Resistive temperature detectors (RTD) measure the change in the electrical
resistance of a wire-wound resistor with temperature, typically, a platinum
resistance element is used with a resistance of about 100 Ω. The resistance
change can be measured in a bridge circuit, but normally the resistor is driven
from a constant current source and the voltage developed across the resistor
measured. Care must be taken with these devices to ensure that the current
flowing through the devices is low to minimize the temperature changes occurring
due to the internal heating of the resistor. Pulse techniques can be used to
prevent internal heating. In this case the current is turned on for a few milliseconds,
the voltage measured and then turned off for, say, a second. Figure 12.13a
shows the simplest connection to the RTD with just two leads, the meter being
connected to the current supply leads. The resistance of long leads between the
detector and the resistor contribute to measurement error, as the meter is measuring
the voltage drop across the current lead resistance and junctions as well
as the RTD.