A dark-themed banner featuring laboratory autoclaves with digital interfaces, suggestive of a high level of precision. Graph lines and a curve on the left hint at a scientific analysis or study related to the use of these sterilization devices in a laboratory setting

Sterilization 101: How Does a Laboratory Autoclave Work?

Amit Gupta
Written by: Amit Gupta

MS Mechanical Engineering, Vice President of Engineering

Steam sterilization is an important process, one that is performed in every laboratory. In this article, we will explore the history of steam sterilization, how a sterilizer works, and emerging trends in sterilizer design.

An Introduction to Steam Sterilization


The terms steam sterilizer and autoclave are synonymous and can be used interchangeably. That said, autoclave is often used in laboratory settings, while sterilizer is more commonly heard in hospitals or pharmaceutical settings.

Two modern steam sterilizer autoclaves with a stainless steel finish on a white background. The one on the left has its door open, displaying the interior shelves, while the one on the right is closed, both featuring control panels with digital displays, indicative of medical or laboratory equipment for sterilizing tools and instruments.

Autoclaves use steam heat to kill any microbial life that may be present on a contaminated load. A load — also known as goods — is considered sterile once it has undergone a full sterilization cycle. Once a load is sterile, it can be used without fear of introducing foreign microorganisms into a sensitive environment, such as a laboratory, hospital operating room, food production facility, and so on. Different types of goods must be sterilized for different lengths of time and at different temperatures. Some autoclaves include additional features, such as vacuum functions, special cycles, and integral electric boilers.

History of the Autoclave

Dr. Charles Chamberland invented the autoclave in 1879, but the concept of using steam in an enclosed space in order to prevent sickness has existed in some form or other since 1679.

The principles and methods for sterilization have remained largely unchanged for the past 150 years. In fact, most major advancements in autoclave technology since 1879 have revolved around sterilization process monitoring, autoclave safety, and sterilization cycle creation rather than alterations to the sterilization process.

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Why Steam?

In order to kill a cell with heat, its temperature must be raised to a degree at which the proteins in the cell wall break down and coagulate. Steam is a very efficient medium for heat transference, which makes it an excellent way to destroy microbes. Air, on the other hand, is a very inefficient way to transfer heat/energy because of a concept known as the heat of vaporization.

It requires 80 kilocalories (kcal) of heat energy to bring one liter of water to its boiling point (100˚C). It would require 540 kcal to convert that liter of water into steam, which means that steam at 100˚C contains seven times as much energy as water at 100˚C.

That energy is what makes steam so much more efficient at destroying microorganisms. When steam encounters a cooler object, it condenses into water. Then, it transfers all of the energy that was used to boil the water directly into the water. This process heats up the cells far more efficiently than air at similar temperatures. This is why we use steam to achieve sterility.

What is Sterility?

Most people have a working understanding that sterile goods are free of microorganisms and are, therefore, safe to use in medical, food production, research or other settings in which the presence of germs would be a significant safety hazard or detriment.

Exactly how many microorganisms will be left alive over time at a fixed temperature is expressed as a probabilistic logarithmic curve — a function that approaches but never reaches zero (see Figure 1).

A probabilistic logarithmic curve.

Figure 1

As the function approaches zero, one would typically choose a level of confidence — called the Sterility Assurance Level (SAL) — for the odds that the last microorganism present will survive. Contrary to popular belief, sterilization is not binary where something is either sterile or non-sterile. Sterilization is a statistical event characterized by this confidence factor (SAL). The general standard for SAL is 10^6, or a one-in-a-million chance of a single viable microorganism surviving. How long sterilization takes depends on the set temperature and SAL level desired; higher temperatures will achieve sterility faster.

How Does an Autoclave Work?

Whether it’s a small tabletop unit or a room-sized bulk unit, all autoclaves operate using principles similar to those of a common kitchen pressure cooker — that is, the door is locked to form a sealed chamber, and all air within that chamber is replaced by steam. The steam is then pressurized in order to bring it to the desired sterilization for the desired duration. Once the cycle is complete, the steam is exhausted, and goods can be removed.

For a more detailed explanation of the various phases of a sterilization cycle, please refer to the list and image (Figure 2) shown below:

  1. Purge Phase: Steam flows through the sterilizer and starts to displace the air; temperature and pressure ramp slightly to a continuous flow purge.
  2. Exposure (Sterilization) Phase: During this phase, the autoclave’s control system is programmed to close the exhaust valve, thereby causing the interior temperature and pressure to increase to the desired setpoint. The program then maintains the desired temperature (dwells) until the desired time is reached.
  3. Exhaust Phase: Pressure is released from the chamber through an exhaust valve and the interior is restored to an ambient pressure (though contents remain relatively hot).
    Chart demonstrating the three phases of a sterilization cycle, with Time / Duration on the x-axis and Vacuum Pressure / Temperature on the y-axis.

