Slope Resistance
Ratio
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Slope:
(Resistance Ratio):
In
considering the
relationship between
resistance and
temperature of
thermistors there
are some important
concepts that are
used in the thermistor
industry. One such
concept is slope,
which is an indication
of the rate of
change of the resistance
of the component
with temperature.
The slope or resistance
ratio for thermistors is
defined as the ratio of
resistance at one temperature
(usually 0°C) to the
resistance at a second
and higher temperature
(usually 70°C).
The concept of resistance
/ slope is demonstrated
in Graph # 2, (relating
to temperature coeffecients),
where the 0/70°C slope
line connects the resistance
value at 0°C to the
resistance value at 70°C.
This provides an indication
of the rate of change of
resistance with temperature
and the potential thermal
sensitivity of the component.
Slope measurements are
used as a qualification
step in the process of
manufacturing "chip" thermistors.
This is a monitoring step
at an early stage of the
process to ensure that
that the thermistor material
will meet required specifications.
The following example illustrates
how the slope value of
a thermistor can be calculated
using the Resistance versus
Temperature tables provided
in the product
section of this website.
Slope: (Resistance Ratio)
Example of calculations:
For BetaTHERM’s 30K5
thermistors (which have
a nominal resistance of
30000 ohms at 25°C)
(Curve 5 Material) the
nominal R0/R70 ratio will
equal 17.73
Per the R-T tables the
nominal 30K5 resistance
at 0°C = 94980 ohms
and the nominal resistance
at 70°C = 5358 ohms:
Slope or resistance ratio
provides an introduction
to the concept of rate
of change of resistance
with temperature and the
sensitivity of the resistance
of thermistors to temperature
change. This concept is
developed further by considering
the more general case of
thermal sensitivity in
terms of percentage resistance
change of a component per
degree centigrade increase
in temperature.
In the thermistor industry,
this topic is dealt with
by the definition of a
material parameter known
as Alpha ( ).
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