Where to place the PT100 probe in an autoclave

When you connect a sterilizer with the AutoclaveXpress Trilogy, the PT100 probe becomes the single point of truth for every cycle you record. Its placement decides whether your dashboard reflects what the load actually experienced or merely what one convenient corner of the chamber felt. This note is the companion to our installation guide: that one tells you how to wire the system, this one tells you where the probe should live.

01What you are really measuring

It is tempting to think of the goal as "measure the chamber temperature." It is not. Sterilization is achieved only when every part of the load is held at the required temperature, in saturated steam, for the required time. So the real question your probe must answer is harder and more specific: is the hardest-to-sterilize point of the load reaching and holding temperature?

That distinction changes everything about placement. A probe sitting in a warm, open pocket of steam near the inlet will sail through every cycle while a dense pack in the corner quietly fails. The probe has to represent the worst case, not the average and certainly not the best case. Good placement is, in essence, a deliberate hunt for the coolest, slowest, most stubborn location — and then sitting your sensor there.

It is worth being honest about why the margin matters so much here. Sterilization parameters are built around the coldest, slowest point reaching temperature for the full hold time; the rest of the load, being warmer, comfortably exceeds it. If your probe sits anywhere warmer than the true cold spot, it quietly eats into that safety margin without anyone noticing, because the curve still looks textbook-perfect. The whole discipline of placement is about refusing that easy, flattering reading and insisting on the honest, demanding one.

02The cold spot and the drain

Every sterilizer has a cold spot: the location that is slowest to reach temperature and first to lose it. Find that point and you have found where the probe belongs, because if the cold spot is satisfied, everything warmer than it is satisfied too.

In most steam sterilizers the cold spot is low in the chamber, near the drain. There is a physical reason for this. Air is heavier than steam and impairs sterilization wherever it lingers; as steam displaces air, the air and the cooler condensate sink and collect at the bottom, around the discharge. That is why standards for steam sterilizers treat the chamber discharge as a reference location, and why the sterilizer's own control sensor is so often placed in or near the drain line. It is the chamber's natural worst case.

03Where never to place the probe

Several locations feel reasonable and are actively misleading. Avoid them all.

• Touching the chamber wall. The wall is heated and holds temperature differently from the steam around the load. A probe against metal reads the wall, not the cycle, and usually reads falsely high.
• In the steam inlet or a direct jet. Incoming steam can be hotter than the chamber and momentarily superheated. A probe in the jet sees a flattering spike that no part of the load ever experiences.
• Hard against the door or its gasket. The door area has its own thermal behaviour and sealing dynamics; it is not representative of the load.
• Near a heating element. As with the wall, you measure the heat source rather than the sterilizing environment.
• Buried where steam cannot circulate freely in a way that is not part of the real load — an artificial dead pocket you created with the cable or a clamp.

04Chamber space vs. in the load

There are two distinct things you might measure, and it helps to be clear about which one your probe is doing.

Free chamber temperature

A probe in the free steam space of the chamber — the reference point near the drain is the classic example — tells you about the sterilizing environment the machine is producing. This is the most common and most robust placement for a single, permanent monitoring sensor, because it is repeatable cycle after cycle and is independent of how today's particular load was packed.

In-load temperature

A probe placed inside the load — at the centre of the densest pack, or inside the lumen of a hollow instrument — tells you what the hardest item actually reached. This is the most demanding test and the truest reflection of sterilization, but it depends entirely on the load being arranged the same way every time, which in real clinical use it rarely is.

For continuous monitoring, the free chamber reference point is usually the right choice: stable, repeatable, and representative of the machine. The in-load worst case belongs to formal performance qualification, where multiple sensors are placed throughout a defined reference load.

One in-load case deserves special mention: hollow instruments and lumens. The inside of a long, narrow channel is the single hardest place for steam to reach, because any trapped air has to be displaced through a small opening. If your facility sterilizes lumened devices, the interior of a representative lumen is the true worst case during qualification — far more demanding than any open chamber location. It is exactly the kind of point a free-space monitoring probe cannot see, which is why continuous monitoring complements, rather than replaces, periodic qualification with load-borne sensors and process-challenge devices.

05Practical placement for the Trilogy

The Trilogy's PT100 is a permanent, single monitoring probe, so in almost every installation the best home for it is the chamber reference point near the drain, or as close to the sterilizer's own validated reference location as you can reach. There it sees the chamber's natural worst case, behaves consistently on every cycle, and correlates closely with the machine's control sensor.

How to mount it well

• Use an existing port or thermowell. Many sterilizers have a validation or measurement port designed exactly for an external sensor. Use it. Never drill into or modify the pressure vessel to create one.
• Suspend the tip in the steam, not against metal. The sensing tip should sit in free steam at the reference location, clear of the wall, the floor and any element.
• Route the cable through a proper gland. Bring the shielded cable out through an existing penetration or gland so the chamber's pressure integrity is never compromised, and leave a small service loop so the tip cannot be tugged out of position.
• Keep it repeatable. Once you have a good location, fix it there. The value of continuous monitoring comes from comparing like with like, cycle after cycle — a probe that moves is a probe you cannot trust over time.

06Sterilizer type and air removal

How a sterilizer removes air shapes where its cold spot sits. Air is the enemy of steam sterilization, and the way it is evacuated determines where any residual air — and therefore the worst case — can hide.

• Vacuum (type B) sterilizers actively pump air out before admitting steam, so they handle wrapped loads and hollow instruments well. The cold spot is more predictable, but the reference point near the drain remains the standard monitoring location.
• Gravity-displacement and type N machines rely on steam pushing air down and out through the drain. Residual air is more likely to linger low and near the discharge, which reinforces the drain area as the location to watch.

Large and small sterilizers are covered by different reference frameworks — broadly, EN 285 for large steam sterilizers and EN 13060 for small ones, with ISO 17665 describing moist-heat sterilization more generally. These standards inform where reference measurements are taken, but they do not override your own facility's validated cold spot. Treat the standards as the map and your performance qualification as the territory.

07Confirming the placement is right

Once the probe is in place, prove it before you rely on it.

1. Compare with the machine. Run a normal cycle and lay the Trilogy's curve against the sterilizer's own printout or display. The plateau temperature and timing should agree closely. A large, consistent gap means the probe is seeing something different from the machine — usually a placement problem.
2. Watch the shape, not just the peak. A good placement produces a clean curve: a steady climb, a stable plateau at the set temperature, and a clean fall. Spikes, a plateau that sits oddly high, or a jagged trace often point to a probe in a steam jet, against a wall, or loose.
3. Repeat across a few cycles. Placement is only validated when the behaviour is repeatable. Run several cycles and confirm the curves are consistent before you treat the readings as your reference.

One subtlety to keep in mind while you compare curves is thermal response. A probe with a heavier sheath, or one tucked inside a pack, will lag the true steam temperature slightly on the way up and on the way down — it takes a moment to catch up. A small, consistent lag is normal and harmless. What you are watching for is not perfect instantaneous agreement but a stable, repeatable relationship between your probe and the machine. Once you know the normal shape of that relationship, any departure from it becomes a genuinely useful early warning.

08Placement checklist

Get the placement right and everything downstream becomes trustworthy: the live curve, the stored record, the AI analysis, the audit trail. Get it wrong and the rest of the chain faithfully records the wrong thing. A few thoughtful minutes deciding where the tip sits is the highest-value decision in the whole installation.