Temperature Sensors Explained: Thermocouple vs PT100 vs NTC

Every temperature control system is only as good as its sensor. Pick the wrong type, or wire the right one badly, and even the best PID controller will hold the wrong temperature. This guide covers the three sensor types we supply — thermocouples, PT100 resistance thermometers and NTC thermistors — how each works, how to choose, and the installation mistakes we see most often.

Short answer: above 400°C, use a thermocouple. Below 400°C where accuracy matters, use a PT100. For simple refrigeration, HVAC and ambient control below about 120°C, an NTC is the low-cost option — but only if your controller has an NTC input.

Quick Comparison

Thermocouples

A thermocouple is two different metals joined at the tip. Heating the junction produces a small voltage — a few hundredths of a millivolt per degree — which the controller measures and converts to temperature. Because the voltage depends on the difference between the hot tip and the controller's terminals, the controller measures its own terminal temperature and corrects for it automatically. This is called cold junction compensation, and every thermocouple controller we sell does it for you.

Thermocouples are rugged, cheap, fast and reach far higher temperatures than any other sensor type. The trade-off is lower accuracy and a signal small enough to be affected by poor wiring.

Which thermocouple type?

The thermocouple type must match the controller's input setting — a Type J setting with a Type K probe will read wrongly across the whole range.

Wiring thermocouples correctly

• Polarity matters. On current IEC cable, green is positive and white is negative. Older UK cable uses brown positive and blue negative; US cable uses yellow positive and red negative. Reversed polarity makes the reading fall as temperature rises.
• Never extend with copper cable. Use Type K extension (KX) or compensating (KCB) cable and thermocouple plugs and sockets. Copper creates extra junctions and errors.
• Keep runs short and shielded. The signal is tiny, so keep it away from mains and motor cables. For runs beyond about 30m, convert to 4-20mA with a temperature transmitter.

Our range: Type K 5mm stainless probe, Type K 4 x 30mm probe and the Type K ceramic kiln probe.

PT100 Resistance Thermometers

A PT100 is a platinum element measuring exactly 100 Ω at 0°C, rising by 0.385 Ω for every degree. The controller passes a small current through it and calculates temperature from the resistance. Because platinum is extremely stable and the relationship is almost linear, PT100s are the first choice wherever accuracy and repeatability matter up to around 450°C.

Accuracy is set by tolerance class under IEC 60751. Class B — the common industrial grade — is ±0.3°C at 0°C and ±0.8°C at 100°C. Class A halves that, at ±0.15°C and ±0.35°C.

2, 3 or 4 wire?

The resistance of the connecting cable adds directly to the reading. With only 0.385 Ω per degree to work with, every extra ohm of cable reads about 2.6°C high.

• 2 wire — no compensation. Fine for very short leads only; a 10m run of 0.5mm² cable adds roughly 2°C.
• 3 wire — the industrial standard. A third lead lets the controller measure and cancel the cable resistance, provided all three cores are the same size and length.
• 4 wire — removes cable resistance completely. Mainly used for laboratory-grade measurement.

Unlike thermocouples, a PT100 can be extended with ordinary copper instrument cable. Under IEC 60751 the cores are red and white, with the two same-coloured wires connected to the same side of the element.

Our range: PT100 3 wire 5mm stainless probe and the PT100 4 x 30mm waterproof probe.

NTC Thermistors

An NTC (negative temperature coefficient) thermistor is a semiconductor whose resistance falls sharply as it warms — a 10K NTC measures 10,000 Ω at 25°C. The large change per degree makes NTCs sensitive and cheap, and cable resistance has almost no effect, so they wire with two ordinary cores and no polarity.

The limitations are a narrow working range, typically up to about 120°C, and a non-linear curve that the controller must be designed for. NTC values are not interchangeable: a controller set up for a 10K sensor will not read correctly with a 5K or 2K one.

NTCs are the standard sensor for refrigeration, cold rooms, HVAC and ambient monitoring, and are used by the Ageon A102 and A103 controllers. Our range: Novus NTC 10K stainless sensor (-30 to 105°C).

How To Choose — Five Questions

1. What is the maximum temperature? Above 400°C, it has to be a thermocouple. Above 120°C, rule out NTC.
2. How accurate does it need to be? If you need better than ±1°C, choose a PT100.
3. What input does your controller have? The sensor must match the controller. Most Novus PID controllers accept thermocouples and PT100; many refrigeration controllers accept NTC only.
4. How long is the cable run? PT100s and NTCs tolerate long runs of ordinary cable. Thermocouples need special cable; beyond about 30m, use a 4-20mA transmitter.
5. What is the environment? Heavy vibration and very high heat favour thermocouples. Liquids and food usually call for a sealed stainless PT100.

Installation Tips

• Immerse the probe properly. As a rule of thumb, insert at least 10 times the probe diameter into the medium. Too shallow and the sensor reads the temperature of the fitting, not the process.
• Get good thermal contact. In a drilled hole, the probe should be a close fit. Air gaps slow the response and cause overshoot.
• Put the sensor where the control matters. Near the heater gives fast, stable control but may not represent the product. Near the product is more representative but slower. If in doubt, ask us.
• Separate signal and power cables. Route sensor leads away from heater, contactor and inverter wiring, and use screened cable in electrically noisy areas.

Common Sensor Faults

• Reading jumps to maximum or shows a sensor error — open circuit: a broken sensor, loose terminal or damaged cable.
• Reading falls as the process heats up — thermocouple polarity reversed.
• Reads a steady few degrees high — PT100 wired 2-wire, or cable resistance not compensated.
• Reads wrong across the whole range — the controller's input type doesn't match the sensor, e.g. Type J selected for a Type K probe, or the wrong NTC value.
• Erratic or noisy reading — sensor cable run alongside power cables, or no screen.

For a step-by-step approach, see our Faultfinding guide.

Frequently Asked Questions

Can I swap a thermocouple for a PT100?
Only if your controller accepts both and you change its input setting. Most Novus PID controllers — N1020, N1030, N1040, N1050, N1200 and N20K48 — accept thermocouples and PT100.

Can I connect one sensor to two controllers?
No — the two inputs will interfere with each other. Use a dual-element sensor, or convert the signal to 4-20mA with a transmitter and loop it through both devices.

How do I test a sensor?
A PT100 should read about 108-110 Ω at room temperature on a multimeter. A 10K NTC reads about 10 kΩ at 25°C. A thermocouple should show continuity (a few ohms) between its two leads; the voltage it produces is too small to check reliably with a basic meter, so the quickest test is to connect it to the controller and warm the tip.

How do I get a sensor signal into a PLC?
Either use a PLC input card that accepts the sensor directly, or convert to 4-20mA — see PLC Signal Conversion and Temperature Transmitter FAQs.

Need Help Choosing?

Browse all sensors and PID temperature controllers, or call us on +44 (0)1824 520565. We'll help you match the sensor to your controller and process. Back to the Knowledge Base.