Resistance Temperature Detector, PT100 – Top 20 Questions

How Does a PT100 Work? Your Top 20 Questions Answered

The PT100 temperature sensor is a cornerstone of precise temperature measurement across countless industrial, scientific, and commercial applications. Its reliability and accuracy have made it a go-to choice for engineers and technicians worldwide. But how exactly does this ubiquitous sensor work? Here are the answers to the top 20 questions about the PT100.

1. What is a PT100 sensor?

A PT100 is a type of Resistance Temperature Detector (RTD). The “PT” signifies that the sensing element is made of platinum, and the “100” indicates that it has a nominal electrical resistance of 100 ohms at 0°C (32°F).

2. What is the fundamental working principle of a PT100?

The operation of a PT100 is based on the principle that the electrical resistance of a metal changes predictably with temperature. As the temperature of the platinum sensing element in a PT100 increases, its electrical resistance also increases. By measuring this resistance, the temperature can be accurately determined.

3. Why is platinum used in PT100 sensors?

Platinum is the material of choice for high-accuracy RTDs for several key reasons:

High Linearity: It has a very linear and well-defined resistance-temperature relationship over a wide temperature range.
Chemical Stability: Platinum is highly resistant to corrosion and contamination.
High Melting Point: This allows for a wide operating temperature range.
Repeatability: It provides consistent and repeatable measurements.

4. What is the relationship between resistance and temperature in a PT100?

The relationship is defined by the Callendar-Van Dusen equation, a complex formula that accurately maps the resistance of platinum to temperature. For general purposes, a temperature coefficient, alpha ( $α$ ), is often used. The most common alpha value for PT100 sensors is 0.00385 Ω/Ω/°C. This means for every degree Celsius rise in temperature, the resistance increases by approximately 0.385 ohms.

5. How is the temperature actually measured from the resistance?

A small, constant current is passed through the PT100 element. According to Ohm’s Law ( $V = I \times R$ ), the voltage drop across the resistor is then measured. Since the current is known and constant, the voltage drop is directly proportional to the resistance. This resistance value is then converted to a temperature reading using calibration tables or mathematical formulas programmed into the measuring instrument.

6. What are the different types of PT100 construction?

There are two primary construction types:

Wire-Wound: A fine platinum wire is wrapped around a ceramic or glass core. This type is very stable and accurate, often used as a reference standard.
Thin-Film: A thin layer of platinum is deposited onto a ceramic substrate and then etched to create a resistive path. These are smaller, more common, and more resistant to vibration.

7. What do 2-wire, 3-wire, and 4-wire PT100 configurations mean?

This refers to the number of wires used to connect the PT100 element to the measuring device. This is crucial for accuracy.

2-Wire: The simplest configuration. However, the resistance of the lead wires is added to the resistance of the sensor, leading to measurement errors, especially with long cable runs.
3-Wire: This is the most common industrial configuration. It uses a third wire to measure the lead wire resistance, allowing the measuring instrument to compensate for this error, leading to significantly higher accuracy.
4-Wire: This is the most accurate configuration. Two wires carry the excitation current, and the other two are used to measure the voltage drop directly at the sensing element. This completely eliminates the effect of lead wire resistance and is used in laboratory and high-precision applications.

8. What is the typical temperature range of a PT100?

A standard PT100 sensor can typically measure temperatures from -200°C to +850°C (-328°F to +1562°F). The exact range can vary depending on the sensor’s construction and sheathing materials.

9. How accurate are PT100 sensors?

PT100 sensors are known for their high accuracy. They are classified into different tolerance classes according to standards like IEC 60751. The most common are:

Class B: ±(0.3 + 0.005 |T|) °C
Class A: ±(0.15 + 0.002 |T|) °C
Class 1/3 DIN (or AA): ±(0.1 + 0.0017 |T|) °C Where |T| is the absolute temperature in degrees Celsius.

10. How do you test a PT100 sensor?

You can perform a basic functionality test with a multimeter set to measure resistance (ohms). At room temperature (around 20-25°C), a functional PT100 should read approximately 107.79 Ω to 109.73 Ω. You can also place the sensor in an ice bath (0°C), where it should read very close to 100 Ω.

11. What is the difference between a PT100 and a PT1000?

The only difference is their nominal resistance at 0°C. A PT1000 has a resistance of 1000 ohms at 0°C. This higher resistance can be advantageous in some applications, as it produces a larger voltage drop, which can lead to better resolution and reduced impact from lead wire resistance in 2-wire configurations.

12. PT100 vs. Thermocouple: Which is better?

The choice depends on the application:

PT100: Generally more accurate, stable, and linear over their temperature range. They are preferred for applications requiring high precision.
Thermocouple: Can measure a much wider temperature range (up to over 2000°C), are more rugged, less expensive, and have a faster response time. They are suitable for high-temperature and harsh environments where extreme precision is not the primary concern.

13. What is self-heating in a PT100?

The small current used to measure the resistance of the PT100 can generate a tiny amount of heat, slightly raising the temperature of the sensing element. This is known as self-heating and can introduce a small error. This error is usually negligible but can be a factor in very high-precision measurements or in static air.

14. How do I choose the right PT100 for my application?

Consider the following factors:

Temperature Range: Ensure the sensor can handle the expected temperatures.
Accuracy Requirements: Select the appropriate tolerance class (A, B, etc.).
Environment: Choose a sensor with a suitable sheath material (e.g., stainless steel for general use, Inconel for high temperatures) and construction that can withstand vibration, moisture, and chemical exposure.
Wiring Configuration: Decide between 2, 3, or 4-wire based on the required accuracy and the distance to the measuring device.

15. What are the advantages of using a PT100?

High accuracy and repeatability
Excellent stability over time
Good linearity over a wide temperature range
Interchangeability between different manufacturers

16. What are the disadvantages of using a PT100?

Higher cost compared to thermocouples
Slower response time than thermocouples
Susceptible to self-heating errors
More complex measurement circuitry is required

17. Can I extend the wires of a PT100?

Yes, you can extend the wires. However, it is crucial to use the correct type of extension cable and consider the added resistance, which can affect the accuracy of 2-wire systems. For 3 and 4-wire systems, the impact is less significant but still present.

18. Do PT100 sensors require calibration?

For applications requiring high accuracy, periodic calibration against a known temperature standard is recommended. While PT100s are very stable, their characteristics can drift slightly over time, especially if subjected to extreme temperatures or mechanical stress.

19. What is a “thermowell”?

A thermowell is a protective tube-like fitting that a temperature sensor like a PT100 is inserted into. It allows the sensor to be removed and replaced without shutting down or draining the process (e.g., a pressurized pipe or tank). It also protects the sensor from the process fluid.

20. Where are PT100 sensors commonly used?

PT100 sensors are found in a vast array of applications, including:

Industrial Automation: Monitoring and controlling temperatures in manufacturing processes.
HVAC Systems: Regulating building temperatures.
Laboratory and Research: High-precision scientific measurements.
Food and Beverage Industry: Ensuring food safety and quality.
Automotive: Engine and exhaust temperature monitoring.
Aerospace: Critical temperature sensing in aircraft and spacecraft.