RTD Pt100

What is an RTD Pt100

An RTD, resistance temperature detector, measures temperature by exploiting the fact that the electrical resistance of a metal rises in a known, repeatable way as the metal gets hotter. The Pt100 is the dominant industrial RTD. The name encodes its two defining facts: Pt is platinum, the sensing material, and 100 is the element resistance in ohms at 0 degrees Celsius, the ice point. Platinum is chosen over copper or nickel because its resistance-temperature relationship is the most linear, the most stable over years of service, and the most reproducible from one batch to the next, which is exactly what a reference instrument needs.

The governing international standard is IEC 60751, which fixes the resistance value at every temperature, the four accuracy classes, and the fundamental temperature coefficient. Under that standard a Pt100 reads 100.00 ohm at 0 degrees, about 18.52 ohm at -200 degrees, 138.51 ohm at 100 degrees, and roughly 390.48 ohm at 850 degrees. The nominal range of the platinum curve itself runs from -200 to +850 degrees, though a given probe is certified only over the narrower window its element and sheath can survive. The 2022 third edition of IEC 60751 made the resistance-versus-temperature formula, rather than the printed tables, the authoritative definition.

It is worth separating three terms that engineers use loosely. The sensing element is the bare platinum chip or coil. The RTD or probe is that element packaged in a protective sheath with extension leads. The transmitter is the electronics that excite the element, linearize its resistance, and output a standardized signal such as 4-20 mA or HART. A Pt100 by itself is a passive resistor: it needs an instrument to read it. The companion device that does the reading and conversion is covered separately in the temperature transmitter guide.

The Pt100 sits inside a family. A Pt1000 is the identical platinum curve scaled to 1000 ohm at 0 degrees, ten times more sensitive, so lead resistance and contact resistance matter ten times less, which is why Pt1000 is favored in 2-wire and battery-powered designs. Pt500 sits between them. Outside platinum, nickel Ni100 and Ni1000 and copper Cu RTDs exist for narrow or low-cost niches, but they are less stable and less standardized. For the bulk of process automation, the Pt100 to IEC 60751 is the default, and the rest of this guide stays with it.

Four engineering metrics decide whether a Pt100 is right for a job: its tolerance class at the operating temperature, the wire count that protects accuracy, the element construction that sets the temperature and vibration limits, and the alpha coefficient that must match the receiving instrument. The chapters that follow take these in turn, because a mismatch in any one of them quietly destroys the accuracy that the platinum element is capable of delivering.

IEC 60751 Tolerance Classes

The single most misread number on a Pt100 datasheet is its accuracy, because accuracy is not a constant. IEC 60751 defines four tolerance classes, AA, A, B, and C, and each is a formula in degrees Celsius that grows as the temperature moves away from the 0 degree ice point. A Pt100 is most accurate at 0 degrees and progressively less accurate above and below it, opening like a funnel. Quoting a class without the temperature is meaningless. The table below gives each formula and its value at two reference points, where t is the absolute value of the temperature in degrees Celsius.

Class B is the industrial default and the cheapest class. At 0 degrees it is plus-or-minus 0.30 degrees, widening to plus-or-minus 0.80 degrees at 100 degrees and plus-or-minus 1.30 degrees at 200 degrees. For most heating, HVAC, and general process loops Class B is entirely adequate, and many off-the-shelf assemblies ship in Class B unless a higher class is ordered. Class A roughly halves the tolerance to plus-or-minus 0.15 degrees at 0 and plus-or-minus 0.35 degrees at 100, and is the usual choice where 0.5 degree process control matters.

Class AA, formerly called Class 1/3 DIN B, is the tightest standard class at plus-or-minus 0.10 degrees at 0 degrees, suited to laboratory, custody, and pharmaceutical work. Vendors also sell fractional grades such as 1/10 DIN B that are tighter still, but these are tested selections, not separate IEC classes, and their improved tolerance is only valid over a narrow band near 0 degrees. Class C is the loosest at plus-or-minus 0.60 degrees at 0 degrees and is reserved for rough or low-cost duties.

A subtlety that catches many buyers is that the temperature range over which a class is valid depends on the element construction, not just the class name. The standard certifies the tighter classes over a narrower window for thin-film elements than for wire-wound elements, because film tracks are harder to hold to tolerance at the extremes. The table below summarizes the validity ranges from IEC 60751.

Two practical conclusions follow. First, for cryogenic service below -50 degrees only Class B or Class C are valid, because the tight classes are simply not certified there. Second, if a process needs Class A at 250 degrees, a thin-film element ordered as Class A will only meet its class up to about 300 degrees and a wire-wound element is the safer specification. The class on the purchase order is a promise the manufacturer can only keep inside these ranges.

