Basic Knowledge RTD Measurement (Con)

The RevPi AIO module has 2 measuring inputs for platinum resistance thermometers (RTD, Pt100/Pt1000). Such sensors change their resistance in proportion to the ambient temperature. The RevPi AIO module measures the resistance of the sensor with high precision and calculates the temperature. The process image (PA) then contains the temperature values of the two measuring inputs in the unit 1/10 °C. At room temperature, for example, the value 235 in the PA could be 23.5 °C. You can also define your own scaling in PiCtory (for example in °F).

There are various measurement techniques for the use of resistance thermometers. We would now like to introduce you to these methods and their fields of application:

A resistance temperature sensor is a metal resistor that changes its value depending on the surrounding temperature. The degree of change depends on the type of metal. The correlation between temperature rise and resistance increase is almost linear, but is determined by a complex formula for particularly accurate measurements, which means also in the AIO module. The metal used in industrial sensors is almost always platinum (Pt), as it has a high long-term stability over a wide measuring range. The typical resistance of the two most common resistance temperature sensors is:

  • Pt100: 100 Ω at 0 °C.
  • Pt1000: 1 kΩ at 0 °C.

These resistance temperature sensors can theoretically measure temperature ranges from -200 °C to +850 °C. However, the largest range of validity for limit deviations provided for in the standard DIN EN 60751 is between the technically feasible test conditions of -196 °C (boiling point nitrogen) and 600 °C. The permissible limit deviations are divided into accuracy classes:

  • Class A (-50 to 250°C scope) permits a limit deviation of 0.1 + 0.0017 * T. At 0°C this means a maximum deviation of 0.1 °C and at 200°C the deviation is already 0.44 °C.
  • Class B (-196 to 600°C scope) permits a limit deviation of 0.3 + 0.005 * T. (therefore max. 3.3 °C at 600 °C).

Tip!: Detailed information on your resistance temperature sensor can be found in the manufacturer’s operating instructions.

Below some examples of resistances of Pt100 and Pt1000 sensors:

Temperature in °C −200 −100 -50 -10 ±0 +10 +50 +100 +600
Pt100 in Ω 18,52 60,26 80,31 96,09 100,0 103,9 119,4 138,5 313,7
Pt1000 in Ω 185,2 602,6 803,1 960,9 1000 1039 1194 1385 3137

In the range from 0 to 100 ohms, the resistance of a Pt100 sensor changes by only 0.4 ohms (for Pt1000 it is 4 ohms) per °C. That’s just 0.4% change in resistance per °C. This shows how much the cable from the measuring device to the Pt100 plays a role, because its resistance is in series with the sensor and adds up to its resistance. A 3.5-metre long copper supply line with a 0.3 mm² cross-section would increase the Pt100 resistance to such an extent that a measuring error of 1 °C would occur. For this reason, there are sophisticated measuring methods to determine the Pt100 resistance precisely. The basis of the measurement is always the same: To measure a resistance, a known current is passed through this resistance. This causes a voltage to drop above the resistor, which is the actual measurement variable for the encoder. The voltage is proportional to the known current and the resistance to be determined according to Ohm’s law.

2-wire RTD measurements

The 2-wire measurement is the most basic measuring method. The two wires are connected to the platinum resistor. These wires conduct the measuring current through the platinum resistance and are also used to measure the voltage at the resistor. In this configuration, however, the line resistances add up to the platinum resistance as described above. Since the line resistances are not generally known, the 2-wire measurement of long lines will quickly result in large measurement errors which cannot be corrected easily. If you need a very accurate measurement result and you have cable lengths of several meters, we do not recommend this method.

PLEASE NOTE!: The RevPi AIO module has no dedicated 2-wire inputs. Of course, you can still use this measurement method. Use the 3-wire inputs and replace the missing cable with a jumper at the AIO connector. Select “3-Wire-Connection” as the measurement method in the configuration in PiCtory.

4-wire RTD measurements

The advantage of 4-wire measurement is that it is not influenced by the line resistances. With this measuring method you get the most accurate measurement of the voltage. For this purpose, 2 wires with the same colour are connected to both ends of the platinum resistor. The supply line therefore has 4 wires (usually 2 x red and 2 x white). The measuring current is passed through the platinum resistor via 1 of these 2 wires. Since the current is known and automatically fixed, it is independent of the resistance of the supply line. The voltage at the platinum resistance is measured via the other two lines using a very high-resistance measuring method. Since virtually no significant current flows through the measuring lines during this voltage measurement, there is no voltage drop across the supply lines that could distort the measurement. If you need a very accurate measurement result and you have cable lengths of several meters, we recommend this method.

3-wire RTD measurements

A cheaper alternative to 4-wire measurement is the 3-wire measurement. When measuring circuits are located far away, it is possible to save on the wiring by eliminating the need for a lead wire compared to the 4-wire measurement.

In general, the measurement works in such a way that one wire is used for the current supply and one wire for voltage measurement. The 3rd. wire is used for both current and voltage measurement. Using an electronic trick, the voltage drop across the shared line can be compensated and does not interfere with voltage measurement over the platinum resistor. However, this electronic trick assumes that all 3 wires have approximately the same resistance – which is usually the case with a multi-core cable. If you need a very accurate measurement result and you are using multi-conductor cables of several meters, we recommend this method.

How to connect and configure a resistance temperature sensor to the RevPi AIO can be found here.

Table overview of AIO connectors for RTDs

According to EN and DIN, the cables attached to one end of the resistor have the same colour red or white. Some manufacturers use black and white as color coding. In any case, however, the wires with identical colours are always connected to the identical resistor pins. The following table shows you how to connect the different connector types with the pins of the AIO module:

RTD graphic