Pick an RTD when the process sits inside the platinum range, from −200 °C to +800 °C, and you need a reading that stays put year after year. Pick a thermocouple when the metal runs hotter, up to +1,600 °C, or when a fast, cheap element matters more than drift. Below +400 °C, most plants should start with Pt100.
An RTD is a piece of pure metal whose resistance you measure; a thermocouple is a junction of two alloys producing its own millivolts. That difference propagates into span, accuracy, drift, wiring and cost.
Five questions, in this order
- What temperature will the element see, including overshoot on a hot run and the coldest point of a cold start?
- What accuracy must hold across that span, not at one convenient point?
- How long can the installation sit between calibrations without the reading being in doubt?
- How much cable runs from probe to transmitter, and how many conductors can you get there?
- What must the element survive — oxidising gas, salt water, vibration, a hazardous-area classification?
Range: where each type runs out
A thermocouple is the only one of the two that reaches furnace temperatures. Thermo Sensor builds them for solid, liquid and gaseous media up to +1,600 °C, and the mineral-insulated sheathed construction is available from 0.15 mm diameter upward. The sheathed range is offered in Type K, Type J and Type N to DIN EN 60584. Above about +1,300 °C the base-metal types run out and platinum-rhodium types take over.
Platinum resistance sensors end where the platinum ends. Thermo Sensor supplies Pt100, Pt500 and Pt1000 elements, plus NTC and PTC types, covering −200 °C to +800 °C depending on the model — a wide band, but a hard ceiling, because no platinum resistance element in production reads +1,200 °C.
Accuracy: read the class, then read the formula
Both technologies are standardised, so the honest comparison is class against class rather than brand against brand. A base-metal thermocouple to DIN EN 60584-2 in Class 1 is ±0.5 °C or ±0.004 × |t|, whichever is larger. A platinum RTD to DIN EN 60751 in Class A is ±(0.15 + 0.002 × |t|) and in Class B ±(0.3 + 0.005 × |t|).
| Temperature | Type K Class 1 | Pt100 Class A | Pt100 Class B |
|---|---|---|---|
| 0 °C | ±0.50 °C | ±0.15 °C | ±0.30 °C |
| 100 °C | ±0.50 °C | ±0.35 °C | ±0.80 °C |
| 200 °C | ±0.80 °C | ±0.55 °C | ±1.30 °C |
| 300 °C | ±1.20 °C | ±0.75 °C | ±1.80 °C |
The pattern matters more than any single row. Through the band where most process work happens, the platinum element holds a tighter tolerance and its error grows with a gentler slope; above roughly +300 °C the thermocouple figures widen faster. Class A is defined to +300 °C and Class B to +400 °C, so quoting a Pt100 class at +700 °C is not a like-for-like claim. Where extra resolution is needed, Thermo Sensor offers Class B elements at one tenth of the class tolerance, giving 0.03 °C, and thermocouples in half-Class 1 material with a maximum deviation of 0.5 °C on request. Those figures cover the element only; transmitter accuracy, lead resistance and the gap between probe tip and measured material add to them.
Stability: the number that sets your calibration interval
This is where the two separate most clearly, and why an RTD wins in a plant that runs continuously. Thermo Sensor states it plainly on its own technical pages: resistance sensors are long-term stable with an approximately linear characteristic curve, while thermocouples reach higher temperatures but show poorer long-term stability and, in some cases, lower accuracy.
In service a thermocouple drifts because its alloys change. Base-metal types suffer oxidation of the nickel-bearing wires, and the drift is not linear, so a spot check at one temperature does not reveal it. On a line running around the clock, that usually outweighs any accuracy advantage a thermocouple can offer at high temperature.
Cost: the element is the cheapest part
The sensor element is rarely what decides the bill. A thermocouple needs two conductors of its own alloy from probe to terminal, and that cable is extension or compensating grade rather than ordinary wire. An RTD in three-wire form needs three conductors, four in four-wire form, and over a long run the cable can cost more than the element. A thermocouple also needs cold junction compensation in the terminal block or transmitter, while an RTD needs lead resistance compensation instead. The genuinely different item is service life, because an element that drifts is recalibrated or replaced on a schedule and one that holds its curve is not. Transmitters for both come as 4–20 mA or digital.
Wiring: two conductors or four
Lead resistance is where RTD installations quietly lose accuracy. Copper at 0.5 mm² adds roughly 0.034 Ω per metre, so a 10 m two-wire loop puts about 0.69 Ω in series with the element. A Pt100 changes about 0.385 Ω per degree near the bottom of the range, turning that into roughly 1.8 °C of error — more than the element tolerance itself. The same lead on a Pt1000, changing about 3.91 Ω per degree, produces roughly 0.18 °C instead. That is why Pt1000 exists.
Three wires fix it in the field: a third conductor lets the circuit assume the leads are equal and subtract the common resistance. The match has to be real, so buy three-wire cable rather than three single cores of different types. Four wires carry separate current and voltage paths, remove the error completely and are reserved for reference and laboratory work.
