What is a PT100?
A PT100 is a temperature sensor with a platinum element that has an electrical resistance of exactly 100 ohms at 0 °C; the resistance rises by about 0.385 ohms per degree, allowing the temperature to be calculated accurately from the measured resistance. The PT100 is the most widely used resistance temperature detector (RTD) in industry and is standardised in IEC 60751. The main alternative is the thermocouple, which works with a voltage generated between two different metals. Both are connected to a PLC or DCS via a temperature transmitter or an analogue input card.
🧠 How does a PT100 work?
The resistance of platinum increases predictably and almost linearly with temperature:
| Temperature | PT100 resistance | Pt1000 resistance |
|---|---|---|
| −50 °C | 80.31 Ω | 803.1 Ω |
| 0 °C | 100.00 Ω | 1000.0 Ω |
| 50 °C | 119.40 Ω | 1194.0 Ω |
| 100 °C | 138.51 Ω | 1385.1 Ω |
| 200 °C | 175.86 Ω | 1758.6 Ω |
The instrument passes a small measuring current through the element and measures the voltage. A Pt1000 works the same way but with 1000 Ω at 0 °C, so cable resistance has less influence.
🔧 What is the difference between 2-, 3- and 4-wire connections?
| Connection | Principle | Accuracy | Application |
|---|---|---|---|
| 2-wire | Cable resistance is included in the measurement | Low; error grows with cable length | Short cables, simple measurements |
| 3-wire | Compensates cable resistance, assuming equal conductors | Good | Most common in industry |
| 4-wire | Measuring current and voltage sensing separated | Very high | Laboratory, calibration, demanding applications |
With a 2-wire PT100, 1 ohm of cable resistance already causes an error of almost 2.6 °C. That is why installations usually use 3- or 4-wire connections, or mount the transmitter directly in the sensor head.
🌡️ How does a thermocouple work?
A thermocouple consists of two wires of different metals joined at one end. A temperature difference between that measuring point and the connection point (the cold junction) generates a small voltage: the Seebeck effect. Because the voltage depends on the temperature difference, the temperature at the connection must be measured and compensated (cold junction compensation).
| Type | Materials | Range (approx.) | Application |
|---|---|---|---|
| K | NiCr – NiAl | −200 to 1250 °C | Most common, general industrial |
| J | Fe – CuNi | −40 to 750 °C | Plastics processing, older installations |
| T | Cu – CuNi | −200 to 350 °C | Low temperatures, food |
| N | NiCrSi – NiSi | −200 to 1250 °C | More stable alternative to type K |
| S / R / B | Platinum – platinum-rhodium | up to 1600–1700 °C | Furnaces, glass, steel |
🔄 PT100 or thermocouple: which should you choose?
| PT100 | Thermocouple | |
|---|---|---|
| Range | Approx. −200 to 600 °C (up to 850 °C possible) | Up to 1700 °C, depending on type |
| Accuracy | High (class A: ±0.15 °C at 0 °C) | Lower (type K class 2: ±2.5 °C) |
| Long-term stability | Very good | Drift at high temperatures |
| Response speed | Moderate | Fast, especially with thin wires |
| Resistance to vibration | More sensitive | Robust |
| Cost | Higher | Lower |
Rule of thumb: up to about 400 °C and where accuracy matters, choose a PT100; for high temperatures, fast response or harsh conditions, choose a thermocouple.
❓ Frequently asked questions
What does the name PT100 mean?
PT stands for platinum, chemical symbol Pt, and 100 for the resistance of 100 ohms at 0 °C. A Pt1000 has a resistance of 1000 ohms at 0 °C. Platinum is used because its resistance changes with temperature in a very stable and predictable way over a wide range.
How accurate is a PT100?
The accuracy of a PT100 depends on its tolerance class under IEC 60751. Class B has a tolerance of ±(0.30 + 0.005·|t|) °C, class A ±(0.15 + 0.002·|t|) °C and class AA ±(0.10 + 0.0017·|t|) °C. At 100 °C, class A therefore means a maximum deviation of ±0.35 °C.
How do you check whether a PT100 is faulty?
You check a PT100 by measuring its resistance with a multimeter and comparing it with the table value at the current temperature; at a room temperature of 20 °C it should be about 107.8 ohms. Infinite resistance indicates an open circuit, very low resistance a short circuit.
What is cold junction compensation?
Cold junction compensation corrects a thermocouple measurement for the temperature at the connection point, where the thermocouple wires change to copper wiring. Because a thermocouple only measures a temperature difference, the temperature of that point must be known. Transmitters and input cards measure it with a built-in sensor and compensate automatically.
Why put a temperature transmitter in the sensor head?
A temperature transmitter in the sensor head converts the weak resistance or millivolt signal directly into a robust 4-20 mA or digital signal. Long cables and electrical interference then no longer affect the measurement. The transmitter can also provide diagnostics, such as a cable break or sensor drift.
📌 In summary
A PT100 measures temperature via the resistance of platinum (100 Ω at 0 °C) and is accurate and stable up to about 600 °C; a thermocouple measures via a thermoelectric voltage and suits higher temperatures up to 1700 °C. In practice, a correct connection (3- or 4-wire) or a transmitter in the sensor head determines how reliable the measurement is.