# Difference Between 2 Wire 3 Wire and 4 Wire RTDs

## Principles of Operation and Types of RTDs

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.

### 2-Wire RTDs

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.

### 3-Wire RTDs

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:

• First, the instrument measures the combined resistance of Wire A, the RTD, and Wire C.
• Next, the instrument measures the resistance of Wire B and Wire C (without the RTD’s resistance since Wire A and B are directly connected at one end).
• The instrument then subtracts the second reading (A+B) from the first reading (A+RTD+C). This essentially removes the resistance of the lead wires from the measurement, leaving only the RTD’s resistance.

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.

### 4-Wire RTDs

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:

• Current Drive: A constant current is passed through the RTD using wires A and D. These wires will have some resistance, but it doesn’t matter in this case since we’re not using them to measure the voltage.
• Voltage Measurement: The voltage drop across the RTD due to the current is measured using wires B and C. Since these wires are only used to measure the voltage (they don’t carry the current), their resistance doesn’t affect the voltage reading. Thus, the measured voltage is purely due to the RTD’s resistance.

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.

• Maximum Accuracy: The 4-wire setup is the most accurate of the RTD configurations because it completely compensates for lead wire resistance.
• Consistency: Even if the lead wires have varying resistance (e.g., due to different lengths or environmental factors), the reading remains accurate.

However, there are some considerations:

• Cost and Complexity: A 4-wire system can be more expensive and might require more sophisticated instrumentation compared to 2-wire or 3-wire setups.
• More Wiring: As the name suggests, you need four wires, which might not be ideal in situations where wiring needs to be minimized.

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.

### How to Choose a RTD Probe for your Wireless Temperature Sensor

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

## Summary and Use-cases

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.

### 2-Wire RTD

1. Configuration: Uses two wires, one connected to each end of the RTD.
2. Advantages: Simplicity and lower cost.
3. Disadvantages: Least accurate of the three configurations because the resistance of the lead wires adds directly to the RTD’s resistance, affecting the temperature reading.
4. Best Used: In applications where high accuracy is not crucial or where the lead lengths are very short. Most cost-efficient option.

### 3-Wire RTD

1. Configuration: Uses three wires. Two are connected to one end of the RTD, and one to the other end.
2. Advantages: Compensates for lead wire resistance, offering better accuracy than the 2-wire system.
3. How it Works: The resistance of two of the lead wires is measured and subtracted out to get the true resistance of the RTD. This assumes that all three wires have the same resistance.
4. Disadvantages: Not as accurate as the 4-wire system, especially if there are slight differences in the resistance of the three wires.
5. Best Used: In most industrial applications where a balance between accuracy and cost is desired.

### 4-Wire RTD

1. Configuration: Uses four wires. Two drive a current through the RTD, and the other two measure the voltage drop across the RTD.
2. Advantages: Provides the highest accuracy by completely eliminating the effects of lead wire resistance on the measurement.
3. How it Works: The current-carrying wires and voltage-measuring wires are separate, so the resistance of the wires doesn’t interfere with the voltage measurement.
4. Disadvantages: More complex and can be more expensive due to additional wiring and more sophisticated instrumentation.
5. Best Used: In high-precision laboratory and industrial applications where maximum accuracy is essential and cost is not a factor.

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.

##### YOU MIGHT ALSO LIKE...​
###### How to Setup the NCD IoT Wireless RTD Temperature Sensor

RTD: An Introduction Resistance Temperature Detectors (RTDs) have emerged as a precise and prevalent method for measuring temperature. While numerous RTD configurations exist, they predominantly adopt either a two-wire, three-wire, or four-wire system. Further variations stem from differences in resistance, typically seen between Pt100 and Pt1000 RTDs. These types of sensors as the name implies work based on the fact that when the temperature of the metal in the sensor