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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