Measuring principle of RTD temperature measurement

Since an RTD sensor changes its internal resistance in a known ratio as a function of temperature, only the resistance of the sensor needs to be measured for a temperature measurement. There are various possibilities for this, which can be divided into direct and indirect resistance measurements.

• Indirect resistance measurement:
  In an indirect resistance measurement, the current and the voltage are measured simultaneously, so that the resistance value can then be calculated using Ohm's law. Here, a distinction is made between a current and a voltage correct measurement.
• Current correct measurement
  In the context of a current correct measurement, the current is tapped via the resistance immediately before or after the resistance. The parallel-connected voltmeter thus measures not only the voltage across the resistance, but also the voltage that drops across the ammeter, so that the voltage measurement is distorted.
• Voltage correct measurement
  In the context of a voltage correct measurement, the voltage that drops across the resistance is measured. In this case, however, the ammeter is connected before the voltmeter and the resistance, thus the current is also measured via the voltmeter. Consequently, the current measurement is distorted. Depending on the selected measuring circuit, the voltage dropping across the ammeter or the current flowing through the voltmeter must be taken into account.
• Direct resistance measurement:
  A direct resistance measurement, also called a comparative or ratiometric measurement, involves a comparative measurement between the unknown and a known reference resistance. Both resistances are connected in series and are connected to a constant current or voltage source (see following figure).
• Then either the voltage or the current of both resistances is calculated so that the unknown resistance can be determined. The reference resistance used here depends on the size of the resistances to be measured.