When you see PT100 stamped on a temperature sensor, a wiring diagram, or a transmitter datasheet, it is telling you two specific things at once. The PT means the sensing element is platinum, and the 100 means that element measures exactly 100 ohms at 0C. Everything else about the sensor - how its resistance climbs with temperature, how accurate it is, and whether one PT100 can swap in for another - follows from that definition and the international standard behind it. This page unpacks what the designation actually means so the number on the label stops being a mystery.
PT100 RTD in one line: A PT100 is a resistance temperature detector whose platinum element measures 100 ohms at 0C and increases in resistance in a nearly linear, repeatable way as temperature rises. Because that behavior is defined by the IEC 60751 standard, any compliant PT100 tracks the same resistance-versus-temperature curve, which is what makes them interchangeable.
The first half of the name is the material. Platinum is chosen for RTDs because its resistance rises with temperature in a smooth, stable, and highly repeatable way, and because it resists corrosion and stays chemically calm across a wide range. The second half is the nominal resistance: a PT100 reads 100.00 ohms at the ice point, 0C. Warm it and the resistance climbs; cool it below zero and the resistance falls. That fixed anchor at 0C is the reference every other reading is measured against.
How fast the resistance changes is captured by the temperature coefficient, usually written as alpha. The common industrial value is 0.00385 ohms per ohm per degree Celsius, often quoted as the 385 curve. In plain terms, a standard PT100 gains roughly 0.385 ohms for every degree it warms, so it reads about 138.5 ohms at 100C. The exact curve is not a perfectly straight line; the standard defines it with the Callendar-Van Dusen equation, which adds small correction terms so a transmitter can convert resistance back to temperature accurately across the full span.
That alpha value matters when sourcing sensors because a few alternative platinum curves exist, such as the 0.003916 coefficient used in some legacy and North American equipment. A transmitter configured for the 385 curve will read a 3916 element slightly off, and vice versa. Confirming that the sensor curve and the transmitter setting match is one of the quiet details that separates a correct loop from one that is consistently a degree or two out.
PT1000 is the same idea with a bigger number: a platinum element that reads 1000 ohms at 0C instead of 100. Because it starts ten times higher, a PT1000 changes about ten times more resistance per degree, which makes lead-wire resistance a smaller fraction of the total signal and can improve resolution in low-power or two-wire designs. The tradeoff is that PT1000 is less universally supported than PT100 in older instrumentation, and self-heating behavior differs because the element dissipates power differently. For most oil and gas field transmitters the PT100 remains the default, with PT1000 showing up in compact sensors and battery-powered devices.
Interchangeability is governed by tolerance class. IEC 60751 defines Class AA (the tightest, sometimes called 1/3 DIN), Class A, Class B (the common industrial grade), and looser classes below that. Each class specifies how far a sensor's actual resistance may deviate from the ideal curve, expressed as a permitted error in degrees that widens as you move away from 0C. A Class B sensor might be allowed roughly 0.3C of error at the ice point and more at the extremes, while a Class A or AA element holds a much tighter band.
The practical consequence is that class, not the PT100 name alone, tells you how accurate the raw sensor is before any transmitter trimming. If a spec sheet just says PT100 without a class, it does not fully define the accuracy. For custody-relevant temperature or for tight energy balances, engineers call out Class A or AA and confirm the sensor is supplied with a matched or characterized transmitter rather than relying on nominal interchangeability alone.
In the field the PT100 element itself rarely travels far as a raw resistance signal. A nearby temperature transmitter excites the element, measures its resistance, applies the Callendar-Van Dusen conversion, and outputs a scaled value - a 4 to 20 mA loop or a digital reading in engineering units - that a PLC or RTU can pick up. A cloud SCADA platform such as Merobix then reads that scaled temperature from the controller over a protocol like Modbus or DNP3, timestamps it, and historizes it so trends and alarms can be built on top of a value that already accounts for the sensor curve.
Knowing that a point is a PT100 helps when a reading looks wrong from a remote screen. A temperature that has drifted a consistent couple of degrees hints at a curve or class mismatch, or a lead-wire compensation problem, rather than a real process change. A reading that has gone dead or pinned to a fault value often points to an open or shorted element, which a transmitter typically reports as an out-of-range or burnout condition that the SCADA layer can alarm on directly.
Because so many wellsite and facility temperatures - separator outlets, heater tubes, glycol loops, bearing housings - ride on PT100 elements, treating them as a known, standardized family simplifies operations. When every platinum RTD on the site shares the same 385 curve and a documented tolerance class, spare sensors are truly interchangeable, remote diagnostics are consistent, and a technician can swap an element in the field with confidence that the historized trend will continue on the same scale.
It is the sensor's resistance at 0C: a PT100 platinum element measures exactly 100 ohms at the ice point. From that anchor its resistance rises predictably as it warms, reaching about 138.5 ohms at 100C on the standard curve. The PT part indicates the platinum sensing material.
PT1000 starts at 1000 ohms and changes about ten times more resistance per degree, which reduces the relative impact of lead-wire resistance and can improve resolution in low-power designs. However, PT100 is more universally supported by existing instrumentation and is the field default in oil and gas. Neither is strictly better; the right choice depends on the transmitter, wiring, and power budget.
The class defines how far the sensor's real resistance may deviate from the ideal curve, expressed as a permitted temperature error. Class B is the common industrial grade with a wider tolerance band, while Class A and Class AA hold much tighter accuracy, especially away from 0C. For tight energy balances or custody-related temperatures, Class A or AA elements are specified.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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