R= ------------
A
Where
R = Resistance (ohms)
r = Resistivity (ohms)
L = Length
A = Cross sectional area
RTD Materials
Some of the common RTD materials are:
Copper, nickel iron are also commonly used RTD materials.
METAL RESISTIVITY OHM/CMF
Because of their lower resistivities, gold and silver are rarely used as RTD elementsThe most common RTD’s are made of either platinum,nickel, or nickel alloys.
Platinum with a temperature coefficientof 0.00385 - 0.003923 //°C and practical temperature range of -452 to +1100°F (-269 to +593°C).The platinum RTD has the best
accuracy and stability.
Leadwire Configurations: RTDs are available with four different leadwire
configurations. The selection of leadwire configuration is based on desired accuracy and instrumentation to be used for the measurement.
Two wire RTD: The two wire RTD is the simplest wire configuration. One wire is attached to each side of the element. A measure can be taken by any device equipped to measure resistance,including basic Volt Ohm Meters (VOM).This is the least accurate way of
measuring temperature, due to the fact that the leadwire resistance is in series with the sensing element. The leadwire is at a different temperature than the sensing element and also has different resistance verses temperature characteristics. The longer the leadwire
the greater the effect on the measurement. A typical two wire bridge looks like the following:
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Three Wire RTD: The three wire RTD is the most popular configuration for use in
industrial applications. When used correctly, the three wire configuration eliminates the series resistance. This permits an accurate measurement of the sensing element. Two of the leads are connected to one side of the sensing element and the single lead to the other
side.The resistance in L1 and L3 should be matched as close as possible, this will cause the lead resistance to cancel themselves.The color code...