Thermal
Time Constant
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Thermal
Time Constant (T.C.):
When a thermistor is
being used to monitor the temperature
of it’s environment then
the accuracy of measurement of
the resistance of the thermistor
is critical.
While the power dissipated in the
thermistor is an important factor
in this measurement as discussed
in the previous section, the thermal
characteristics of the system and
the thermistor are important also.
This is especially relevant in
systems where the temperature is
changing with time. The dynamic
thermal response of
the thermistor must be considered
in these situations. To quantify
this dynamic response, the concept
of a Thermal
Time Constant (T.C.) is
used in the thermistor industry
and it is defined as follows:
The
Thermal Time Constant
for a thermistor is
the time required for
a thermistor to change
its body temperature
by 63.2% of a specific
temperature span when
the measurements are
made under zero-power
conditions in thermally
stable environments.
This concept is illustrated in
the example below:
Example: A
thermistor is placed in an oil
bath at 25°C and allowed to
reach equilibrium temperature.
The thermistor is then rapidly
moved to an oil bath at 75°C.
The T.C. is the time required for
the thermistor to reach 56.6°C
(63.2% of the temperature span).
| The
dominant
factors that
affect the
T.C. of a
thermistor
are: |
 |
The
mass and the
thermal mass
of the thermistor
itself |
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Custom
assemblies
and thermal
coupling agents
that couple
the |
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thermistor
to the medium
being monitored. |
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Mounting
configurations
such as a probe
assembly or
surface mounting. |
|
Thermal
conductivity
of the materials
used to assemble
the thermistor |
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in
probe housings. |
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The
environment
that the thermistor
will be exposed
to and |
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the
heat transfer
characteristics
of that environment.
Typically,
gases are less
dense than
liquids so
that thermistors
have greater
time constants
when monitoring
temperature
in a gaseous
medium than
in a liquid
one. |
The
definition of Thermal
Time Constant arises
from the exponential
nature of the rate
of transfer of
heat between the
thermistor and
the medium that
it is monitoring.
It is similar in
principle to the
definition of time
constants in describing
the responses of
systems where physical
effects have an
exponential response
with respect to
time.
BetaTHERM
offers a wide variety
of thermistor devices
with T.C.s ranging
from 100 milli-seconds
to 10 or even 20
seconds depending
on test conditions.
Graph
# 8 illustrates
determination
of T.C. for the
thermistor of
the previous
example using
a strip chart
recorder. When
the thermistor
is transferred
from a 25°C
oil bath to a
75°C oil
bath it’s
resistance will
change and the
voltage drop
across it can
be measured using
the chart recorder.
By measuring
the graph and
the speed of
the chart recorder
the T.C. for
the device in
a stable oil
bath environment
can be determined.
Time
Constant recording
of a thermistor
element
using a strip chart recorder.
Graph
# 8
The
value of resistance
of a thermistor
that is measured
in a physical system
depends on the
power dissipated
in the thermistor
due to the measurement
method and also
on the thermal
characteristics
of a dynamic temperature
system. It is important
to consider both
effects in implementing
thermistor sensing
systems.
It is useful to combine aspects
of both effects in a single parameter
and this can be achieved by definition
of a Thermal
Dissipation Constant as
described in the next section.
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