What is RTD (Resistance Temperature Detector)?

A resistance temperature detector is a passive electronic device or a sensor. The name itself suggests that it is related to resistance, temperature, and detection. Similarly, its resistance changes when there is a temperature change; the resistance increases when the temperature of the RTD device increases or vice-versa. It is also known as a resistance thermometer. Its working is similar to the thermocouple.

What is the principle of RTD?

Resistance temperature detector works on a simple principle. It probes the changes between them. As the temperature rises, the resistance also increases. In other words, when the temperature of the metal increases, the resistance offered in the flow of electricity also increases. When the electricity passes through the sensor, it offers resistance which increases according to the temperature of the metal. It shares a directly proportional relationship between temperature and resistance change.

Change in temperature $\alpha$ change in offered resistance. This makes an RTD a positive temperature coefficient.

How is RTD related to temperature coefficient?

The temperature coefficient is defined as the change in temperature that causes the change in the resistance experienced by the material. If the temperature rise increases the resistance, it is called temperature coefficient; that’s why RTD defines the temperature coefficient or positive temperature coefficient.

If the resistance decreases with the increase in temperature, it is called the negative temperature coefficient.

Construction of RTD

There is no such complicated way for the construction of the RTD. A wire is wound or coiled on a ceramic or mica frame for the sake of small size. They help to improve the thermal conductivity and decrease the response time. A protective tube made up of stainless steel covers the wounded wire and ceramic for protection cream. Ceramic or mica is also fixed between the protective tube or sheath and wire to provide better insulation.

Types of RTD

Types of RTD-

• Thin-filmed
• Wire-Wound

Thin-filmed RTD

This type of RTD is made up of platinum films on a ceramic substrate which works as resistive material. Thin-film RTD shows a linear relationship between temperature and resistance. It gives a very high accuracy value.

Wire-wound RTD

This type of RTD is a build-up of small radius wire made of platinum. This wire is wound like a coil and packed in a ceramic insulator.

Benefits of using platinum in RTD

There are various benefits of using platinum in RTD-

• Linear relation between temperature and resistance.
• Highly sensitive
• Stability

Classifications of RTD in terms of the number of wires

The most commonly used RTD has two wires. Some RTD also has three or four wires. RTD consists of a resistance element (mica or ceramic) and insulated wire (Platinum). There are two ports for connecting through the circuit in two wired RTD. The two-wire configuration of RTD is the simplest configuration among the three and four-wire configurations.

The three-wire configuration of RTD has three wires. This type of configuration is the most commonly used configuration. In this arrangement, two wires are for sensing elements to connect with the monitoring device from one side, and the remaining wire connects to the other side.

The four-wire configuration of RTD has four wires, two wires on each side. This configuration is the most complex. Two wires are used for connecting the two sensing elements. Among two sets, one is used to measure the current, and the other is used to measure the voltage drop over the resistor.

Main categories of RTD sensors

There are two main categories of RTD sensors-

• PT-100
• PT1000

PT100

It is the most commonly used RTD, made of up platinum having a resistance value at $0°\mathrm{C}$ is 100 ohm.

PT1000

It is also made up of platinum. Its resistance value at $0°\mathrm{C}$ is 1000 ohm.

If we compare PT100 and PT1000, then PT1000 is more accurate and works better. PT1000 is more précised or accurate for small temperatures. It shows a large resistance change with the small temperature change.

Difference between RTD and thermocouple

The RTD works on two-parameter, i.e., temperature and resistance. When temperature rises which result, a rise in resistance is seen. Similarly, a thermocouple is defined as a thermoelectric sensor. Thermocouple also works on two parameters, i.e., voltage and temperature. For thermocouples, change in generated voltage is responsible for the temperature change. So we can say that they both are similar. They have one same parameter; that is temperature.

Comparison on the basis of other factors:

Range of operating temperature-

• RTD: $-200°\mathrm{C} \mathrm{to} 850°\mathrm{C}$
• Thermocouple: $-200°\mathrm{C} \mathrm{to} 1000°\mathrm{C}$

Sensitivity-

• RTD- Low
• Thermocouple- High

Size-

• RTD- large
• Thermocouple- small

Stability-

• RTD- More Stable
• Thermocouple- Less stable

Output-

• RTD- Linear
• Thermocouple- Non-Linear

Advantages of the RTD

Advantages of RTD are-

• Repeatability
• High Accuracy
• Consistent
• Stability
• The use of platinum in RTD gives a high range of temperature.

Disadvantages of RTD

Disadvantages of RTD are-

• The initial cost is high
• Response time is low
• Not suitable for higher temperature ($>800°\mathrm{C}$)

Uses of the RTD

Applications of the Resistance temperature detector are-

• RTD sensors are used to measure the temperature of an automotive engine.
• Used as intake air temperature sensor
• Used in food handling and process industry
• Used in medical equipment
• Used in aerospace
• Used in industries

Common Mistakes

Remember the difference between the operating temperature of RTD and Thermocouple.

Context and Applications

In each of the expert exams for undergraduates and graduates, this topic is huge and is mainly asked in

• Bachelor of technology in the electrical and electronic department
• Masters of technology in the electrical and electronic department

Related Concepts

• Thermometer
• Resistor

Practice Problems

Q1. What is the relation between temperature and resistance in RTD?

1. Directly proportional
2. Inversely  proportional
3. No relation
4. None of them

Correct option-(a)

Explanation- Both are directly proportional to each other.

Q2. What is the temperature coefficient of RTD?

1. Negative
2. Positive
3. Can’t say
4. None of these

Correct option-(b)

Explanation- The RTD is a positive temperature coefficient because the temperature rise tends to the rise in resistance.

Q3. What is the range of operation temperature of RTD?

1. $10°\mathrm{C} \mathrm{to} 100°\mathrm{C}$
2. $-50°\mathrm{C} \mathrm{to} 500°\mathrm{C}$
3. $-200°\mathrm{C} \mathrm{to} 600°\mathrm{C}$
4. None of the above

Correct option- (c)

Explanation- Operating temperature range of RTD is $-200°\mathrm{C}-600°\mathrm{C}$.

Q4. Which one is not an RTD configuration?

1. 2 wire
2. 3 wire
3. 4 wire
4. 5 wire

Correct option- (d)

Explanation- There is no configuration of RTD having 5 wires.

Q5. What is the common parameter between the RTD and Thermocouple?

1. Voltage
2. Current
3. Temperature
4. Resistance

Correct option- (c)

Explanation- In RTD and thermocouple the common parameter is temperature and the non-common parameter are voltage and resistance.