How a PT100 Probe Works
A PT100 is one of the most boring instruments in a sterilization room, and that is precisely its virtue. It has no electronics, no firmware and no calibration menu. It is a piece of platinum whose resistance changes with temperature in a way that has been standardised internationally — which is why two probes from two manufacturers agree with each other.
What the name means
PT is platinum. 100 is the resistance of the element at 0 °C: exactly 100 Ω. Everything else follows from those two facts.
Platinum is used rather than a cheaper metal for three reasons: its resistance changes with temperature in a nearly linear and highly repeatable way, it resists oxidation, and it can be produced to a purity that makes one batch behave like the next. That reproducibility is what allows a standard to exist at all.
The physics, in one paragraph
Metals conduct less well when they are hot: the atoms vibrate more, electrons collide more often, resistance rises. In platinum this relationship is stable and repeatable enough to be used as a measurement rather than an indication. You do not read a temperature from a PT100; you read a resistance, and convert it using a curve that everyone agrees on.
The curve that everyone agrees on: IEC 60751
The international standard IEC 60751 defines the reference relationship between resistance and temperature. Above 0 °C it is a second-order polynomial:
with R₀ = 100 Ω, A = 3.9083 × 10⁻³ °C⁻¹, B = −5.775 × 10⁻⁷ °C⁻²
The B term is tiny and negative, which is why the curve looks almost straight but bends very slightly downward as temperature rises. Applied to the temperatures that matter in steam sterilization:
| Temperature | Resistance (IEC 60751) |
|---|---|
| 0 °C | 100.00 Ω |
| 100 °C | 138.51 Ω |
| 121 °C | 146.44 Ω |
| 134 °C | 151.33 Ω |
Notice the sensitivity: between 121 and 134 °C, thirteen degrees, the resistance changes by less than 5 Ω. That is roughly 0.375 Ω per degree in this range. Everything about good PT100 practice follows from that number, because it tells you how little stray resistance it takes to ruin a reading.
Accuracy classes: AA, A, B
The same standard defines tolerance bands. They are not marketing grades; each is a formula, and the permitted error grows with temperature:
| Class | Tolerance formula | At 121 °C | At 134 °C |
|---|---|---|---|
| AA | ±(0.10 + 0.0017·|t|) | ±0.31 °C | ±0.33 °C |
| A | ±(0.15 + 0.0020·|t|) | ±0.39 °C | ±0.42 °C |
| B | ±(0.30 + 0.0050·|t|) | ±0.91 °C | ±0.97 °C |
Read the last column again. At 134 °C, a class B element may legitimately be off by nearly a full degree while remaining perfectly within specification. A class AA element stays inside a third of a degree. When the question you are asking is whether a plateau held its value, that difference decides whether your record means anything.
This is why the chain we describe uses a class AA, 4-wire element. It is not gold plating; it is the difference between a curve you can reason about and a curve you can only glance at.
Why the number of wires matters more than people expect
A resistance is measured by passing a small known current through the element and reading the voltage across it. The instrument cannot tell the difference between the resistance of the platinum and the resistance of the wires leading to it.
2-wire
The cable resistance is added directly to the measurement. Copper is not free of resistance, and at roughly 0.375 Ω per degree, one ohm of cable adds about 2.7 °C of error. A few metres of thin cable can therefore wipe out the entire benefit of a class AA element.
3-wire
A third conductor lets the instrument estimate and subtract the cable resistance, assuming all conductors are identical. It usually is a good assumption, and 3-wire is the common industrial compromise.
4-wire
Two wires carry the current, two others measure the voltage right at the element while carrying almost none. The cable drops out of the measurement entirely, without assumptions. This is the arrangement to use when the accuracy of the element is the point of the exercise.
Two effects worth knowing about
Self-heating
The measuring current warms the element slightly. Instruments use a small excitation current precisely to keep this negligible, and it is one reason a PT100 should be read by a proper transmitter rather than an improvised circuit.
Response time
A probe inside a stainless thermowell does not react instantly: the heat has to cross the wall of the well and reach the element. In a sterilization cycle, which lasts minutes rather than seconds, this lag is not a problem — but it does mean the recorded rise looks slightly smoother than reality. Knowing that avoids misreading a normal curve as a slow machine.
From resistance to a record
The element produces a resistance. Turning it into something usable requires a converter that excites the element, measures the voltage, applies the IEC 60751 curve and publishes a digital value — in our chain, over RS-485 using Modbus RTU. From there a gateway carries the value to the platform.
The important consequence for anyone evaluating a system: the accuracy of the record is set by the element and the wiring, not by the software. No amount of cloud processing recovers information that the cable already destroyed.
What a PT100 does not tell you
- It does not measure pressure. Not indirectly, not by calculation. Pressure can only come from a separate sensor or from a digital output published by the autoclave itself and read by the gateway.
- It does not know what it is touching. It reports the temperature at the element. What that temperature represents depends entirely on where the probe sits — which is a subject in its own right, covered on our page about PT100 sensor placement in autoclave sterilization.
- It does not judge a cycle. It produces numbers. Whether those numbers mean a load may be released is a decision for the qualified person responsible for it.
Why this matters for sterilization records
Steam sterilization is defined by holding a temperature for a duration. A record of that process is therefore only as good as the temperature measurement underneath it. A class AA 4-wire PT100, read through a proper converter, gives you a curve where a third of a degree of drift is visible — and drift of that size, sustained over months, is exactly the kind of change nobody notices from a printed strip.
For how that measurement chain is assembled in practice, see how to convert an existing autoclave to IoT.
Frequently asked questions
What does the 100 mean?
The element measures 100 Ω at 0 °C. PT is platinum.
What resistance corresponds to 121 and 134 °C?
About 146.4 Ω and 151.3 Ω respectively, on the IEC 60751 curve.
Is class AA worth it over class B?
At 134 °C the permitted deviation is about 0.33 °C for AA against 0.97 °C for B. If you want drift to be visible, yes.
Why 4-wire?
It removes cable resistance from the measurement. In 2-wire, one ohm of cable is worth roughly 2.7 °C of error.
Can it measure pressure?
No. Temperature only.
Talk to our team about your installation
IOT.AUTOCLAVEXPRESS.COM is a division of SCMDSD LLC (USA). The platform records and documents sterilization cycles. It does not determine the conformity of a cycle and does not authorise the release of a sterilized load; that decision remains with the qualified personnel responsible for it. All technical resources