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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