Selecting an RTD

RTD Element Types

When deciding the RTD element type, first consider what instrument you will be reading the sensor with. Choose an element type that is compatible with the instrument’s sensor input. By far the most common RTDs are 100 Ohm Platinum with .00385 temperature coefficient. Type PB is an example

Type Element Material Base Resistance (Ohms) TCR (Ohm/Ohm/C) Base Resistance Tolerance +/- TCR Tolerance %
CB COPPER 10 OHMS AT 25C .00427 0.2% 1%
CC COPPER 10 OHMS AT 25C .00427 0.5% 1%
NA NICKEL 120 OHMS AT 0C .00672 0.5% 1%
PA PLATINUM 100 OHMS AT 0C .00385 0.06% 0.12%
PB PLATINUM 100 OHMS AT 0C .00385 0.12% 0.35%
PC PLATINUM 100 OHMS AT 0C .00385 0.5% 1%
PE PLATINUM 100 OHMS AT 0C .00391 0.12% 0.35%
PF PLATINUM 100 OHMS AT 0C .00391 0.5% 1%
PG PLATINUM 100 OHMS AT 0C .00375 0.12% 0.35%
PH PLATINUM 100 OHMS AT 0C .00392 0.5% 0.1%

Accuracy

Secondly, decide what accuracy is needed in your measurement. Accuracy is a combination of both base resistance tolerance (resistance tolerance at the calibration temperature) and temperature coefficient of resistance tolerance (tolerance in the characteristic slope). Any temperature above or below this temperature will have a wider tolerance band or less accuracy (see graph below). The most common calibration temperature is 0°C.

Platinum RTD Standard Accuracy Resistance vs. Temperature


Leadwire Effects

Measurement of temperature with a resistance temperature detector is a matter of measuring resistance. Unbalanced Wheatstone bridges are invariably used to measure the resistance. When measuring the resistance of the sensing element, all external factors must be minimized or compensated for, in order to obtain an accurate reading.

A major cause of error can be the resistance of the leadwires, especially in two lead configurations.



The resistance is in series with the sensing element, so the readout is the sum of the resistances of the sensing element and the leadwires. Two-lead RTD’s are possible when the sensing element has a high resistance and the leadwires have a low resistance.

When the leadwire resistance is comparably high, however, it must be compensated. Compensation can be achieved with a three-lead configuration. As shown in the three-lead diagram, one side of the power supply is taken to one side of the RTD via L3. This puts L1 and L2 in opposite arms of the bridge so they cancel each other out and have no effect on the bridge output voltage.



Three-lead connections are recommended for RTDs, especially with low sensing element resistance, where a small leadwire resistance can have a large effect on readout accuracy.