Resistance Temperature Devices (RTD) Design

Sunday, November 19th, 2017 - Passive Transducers, Resistive Transducers, Temperature

Resistance Temperature Devices (RTD) Design

RTD Constructions

RTDs are constructed in two forms: wire-wound (Figure 1.a) and thin film (Figure 1.b). Wire-wound RTDs are made by winding a very fine strand of metal wire (platinum, typically 0.0005–0.0015 in. diameter) into a coil and packaged inside a ceramic mandrel, or wound around the outside of a ceramic housing and coated with an insulating material to prevent the sensor from shorting. Larger lead wires (typically 0.008–0.015 in. diameter) are connected to the wound wires. Wire-wound RTDs provide superior interchangeability and stability to the highest temperatures.

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Figure 1. (a) Wire-wound RTD; (b) thin-film RTD. (Courtesy of RdF Corporation, Hudson, New Hampshire, USA.)

Thin-film RTDs are produced using thin-film lithography that deposits a thin film of metal (e.g., 1 μm platinum) onto a ceramic substrate through the cathodic atomization or sputtering process. The metal is deposited in a specific pattern and trimmed by a laser to its correct resistance value. The elements are coated with a glasslike material for mechanical and moisture protection. Primary advantages of thin-film sensors are:

  1. a greater resistance can be placed in a smaller area in more versatile shapes and designs. 100, 500, and 1000 Ω thin films are available;
  2. they can be made much smaller than their wire-wound counterparts and can achieve higher sensitivity;
  3. they have higher resolution per degree of temperature, and errors due to lead wire resistance are minimized;
  4. they are relatively inexpensive;
  5. they can also function as strain gauges.

However, film sensors are less accurate than wire types.

RTD Wiring Configurations

Two-, three-, and four-wire configurations RTD

Figure 2. Two-, three-, and four-wire configurations RTD

RTDs come with two-, three-, or four-lead wires per element (Figure 2). A two-wire RTD is the least expensive, but the lead wire resistance unavoidably affects the measurement results. Two-wire RTDs are mostly used with short lead wires or where high accuracy is not required. Three-wire RTDs remove the lead wire resistance from the measurement by adding a third lead wire. This works by measuring the resistance between lead 1 and lead 2 (R1+2) and subtracting the resistance between lead 2 and lead 3 (R2+3), which leaves just the resistance of the RTD bulb (Rb) assuming that lead wires 1, 2, and 3 are all the same resistance. Three-wire configurations are most commonly used in industrial applications. When long distances exist between the sensors and measurement instruments, significant savings can be made by using the three-wire configuration instead of the four-wire configuration. Errors caused by resistance imbalance between leads can be cancelled out in a four-wire RTD circuit that is similar to a Wheatstone bridge—using wires 1 and 4 to power the circuit and wires 2 and 3 to read. Four-wire RTDs are used where superior accuracy is critical.

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