Figure 12.13b shows a 4-wire connection to an RTD. The meter connects
directly to the RTD so that only the voltage drop across the RTD is measured;
no error is introduced due to the resistance of the current supply contacts or lead
resistance to the RTD. Platinum is the material of choice for an RTD. A linearity
of 3.6 percent can be obtained from 0 to 850°C without signal conditioning.
The RTD has a constant current flowing through it giving an offset zero, so that
zero level correction and span conditioning are required. Direct connection of
the resistive element to the controller will be further discussed in Chap. 13.