The RTD (more commonly known as PT100) is one of the most used temperature sensors in industry. It is known to be the most accurate and repeatable sensor for low to medium temperatures (-300 to + 600 ° F.)
RTD stands for Resistance Temperature Device. Quite simply, the sensor comprises of a resistor that changes value with temperature. The most common RTD by far is the PT100 385. This element measures 100 Ohms @ 0 degrees C (32 °F) and 138.5 Ohms @ 100 °C (212.0 °F).
One of the greatest challenges for instrument engineers is dealing with the relatively low resistance of the device. This is because any stray resistance (in particular lead resistance) of RTD assembly can add a significant error to the measured resistance.
To combat this, different lead compensation schemes were invented and have come to be known as 2 wire, 3 wire and 4 wire.
The 2 wire technique
The two wire RTD is the simplest form.
The Lead R is the lead resistance of the wire connecting the RTD to the instrument. In this scenario the instrument is going to read a higher temperature than the true RTD temperature because the instrument measures:
RTD + 2x Lead R
For example if the lead resistance was 0.5 Ohms then the instrument would read 2.6˚C (4.7F) higher than it should. The only way to compensate this error is to manually adjust the offset of the instrument. This of course becomes tedious and prone to human error. Automatic lead compensation instruments were invented to address this problem. The compensation techniques use additional wires connected to the sensor to measure the lead resistance and negate its effects.
The 3 wire technique
The three wire lead scheme requires two measurements, the first measurement is V1 which gives a result for RTD + Lead R. The second measurement gives a result V2 for R Lead. Hence to get the true RTD measurement we simply subtract V lead from V lead + RTD leaving RTD.
Hence for any Lead R value this scheme will automatically compensate out the lead resistance and give you the correct temperature.
The assumption this technique makes is that the lead resistance is the same in each of the three wires. This is a very safe assumption to make in particular with modern manufacturing techniques used in wire production. In the practical examples section you will get more of a feeling how these errors stack up.
The 4 wire technique
This technique relies on a very high input impedance of the modern instrument so that in the sensor wire there is practically no current flow: this is a very valid assumption today.
The RTD is sensed in the scheme with no error by measuring VRTD in one measurement. The advantage of this scheme is that it also compensates out any lead wire imbalances.
Historically the 4 wire technique has been popular in Europe led by the German influence for absolute precision.
In the North American market the 3 wire technique has been much more widely deployed in the past and even today outsell the 4 wire sensors by 3 to 1. This has been led by cost and practicality.
Practical examples
Headmount transmitter using 24 AWG wire to connect the RTD sensor to the transmitter with a probe length of 12”
Headmount transmitter using 24 AWG wire to connect the RTD sensor to the transmitter with a probe length of 12”
From the results above for headmount applications both 3 and 4 wire are excellent techniques for eliminating lead resistance effects.
Furthermore, the long cable test also shows 3 to 4 wires to be perfectly adequate. Even if there is some wire imbalance, the calculated error puts it firmly in the uncertainty band of almost all industrial applications.
Another question that is sometimes asked is: “ If I have a 4 wire RTD can it be used as a 3 wire RTD?” The answer is yes, leaving one wire disconnected from the 4 wire sensor will not add any error to the 3 wire system of lead compensation.
However the opposite is not true.
You cannot use a 3 wire RTD with a 4 wire instrument by simply shorting the 3rd and 4th wire together at the instrument. This will result in substantial lead wire resistance error.
In conclusion
2 wire RTD inputs should be avoided altogether unless the wire lengths are short and you are using a low gauge wire to reduce the lead resistance.
3 and 4 wire compensation techniques have been proven over many years to provide an excellent means to automatically compensate lead wire resistance in RTDs.
You would choose 4 wire if you are concerned with absolute precision over long lead lengths. Whereas it has been shown that the 3 wire technique is accurate for all practical industrial purposes and that it saves around 20% in wire cost over the 4 wire technique.