Resistance Curve and Wire Compensation

Everything a Pt100 does rests on a single curve: how resistance maps to temperature. IEC 60751 fixes that curve through the Callendar-Van Dusen equation. Above 0 degrees the equation is R(t) = R0 (1 + A·t + B·t²), and below 0 degrees a third term, C·(t - 100)·t³, is added because the curve bends slightly at low temperature. R0 is 100 ohm. For the standard platinum grade the coefficients are A = 3.9083e-3, B = -5.775e-7, and C = -4.183e-12. A transmitter or data acquisition card stores these coefficients and inverts the equation to turn measured ohms into a temperature reading.

The headline sensitivity of a Pt100 is about 0.385 ohm per degree Celsius near room temperature, which is the source of the fundamental temperature coefficient, alpha. Alpha is defined as the mean fractional resistance change between 0 and 100 degrees, and for the international standard grade it is 0.003851 per degree, almost universally written as alpha 0.00385. This number is the most important compatibility check in the whole field. Other historical grades exist, notably 0.003902 used in parts of the US market and 0.003916 in the old JIS standard, and a sensor of one grade read by an instrument expecting another will be wrong by up to a degree across the span. Always confirm both ends are alpha 0.00385.

Because the element changes only 0.385 ohm per degree, the resistance of the copper leads between the element and the instrument is a direct error source. A lead that adds 0.385 ohm adds a full degree of error. This is why wire count is a specification, not a wiring detail. The table below compares the three configurations.

A 2-wire connection runs one lead to each end of the element, so the instrument sees the element plus both leads as a single resistance and cannot separate them. Roughly 0.4 ohm of copper, common on a few meters of thin cable, already produces about 1 degree of error, and the error drifts with cable temperature. A 2-wire Pt100 is acceptable only for short runs, for the higher-resistance Pt1000 where lead error is ten times smaller, or where a degree does not matter.

A 3-wire connection adds a third lead so the instrument can measure the resistance of one lead and subtract it from the loop, on the assumption that all three leads are the same gauge and length and therefore equal. This compensation is excellent in practice and 3-wire is the industrial default, the configuration most field transmitters expect. Its only weakness is that an imbalance between the leads, from a poor crimp or a partial corrosion, reappears as error. A 4-wire connection drives a precise current through two leads and senses the voltage across the element through the other two, which carry almost no current, so lead resistance drops out of the measurement entirely regardless of length or balance. This Kelvin connection is the choice for calibration laboratories and reference instruments. IEC 60751 requires 3-wire or 4-wire for Class A and Class AA, because 2-wire lead error would swamp the tighter tolerance.

Element Construction and Probe Assembly

The platinum element comes in two constructions, and the choice between them sets the temperature limit, the vibration tolerance, the response speed, and the price. The two are wire-wound and thin-film. Beyond the bare element, a usable Pt100 is an assembly: the element sits inside a sheath, packed against a tip, with extension leads and often a thermowell. Each layer affects accuracy and reliability.

A wire-wound element coils a fine platinum wire, only tens of micrometers thick, around a ceramic or glass former, leaving the wire free to expand without stress. This construction reaches the lowest tolerances, holds Class AA and Class A over the widest range, and survives long-term to roughly 600 degrees and, in special designs, toward 850 degrees. Its weaknesses are size, fragility, and cost: the free-moving wire is sensitive to mechanical shock and vibration, which can shift the resistance or break the coil, so wire-wound probes need careful mounting. A glass-encapsulated subtype trades a little high-temperature range for better moisture sealing and vibration resistance.

A thin-film element sputters a thin platinum track in a serpentine pattern onto a flat ceramic substrate, laser-trims the track to set the exact resistance, and seals it under a glass passivation layer. The result is small, often only a couple of millimeters, fast responding because of its low thermal mass, far more tolerant of vibration because nothing moves, and cheap to make in volume, which makes thin-film the standard for OEM and automotive use. Its tradeoffs are a narrower certified range for the tight classes, as the previous chapter showed, and a slightly higher long-term drift at elevated temperature. The table below compares the two.

The assembly around the element matters as much as the element. The sheath is usually 316 or 316L stainless steel for general process duty, Inconel for high temperature or aggressive gas, and a hygienic electropolished finish for food and pharmaceutical work. Sheath diameter, commonly 3 to 6 millimeters, trades ruggedness against response time. A thermowell, a closed protective pocket welded or threaded into the pipe, lets the sensor be removed without breaching the process and protects it from flow forces and pressure, but it adds thermal lag and must be sized against flow-induced vibration per ASME PTC 19.3 TW. A mineral-insulated cable construction, with the platinum element and magnesium-oxide-packed leads in a single bendable metal tube, gives a vibration-proof, formable probe favored in engines and turbines.

Tip design closes the loop on response time. A tightly packed, spring-loaded tip in good contact with the sheath wall conducts heat to the element quickly, while an air gap slows it. For fast control loops, specify a reduced-tip or grounded design and a small diameter; for rugged survival, accept the slower response of a heavier sheath and thermowell.

Key Specification Parameters

A Pt100 datasheet carries a dozen parameters, but only a handful change the selection. Reading them correctly is the core skill. The parameters that matter are tolerance class and its valid range, alpha coefficient, wire count, temperature range, self-heating, response time, insulation resistance, and long-term stability. Each is explained below so a spec sheet can be decoded line by line.

