R ESISTANCE T E M P E R AT U R E D ETECTORS ( RT D S )

R ESISTANCE T E M P E R AT U R E D ETECTORS ( RT D S )

Resistance temperature detectors (RTDs) are temperature transducers made of conductive wire elements. The most common types of wires used in

RTDs are platinum, nickel, copper, and nickel-iron. A protective sheath material (protecting tube) covers these wires, which are coiled around an insulator that serves as a support. Figure 13-10 shows the construction of an RTD. In an RTD, the resistance of the conductive wires increases linearly with an increase in the temperature being measured; for this reason, RTDs are said to have a positive temperature coefficient.

Insulator

Resistive Element

Protective Sheath

Figure 13-10. Resistance temperature detector.

RTDs are generally used in a bridge circuit configuration. Figure 13-11 illustrates an RTD in a bridge circuit. As mentioned in the previous section,

a bridge circuit provides an output proportional to changes in resistance. Since the RTD is the variable resistor in the bridge (i.e., it reacts to temperature changes), the bridge output will be proportional to the tempera- ture measured by the RTD.

As shown in Figure 13-11, an RTD element may be located away from its bridge circuit. In this configuration, the user must be aware of the lead wire resistance created by the wire connecting the RTD with the bridge circuit. The lead wire resistance causes the total resistance in the RTD arm of the bridge to increase, since the lead wire resistance adds to the RTD resistance. If the RTD circuit does not receive proper lead wire compensation, it will provide an erroneous measurement.

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R RTD

Figure 13-11. RTD in a bridge circuit.

Figure 13-12 presents a typical wire compensation method used to balance lead wire resistance. The lead resistances of wires L1 and L2 are identical because they are made of the same material. These two resistances, R L1 and

R L2 , are added to R 2 and R RTD , respectively. This adds the wire resistance to two adjacent sides of the bridge, thereby compensating for the resistance of the

lead wire in the RTD measurement. The equations in Figure 13-12 represent the bridge before and after compensation. Note that R L3 has no influence on the bridge circuit since it is connected to the detector (e.g., input module, amplifier, etc.).

without lead wire consideration

R 2 R RTD

R 1 R 3 = taking lead wire into consideration

R 2 R RTD + R L 1 + R L 2 (no compensation)

R 1 R 3 = taking lead wire into consideration R 2 + R L 1 R RTD + R L 2 (with compensation)

Figure 13-12. RTD bridge configuration with lead wire compensation.

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As mentioned previously, the changes in RTD resistance are proportional to changes in temperature. The following equation defines these resistance changes:

where: R T = the change in resistance at temperature T

R T 0 = the RTD resistance at a reference temperature point T 0

(e.g., copper is 10 at 25 C) Ω ° α 1 = a constant per degree Celsius that varies with the first

RTD material α 2 = a constant per degree Celsius that varies with the second

RTD material

When an RTD is connected to a PLC’s RTD input module, the interface determines the temperature (T) based on changes in resistance R T . The module stores this value, which is calculated through an equation that corresponds to the RTD’s type of input (e.g., copper), in a table. During this process, the input module also compensates for lead wire connections.

If an RTD is used with a standard analog input module, the user must design the bridge circuit, as well as the amplifier, so that the signal matches that of the input module range (e.g., 0 to 10 VDC). To do this, the PLC must compute the temperature by determining the temperature-versus-voltage curve. It determines this linear curve by analyzing another curve, the resis- tance-versus-temperature curve. It then computes the temperature using the temperature-versus-VDC equation or the linear interpolation look-up table for the input count value of the analog input voltage. This technique can be used with any transducer that uses a bridge circuit for signal detection (e.g., thermistor, strain gauge, etc.). If the transducer’s temperature detection range is linear with respect to resistance, the PLC can use an equation to compute the temperature. If the transducer’s temperature detection range is not linear, the PLC must perform a linear interpolation based on a look-up table. Chapter

7 explains linear equations in analog readings, while Chapter 11 provides examples of linear interpolations of analog readings.