What Is a Class B Autoclave? Chamber Design and Cycle Types

September 12, 2026

A Class B autoclave is a benchtop sterilizer that can process wrapped, porous and hollow loads, not just bare metal instruments. Under EN 13060, the "B" designation means the chamber is fitted with a vacuum pump and the cycle actively removes air from the load before admitting steam. If your clinic wraps instruments in pouches or sterilizes handpieces, suction tips or lumened devices, Class B is the performance level that matters.

CliniEco incubator used to read biological indicator vials after a benchtop sterilizer cycle

Class N, Class S and Class B: What EN 13060 Actually Separates

EN 13060 is the European standard for small steam sterilizers, and it sorts benchtop units by the load types they are validated to sterilize, not by size or price. The classification exists because air is the enemy of steam: a pocket of air trapped inside a load insulates it from the saturated steam that does the killing.

Class N: unwrapped solid instruments

Class N sterilizers use a simple gravity displacement cycle. Steam enters at the upper end of the chamber, pushes cooler air out through a drain at the bottom, and that is the entire air-removal strategy. It suits bare, solid, unwrapped instruments in a single layer, but not wrapped items, porous materials or anything with a lumen, where air has nowhere to escape.

Class S: the loads the manufacturer names

Class S is a middle category with no fixed load list. The manufacturer specifies exactly which load types the unit is validated for, for example unwrapped solid instruments plus one specific single-wrapped item and nothing else. Two Class S machines from different makers may not accept the same loads, so you read the manual rather than the badge.

Class B: wrapped, porous and hollow loads

Class B sterilizers must pass the full EN 13060 test programme, including the hollow-load and porous-load tests, using fractionated pre-vacuum cycles to clear air a gravity cycle cannot reach. In practical terms, a Class B benchtop sterilizer is the classification that covers pouched instruments, wrapped cassettes, textile packs and lumened devices such as handpiece tubing and cannulated instruments.

Class	Load types covered	Strengths	Limits
Class N	Unwrapped solid instruments in a single layer	Short, simple cycles; lowest cost; easy for new staff to run	No wrapped, porous or hollow loads; gravity air removal only
Class S	Only the load types named by the manufacturer	Faster and cheaper than Class B when your load matches the specification	No universal load list; changing wraps or adding lumens can move you outside validation
Class B	Wrapped, porous and hollow loads, plus unwrapped solids	Passes EN 13060 hollow and porous load tests; pre-vacuum cycles; suits dental and clinic-wide use	Higher purchase price; needs vacuum pump maintenance and daily verification

Quick facts: EN 13060 is the standard that defines Class N, S and B for small steam sterilizers.
Class B is the class validated for wrapped, porous and hollow loads across the board.
Fractionated pre-vacuum, often shortened to pre-vac, removes air in pulses before steam is admitted.
Class B does not remove the need for daily biological monitoring.

Why Fractionated Pre-Vacuum Reaches Inside Lumens and Pouches

A gravity cycle can only push air downward out of the chamber. A lumen, a folded wrap or a dense textile pack traps air where that downward flow never reaches, and steam meeting an air pocket stalls at the boundary instead of condensing on the instrument.

Fractionated pre-vacuum solves this by pulling a deep vacuum, breaking it with steam, and repeating, so each pulse dilutes and removes more of the residual air. Once the air is gone, admitting steam produces a rapid pressure and temperature rise that drives saturated steam into narrow channels and through wrap material. The result is direct steam contact on the instrument surface, which transfers the latent heat that kills microorganisms.

Self-sealing sterilization pouches used for wrapped loads in a Class B benchtop sterilizer

The Cycle Types You Will Actually Run

The cycle you choose follows the load, not the label on the door. For the longer comparison of how air removal changes what a cycle can sterilize, see our guide to autoclave cycles: gravity vs vacuum pre-vac.

134 C fast cycle for wrapped instruments

The workhorse cycle on most Class B units. Roughly 3 to 4 minutes at 134 C, plus pre-vacuum and drying. Run it for pouched instruments and handpieces; it is the fastest option that still meets wrapped-load requirements.

121 C gravity cycle for heat-sensitive or bulk items

A longer, gentler cycle, typically 15 to 30 minutes at 121 C depending on load mass. It suits items that cannot tolerate 134 C and some bulk loads. This is a different cycle type, not a different class: on a Class B unit the cycle still has to be validated for the load inside it.

