Whilst it is perfectly possible to connect a Pt100 with a two-wire connection it is important to recognise that this will introduce measurement errors due to the resistance of the lead wire. This is because the measuring instrument will measure the total resistance of the measuring circuit and not just that of the sensing element itself.
If you consider that the resistance changes per degree Celsius of temperature change is just 0.3851Ω, then it can be seen that by adding in connection wires with a resistance of even just 1Ω would lead to an error approximately +2.6°C. This falls outside of the published tolerances for this type of sensor.
Two wire connection is often used and Process Parameters many sensors with this type of connection. It is often used as a lower-cost solution They are however suited to a few special cases as follows: –
- Where the application does not have a high demand for accuracy.
- Where the sensor cable is very short.
- Where the error is determined by testing and an offset applied to the measuring instrument.
If your application demands do not fall into these then there are standard methods of compensating for the lead resistance.
The addition of a third wire, connected to one side of the measuring element, helps to compensate for the lead resistance. It is very important that each of the three wires used in the measuring circuit are equal in terms of both conductor size and length. This is because the measured result is averaged and only gives good accuracy where all three wires have the same resistance.
The 3 wire connection specification works by measuring the resistance value through the detector and also taking a second resistance value through the pair of wires joined on one side of the detector. The subtraction of this resistance value from the total gives the resistance value for the measuring element in isolation.
Three wire connection is by far the most common of all wiring types used in Pt100 thermometry. Many instruments utilise this method of connection including temperature transmitters, temperature controllers, panel displays and data loggers.
For the greatest accuracy, you should choose a four wire Pt100 RTD specification. This measuring system is the only way of fully compensating for all lead resistance in the measuring system, even if each wire has a different resistance.
The measuring system using one pair of wires to carry the excitation current used for the measurement and the second pair is used to measure the resistance of the sensing detector by measuring the voltage drop.
As the 4-wire connection method fully compensates for all lead resistance we strongly recommend that this is used when using a high specification of Pt100 such as 1/5 or 1/10 DIN tolerance. We believe it is a false economy to specify a high tolerance temperature sensor with its associated cost and use an inferior measuring system. Four wire connection is predominantly used in laboratories and calibration applications and anywhere the highest accuracy is required.
The final method of connection is now extremely rare but is worthy of a mention. A 4-wire blind loop connection utilises a simple 2-wire connection on the sensor and a separate pair of wires joined as a closed loop connection. The functionality of this system is similar to the 3-wire system and therefore demands that each conductor has the same resistance.