Self
Heating Sensing Applications
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Self-Heat
Sensing Applications:
Overview
Thermistor self heat mode sensing
applications are based on the voltage-current
characteristics of the thermistor,
and on the thermal characteristics of
the thermistor device and it’s
environment.
Applications involving both static
and time dependent conditions
have been developed. This section
will describe the principles
of the use of thermistors in
self-heat mode for liquid level
sensing, flow sensing and voltage
regulation.
Liquid
Level Sensing
When a thermistor is in self-heat
mode it heats up and dissipates
energy, and it will have thermal
characteristics that depend on
it’s environment, as discussed
earlier. For example, if the
thermistor is in a static
gas environment, the thermistor
might have a voltage-current
characteristic similar to Curve
Go of Graph # App 1.
Heat
dissipation in static
liquids is roughly ten
times greater than
that of a static
gas environment,
and Curve Lo of Graph
# App 1. represents
the voltage-current
characteristic of
a thermistor in a
static liquid.
If
the thermistor sensor
is immersed in such
a liquid, the increased
heat dissipation
will cool the thermistor
and it’s resistance
will increase. This
difference in resistance
value of the thermistor,
due to the difference
in thermal characteristics
in self heat mode,
between being in
a static gas (air)
and being in a static
liquid is the basis
of self-heat liquid
level sensing applications.
Fluid
Flow Sensing
An important range of thermistor
applications are based on heat
transfer differences between static and moving
fluids. These applications
depend on an understanding of
mathematical expressions describing
the heat transfer behaviour at
the interface of the sensing
element and the fluid. Heat transfer
behaviour at such fluid/sensor
interfaces has been determined
for many gases and liquids under laminar
flow conditions.
The
difference in voltage-current
characteristic of
a thermistor sensor
in self-heat mode
between static and
moving fluids can
be significant. This
is indicated in Graph
# App 1, where Curve
Go represents
the thermistor in
a static gas environment, Curve
G1 represents
the thermistor in
a flowing gas, and Curve
G10 represents
the thermistor in
a faster flowing
gas, for laminar
flow conditions.
In
flow sensing applications,
voltage levels may
be used to govern
power dissipation
when a constant current
is applied to the
thermistor element.
This principal is
the basis for computerized
automotive air/fuel
control and fluid
flow monitoring.
Thermistors can also be used
for measurements where the pressure
of the fluid medium may vary.
Such applications include measurements
on low pressure gasses and at
vacuum levels. Curve Gv of Graph
# App 1 represents typical
voltage-current characteristics
for a thermistor in self-heat
mode in a vacuum environment.
It shows that heat dissipation
is substantially reduced compared
to a static atmospheric air environment
which is represented by Curve
G0.
Voltage-Current
Behaviour for Level
and Flow Sensing Applications:
Graph
# App 1
Gas
Analysis using
thermistors in
self heat mode:
A modified Wheatstone bridge
circuit utilizing two extremely
small, fast response "matched" self-heated
thermistor elements has been
developed for gas thermal conductivity
and chromatography analysis.
Gasses of different molecular
weights have different dissipation
constants if other conditions
are kept constant. This is an
important principle of gas analysis
using thermistors.
The
general principle
is that one of the
sensors is placed
in a reference gas
while the other is
used to monitor a
gas that is to be
analyzed. The use
of a matched pair
of sensors ensures
that the deflection
of the bridge is
due to differences
in thermal characteristics
between the reference
gas and the gas being
analyzed, rather
than being due to
differences in the
thermistors. Properties
of the gas being
analyzed can then
be compared with
properties of the
reference gas for
characterization
of it’s parameters
or of it’s physical
state. Practical
implementation details,
such as instrumentation selection
and calibration of
the measurement system
for applications
of this nature are
beyond the scope
of this catalog,
but relevant information
can be found in literature
on thermistor applications.
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