Resistance Temperature Detectors
RTD's function as a result of
increasing resistance in proportion to increasing temperature: i.e.,
with a known current, as temperature increases, voltage increases.
As a known current is placed through the RTD, the output voltage
indicates RTD resistance. Since the relationship between the
resistance and the temperature is known, the temperature can thus be
The resistivity of the metal
used in an RTD is dependent upon the temperature. Platinum has the
best range and is the most common metal used.
The platinum RTD element is
manufactured to have a set resistance at 0° C; however, once
incorporated into an instrument loop, the lead resistance of the loop
must be compensated for in order to achieve an actual temperature
reading. For this reason, while the 2-wire RTD probe may be
adequate for very short lead lengths, it is inherently inaccurate and
The most commonly used RTD is
the 3-wire probe, which allows for the compensation of loop resistance
by introducing a third wire, known as a reference wire, that is equal in
length and size to the two wires connecting the RTD to the readout
device. This third wire bypasses the RTD at its junction to one of
the other two wires, and allows the readout device to automatically
subtract the lead resistance from the overall resistance of the circuit.
Offering even greater
potential for accuracy is the 4-wire RTD, which uses two reference
wires. However, the system must be designed to accommodate this
The usual nominal resistance
of platinum RTDs is 100ohms at 0° C. Unfortunately, standards are
not identical worldwide, which presents a problem when a RTD built to
one standard is used with an instrument designed to a different
standard. In addition, manufactured tolerances must be
considered. Not only do they vary with the manufacturer and the
standard, but the tolerances are also affected by the manufacturing
GAYESCO offers platinum RTDs
to all standards; the DIN and American standards the most
available. Our normal tolerance is 0.1% of the reading. The
following table delineates the most common standards.
GAYESCO RTD assemblies are TIG-welded to
avoid possible contamination from brazing flux. Our strict
manufacturing procedures are designated to prevent element contamination
from chemicals and moisture.