Last Updated on August 12, 2026
TL;DR: Understanding the different types of autoclaves is essential for selecting the right sterilization solution for your laboratory or clinical environment. This guide breaks down the most common types of autoclave sterilization, explaining how each system works and what materials they are best suited for.
Key takeaways include:
- The two types of autoclaves based on sterilization method: gravity displacement and vacuum-assisted
- How Class N, Class S, and Class B autoclaves differ in air removal, steam penetration, and load compatibility
- Which different types of autoclave machines are best suited for solid, porous, wrapped, or hollow loads
- The advantages and limitations of each autoclave class across laboratory and clinical settings
- How autoclave selection aligns with application requirements and regulatory standards
What is an autoclave?
An autoclave is a steam sterilizer that uses saturated steam under pressure to destroy microorganisms on laboratory materials, instruments, liquids, and waste. Autoclaves differ mainly in how they remove air from the chamber and allow steam to reach the load.
Laboratory autoclaves are available in several configurations, each designed for specific sterilization requirements. Differences in air removal, steam penetration, and validated load types affect which system is appropriate for laboratory, clinical, or research applications.
This guide explains how gravity displacement, Class N, Class S, and Class B autoclaves differ, helping you identify the type that best matches your materials, workflow, and compliance requirements. You can also review five factors to consider when choosing an autoclave.
Comparison of common autoclave types
Autoclaves are commonly grouped by how they remove air from the chamber and
allow steam to reach the load. The table below summarizes the main
differences between gravity, vacuum, and Class S autoclaves.
| Autoclave type | Air-removal method | Common applications | Main advantages | Limitations |
|---|---|---|---|---|
| Gravity displacement / Class N | Steam enters the chamber and pushes heavier air downward and out through the drain. | Liquids, culture media, glassware, laboratory waste, and solid unwrapped instruments. | Simple operation, dependable performance, and fewer complex components. | Generally not suitable for wrapped, porous, or narrow hollow loads. |
| Pre-vacuum / Class B | A vacuum pump removes air before steam enters the chamber. | Wrapped instruments, porous materials, textiles, and hollow instruments. | Improved air removal and steam penetration for complex loads. | More complex equipment, higher maintenance needs, and greater cost. |
| Class S | Uses a manufacturer-defined air-removal and sterilization process. | Specific wrapped, unwrapped, porous, or hollow loads approved by the manufacturer. | Designed for selected applications that fall between Class N and Class B capabilities. | Load compatibility varies by model and must be confirmed with the manufacturer. |
How different types of autoclaves work
The primary distinction among the different types of autoclave machines is their sterilization method. Broadly speaking, there are two types of autoclaves based on sterilization method:
- Gravity displacement autoclaves
- Vacuum-induced/pre-vacuum autoclaves
Some laboratory autoclaves incorporate features from both gravity displacement and vacuum-assisted sterilization to support specific applications. Although all steam autoclaves use saturated steam under pressure, the method of air removal affects steam penetration and the types of loads each system can process effectively. Learn more about the importance of steam in laboratory autoclaves and what can and cannot be autoclaved.
Class N gravity autoclaves: simple and dependable
Class N autoclaves commonly use gravity displacement to remove air from the chamber. Their simple design, dependable operation, and relatively low maintenance requirements make them common in laboratory settings.
Steam generation and circulation
Gravity autoclaves work by pumping steam into the autoclave chamber, which displaces ambient air and forces it out through exhaust valves. Once the air is removed, saturated steam surrounds the load to achieve sterilization.
Suitable materials
Due to their use of gravity-based air removal, Class N autoclaves are commonly used for compatible liquids, glassware, laboratory waste, and solid, non-porous unwrapped items, such as:
- Laboratory glassware
- Culture media
- Biohazard waste
- Unwrapped metal instruments
- Laboratory utensils
They are not recommended for sterilizing porous loads, wrapped items, or surgical instruments with internal channels. With these materials, trapped air can prevent effective steam penetration.
Use cases
Class N autoclaves are commonly used where routine sterilization involves liquids, culture media, glassware, laboratory waste, and unwrapped solid items. Typical settings include:
- Academic and research laboratories
- Biotechnology laboratories
- Pharmaceutical research and quality control laboratories
- Food and environmental testing laboratories
- Teaching laboratories
They are particularly well suited for laboratories that do not routinely sterilize wrapped, porous, or narrow-lumen instruments.
Limitations
Without a vacuum phase, Class N autoclaves cannot reliably sterilize:
- Porous materials
- Layered or wrapped loads
- Hollow instruments with lumens
Standards and regulations
While suitable for many applications, specific healthcare and laboratory settings may require autoclaves that meet more stringent sterilization standards. These regulated environments may necessitate the use of Class B autoclaves.
