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Where a thermocouple senses temperature as a function of the voltage generated from the
difference in temperature between the open and closed ends, a Resistance Temperature
Detector (RTD) senses temperature as a result of the change in resistance that occurs when
it changes temperature.
Platinum is the material used in most RTDs. It is highly resistant to contamination and
has a predictable resistance change as a function of temperature. Classically, platinum
wire is wound around a ceramic bobbin.
Other materials used in RTDs are tungsten, copper, nickel and nickel alloys. The common
characteristic of most of these materials, including platinum, is high resistivity (copper
is the exception). The higher the resistivity, the less metal that needs to be used.
Because of its stable and predictable characteristics, platinum is the first choice for
use in RTDs.
Some RTDs are constructed by depositing a platinum film on a substrate. It is then
encapsulated for protection. Because these thin-film devices are small, they respond
rapidly to changes in temperature.
RTD temperature measurements might be made by measuring the resistance of the sensor
and using that reading to determine its temperature. Because the resistance of an RTD is
quite low, however, the resistance of the wires connecting the RTD to the meter can be
significant. To eliminate the error introduced by the lead resistance, a Wheatstone Bridge
or similar circuit is used. When the RTD is connected in this manner, it is possible to
read only the resistance of the RTD. This is why most RTDs have three or more leads.
Unless otherwise stated, all SensorPulse RTDs are made for three wire operation.
In the bridge circuit, the RTD is one resistor in the group of resistors which comprise
the bridge. A small regulated current, sometimes known as a "search current" is
transmitted through the device. At a constant current, the voltage measured across the
bridge will vary with the resistance and provide a value that can be interpreted as
temperature.
RTDs are generally less rugged and more expensive than other sensors. They offer,
however, higher accuracy (typically ±0.10% of reading), repeatability and long-term
stability.
RTDs are typically used in food, laboratory and pharmaceutical applications, and in
other applications requiring high accuracy.
All SensorPulse RTDs are manufactured to industry standards and meet stringent ANSI standards.
This assures interchangeability with other standard RTDs without requiring additional
instrument recalibration.
SensorPulse RTDs are used in all types of applications, can measure wide temperature ranges,
and are offered in a large variety of standard configurations.
All SensorPulse RTD assemblies are designed for process temperatures up to 850° F (500°F for
teflon wire). If higher service temperatures are required, please consult the factory. See
Applications Guide for additional information
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