'open coil'? That doesn't NOT mean the length of the wire when strecthed out and uncoiled into straight wire. All of John's math is nice, but I think it is overkill here. Here's why:

When this company makes and ships coiled wire the coils are touching. This is called closed coil. If you were to measure its resistance it would be very small because the coils are touching and the conductor is no longer fine wire, but a thin walled tube.

You NEVER use these heaters with the coil closed since applying electricity to it would essentilly be creathing a short circuit. Say goodbye to your nichrome.

Instead, the coils are opened. It should be stretched out until the there is 3 wire diameters of space between each coil.

How to measure the right amount of wire if each person stretches the wire to different lenghts? Simple, measure the wire when the coils are still closed. This is why they are telling you the closed coil resistance.

For my heater using 19ga wire, I measure 6.75" of closed coil, straighten the last two coils at each end to clamp in the terminal block, then stretch out the remainder to fit my ceramic blocks. A perfect 1600W heater every time!

Hope this helps, Dan
 
Alchemist
Dan, I agree, they are completely overkill. But I find that kind of math helps me fully understand what is going on.

He didn't ask, so I didn't answer, but you absolutely have to measure the closed coil. And that was also why I gave the practical answer of ohms per inch of closed coil.
 
Dan
LOL! I'm so freakin' goal oriented that I go right to the 'What practical thing is needed to be done NEXT." The fun, for me, is making a project go.
 
Alchemist
Oh, and I just adore the journey. The destination has to be worth getting to, but I love getting there, including all the really interesting side trips (unless they are deadend paths, in which case I ignore them).
 
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