    Figure 2

Critical Components of an Autoclave

The typical laboratory autoclave is comprised of the following components (Figure 3):

A graphic of a steam sterilizer autoclave with numbered parts. On the left, the autoclave appears closed, with a digital control panel labeled '2'. On the right, the same autoclave is shown open, revealing its internal components including the chamber '3', control valves '4', and a drainage system '5', among others. The image has a clean, technical look with a pale blue and white color scheme, indicating a focus on the equipment's features and functions.

Figure 3

1. Vessel

The vessel is the main body of the autoclave and consists of an inner chamber and an outer jacket. Laboratory and hospital autoclaves are constructed with “jacketed” chambers (see Figure 4), where the jacket is filled with steam, reducing the time that it takes to complete a sterilization cycle and reducing condensation within the chamber. A vessel designed and manufactured with a full jacket is superior to that of a partial jacket or blanketed jacket for the following reasons:

  • Improved temperature uniformity within the chamber
  • Reduced likelihood of wet packs
  • Minimizes wet steam, which is not good for sterilization

In the United States, every autoclave vessel is inspected and tagged with an American Society of Mechanical Engineers (ASME) nameplate that includes a National Board number. Manufacturers are required to hydrostatically test each vessel and apply an ASME nameplate before putting an autoclave into use. This inspection and the ASME nameplate are key indicators of a properly functioning autoclave.

Laboratory and hospital autoclave vessels can vary in size, from 100L to 3,000L, and are typically constructed from 316L stainless steel. Inner chambers are either 316L or nickel-clad, and outer jackets are made of 316L, 304L stainless steel, or carbon steel.

Diagram of a jacketed autoclave chamber that shows the movement of steam supply, treated water supply, and cold water supply throughout the autoclave.

Figure 4

2. Control System

All modern autoclaves are equipped with a controller interface, not unlike what you would find on a microwave or oven. That said, autoclave control systems tend to be a bit more sophisticated than those of household appliances. A sterilization cycle follows a preprogrammed software formula that opens and closes valves and other components in a specific sequence. Therefore, all autoclaves require some form of control system, whether it’s as simple as a “push button” system with a microprocessor, or as complex as a programmable logic controller with a color touch screen.

3. Thermostatic Trap

All autoclaves feature some form of thermostatic trap or steam trap, a device designed to allow air and water (condensate) to escape from the chamber. Although a steam delivery system/steam autoclave might use a variety of traps, they all perform the same basic function: removing condensate while preventing the passage of dry steam. Most often, steam traps are temperature-sensitive valves that close when heated past a certain setpoint. Thermostatic traps are a critical component of any well-designed autoclave.

4. Safety Valve

All autoclaves operate under elevated pressure (14–45 pound-force per square inch gauge) and must therefore be manufactured with an incredibly robust construction and fitted with a number of safety features and devices to ensure they present no danger to users. One of these devices is the safety valve, which is the final fail-safe device for the pressure vessel should all electronic controls fail. It is imperative that the safety valve be inspected, tested, and verified to be in proper working condition based on the recommendations of the sterilizer and/or valve manufacturer, as well as local inspection and insurance agencies.

5. Waste-Water Cooling Mechanism

Many autoclaves are equipped with a system to cool effluent (air, steam, and condensate) before it enters the drain piping. Many municipalities and buildings do not allow effluent above 140˚F to enter the floor drain. In order to avoid damage to the facility’s drain piping, the steam must be cooled before it can be sent down the drain. The simplest method for cooling this steam is to mix it with additional cold tap water, but the amount of water required can cause an autoclave to be a major contributor to a building’s water usage. Some autoclaves come equipped with systems designed to reduce, or even eliminate, water consumption.

6. Vacuum System (if applicable)

In order to ensure proper sterilization, it’s vital that all air inside the autoclave chamber be replaced with steam. Certain commonly sterilized goods — especially porous materials, such as animal bedding or cloth or containers with small openings, such as flasks or goods in bags — tend to retain air pockets. If an air pocket is present during the cycle, any microorganisms within that pocket will survive, and the goods will not be sterile.

For this reason, many sterilizers will include a vacuum system. This not only enables the user to forcibly remove air by using a vacuum on the chamber before a cycle (known as pre-vacuum), it also enables them to use a vacuum after the cycle (known as post-vacuum) to remove any steam that remains in the chamber, and to dry off the goods inside the autoclave.

7. Steam Generator (if applicable)

A central “house” boiler is the most common steam source for an autoclave. However, if house steam is unavailable or insufficient for the autoclave, one must resort to using an electric steam generator, also known as a boiler. These boilers typically sit underneath the autoclave chamber and utilize electric heating elements to heat water and generate steam.

