Resistance Temperature Detectors (RTDs) are a type of temperature measurement device that relies on the ability of certain metal materials (most notably Platinum) to increase their resistance with the increase in temperature. These are utilized together with a current source, where as the temperature changes so does the current (as a result of the changing resistance) in order to produce temperature measurements.
Depending on the how “good” of a reading one needs, the number of wires connected to the sensor varies. You can have RTDs with 2, 3, or 4 wires, which leads to 3 distinct type with their own characteristics, which we will compare in detail in this articles.
These are the most straight forward ones, they are the easiest to build and operate:
1. Connection: You connect the RTD to your measuring instrument (like a digital multimeter or a specialized RTD reader) using two wires.
2. Measurement: A small current is passed through the RTD, and the resulting voltage is measured. Using Ohm’s Law (V = IR), you can find the resistance of the RTD.
3. Temperature Calculation: By knowing the resistance, you can determine the temperature of the RTD using its predefined resistance-temperature relationship.
As straight forward as this setup is it comes with a certain disadvantages (simplicity always comes a cost):
Lead Wire Resistance: The two wires connecting the RTD to the instrument have their own resistance. Let’s say each wire has a resistance of 0.5 ohms. When you’re measuring the resistance of the RTD, you’re also measuring an extra 1 ohm (0.5 ohms from each wire). This means if your RTD reads 101 ohms, the actual resistance of the RTD might only be 100 ohms, and the extra 1 ohm is coming from the lead wires.
Why is this a problem? Well, even a slight change in resistance in an RTD can correspond to a significant temperature change. So, if you don’t account for the lead wire resistance, your temperature reading can be off.
In many practical applications, especially where high precision is required, a 2-wire setup might not be ideal because of this lead resistance issue. That’s why there are also 3-wire and 4-wire RTD setups that help compensate for this problem. But for applications where slight inaccuracies are acceptable or where cost is a concern, a 2-wire RTD can still be a good choice.
This RTD type adds an additional conductor to the sensor probe:
1. Connection: The RTD element still has two connection points, but now you have three wires: two of them (let’s call them A and B) connect to one end of the RTD, and the third wire (let’s call it C) connects to the other end.
2. Measurement:
3. Temperature Calculation: Just like before, once we have the accurate resistance of the RTD, we can determine its temperature based on its resistance-temperature relationship.
Why isn’t it perfect? For the 3-wire setup to work effectively, Wires A, B, and C should have identical resistances. This means they should be made of the same material, be of the same length, and have the same cross-sectional area. In reality, there might be slight differences, which can introduce minor errors. However, in most practical scenarios, the 3-wire configuration is still much more accurate than the 2-wire setup.
In summary, the 3-wire RTD configuration compromises accuracy and cost. It provides more accurate temperature measurements than the 2-wire setup by compensating for lead wire resistance but might not be as precise as the 4-wire configuration.
The 4-wire RTD configuration takes things a step further in accuracy. It’s designed to entirely eliminate the effects of lead wire resistance, which can introduce errors in the temperature reading.
Here’s a breakdown of the 4-wire RTD setup:
1. Connection: The RTD still has its two connection points. This time, you have four wires connected: two for driving the current through the RTD (let’s call them A and D) and two for measuring the voltage across the RTD (let’s call them B and C).
2. Principle: In the 4-wire setup, the main idea is to separate the current-carrying wires from the voltage-measuring wires. By doing this, we can ensure that the resistance of the lead wires doesn’t impact the voltage measurement.
3. Measurement:
4. Calculation: Using Ohm’s Law (V = IR), the accurate resistance of the RTD can be calculated, and based on this resistance, its temperature is determined.
Advantages of the 4-wire system:
However, there are some considerations:
In essence, the 4-wire RTD configuration is the go-to choice when the highest level of accuracy is needed, and any potential source of error, like lead wire resistance, must be eliminated.
Types of RTD Sensor RTDs (Resistance Temperature Detectors) come in various types based on their construction, resistance values, and the material used. Here are the
Let’s summarize the differences between the 2-wire, 3-wire, and 4-wire RTD configurations and see what each best setup is best at.
In essence:
2-Wire is simple and cost-effective but least accurate.
3-Wire is a compromise, balancing accuracy and cost.
4-Wire offers the highest accuracy, compensating for all lead wire resistances, but comes at the highest cost.
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