Thermal
Dissipation Constant
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Thermal
Dissipation Constant (D.C.):
Because
the measured resistance
of a thermistor at
a particular time
depends on the power
dissipated in the
thermistor during
measurement and on
the thermal dynamics
of the system being
measured, it is useful
to quantify the combined
effect of these two
factors. This leads
to the concept of Thermal
Dissipation Constant
(D.C.), which
is defined as follows:
The
Thermal Dissipation
Constant of a thermistor
is defined as the
power required
to raise the thermistor’s
body temperature
by 1°C in a
particular measurement
medium. The D.C.
is expressed in
units of mW/°C
(milliWatts per
degree Centigrade).
BetaTHERM
specify the D.C.
for Epoxy Coated
BetaCURVE and BetaCHIP
Thermistor series
(which are described
later) as typically
0.5mW/°C to 1.0mW/°C
in still air at 25°C,
and 7mW/°C to
8mW/°C in a well
stirred oil bath
at 25°C.
| The
D.C. is a very
important parameter
in circuit design
and application
considerations.
In practical
applications
the D.C. will
be affected by: |
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the
mass or thermal
mass of a thermistor. |
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the
mounting of the
thermistor in
a probe assembly. |
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the
thermal dynamics
of the environment
that the thermistor |
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is
to monitor. |
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the "ranging" of
measuring instruments
that change current |
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levels
as measurement
ranges change
to track resistance
changes of thermistors. |
The
D.C. is an important
factor in applications
that are based on
the self-heating
effect of thermistors.
In particular, the
resistance change
of a thermistor due
to change in D.C.
can be used to monitor
levels or flow rates
of liquids or gasses.
For example as flow
rate increases, D.C.
of a thermistor in
a fluid path will
increase and the
resistance will change
in a manner that
can be correlated
to flow rate.
The
three factors, zero-power
resistance (Ro),
time constant (T.C.),
and dissipation constant
(D.C.) influence
the measured value
of the resistance
of a thermistor which
will affect temperature
values that are calculated
from the resistance
measurements. An
understanding of
these factors is
critical in developing
thermistor applications
and in measurement
of thermistors.
The
importance of the
three factors discussed
previously can be
understood more completely
by studying the Voltage-Current
characteristics of
thermistors.
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