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Instrumentation Table of Contents
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Resistance Temperature Detectors (RTD's)
incorporates pure metals or certain alloys that increase in
resistance as temperature increases and, conversely, decrease
in resistance as temperature decreases. RTDs act somewhat
like an electrical transducer, converting changes in
temperature to voltage signals by the measurement of
resistance. The metals that are best suited for use as RTD
sensors are pure, of uniform quality, stable within a given
range of temperature, and able to give reproducible
resistance-temperature readings. Only a few metals have the
properties necessary for use in RTD elements.
RTD elements are normally constructed of
platinum, copper, or nickel. These metals are best suited for
RTD applications because of their linear
resistance-temperature characteristics (as shown in Figure
1), their high coefficient of resistance, and their ability
to withstand repeated temperature cycles. The coefficient of
resistance is the change in resistance per degree change in
temperature, usually expressed as a percentage per degree of
temperature. The material used must be capable of being drawn
into fine wire so that the element can be easily
constructed. RTD elements are usually long, spring-like
wires surrounded by an insulator and enclosed in a sheath of
metal. Figure 2 shows the internal construction of an RTD.
This particular design has a platinum element
that is surrounded by a porcelain insulator. The insulator
prevents a short circuit between the wire and the metal
sheath. Inconel, a nickel-iron-chromium alloy, is normally
used in manufacturing the RTD sheath because of its inherent
corrosion resistance. When placed in a liquid or gas medium,
the Inconel sheath quickly reaches the temperature of the
medium. The change in temperature will cause the platinum
wire to heat or cool, resulting in a proportional change in
resistance. This change in resistance is then measured by a
precision resistance measuring device that is calibrated to
give the proper temperature reading. This device is normally
a bridge circuit, which will be covered in detail later in
this text.
The illustration below, shows an RTD
protective well and terminal head. The well protects the RTD
from damage by the gas or liquid being measured. Protecting
wells are normally made of stainless steel, carbon steel,
Inconel, or cast iron, and they are used for temperatures up
to 1100°C.
RTD Protective Well and Terminal Head
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