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Sterilization Cycles

There are, in general, four standard sterilization cycles: gravity, pre-vacuum, liquids, and flash (also known as immediate use). The chart shown below explains these cycles in greater detail.

 Basic Cycles Description Typical Application or Load Type
Gravity The most basic sterilizer cycle. Steam displaces air in the chamber by gravity (i.e. without mechanical assistance) through a drain port. Glassware, unwrapped goods, waste, utensils, redbags.
Pre-vacuum and/or Post-Vacuum Air is mechanically removed from the chamber and load through a series of vacuum and pressure pulses. This allows the steam to penetrate porous areas of the load that couldn’t otherwise be reached with simple gravity displacement. Wrapped goods, packs, animal cage bedding, cages, porous materials, redbags.
Liquids A gravity cycle with a slower exhaust rate to minimize boil-over. Media, LB broth, water, etc.
Immediate Use/Flash (Healthcare sterilizers only) High temperature cycle (over 270°F) for a shorter period of time. Unwrapped goods

Some autoclaves also have the ability to perform specialty cycles. These are designed to avoid damage to delicate goods that need to be sterilized, but would be damaged or destroyed by the rapid changes in temperature and pressure in a normal cycle. These specialty cycles include much longer cycles at lower temperatures, steam-air mix cycles (with special pressure controls to avoid breaking sealed test tubes), and cycles that use special instrumentation to ensure full sterilization temperature is achieved.

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Common Autoclave Temperature Ranges

Autoclaves are typically designed to reach a maximum temperature that ranges between 250°F and 275°F (121°C and 135°C). This is because most contaminants must be exposed to these high temperatures for a certain amount of time to be destroyed, thus achieving sterilization.

That said, some autoclaves can be configured to run what are called “low-temperature cycles.” These cycles top out in the 158°F–212°F (70°C–100°C) range and are used to accommodate load types that aren’t capable of withstanding the harsh conditions that a standard sterilization cycle creates within the chamber.

What Are Common Autoclave Sizes?

Autoclaves come in a wide range of sizes and configurations, with different combinations serving different purposes. The most compact models can easily sit on a countertop and are mostly found in dental and some laboratory settings, whereas smaller instruments, devices, or pieces of equipment are used as part of the day-to-day.

Large autoclaves, which can stand over six feet high, are used in environments where larger devices are called for or where load volume is high — typically in a healthcare facility’s sterile processing department (SPD) or a laboratory that focuses on medical research, life sciences, or food safety.

The largest autoclaves, which can reach the size of a tractor trailer, are typically found in industrial manufacturing settings.

What Can Be Autoclaved?

Because of the extreme heat, high pressure, and moisture present during a sterilization cycle, only certain items are autoclave-safe. Materials that are autoclavable include:

  • Glass — While Pyrex is generally considered the safest to autoclave as it doesn’t expand to the same extent as other types of glass when heated, most glass can withstand sterilization cycles. Still, it’s vital to follow manufacturer guidelines before autoclaving any glassware.
  • Polycarbonate plastics — These plastics remain stable at high temperatures, making them a good candidate for steam sterilization. However, it’s important to note that items made from this material can only tolerate between 30 and 50 cycles before they lose integrity.
  • Polypropylene — This plastic resin is commonly used to manufacture pans, trays, and bags and is valued for its durability and relatively low cost. Items made from polypropylene, including some wipes, are generally considered safe to autoclave.
  • Certain metals — Stainless steel grades 304, 316, 304L, and 316L as well as aluminum that’s been coated in medical-grade anodizing can safely be autoclaved.
  • Latex and vinyl gloves — These are only safe to sterilize when placed in an autoclavable waste bag. Otherwise, they may burn.
  • Paper — But only if placed in a suitable waste bag and sterilized on a steam cycle.

How Much Does an Autoclave Cost?

As autoclaves come in a variety of sizes; have wide-ranging functionalities and features; and are designed for distinct use cases, settings, and industries, pricing is largely unique to individual units. When budgeting for any autoclave, it’s important to consider more than only the initial investment. Like all machinery, an autoclave must undergo routine maintenance and may eventually need more extensive repairs during its lifetime. Additionally, your sterilizer requires utilities such as water and electricity to run and is subject to sterility assurance testing, so it’s important to factor these expenses into the total cost of ownership.

Lab Autoclaves vs. Industrial Autoclaves vs. Medical Autoclaves

As we mentioned previously, autoclaves are used across different environments and industries — most commonly in labs, industrial applications, and medical facilities. While all autoclaves rely on moisture, pressure, and heat to sterilize loads, there are notable differences between the machines used in each of these three settings. These are outlined in the chart below.

