How to measure the length of metal sheet coil?

1. Nov 5, 2014

qnikchen

Hi everyone,

I have been trying to find an equation to measure the length of a coil. The coil is a metal sheet that wraps around the loops as it creates. It looks like a roll of tape. I have been using two different formula, but it is not giving me the right results.

1. L = 0.065449 (OD^2 - ID^2) / T; where OD is the outer diameter, ID the inner diameter, and T the thickness of the sheet.

2. L = ((ID + OD)/2 X 3.14 * # of wraps on the coil )/12.

I am not sure if I am using the right formula, but can some please give me a hint on this?

Thank you

2. Nov 5, 2014

Dr.D

Where did you get those equations? Did you derive them?

3. Nov 6, 2014

qnikchen

Hi Dr. D.

Those are found online. Both formulas takes the number in inches, and convert them in feet.

4. Nov 6, 2014

Dr.D

To get a better result andto gain more understanding, try working the problem from first principles using integration.

5. Nov 11, 2014

Baluncore

The area of metal at the end of the roll is; Area = (π * (OD/2)2) – (π * (ID/2)2)
Therefore, Area = π/4 * ( OD2 – ID2 )

The length of the coil will be Area / T
Therefore, Length = π/4 * ( OD2 – ID2 ) / T

The 0.065449 in your first equation is π/4, divided by twelve to convert from inches to feet.

6. Nov 13, 2014

Low-Q

L=[square root (OD^2 - ID^2)] x PI x turns

turns= (OR-IR)/foil thickness
Remember that most foil coils do have a foil of insolation parallell with the copper foil (If it is intended to be an inductor. So foil thickness should be both combined.

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