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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