Lab Autoclaves Industrial Autoclaves Medical Autoclaves
Sizes Ranges from bench-top to large floor loaded, bulk models  Extra large, can be comparable to the size of a tractor trailer Ranges from bench-top to large floor-standing models 
Common Load Types Glassware, growth media, contaminated waste material, animal cages, pipette tips, liquids, culture tubes and plates, wipes, gloves Pressure-treated wood, vulcanized rubber, food cans and jars, composite materials, laminate surfaces  Surgical instruments, implants, reusable surgical drapes and linens, syringes, personal protective equipment (PPE)
Typically Used In… Research labs (academic, government, pharmaceutical, medical), animal science labs and food safety testing labs. Manufacturing, aerospace, ballistics, and chemical industries Dental offices, hospitals, and ambulatory surgery centers

Emerging Autoclave Trends

Autoclaves may be considered ancient devices by the standards of modern science, but this does not mean that they lack innovation, especially when it comes to controls, cloud connectivity, and ecological impact.

As mentioned earlier, autoclave controls have advanced greatly in the age of computers, progressing from manual controls and simple timers to computer automation that minimizes or eliminates the need for user input. Computerized controls (also known as PLCs) have also led to advances in data control, record keeping, and remote monitoring via mobile devices. Autoclaves with automatic printers that record data for the purpose of verifying successful sterilization have since been replaced by new models that connect to the cloud to store cycle records on the Internet. The use of PLCs has also enabled the development of autoclaves that warn users when maintenance is needed and to create preventative maintenance schedules and replacement parts lists.

Another trend in autoclave design is sustainability. Autoclaves consume vast amounts of water and energy, both in laboratories and hospitals. In recognition of this, many manufacturers have found innovative ways to reduce autoclaves’ environmental impact. Green autoclaves that reduce water use — in some cases, from 1,500 gallons a day to less than one gallon a day — or even fully recycle the consumed water are critical to creating an environmentally friendly laboratory. Control systems that automatically turn off the autoclave when not in use can also significantly reduce energy consumption — in some cases, from 80 kilowatt-hours per day to 20 kilowatt-hours per day.

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Consolidated Sterilizer Systems has a rich heritage in the steam sterilization industry, with over 75 years of experience. We strive to deliver manufacturing excellence and are committed to supplying high-quality, high-performance sterilization and distillation solutions. If you’re interested in learning more about the steam sterilization process, or have other autoclave-related questions, contact us today.

Frequently Asked Questions

Q: How does an autoclave work?
A: Autoclaves use extreme heat in the form of pressurized steam in order to sterilize goods. Similar to a pressure cooker, an autoclave uses a locked door to create a sealed chamber. The air within that chamber is then replaced by steam, which is pressurized until the goods within the chamber have been sufficiently sterilized.

Q: How does autoclaving kill bacteria?
A: Autoclaves use steam heat to raise temperatures to such a degree that proteins within the cell walls of a microbe break down and begin to coagulate, thereby killing the bacterium and achieving sterilization.

Q: Why is autoclaving items better for sterilization purposes than boiling them?
A: Steam is a very efficient medium for heat transference. As a result, you can achieve higher temperatures using steam than boiling water, which makes it a more effective method of killing bacteria and other microorganisms.

Q: How long does it take an autoclave to sterilize goods?
A: How long it takes to sterilize a load depends entirely on the content of the load, the set temperature of the autoclave, and the Sterility Assurance Level desired. Generally speaking, the higher the temperature, the faster a load will achieve sterility.

Q: What temperature(s) can an autoclave reach?
A: Autoclaves are typically designed to reach temperatures between 250°F and 275°F (121°C and 135°C).

Q: What are the phases of sterilization?
A: The general process for using an autoclave to sterilize goods breaks down into three basic phases:

  1. The purge phase, during which steam displaces air within the autoclave chamber and temperature and pressure steadily increase.
  2. The exposure (sterilization) phase, during which the autoclave’s control program closes the exhaust valve, causing its interior temperature and pressure to increase to the desired setpoint, and stay there for the desired amount of time.
  3. The exhaust phase, during which pressure is released from the chamber via the exhaust valve, and the interior is restored to ambient pressure.

Q: How long do items stay sterile after autoclaving?
A: It depends entirely on how items are packaged after sterilization. Generally speaking, items should be re-sterilized after each use, but items packaged in double-wrap linen packs or an inner layer of paper and outer layer of plastic have been proven to remain sterile for up to 96 weeks.

Q: What are the different types of autoclaves?
A: The two most common types of steam sterilizers are gravity displacement autoclaves and high-speed pre-vacuum autoclaves. Both types of autoclave come in a variety of shapes and sizes, ranging from tabletop units to room-sized bulk units, with a vast array of customization options, such as vertical sliding doors, double-door pass-thru chambers, and stackable dual chambers.

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