RTD elements take either of two forms: wire wound (see Figure 1) or thin film. Wire-wound elements are made primarily by winding a very fine strand of platinum wire into a coil until there is enough material to equal 100 Ω of resistance. The coil is then inserted into a mandrel and powder is packed around it to prevent the sensor from shorting and to provide vibration resistance. This is a time-consuming method and all work is done manually under a microscope, but the result is a strain-free design.
|Figure 1. The wire-wound element is built by winding a small-diameter platinum sensing wire around a nonconducting mandrel.||Figure 2. The thin film sensing element is made by depositing a thin layer of platinum in a resistance pattern on a ceramic substrate. A layer of glass or epoxy is applied for moisture protection.|
When discussing RTDs, several specifications must be considered:
- Wiring configuration (two-, three-, or four-wire)
- Response time
Serious lead wire resistance errors can occur when using a two-wire RTD especially in a 100 Ω sensor. In a two-wire circuit, a current is passed through the sensor. As the temperature of the sensor increases, the resistance increases. This increase in resistance will be detected by an increase in the voltage ( V = I � R). The actual resistance causing the voltage increase is the total resistance of the sensor and the resistance introduced by the lead wires. As long as the lead wire resistance remains constant, it will not affect the temperature measurement. The wire resistance will change with temperature, however, so as the ambient conditions change, the wire resistance will also change, introducing errors. If the wire is very long, this source of error could be significant. Two-wire RTDs are typically used only with very short lead wires, or with a 1000 Ω element.
In a three-wire RTD there are three leads coming from the RTD instead of two. L1 and L3 carry the measuring current, while L2 acts only as a potential lead. Ideally, the resistances of L1 and L3 are perfectly matched and therefore cancelled. The resistance in R3 is equal to the resistance of the sensor, Rt , at a given temperature (usually the midpoint of the temperature range). At this point, no current passes through the centre lead. As the temperature of the sensor increases, the resistance of the sensor increases, causing the resistance to be out of balance. Current then flows in the centre lead and will indicate an offset temperature.
The optimum form of connection for RTDs is a four-wire circuit It removes the error caused by mismatched resistance of the lead wires. A constant current is passed through L1 and L4; L2 and L3 measure the voltage drop across the RTD. With a constant current, the voltage is strictly a function of the resistance and a true measurement is achieved. This design is slightly more expensive than two- or three-wire configurations, but is the best choice when a high degree of accuracy is required.
Self-Heating. To measure resistance, it is necessary to pass a current through the RTD. The resultant voltage drop across the resistor heats the device in an effect known as the I 2 R, or Joule heating. The sensor's indicated temperature is therefore slightly higher than the actual temperature. The amount of self-heating also depends heavily on the medium in which the RTD is immersed. An RTD can self-heat up to 100 3 higher in still air than in moving water.