Choosing an industrial temperature sensor looks simple until you compare the options side by side. PT100 resistance thermometers, thermocouples and digital sensors all measure temperature, but they do it in very different ways. Each technology has a range, accuracy level, wiring style and cost that suits some applications far better than others.
How a PT100 RTD Works
A PT100 is a resistance temperature detector (RTD) made from platinum. Its resistance is 100 ohms at 0 °C and rises in a predictable, almost linear way as temperature increases. A PT1000 works on the same principle but has 1000 ohms at 0 °C, which makes it less sensitive to lead wire resistance.
The measuring instrument passes a small current through the element and reads the resulting voltage. Because the wires themselves add resistance, PT100 sensors come in different wiring configurations:
- 2-wire: simplest and cheapest, but lead resistance adds directly to the reading. Best for short cable runs.
- 3-wire: the most common industrial choice. It compensates for lead resistance as long as the wires are matched.
- 4-wire: the most accurate option, used in laboratories and calibration work, because lead resistance is almost fully cancelled.
RTDs are also classified by tolerance class, such as Class A and Class B under IEC 60751. Class A offers tighter tolerance than Class B.
Strengths: high accuracy, good stability over time, excellent repeatability.
Limitations: narrower temperature range than thermocouples, slower response in larger probes, and higher cost per point.
How a Thermocouple Works
A thermocouple is made of two different metal wires joined at one end. When the junction is heated, a small voltage appears between the wires. This is the Seebeck effect. The voltage depends on the temperature difference between the hot (measuring) junction and the cold (reference) junction at the instrument, so the instrument must apply cold junction compensation.
Thermocouples are identified by letter types. Common ones include:
- Type K (nickel-chromium / nickel-aluminium): general-purpose, wide range, very widely used.
- Type J (iron / constantan): suited to lower temperature ranges and reducing atmospheres.
- Type T (copper / constantan): good for low and sub-zero temperatures.
- Types R, S and B (platinum-rhodium): for very high temperatures such as furnaces and kilns.
Thermocouple wiring must use matching extension or compensating cable. Ordinary copper wire introduces extra junctions and measurement errors.
Strengths: very wide temperature range, rugged, fast response with small junctions, low cost.
Limitations: lower accuracy than RTDs, millivolt signals that are sensitive to electrical noise, and drift over time at high temperatures.
How Digital Temperature Sensors Work
Digital temperature sensors combine a sensing element with electronics that convert the measurement into a digital value. The output may be a protocol such as 1-Wire, I²C or SPI for embedded systems, or RS485 Modbus for industrial transmitters.
Many industrial “digital” sensors are actually an RTD or thermistor with a built-in transmitter. The advantage is that the signal leaves the probe already converted, so cable length and noise matter much less.
Strengths: easy integration with controllers and IoT systems, built-in calibration, multiple sensors on one bus, and extra data such as diagnostics.
Limitations: the electronics limit the operating temperature, response depends on the housing, and you must match the protocol to your system.
Comparison Table
| Criterion | PT100 RTD | Thermocouple | Digital sensor |
|---|---|---|---|
| Typical range | Moderate (roughly cryogenic to several hundred °C) | Very wide, up to very high temperatures | Limited by electronics |
| Accuracy | High | Moderate | Good to high |
| Long-term stability | Excellent | Fair, drifts at high heat | Good |
| Response time | Medium | Fast (small junction) | Medium |
| Signal | Resistance | Millivolts | Digital data |
| Noise sensitivity | Low to medium | High | Low |
| Cost per point | Medium | Low | Low to medium |
| Best for | Process control, food, pharma, HVAC | Furnaces, exhausts, high heat | Building automation, IoT, embedded |
Key Questions Before You Buy
What temperature range do you need?
Start with the minimum and maximum process temperature, plus a safety margin. If the process runs at very high temperatures, a thermocouple is usually the only practical option. For moderate ranges, an RTD typically gives better accuracy.
How accurate does the measurement need to be?
Quality-critical processes, such as sterilisation, pasteurisation or calibration baths, generally call for an RTD. If a few degrees of uncertainty is acceptable, a thermocouple can do the job at lower cost.
How far is the sensor from the controller?
Long cable runs favour a 4-20 mA transmitter or a digital output. Raw thermocouple and 2-wire RTD signals degrade with distance and electrical noise.
What is the environment like?
Consider vibration, moisture, chemicals and pressure. Choose the right sheath material, such as stainless steel, and a suitable protection rating like IP65 or IP67. For pressurised or corrosive fluids, a thermowell protects the sensor and allows replacement without stopping the process.
What does your control system accept?
Check the input cards on your PLC or controller. Some accept RTD and thermocouple inputs directly, while others only take 4-20 mA or a fieldbus such as Modbus.
Installation Tips
- Insert the probe deep enough. A common rule of thumb is an immersion depth of around 10 times the sheath diameter, so that heat conducted along the stem does not distort the reading.
- Keep sensor cables away from power cables and motor drives.
- Use shielded cable and ground the shield at one end only.
- For thermocouples, always use the correct extension cable type.
Choosing the Right Industrial Temperature Sensor
There is no single best industrial temperature sensor. A PT100 is the go-to choice for accuracy and stability at moderate temperatures. A thermocouple wins at extreme heat and fast response. A digital sensor is ideal when you want easy integration and clean data over long distances.
Define your range, accuracy, environment and interface first, and the right option usually becomes clear. Browse our temperature sensor category to compare probes and transmitters, or contact our team if you would like help selecting the right model for your application.