Application Guide catalog - Watlow - #50

Reference 33 shows a two-wire RTD connected lo a typical Wheatsione bridge circuit. l_£ is the supply current; E₀ is the output voltage; Ri, Rg, F^are fixed resistors; and RT is the RTD. In this circuit, lead resistances l_i and L2 add directly to RT.

The Art of Temperature Sensing

Resistance Temperature Detectors—RTDs

Continued

The majority of RTD applications throughout industry today standardize on the three-lead wire systems. Frequently the question arises— what is the difference in two-, three-or four-wire BTDs and how does the system operate?

The key Issue Is accuracy. Most manu­facturers will specify accuracy as 0.1 percent or a similar figure. This number only refers to how tightly the element (resistor) Is calibrated at one temperature and does not reflect the total sensor accuracy after lead wire has been added to the resistance element. Understanding how the bridge circuit operates should answer these questions and further empha­size the value of three- and four-wire systems, when accuracy is important to the user.

Because an RTD is a resistance type

wire between the resistive element and control instrument will add to (he readings. Furthermore, this added resistance is not constant, since the conductor in lead wires changes resistance with changing ambient temperature. Fortunately, errors may be nearly canceled by using a three-or four-wire system.

celing their resistance, since they're in two separate arms of the bridge. \-2. connected to E₀, is used only as a potential lead; no current flows through it when the bridge is balanced. This method of lead wire compen­sation depends on close matching of the resistance in Li and L3 and high impedance at E₀, since any current flow in L2 will cause errors. The two common leads. L₂ and L3, are normally the same color for easy identification.

in the three wire circuit shown in Reference 'M tie dentica: measuring current flows through Li and L₃, can-

resistance become significant. Although many laboratory systems employ resis­tive networks for four-wire compensa­tion, the most common industrial circuit drives a constant current through two leads, and measures current drop across the remaining two (see Reference 35). Assuming that input impedance pre­vents current flow in |_2 and L3, the o"ly significant source of error is variation in the measuring current.

Four-wire circuits offer the ultimate or where small errors such as contact

l^; necessary, you can connect a two wire RTD to a three-wire circuit, or a three-wire RTD to a four-wire circuit. Just attach the extra extension wires to the ends of the RTD leads, as shown in Reference 36. As long as these connections are close to the sensing element, as in a connection head, errors should be negligible.

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