Tolerance class and valid range were covered in Chapter 2 and are the headline accuracy. Always read the class together with the temperature where it applies, and confirm the element construction supports that class at that temperature. Alpha coefficient must read 0.00385 (more precisely 0.003851) to match standard transmitters; any other value signals a non-IEC grade that will not interoperate. Wire count, 2, 3, or 4, is the lead-compensation specification from Chapter 3 and must match the transmitter input, since a 3-wire transmitter cannot fully use a 2-wire sensor and vice versa.

Temperature range has two figures that are easy to confuse. The measuring range is where the element reads at its rated class. The maximum or storage temperature is where it survives without permanent damage but not necessarily within tolerance. A probe rated to measure to 400 degrees may tolerate brief excursions higher, but repeated overrange shifts the resistance and degrades the class. Self-heating is the error from the measuring current warming the platinum. The accepted rule is to keep dissipation below 0.01 watt to hold self-heating under about 0.2 degrees, which is why transmitters use an excitation current of 0.1 to 1 milliampere; at 1 milliampere a 100 ohm element dissipates only 0.1 milliwatt. Self-heating is worse in still gas than in flowing liquid.

Response time is quoted as t50 and t90, the time to reach 50 and 90 percent of a step change, measured under defined flow such as water at 0.4 metres per second. It depends on sheath diameter, tip contact, and whether a thermowell is fitted. Insulation resistance is the resistance between the element circuit and the sheath, typically specified as at least 100 megaohm at room temperature; a falling insulation resistance signals moisture ingress and a probe near failure. Long-term stability, or drift, is the slow shift in the ice-point resistance over years; a quality Pt100 drifts well under 0.1 degree per year, far better than a thermocouple, which is the main reason to choose a Pt100 for stable, traceable measurement.

Two further fields appear on industrial spec sheets and deserve a note. The process and electrical connection covers the thread or flange that seals to the pipe (for example G1/2, NPT1/2, or a sanitary clamp) and the terminal head or plug that carries the leads (DIN Form B head, M12 connector, or flying leads). The ingress and hazardous-area rating covers the housing IP class, commonly IP65 to IP68, and explosion-protection certification such as ATEX, IECEx, or NEPSI for use in flammable atmospheres. These do not change the temperature reading but determine whether the probe can be installed at all.

Selection Decision Factors

To turn the preceding five chapters into a specific model, follow the decision sequence below. Most Pt100 selection mistakes come not from a single wrong number but from skipping a step and discovering the conflict after installation. These eight steps make a reliable RFQ template.

1. Temperature range and headroom: Establish the normal, minimum, and maximum process temperatures, then pick a measuring range that covers the maximum with margin. Below -50 degrees only Class B or C are valid; above 500 to 600 degrees a wire-wound element is usually required.
2. Tolerance class at the operating temperature: Decide the accuracy you actually need at the temperature you actually run, not at 0 degrees. Class B for general loops, Class A for tight process control, Class AA or fractional DIN for laboratory and custody. Confirm the chosen element construction holds that class across your range.
3. Wire count: Use 3-wire as the industrial default, 4-wire for calibration and reference accuracy, and 2-wire only for short runs or Pt1000. Match the wire count to the transmitter input.
4. Element construction: Wire-wound for cryogenic, high-temperature, and reference duty; thin-film for vibration, fast response, and OEM cost. Specify mineral-insulated cable where the probe must bend or survive heavy vibration.
5. Sheath material and thermowell: 316L for general process, Inconel for high temperature or aggressive gas, electropolished for hygienic duty. Add a thermowell where the sensor must be removable under pressure, and size it against flow-induced vibration per ASME PTC 19.3 TW.
6. Response time and sheath diameter: Small diameter and a spring-loaded tip for fast control loops; larger diameter and a thermowell for ruggedness, accepting the slower t90.
7. Connection and environment: Process thread or flange, terminal head or M12 plug, housing IP rating, and hazardous-area certification such as ATEX, IECEx, or NEPSI where the atmosphere is flammable.
8. Calibration and traceability: Decide whether a factory calibration certificate, a multi-point certificate, or accredited ISO 17025 calibration is required, and at what points. Reference and custody applications usually demand traceable, dated certificates.

One last dimension is easy to overlook: serviceability and interoperability over the life of the loop. A Pt100 lives in a plant for ten to twenty years, so local spare-element availability, the ease of swapping the insert without disturbing the thermowell, confirmed alpha 0.00385 compatibility with whatever transmitter is in stock, and a stable supplier all matter more than a small price difference at purchase. WIKA, Endress+Hauser, Emerson Rosemount, ABB, Honeywell, and Yokogawa supply finished Pt100 assemblies with documented certificates and global service, while the bare elements inside most of them come from a few specialists such as Heraeus Nexensos, IST, and JUMO. Matching class, range, wire count, and alpha at order time prevents the silent accuracy loss that no amount of careful wiring can recover.