Electrical connection of Pt100 temperature sensorsIf the temperature changes, the resistance of the Pt100 temperature sensor also changes.
To detect the change in resistance of the Pt100 when the temperature changes, a constant current is applied to the sensor usually between 1 mA and 3 mA).
According to Ohm's law U= R x I the resistance of the Pt100 generates a voltage drop which is measured and evaluated.
A Pt100 resistor has a resistance of 100 ohms at a temperature of 0°C.
At a current of 1 mA, this results in a voltage of
U[V]= 100 [Ohm] x 0,001[A]= 0,1 Volt
This voltage drop must now be transmitted to the evaluation unit with as little loss as possible. Commercially available copper cables can be used for this purpose. It should be noted that the resistance of the connecting cables is also included in the measurement and can lead to measurement errors, especially with long cables and temperature fluctuations at the connecting cables.
The resistance of the connecting cable can be calculated using the following formula.
Copper cable with wire cross-section: 0,25 mm²
Spec. resistance copper (at 20°C): 0,017 Ohm x mm²/ m
Cable length (distance between sensor and evaluation unit): 30 m (add outgoing and return cable lengths).
R= 0,017 x (2 x 30 m) / 0,25mm² = 4,08 Ohm
Please note: The spec. resistance depends on the purity of the copper and is between 0.017 and 0.018). For exact calculations, use the cable supplier's specifications.
There are several possibilities for connecting the Pt100 sensor to an evaluation unit.
These differ in the number of electrical wires used to connect the sensor to the evaluation unit.
In modern evaluation devices, the resistance of the line can be compensated. Temperature fluctuations on the line, however, still influence the measurement result.
The use of a Pt1000 sensor (1000 Ohm at 0 °C) reduces the influence of the connecting cable by a factor of 10. A cable with a larger conductor cross-section also reduces the cable resistance.
This allows the resistance of the connecting line to be determined and compensated.
The 3-wire circuit is a common connection method and offers a good compromise between cost and measurement accuracy.
The influence of the connecting leads is thus fully compensated.
The 4-wire circuit enables the most accurate measurement, but because of the higher cost of the additional connecting leads, it is only used for special applications where high accuracy is required.
TransmitterTo avoid the problems of cable resistance, it is recommended to use a transmitter for large distances between the Pt100 sensor and the evaluation unit.
This converts the resistance of the Pt100 sensor near the measuring point into a standardized 0(4)-20 mA or 0-10 Volt signal.
This signal can then be transmitted easily and cost-effectively over long distances with a 2-wire connection to the display unit.
So-called "head transmitters", which are installed directly in the connection head of the Pt100 resistance thermometer, are ideal. Alternatively, a variety of devices for panel mounting are also available.
External wiring of the Pt100 resistance thermometerThe electrical connection of the Pt100 resistance thermometers to the evaluation units is made with:
The version with cable or stranded wires is normally used for small distances between the Pt100 resistance thermometer and the evaluation unit. The cable is permanently connected to the Pt100 resistance thermometer at the factory. To keep the resistance of the cable as low as possible, the cable is normally made of copper or another highly conductive metal or metal alloy.
When selecting the cable, please note:
- Sufficient cable cross-section to keep the resistance of the cable as low as possible
- Number of wires according to the desired connection type (2-,3-,4-wire) and number of sensors (1x Pt100, 2x Pt100)
- The cable should be sufficiently heat-resistant to withstand the sometimes high temperatures near the measuring point.
- Sufficient chemical resistance of the cable against the ambient conditions and eventually the medium
- In case of possible electromagnetic interference, a shielded cable must be used
Since the cable is mounted at the factory for these resistance sensors, the above points should be clarified in advance between the manufacturer and the user.
- Connection heads The Pt100 resistance thermometer is supplied from the factory with a connection head. A further cable is wired in the connection head by the customer.
Some of the connection heads are standardized in DIN. SThis allows a manufacturer-independent exchange of standardized connection sockets or head transmitters.