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A G R E AT E R M E A S U R E O F C O N F I D E N C E Resistance temperature detectors
(RTDs)
RTDs measure temperature by monitor-
ing changes in the electrical resistance of
metal wires or films. The wire used is usu-
ally platinum, although other metals, includ-
ing nickel, a nickel/iron alloy, and copper are
also available. Like T/Cs, RTDs are nonlin-
ear, and must be compensated.
RTD errors
Self-heating not accounted for--An RTD
Tips for
must conduct a current in order to read tem-
perature, and this current will heat up the
sensor or RTD. The current through a Pt100
temperature testing
RTD, for example, must be kept below 1mA
to avoid problems with self-heating, and the
current must be even smaller for nickel or
nickel/iron alloy RTDs.
Test lead compensation--The resistance
Dale Cigoy, Keithley Instruments, Inc. of the test leads can have a great effect on the
accuracy of an RTD temperature measure-
E
ment, and when leads are longer than a few
NGINEERS have a wide choice of very small output voltages, so appropriate inches it is usually better to use a three-wire
ways to measure temperature, instrumentation must be used with them. or four-wire (Kelvin) connection.
including thermocouples (T/Cs), T/Cs are nonlinear, and require specialized Using the wrong type of RTD for the tem-
resistance temperature detectors instrumentation, although many digital mul- perature range--While a platinum wire RTD
(RTDs), thermistors, solid state timeters (DMMs) are now available to work can be used from