Thermocouple wiring has its own discipline. Extension cable is designated for the pair, polarity matters, and Thermo Sensor’s connectors carry polarity marks and reverse-polarity-protected contacts. The housing is chosen for the temperature it will see, in standard, high-temperature and ceramic versions rated to +250 °C, +350 °C and +650 °C, colour coded to IEC, DIN and the equivalent national standards.
Measuring current also heats an RTD, and the heating rises with the square of that current. A Pt100 dissipates 0.1 mW at 1 mA and 10 mW at 10 mA, so an instrument that drives too hard can shift its own reading. Keep the excitation modest and let the transmitter handle the amplification.
RTD and thermocouple side by side
| Pt100 / Pt1000 RTD | Thermocouple | |
|---|---|---|
| Measuring principle | Resistance of pure platinum, per DIN EN 60751 | Voltage from a junction of two alloys, per DIN EN 60584 |
| Practical span | −200 °C to +800 °C depending on model | up to +1,600 °C |
| Element accuracy | Pt100 Class A or B, down to 0.03 °C | Class 1 as standard, half-Class 1 on request |
| Error growth | Gentle, roughly 0.002 to 0.005 × | t |
| Long-term stability | Holds its curve over years | Drifts with the alloys, especially base metals |
| Response | Governed by sheath diameter | Can be very quick, from 0.15 mm diameter |
| Conductors to the field | Two, three or four | Two, in thermocouple-grade cable |
| Reference compensation | Lead resistance compensation | Cold junction compensation |
| Lead error at 10 m, 0.5 mm² | about 1.8 °C on Pt100 in two-wire, nil in three-wire | Polarity errors instead |
| Atmosphere | Platinum is inert, the sheath does the work | Must match the gas; oxidising gas attacks base metals above about +500 °C |
| Element cost | Platinum, higher | Base metals, lower |
Which one fits which job
- Mould and tool temperature in injection moulding. A sheathed Type J or Type K against the tool surface answers fast enough to correct the cycle; a Pt100 in a drilled pocket averages the steel and arrives late.
- Food processing, chillers and ovens. Pt100 Class A or B in three-wire form. Stability matters more than the last fraction of a degree, and the reading has to survive a hygiene regime that forbids regular recalibration.
- Kilns, furnaces and heat treatment above +400 °C. Type K for the ordinary range, Type J for reducing atmospheres, platinum-rhodium types above +1,300 °C.
- Exhaust, turbines and engine test beds. A sheathed thermocouple in a high-temperature alloy sheath, sized so the sheath survives the gas and the element still follows the change.
- Laboratory and reference measurement. Pt1000 or Pt100 in Class AA, four wires, transmitter calibrated no worse than the sensor.
- Long cable runs across a plant. Pt1000 in two-wire, or Pt100 in four-wire; both keep the lead error small without pulling extra cable.
- Hazardous areas. Either type, but only the certified version. Thermo Sensor supplies ATEX and IECEx thermocouples and resistance sensors, with Zone 2 approval to the nA standard.
- Below −200 °C. Neither in standard form; both need specialist low-temperature elements specified for the duty.
Where Thermo Sensor fits
Thermo Sensor is a German manufacturer founded in 1994, with around 230 staff at its site in Werne. It has held ISO 9001:2015 continuously since 1996, runs its own DAkkS-accredited calibration laboratory, and supplies both sides of this comparison from one source — which matters when a line mixes a furnace thermocouple with Pt100s on the machine.
The thermocouple range runs to +1,600 °C in Class 1 material as standard, with mineral-insulated sheaths from 0.15 mm diameter in Type K, Type J and Type N. The resistance range covers Pt100, Pt500 and Pt1000 as well as NTC and PTC across −200 °C to +800 °C, with elements at one tenth of Class B tolerance and mountings including screw-in, protective tube, adhesive pad, magnet and bayonet cap.
The company keeps its own thermocouple connector programme with reverse-polarity-protected contacts, colour coded to IEC, DIN, ANSI, NFE, BS, NF and JIS, in standard, high-temperature and ceramic versions for +250 °C, +350 °C and +650 °C. Cable assemblies — thermocouple, compensating, temperature-resistant, coaxial and shielded — are made in-house, and a high-voltage sensor system with contact protection up to 6 kV covers work near busbars and supply equipment.
What to send us for a quotation
Six items decide the answer, and any one can change the recommendation: the minimum and maximum temperature including overshoot, the accuracy needed there, how fast the temperature moves, the lead length and conductor count, the mounting style and sheath material, and the atmosphere or hazardous-area classification. With those figures we can quote the element and a matching transmitter as one package.
Browse the rest of the sensor category or the full brand list, and send the details above through the inquiry form. More background is in the insights section, and the Thermo Sensor brand page lists the series we hold.