To compensate for cable lead resistances and avoid measurement errors, the Pt100 sensor is connected using 2, 3 or 4 wires. What is the difference between these methods? Which one should you choose for your application? You will find the answers in the article, read on!
Resistive temperature sensors, such as the Pt100 and Pt1000, measure temperature using the phenomenon of the metals resistance changes under the influence of temperature. The measuring element of these is a thermoresistor made of platinum. They come in different types of temperature sensors, including the steam-tight JUMO STEAMtemp, the push-in JUMO Etemp B, with connecting cable, or as a portable indicator for measuring and monitoring temperature.
Electrical wires have a resistance – the longer the wire, the greater the wiring resistance. For example, each metre of 1 mm2 copper wire is 0.017Ω. For 3km cables, this gives a measurement error of 10.2 Ω, which means a temperature change of 4°C.
So that the resistance of the cables can be compensated for, the temperature sensors are connected in three ways – with 2, 3 or 4 wires.
The 2-wire connection is the simplest circuit configuration for Pt100 resistance temperature sensors. In this series configuration, a single lead wire connects each end of the resistive temperature sensor to the monitoring device.
With only two wires, it is not possible to compensate for the additional resistance caused by other components in the circuit and for changes in cable resistance as a function of ambient temperature. These are added as a measurement error. This is because the measuring instrument will measure the total resistance of the measuring circuit and not just the resistance of the measuring element itself.
This type of connection is used in applications where:
there are no high accuracy requirements
the sensor cable is very short
the error is determined by the tests and the offset used in the measuring instrument
The three-wire Pt100 is the most common temperature sensor. The addition of a third wire, connected to one side of the measuring element, helps compensate for the resistance of the wire. The result is two measuring circuits, one of which is responsible for measuring the temperature and the other for compensating for it.
It is very important that each of the three wires used in the measuring circuit is the same size and length. This is because the measurement result is averaged and only provides good accuracy if all three wires have the same resistance.
A 3-wire connection involves measuring a resistance value through the sensor and measuring a second resistance value through a pair of wires connected to one side of the sensor. Subtracting this resistance value from the total value gives the resistance value for the measuring element in isolation.
The measurement system uses one pair of wires to carry the excitation current used for measurement, and the other pair is used to measure the sensor resistance by measuring the voltage drop.
Connecting resistance thermometers with the Pt100 sensors in a 4-wire system completely eliminates the influence of the connection wire on the temperature measurement result. It compensates for all possible asymmetries in the resistance of the connection wire, also any possible asymmetries in the lead resistance .
This is the most effective solution, but also the most complicated and costly. The 4-wire connection is most often used in laboratory applications where measurement accuracy is crucial and in applications with long cables.