Bowie-Dick test cycle for the vacuum itself

The Bowie-Dick test runs in an otherwise empty chamber on the first cycle of the day to confirm that the pre-vacuum system is removing air properly. It tests the sterilizer's air removal, not a production load. An uneven or failed result points at a leak, a blocked drain or a vacuum fault, see our breakdown of why autoclave cycles fail the Bowie-Dick test.

Drying stage

The post-cycle drying phase pulls a vacuum to evaporate residual condensate. Shorten it and wrapped packs come out wet; wet packaging compromises the sterile barrier and can let microorganisms wick through. Drying is both a cycle and a design function.

Chamber and Door Design: What Decides Steam Penetration and Drying

A Class B chamber is engineered around vacuum, not only pressure. Three features do most of the work.

Vacuum-rated chamber and door seal. The chamber and door hold a vacuum without leaking, because a leak lets air back in during pre-vacuum pulses. A torn or hardened door gasket is a common cause of a marginal cycle.
Chamber geometry and tray design. A shallow, wide chamber and a slotted or perforated tray let steam circulate around the load. Stacking instruments or overfilling a tray blocks that circulation.
Drain and steam-path design. The position of the drain affects how quickly condensate leaves the chamber and how thoroughly air is displaced at the start of a cycle.

Door design matters for drying too. A door that seals tightly against a flat chamber face holds the vacuum needed to boil off condensate at low temperature without crushing the gasket. Whatever the design, the load must be arranged so steam can reach every surface, our autoclave load preparation checklist for steam contact walks through this step by step.

Class B Still Needs Verification

Class B tells you what the machine is capable of, not that this morning's cycle worked.

In Ontario, the RCDSO requires a biological indicator for each sterilizer on every day it is used: daily monitoring, not weekly. That is one biological indicator per sterilizer per day, and if you run more than one cycle type, one for each cycle type used that day. Across a typical year, daily monitoring works out to roughly 250 indicator vials per sterilizer (5 days a week x 50 weeks). For comparison, CDC and AAMI ST79 use weekly biological monitoring as the baseline in most U.S. states, a useful reference point but not the Ontario requirement. Provincial regulatory colleges publish their own IPAC standards, so confirm against your own province's college.

Biology alone is not enough. Pair biological monitoring with a Class 5 or Class 6 chemical indicator in every load, and run a Bowie-Dick test on the first cycle of each day a vacuum sterilizer is used. ISO 17665 describes the validation and routine control framework, while the CSA Z314 series sets out Canadian requirements for sterilization in health care settings.

Common Questions About Class B Autoclaves

Can a Class B autoclave sterilize unwrapped instruments?

Yes. A Class B unit can process unwrapped solid instruments as well as wrapped, porous and hollow loads, making it the broadest of the three EN 13060 classes.

Does Class B mean I can skip biological monitoring?

No. Class B describes the sterilizer's validated load range, not ongoing verification. In Ontario, the RCDSO requires a biological indicator for each sterilizer on every day it is used, alongside chemical indicators and a daily Bowie-Dick test on vacuum units.

What is the difference between a pre-vacuum and a gravity cycle?

A gravity cycle lets steam push air out of the chamber by displacement. A pre-vacuum cycle uses pulses of vacuum and steam to remove air actively, which is what allows steam to penetrate lumens, wraps and porous packs that gravity cycles cannot reliably clear.

How long does a Class B cycle take?

A 134 C wrapped-instrument cycle is typically 3 to 4 minutes of exposure plus pre-vacuum and drying, so a full cycle usually finishes well under an hour. A 121 C cycle runs longer, often 15 to 30 minutes of exposure.

Do I still need a Bowie-Dick test on a Class B sterilizer?

Yes. The Bowie-Dick test checks air removal and steam penetration in an otherwise empty chamber, which biological and chemical indicators do not confirm on their own. Run it on the first cycle of each day the unit is used.

Related supplies for this workflow: Sterilization Roll, Pre-Cut Sheets (50 m) is stocked in our Ontario warehouse and ships across Canada.

Not sure which monitoring consumable fits your cycles? Start with the 5-pack biological indicator trial, or request a quote for volume pricing.

Ordering for a clinic, lab or care home? Wholesale and multi-site ordering covers case pricing and account setup, and the B2B wholesale collection lists the lines stocked for institutional buyers.

What Is a Class B Autoclave? Chamber Design and Cycle Types