PT100 and Thermocouples: What is the Difference? - ICS Schneider Messtechnik GmbH

Temperature measurement is essential in many industrial processes and scientific applications. Two of the most commonly used types of sensors are PT100 and thermocouples. Both sensors have their own strengths and weaknesses and are used in different scenarios. In this article, we will explore the differences between PT100 and thermocouples, explain how they work, and discuss when each type of sensor is best suited.

Introduction to PT100 and Thermocouples

PT100 and thermocouples are the most commonly used temperature sensors in industrial applications. Both have their origins in the early days of electronics and metrology and are still essential tools for temperature monitoring.

PT100 is a platinum resistance thermometer named after its material and its resistance value at 0°C (100 ohms). The resistance increases linearly with temperature, making PT100 sensors very accurate and stable.
Thermocouples, on the other hand, consist of two different metal alloys joined at one end, generating an electric voltage with temperature changes. This voltage can be measured and converted into a temperature.

How Do PT100 and Thermocouples Work?

Operating Principle of a PT100 Sensor

A PT100 sensor operates on the principle that the electrical resistance of platinum increases with temperature. This resistance can be measured and converted into a temperature. The PT100 is capable of accurately measuring temperatures over a wide range.

Simply put:

At 0°C, a PT100 sensor has a resistance of 100 ohms.
As temperature increases, the resistance increases linearly, allowing this value to be used to determine temperature.

PT100 sensors are often found in industrial applications where precision is crucial, such as in the chemical and food industries.

Operating Principle of a Thermocouple

A thermocouple operates based on the Seebeck effect. This effect states that in a circuit made up of two different metals, a voltage is generated when the two metals are exposed to different temperatures. This voltage is proportional to the temperature difference and can be measured to determine temperature.

Thermocouples are available in various types (e.g., Type K, J, T), which differ in the metal combinations used and their temperature ranges. They are robust and suitable for extremely high temperatures.

Comparison of PT100 and Thermocouples

The choice between a PT100 or a thermocouple often depends on the specific requirements of the application. The following table summarizes the key differences:

Feature	PT100	Thermocouple
Measurement Principle	Resistance Change (RTD)	Seebeck Effect (Thermoelectric)
Material	Platinum	Various Metal Combinations
Typical Temperature Ranges	-200°C to 850°C	-200°C to 1800°C
Accuracy	Very high (±0.1°C to ±0.5°C)	Varies by type, generally lower
Response Time	Slow to Medium	Fast
Cost	Higher	Lower
Calibration	More frequently required	Generally stable
Application Areas	Precise measurements, laboratories	High temperatures, harsh environments
Typical Usage Areas	Chemical industry, pharmaceuticals, food	Metallurgy, glass production

When Should You Use PT100?

PT100 sensors are ideal for applications requiring high precision and stability. Examples include:

Laboratories: Accuracy is essential in scientific research.
Food industry: Temperature control is important for product quality and safety.
Pharmaceutical industry: Many pharmaceutical processes require strict temperature control.

Another advantage of PT100 sensors is their linearity. This makes them easy to calibrate and allows for accurate temperature measurements without complicated calculations.

When is a Thermocouple the Better Choice?

Thermocouples are ideal for environments where high temperatures and rapid temperature changes occur. They are robust and resistant to vibrations and shocks, making them a good choice for harsh industrial environments.

Metallurgy: The manufacture and processing of metals often require monitoring extremely high temperatures.
Glass and ceramics production: High temperatures are also needed here, and the speed of thermocouples is an advantage.
Engines and turbines: Temperature monitoring in areas with high vibrations and rapid temperature changes is important.

Differences in Accuracy and Reliability

One of the biggest differences between PT100 and thermocouples is accuracy. PT100 sensors provide high accuracy and stability, making them the first choice for precise applications. Thermocouples, on the other hand, are less accurate but offer a fast response time and are suitable for temperatures that a PT100 sensor cannot cover.

Accuracy and Stability

PT100 sensors are precise and stable but more expensive. They offer very low measurement deviation and can remain calibrated over a long period. This is important in processes where temperature plays a critical role.

Thermocouples are less stable and can drift over longer periods, requiring regular calibration. Nevertheless, they are often preferred for extremely high temperatures and fast temperature measurements.

Installation and Handling

PT100 sensors require careful installation due to their delicate structure. The thin wires and sensitive platinum structure must be protected from mechanical stress.

Thermocouples are generally more robust and can be installed directly in hot environments. They are more resistant to vibrations and shocks, making them better suited for environments where mechanical stresses occur.

