What is an RTD?

The RTD has long been the workhorse of the temperature measurement world. Though thermocouples are inexpensive and popular, they often do not have the accuracy or resolution needed for industrial or laboratory processes. While the thermocouple works on the Seebeck Effect and produces (and therefore requires the measurement of) very small voltages, the RTD is different. It is all in the name, the Resistance Temperature Detector (or Resistance Thermal Device). 

RTDs have a small coil of metal, often Platinum, in the sensor head whose resistance increases with increasing temperature. This is a very fast responding system with a relatively large and easy to measure change in resistance. This means precision measurement and high resolution readings are much more possible from an RTD than some other temperature measurement devices.

Why are there 2, 3, and 3 wire RTDs?

To measure the resistance of something, we commonly will apply a fixed excitation current and measure the resulting voltage drop across the device. We all remember Ohm’s Law from physics 101 right? It tells us that the resistance of the device is simply equal to the voltage divided by the current. The challenge comes from the fact the our measurement system doesn’t know how much resistance is the RTD and how much is the wires leading from the system to the RTD.

Surely wire doesn’t add that much resistance you say. Well it may not, but long runs and smaller gauge wire can result in large errors making that precise measurement not so precise. RTDs are commonly manufactured in 100, 500, and 1000 Ohm variants, so the lower resistance versions are more susceptible to lead wire errors as the wire resistance is a larger percentage of the total resistance. For example, 22 gauge sensor wire has a resistance of about 1.6 Ohms per hundred feet, so if your sensor is 300 feet from the control room and a 100 Ohm RTD is in use, that’s nearly 5% of the total resistance in the wire!

2 wire RTD configuration

2 Wire Sensors (The Ugly)

The 2 wire RTD is the most basic setup where one wire is attached to each end of the resistance element. Current is fed through the wires and the voltage drop measured. If the lead wires are short this can be an acceptable setup, but is by far the least accurate. Using larger gauge lead wires can help as well since the resistance of the wire per foot is less than smaller wires. Sometimes the lead wire resistance will be measured and corrected for the increase the accuracy of the system, but that can change unexpectedly due to corrosion of the wire and connections.

So if you have a high resistance RTD using short and large leads you may be able to use the 2 wire setup. Overall this is not the recommended configuration though as the sensor is more expensive than a thermocouple, but not really providing much better information.

3 Wire Sensors (The Bad)

Adding one wire to the system adds a bit of cost and complexity, but can greatly improve the results. The third wire is used to measure the resistance of the leads. The third wire is attached to the same side of the resistance element as one of the other wires. The resistance of the element is measured through two wires, then the resistance of the leads is measured between the two wires on the same side and subtracted from the measured total. In reality this is the average lead resistance as we are assuming that all three wires are identical lengths, gauges, and resistances. If you are careful to keep the lengths and types/sizes the same, it isn’t that bad of an assumption.

The 3 wire setup is probably the most common in industrial applications. Why? Well, oftentimes there are long runs of wire to a control room making the 2 wire setup inaccurate, but the additional cost of a 4 wire system would be significant.

3 wire RTD configuration
4 wire RTD configuration

4 Wire Sensors (The Good)

Finally, we can attach two wires to each side of the element with two used for excitation and two for measurement. This 4 wire measurement (engineers would geek out and call it a Kelvin connection) is the most accurate measurement you can get. Lead wire effects are basically completely eliminated for mixed gauges and lengths of wire even.

Because of their high accuracy, this setup is most often used in the laboratory setting where precision measurement is required. Generally in the lab the runs of wire are shorter so the cost isn’t a factor. Also, most laboratory setups may have a few sensors as compared to hundreds in an industrial setting, so again the cost per unit is less of a concern.

So Which is Right for You?

To choose the appropriate configuration for your application you need to consider the requirements, cost, and measurement system to be used. First, the requirements are dictated by your process. Do you need milliKelvin measurements or is the nearest few degrees good enough? Cost is often the next driver as depending on the number of sensors, distance to measurement points, and environment the lower wire count systems can offer a substantial price advantage. 

Last is the measurement system to be used. Whatever temperature transmitter you choose must support the appropriate 2, 3, or 4 wire configuration. Many systems only support one or two configurations. Our 4-channel RTD signal conditioner actually supports all three and outputs the temperature as an amplified analog and digital signal! If you are using RTDs, be sure to see how it can make your system faster to set up. We see lots of applications of temperature sensors ranging from pipe and tube plants to precision laboratory testing of materials, so we designed the device to be the perfect blend of versatility, simplicity, and cost effectiveness.

Summary

To review, RTDs measure temperature by using a metal whose resistance changes in a well known way with temperature. The 2 wire configuration is probably the least common as it is the least accurate. It is really only used when a “rough idea” of temperature is needed, and in those applications there are often more cost effective sensors. The 3 wire configuration is much more accurate and is most often used in industrial applications with long leads and lots of sensors where costs can add up. Finally the 4 wire configuration is what we see is almost all laboratory applications. No matter what configuration you are using, be sure to try it with our RTD signal conditioner and don’t hesitate to ask us for help designing and integrating your next precision system!

John Leeman
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