Class B vacuum autoclaves: deeper sterilization for challenging media
Class B autoclaves are designed for applications requiring reliable steam penetration into wrapped, porous, and hollow loads.
Vacuum function
Unlike gravity systems, Class B autoclaves use a powerful vacuum pump to remove air from both the chamber and the load before steam is introduced. By creating a pre-vacuum and evacuating the ambient air within the chamber, high-temperature steam can fully penetrate and sterilize even the most challenging items.
Steam penetration
Creating a vacuum before and during the sterilization cycle allows for effective steam penetration throughout the load, including:
- Wrapped instruments
- Hollow instruments
- Porous materials
- Laboratory textiles
- Certain liquid containers where vacuum-assisted air removal improves steam penetration
Versatility
Class B systems are designed for a broad range of load types, including wrapped, porous, and hollow items. This makes them well suited for medical, dental, and laboratory environments where vacuum-assisted air removal and deeper steam penetration are required.
Standards compliance
These autoclaves are designed to comply with rigorous international sterilization standards, including EN 13060 for small steam sterilizers, where applicable. This compliance ensures that Class B autoclaves can meet the demands of high-standard healthcare practices.
Efficiency and environmental considerations
Modern Class B autoclaves are designed with efficiency in mind regarding energy consumption and water usage. Some models incorporate eco-friendly features to minimize their environmental impact.
Class S: the in-between option
Class S autoclaves bridge the gap between Class N and Class B systems by providing enhanced air removal while remaining validated only for the load types specified by the manufacturer.
They can sterilize a broader range of items than Class N, including some wrapped materials and products with hollow sections. This makes them a good fit for environments with more varied sterilization needs that don’t require the full performance of a Class B autoclave.
Sterilization capabilities
Class S autoclaves can sterilize:
- Solid instruments
- Certain porous materials
- Some hollow instruments
- Selected wrapped items (depending on model specifications)
Air removal techniques
Instead of full vacuum systems, Class S autoclaves use techniques such as steam pulsing to improve air removal beyond basic gravity displacement. This allows for better penetration of steam into the load when compared with Class N autoclaves.
Standards and regulations
Class S autoclaves meet specific international standards but may not satisfy all requirements mandated for Class B sterilization. Verification against application-specific regulations is essential. Unlike Class B autoclaves, Class S systems are validated only for the specific load types defined by the manufacturer.
Use cases
Class S autoclaves are commonly used in dental practices, smaller medical facilities, and laboratories that need more flexibility than Class N autoclaves can provide but do not require the full capabilities of Class B autoclaves. They are particularly useful for practices that need to sterilize wrapped items or instruments with small lumens.
Which autoclave type is right for your application?
The most appropriate autoclave depends on the materials you sterilize, your laboratory workflow, and any regulatory requirements. The table below provides a general comparison of common laboratory applications.
| Application | Recommended autoclave type | Reason |
|---|---|---|
| Laboratory glassware | Class N (Gravity) | Simple solid loads allow effective steam penetration using gravity displacement. |
| Culture media and aqueous solutions | Class N (Gravity) | Commonly used for routine laboratory sterilization of liquids. |
| Biohazard laboratory waste | Class N (Gravity) | Suitable for many laboratory waste sterilization procedures. |
| Wrapped instruments | Class B | Vacuum-assisted air removal improves steam penetration through packaging. |
| Porous materials or laboratory textiles | Class B | Vacuum cycles remove trapped air before sterilization. |
| Hollow instruments | Class B | Vacuum-assisted air removal supports steam penetration into internal channels. |
| Selected wrapped, porous, or hollow loads | Class S or Class B | Suitability depends on the manufacturer’s validated load specifications. |
Planning for a new autoclave?
Use our laboratory autoclave planning checklist to review load size,
capacity, installation requirements, utilities, and workflow before
comparing equipment.
Choosing the right autoclave for your needs
Selecting the right autoclave begins with identifying which class best matches your sterilization needs.
Beyond autoclave class, laboratories should also consider chamber capacity, installation requirements, programmable cycles, cooling performance, validation options, and the types of materials sterilized each day. The best autoclave is the one that supports your workflow while meeting your sterilization requirements.
- Class B autoclaves are ideal for medical, dental, and research environments requiring comprehensive sterilization across diverse materials.
- Class S autoclaves offer a balance of flexibility and performance for mixed-use settings.
