Last Updated on March 7, 2024

When researching autoclaves, you may find yourself confused and overwhelmed by the different types of autoclaves and the terminology surrounding them. Why are there different types of sterilizers? What is a type S autoclave? And how big of an impact does each choice make?

We’re here to answer these questions and more so you can determine the right autoclave for you.

Differences Between Types of Sterilizers

The most significant differentiating factor between types of autoclaves is their sterilization method. The two principal types of sterilizers are gravity or vacuum-induced/pre-vacuum. That said some autoclaves use both methods to sterilize.

Both types of autoclaves sterilize through high-temperature steam and pressure, but the exact mechanisms differ, which affects what articles each type can respectively autoclave.

Class N Gravity Autoclaves: Simple and Dependable

Gravity autoclaves, also known as gravity displacement autoclaves and Class N autoclaves, are the most common and affordable options.

Gravity autoclaves are well-suited for sterilizing items that are simple in design and do not contain spaces or obstacles for steam penetration. Their simplicity of design and independence from peripheral mechanisms make them highly dependable and cost-effective.

Steam Generation and Circulation:

Gravity autoclaves work by pumping steam into the autoclave chamber, displacing the ambient air and forcing it out through exhaust valves. This process allows the remaining steam to sterilize the contents effectively.

Suitable Articles

Given their method of air removal, Class N autoclaves are best suited for sterilizing simple, solid instruments and materials that do not require air removal from internal channels, such as:

  • Stainless steel surgical instruments and lab utensils
  • Polypropylene Pyrex® or Type I borosilicate glassware
  • Biohazard waste
  • Unwrapped goods

They are not recommended for sterilizing porous loads, wrapped items, or surgical instruments with internal channels because the steam may not penetrate them effectively.

Use Cases

Class N autoclaves are commonly used in settings where the sterilization requirements are straightforward and do not involve complex instruments, such as some dental clinics, tattoo parlors, and beauty salons. They are also found in small laboratories for sterilizing simple laboratory glassware and instruments.


Due to the lack of a vacuum phase, Class N autoclaves cannot effectively sterilize porous loads or items packed in layers. They also cannot ensure the sterilization of items with hollow sections or lumens, such as specific medical devices.

Standards and Regulations

While Class N autoclaves are suitable for many applications, specific healthcare and laboratory settings may require autoclaves that meet more stringent sterilization standards, such as Class B autoclaves, which offer vacuum-assisted air removal and can sterilize a wider range of items.

Class B Vacuum Autoclaves: Deeper Sterilization for Challenging Media

Class B autoclaves are advanced steam sterilizers designed to meet the stringent sterilization standards required in medical, dental, and laboratory settings. They are suitable for sterilizing large or porous items like animal cages, bedding, and wrapped surgical kits.

The vacuum function in these autoclaves ensures a more thorough sterilization process. By evacuating the ambient air within the chamber, high-temperature steam can penetrate and sterilize areas otherwise occupied by air. This mechanism is particularly efficient for sterilizing items with hard-to-reach areas.

Vacuum Function

Unlike Class N autoclaves, Class B autoclaves are equipped with a vacuum pump that creates a pre-vacuum to efficiently remove air from the sterilization chamber and the load itself. This vacuum function is crucial for ensuring complete steam penetration even in the most challenging items, such as porous materials or instruments with internal channels.

Steam Penetration

Creating a vacuum before and during the sterilization cycle allows for effective steam penetration throughout the load, including complex instruments with hollow spaces, wrapped items, and surgical packs. These include:

  • Media solutions in containers, like tissue culture flasks with loose caps
  • High-density polyethylene products, such as pipette tips and syringes
  • Wrapped dry items that can trap air
  • Animal cages and bedding


Class B autoclaves are versatile and suitable for sterilizing all load categories, making them the preferred choice in settings where a wide range of instruments and materials need sterilization, such as hospitals, dental clinics, and scientific laboratories.

Standards Compliance

These autoclaves are designed to comply with the most rigorous international sterilization standards, including EN 13060, a specific standard for small steam sterilizers. 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 represent an intermediate category between Class N and Class B autoclaves. They are more flexible than Class N autoclaves but less comprehensive than Class B autoclaves. They are designed to sterilize a broader range of items than Class N autoclaves can, including certain types of wrapped materials and products with hollow sections, under specific conditions.

Sterilization Capabilities

Class S autoclaves are suitable for sterilizing solid instruments, certain porous materials, and some hollow instruments. They may also be used for wrapped items, depending on the specific design and function of the autoclave.

Air Removal Techniques

While not as advanced as the vacuum systems found in Class B autoclaves, Class S autoclaves often use steam pulsing or other methods to remove air from the chamber more effectively than gravity displacement alone (used in Class N autoclaves). This allows for better penetration of steam into the load.


Class S autoclaves are designed to offer more flexibility than Class N models. They can accommodate a wider variety of sterilization needs, making them suitable for settings where the requirements are more varied but do not necessarily demand the full capabilities of a Class B autoclave.

Standards and Regulations

These autoclaves are designed to meet specific international standards but may not fulfill all the criteria required for Class B autoclaves. Users should verify that a Class S autoclave meets the particular standards and regulations applicable to their needs, especially for critical sterilization processes.

Use Cases

Class S autoclaves are often found in dental practices, smaller medical facilities, and laboratories where the sterilization needs are broader than what Class N autoclaves can handle but do not require the full spectrum of Class B autoclave capabilities. They are particularly useful for practices that need to sterilize wrapped items or instruments with small lumens.

Choosing the Right Autoclave for Your Needs

While there are many factors to consider when choosing the best autoclave, it’s good to determine which class of autoclave you need first.

Class B vacuum autoclaves are ideal for industries like medical and dental, where you need perfect sterilization for various materials. Class S autoclaves fill a niche between Class N and Class B autoclaves, offering greater flexibility and capability for sterilizing a wider range of items than Class N autoclaves without the comprehensive features and standards compliance of Class B autoclaves.

However, Class N gravity autoclaves, Such as TOMY’s vertical SX-series autoclaves, are the recommended choice for most applications due to their reliability and widespread availability.

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