Quick answer: If you sterilize wrapped or pouched instruments, you need a type B sterilizer. Type N units process unwrapped solid instruments only and cannot penetrate packaging. Type S units sit in between and are validated only for the specific load types the manufacturer lists. Choosing a type N machine for a clinic that pouches its instruments is one of the most common and most consequential specification mistakes.
What do the sterilizer types actually mean?
The type letter describes what the machine is able to process, not how large it is. Type N sterilizers use gravity displacement and are intended for unwrapped solid items. Type B sterilizers use a fractionated vacuum and are able to process wrapped instruments, porous loads and hollow items. Type S sterilizers are designed for a defined subset of loads that the manufacturer specifies, which may or may not include wrapped instruments.
Because the letter is a capability statement, it drives everything downstream: how you package instruments, which indicators you use, and whether a Bowie-Dick test applies.
Which type do wrapped instruments require?
Wrapped instruments require a type B sterilizer. Steam must penetrate the packaging as well as the instrument, and air must be removed from inside the pouch and from hollow items. A fractionated vacuum sequence achieves that; gravity displacement cannot do it reliably, which is why a type N machine is not suitable for pouched loads.
Where a clinic uses pouches or wrap for its everyday instruments, the sterilizer specification has to be type B. Buying a type N unit to save on capital cost removes the option of packaging instruments at all, which most clinics cannot accept.
| Feature | Type N | Type B | Type S |
|---|---|---|---|
| Air removal method | Gravity displacement | Fractionated vacuum | Manufacturer-defined |
| Wrapped instruments | No | Yes | Only where the maker lists it |
| Porous and hollow loads | No | Yes | Only for validated load types |
| Bowie-Dick test applies | No | Yes, daily pre-use | Depends on the air-removal design |
| Typical use | Unwrapped solid items only | General clinic processing | Narrow, specified loads |
| Cycle documentation | Standard cycle record | Standard cycle record plus air-removal test | Follow maker's validation list |
Why is type S the one people get wrong?
Type S is defined by the manufacturer, not by a universal standard. That means a type S unit is validated for the exact loads its maker lists, and anything outside that list is unsupported. A clinic that buys a type S machine and then processes whatever fits inside is using the sterilizer outside its validated scope.
The practical test is simple: ask for the manufacturer's validated load list and compare it to what the clinic actually processes. If pouched handpieces or wrapped trays are not on the list, the machine is the wrong type for the work.
What does a type B unit add to the monitoring routine?
A type B unit is a pre-vacuum machine, so it needs a daily air-removal test before the first load in addition to the daily biological indicator. The air-removal test confirms the vacuum stage worked; the biological indicator confirms the cycle killed the spores. Both are required, and they answer different questions.
In Ontario, the RCDSO expects a biological indicator for each sterilizer on every day it is used — daily monitoring, not weekly. The weekly baseline is the CDC and AAMI ST79 pattern in most US states, and it is often the reason a clinic under-orders consumables.
How should a clinic specify a new sterilizer?
Start with the load, not the machine. List what the clinic processes: wrapped trays, pouched instruments, hollow items, porous textiles. Then require the supplier to confirm the machine is validated for every item on that list, in writing. A specification that begins with a room size or a price ends with an unsupported process.
Confirm also the cycle documentation the machine produces, the consumables the validated cycles require, and the maintenance schedule. A sterilizer is not a standalone purchase; it commits the clinic to a monitoring routine that has to be resourced for years.
It is worth building the monitoring cost into the specification as well. A type B machine adds a daily air-removal test on top of the daily biological indicator, so the annual consumable plan is larger than a gravity unit would need. Clinics that compare only capital cost between a type N and a type B unit miss this recurring difference, and it changes the five-year cost of ownership noticeably. Buying the wrong type to save money on the machine usually costs more once the clinic realises it cannot package its instruments at all.
Related reading
Sterilization compliance hub; Canadian sterilizer guide for class B, N and S; air removal testing on pre-vacuum autoclaves; inside a sterilization pouch; dual-indicator sterilization pouches; BI 5-pack trial.
Download the sterilizer specification checklist from the compliance log centre, run the sterilization self-check to confirm your current unit's validated loads, and book a sterilization compliance consultation if you are specifying a replacement.
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. Reselling? become a distributor.
Frequently Asked Questions
Can a type N sterilizer process pouched instruments?
No. Type N units use gravity displacement and are intended for unwrapped solid items. Wrapped or pouched loads require a type B machine.
Is type S always able to process wrapped instruments?
Only if the manufacturer's validated load list includes them. Type S is defined by the maker, so anything outside that list is unsupported.
Does a type B sterilizer need a Bowie-Dick test?
Yes, daily before the first load, because it is a pre-vacuum unit with an air-removal stage. The type N does not have that stage.
What should I ask a supplier before buying?
Ask for written confirmation that the machine is validated for every item you process, plus the cycle documentation and consumables the validated cycles require.
CliniEco Medical is a licensed medical device establishment (MDEL #35334).
0 comments