- Class N gravity autoclaves are a practical choice for compatible liquids, glassware, waste, and unwrapped solid items. TOMY SX-Series gravity displacement autoclaves are designed for laboratories sterilizing liquids, media, glassware, and laboratory waste. Their compact footprint, no-plumbing installation, and programmable cycles make them well suited for research and quality control laboratories.
By understanding the different types of autoclaves, you can confidently choose equipment that best aligns with your workflow.
The best autoclave type depends on the load. Gravity autoclaves are a dependable choice for many laboratory liquids, glassware, solid instruments, and waste loads, while vacuum systems may be required for wrapped, porous, or hollow items.
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Can you tell me or point me in the right direction for documentation for all the different terms used for vacuum sterilizers? Gravity sterilization generally is a much longer cycle. We have Pre Vac and Steam Flush Pressure Pulse in our hospital. The issue we have is when looking at Manufacture’s Instructions for Use they have many different terms for vacuum sterilizers. For example: Pre Vac, Dynamic Air Removal, Fractioned Air, Steam Vacuum. can we assume that any vacuum type sterilizer would be interchangeable with the time and temp directions from manufacturers? For example: Gravity method 20 minutes at 250 degrees F. Pre-Vac method: 4 minutes at 270 degrees F.
Hi,
thank you for your inquiry.
Regarding terminology for vacuum-driven steam sterilization technology, the most common terms are “vacuum sterilization” “pre vacuum sterilization” and derivatives of these terms. There are several other less common terms that are used, including the ones that you mentioned, as manufacturers often use alternative product descriptions to distinguish their products from others. However, the “vacuum” and “prevacuum” are most commonly used. Please note that SFPP (“Steam Flush Pressure Pulse”) is also classified as a type of dynamic air removal along with vacuum or pre vacuum autoclaves by the AAMI (Association for the Advancement of Medical Instrumentation), but have slightly different functionality than vacuum/pre vacuum autoclaves.
For more information on steam sterilization in healthcare facilities, please refer to the AAMI Comprehensive Guide to Steam Sterilization and Sterility Assurance in Health Care Facilities.
Hi , how long does it take to complete a sterilisation run (time from start to finish)?
Hello Rachael,
Thank you for your inquiry.
Though total sterilization cycle time varies depending on sterilization temperature, time, and the sterilzation environment (temperature) for a typical setting of 20-minutes at 121°C, the total cycle time is around 70 minutes including heat up and cool down time.
If you have any questions, please email us at tomy@amuzainc.com or call us at 858-800-3900.
Thank you,
Jason
858-800-3900
Can an autoclave reach a temperature of 280 degree Celsius? If yes, please specify.
Hi,
thank you for your inquiry.
The maximum temperature that TOMY autoclaves can reach is 135°C (or 275°F), as sterilization is typically carried out at between the standard 121°C and 134°C.
Please refer to our product page for other specifications and download product information: https://tomy.amuzainc.com/autoclave/
Thank you,
Jason
858-800-3900
Are Tomy autoclaves the gravity type?
Hello Shinji,
thank you for your inquiry- yes, the Tomy SX-Series Autoclaves are a gravity displacement type steam autoclave sterilizers.
You can find out more info about Tomy SX-Series Autoclaves here: https://tomy.amuzainc.com/autoclaves/
When is the vacuum function applied in an autoclave..Is it before the sterilisation hold or after the sterilisation hold or in between sterilisation hold temperature…?
Hello Vikas,
thank you for your inquiry.
In most vacuum autoclaves, the vacuum function is applied first to remove the air in the chamber and then steam is added until the target temperature and pressure is achieved to start the cycle.
In gravity autoclaves or gravity displacement / downward displacement autoclaves, the pressurized steam does the work of pushing out and replacing the ambient air until the chamber is saturated with steam. The autoclave shuts its valves and allows the pressure to build until the target pressure and temperature is achieved, at which point the cycle starts.
If you have any further questions, or are interested in learning more about autoclaves, please contact us at tomy@amuzainc.com, or fill out the contact form and we will get back to you soon.
Thank you for visiting,
The TOMY Team
Hello sir, I would like to know which autoclave will be best for sterilization of dental equipments,,,, gravity or vacuum and also whether front loading or top loading? And also what are their prices
Hello Anjali,
thank you for reaching out with your question.
For dental equipment, if you are using sterlization pouches, you would likely want to use a vacuum autoclave to allow the steam to penetrate the pouch and sterilize the contents within.
Please have a look at the video on this page. You can access it in this link <https://tomy.amuzainc.com/blog/types-autoclaves-gravity-vs-vacuum-autoclaves-advantages/>.
Thank you and all the best in your autoclave search!