Datasheet

Kanthal Appliance Alloys Handbook 69
10
R
T
= C
t
· R
20
[2]
The following formulas and definitions are
applied to all applications.
Definition :
Resistivity R [Ωmm
2
/m] or [Ω/cmf]
The resistance of a conductor, R
20
, is
directly proportional to its length, L and
inversely proportional to its cross-sectional
area, q:
[Ω]
The proportional constant, ρ is defined as
the resistivity of the material and is
temperature dependent. The unit of ρ is in
metric system [Ωmm
2
/m] respectively for
imperial system [Ω/cmf].
[Ω]
Definition :
Temperature factor C
t
[--]
Resistivity or change in resistance with
temperature, is non-linear for most resis-
tance heating alloys. Hence, the temperature
factor, C
t
, is often used instead of tempera-
ture coefficient. C
t
is defined as the ratio
between the resistivity or resistance at some
selected temperature θ °C and the resistivity
or resistance at 20 °C (68 °F).
R
20
= ρ
L
q
[1]
[3]
C
t
=
R
T
R
20
[--]
C
t
= 1 + (θ – 20)α (Where θ is in °C)
[4]
C
t
= 1 + (θ – 68)α (Where θ is in °F)
[5]
2. Formulas and Definitions
Definition :
Surface load p [W/cm
2
] or [W/in
2
]
The surface load of a heating conductor, p, is
its power, P, divided by its surface area, A
C
.
[7]
A
C
= π · d · L · 10
metric / imperial
Wire
strip / ribbon
A
C
= π · d · L · 12
[6]p =
P
A
c
[W/cm
2
] or [W/in
2
]
[8]
A
C
= 2 · (b + t) · L · 10
A
C
= 2 · (b + t) · L · 12