Cost Comparison

Cost is another factor to consider when choosing between PT100 and thermocouples. PT100 sensors are more expensive to purchase because platinum is a costly material, and the production of precise resistors is expensive. Thermocouples, on the other hand, are cheaper and suitable for mass applications.

Conclusion: Which Sensor is Right?

The choice between a PT100 sensor and a thermocouple largely depends on the requirements of the specific application. If precision and stability over long periods are required, a PT100 sensor is the better choice. If, on the other hand, rapid temperature changes and high temperatures are a priority, the thermocouple offers clear advantages.

Both sensors have their place in modern industry, and it is often sensible to use both in different areas to achieve the best results.

PT100 Resistance Thermometers

SITRANS TS100

The temperature sensor line is delivered with direct connected cable. With its construction based on mineral insulated cable (MIC) supports a wide range of applications. Due to compression fittings or soldering nipples the installation is simple and flexible. With the optional surface adapter the measuring of surface temperature are supported. The certificate for intrinsic safety allows the operation even in Zone 0 without the need of additional thermowell. This application grands an outstanding reaction time.

Catalog Excerpt
User Manual

Housing	96 x 48 mm
Display	5-digit 14 mm digit height Colour: red, green, orange or blue display
Range displayed	-200°C…850°C / -328°F…1562°F
Operation	front panel keyboard protection class IP65
Input	Pt100 3-/4-wire
Analogue Output	2 analog outputs 0/4-20 mA / 0-10 VDC / 16 Bit
Switch point	2 or 4 relays (change-over contacts) 250 V / 5 AAC, 30 V / 5 ADC or 8 PhotoMos outputs (NOC) 30 V / 0.4 A AC/DC
Interface	RS232 or RS485 galvanic isolated Alternatively to analog output 2
Power supply	100-240 VAC 50/60 Hz / DC +/- 10% 10-40 VDC / 18-30 VAC 50/60Hz
Sensor supply	–
Software properties	min/max-memory with adjustable permanent display display flashing at threshold value exceedance/undercut flexible alarm system with adjustable delay times brightness control via parameter or front keys programming interlock via access code

Housing	96 x 48 mm
Display	5-digit 14 mm digit height Colour: red, green, orange or blue display
Range displayed	-200,0°C…850,0°C / -328,0°F…1562,0°F
Operation	front panel keyboard protection class IP65
Input	Pt100 3-/4-wire
Analogue Output	0/4-20 mA / 0-10 VDC / 16 Bit Alternatively digital input
Switch point	2 relays (change-over contact) 250 V / 5 AAC, 30 V / 5 ADC
Interface	–
Power supply	230 VAC 10-30 VDC galvanic isolated Other voltage supplies on demand!
Sensor supply	–
Software properties	min/max-memory with adjustable permanent display display flashing at threshold value exceedance/undercut flexible alarm system with adjustable delay times brightness control via parameter or front keys programming interlock via access code

Housing	96 x 48 mm
Display	4-digit 14 mm digit height Colour: red, green, orange or blue display
Range displayed	-200°C…850°C / -328°F…1562°F
Operation	front panel keyboard protection class IP65
Input	Pt100 2-/3-wire
Analogue Output	–
Switch point	–
Interface	–
Power supply	230 VAC 24 VDC +/-10% galvanic isolated Other voltage supplies on demand!
Sensor supply	–
Software properties	Min/Max-value recording display in °C or °F display flashing at threshold exceedance/undercut navigation keys for polling of min/max-values or for threshold corrections during operation impedance matching programming interlock via access code

Housing	96 x 24 mm
Display	5-digit 14 mm digit height Colour: red, green, orange or blue display
Range displayed	-200°C…850°C / -328°F…1562°F
Operation	front panel keyboard protection class IP65
Input	Pt100 3-/4-wire
Analogue Output	0/4-20 mA / 0-10 VDC / 16 Bit Alternatively interface RS232/RS485
Switch point	1 or 2 change-over relays 250 VAC / 2 AAC ; 30 VDC / 2 ADC
Interface	RS232 or RS485 galvanic isolated Alternatively analog output
Power supply	10-40 VDC / 18-30 VAC 50/60Hz 100-240 VAC 50/60 Hz / DC +/- 10%
Sensor supply	–
Software properties	min/max-memory with adjustable permanent display display flashing at threshold value exceedance/undercut flexible alarm system with adjustable delay times brightness control via parameter or front keys programming interlock via access code