Zero-power
resistance
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Zero-power
resistance characteristic
The "zero-power
resistance characteristic" is
a description of "ideal" conditions
for resistance
measurement - it
can be defined
as follows:
The
Zero-Power Resistance
(Ro) at a specific
temperature T,
is the measured
DC (Direct Current)
resistance when
the power dissipation
is negligible.
Mil-T-23648 considers the
power to be negligible
when "any further
decrease in power will
result in not more than
a 0.1% change in resistance".
In
practical terms,
a thermistor is
generally considered
to be dissipating
Zero-power when
the current through
it is such that
the power dissipated
is less than 100
micro-Watts(µW). A
current of less
than 100 micro-Amps
(µA) will
generally meet
these requirements
in typical applications.
On modern multimeters
resistance measurements
in the kilo-Ohm
range can be performed
with adequate resolution
(+/- 0.1 Ohm) with
measuring currents
of the order of
tens of micro-amps.
There is generally
a compromise in
measuring instruments
or measurement
circuits between
resolution and
magnitude of measuring
current, but for
thermistor measurements
the self-heating
effect must be
considered also.
Zero-power
sensing refers
to applications
that use thermistors
in such a way
that the resistance
of the thermistor
will reflect
the temperature
of the medium.
Zero-power sensing
can be based
on the published
R/T data for
a thermistor,
or on the use
of the Steinhart-Hart
equation to relate
Resistance to
Temperature.
The
measured resistance
value of a thermistor
in a medium is
affected by the thermal
characteristics of
the system which
is comprised of
the thermistor
coupled with the
medium being measured.
This topic is discussed next.
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