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. |
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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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