Datasheet

72 Kanthal Appliance Alloys Handbook
Relative pitch s/d
The ratio s/d is often used. It is called the
relative pitch or the stretch factor, and may
affect the heat dissipation from the coil.
The ratio D/d is essential for the coiling
operation, as well as the mechanical stability
of the coil in a hot state.
Example – wire calculation:
Calculate the resistance of a 3-foot-long
KANTHAL D wire, 22 B & S (0.02535 in
diameter).
Combining equation [1] and [12]:
d
s
r =
[--]
R
20
= ρ
and
L
q
q =
π
4
· d
2
R
20
=
ρ · L · 4
π · d
2
ρ = 1.35 Ω mm
2
/m = 812 Ω/cmf =
= 637.79 Ω/smf
L = 3 foot = 3 · 0.305 m
d = 0.02535 in = 25.35 mil = 0.644 mm
Metric units:
Imperial units (cmf):
= 3.79 Ω
R
20
=
1.35 · 3 · 0.305 · 4
π · 0.644
2
R
20
=
812 · 3
π · (0.02535)
2
R
20
=
812 · 3
25.35
2
= 3.79 Ω
The unit Ω/smf is used principally for con-
ductors with rectangular cross sections. Even
here length is given in feet and width and
thickness in mils.
Example – ribbon calculation:
Calculate the resistance of a KANTHAL D
ribbon 10 feet long, where t = 0.04 in and b
= 0.5 in.
Combining equation [1] and [14]:
R
20
= ρ
and
L
q
q = 0.92 · b · t
R
20
=
ρ · L
0.92 · b · t
ρ = 1.35 Ω mm
2
/m = 812 cmf
L = 10 foot = 10 · 12 in = 10 · 0.305 m
t = 0.04 in = 40 mil = 0.04 · 25.4 mm =
= 1.016 mm
b = 0.5 in = 500 mil = 0.5 · 25.4 mm =
= 12.7 mm
Metric units:
Imperial units (smf):
= 0.346 Ω
R
20
=
1.35 · 10 · 0.305
0.92 · 12.7 · 1.016
R
20
=
812 · 10
0.92 · 500 · 40 · 1.2732
= 0.346 Ω
π · (D – d) · L
e
s =
L
[17]