Autoclave Principle and Working in Laboratory

Autoclave principle and working in laboratory based on principle of gas laws, this can be achieved by raising the pressure inside the device.

Definition of Autoclave

Autoclave instrument, often known as steam sterilizers, are commonly employed in medical and industrial area (Figure 1). Autoclave instrument is a machine that kills hazardous bacteria, viruses, fungus, and spores on goods that are placed inside a pressure vessel using steam under pressure. The equipment’s and glassware’s are heated to a sterilizing temperature and held there for a set period of time. The moisture in the steam efficiently transfers heat to the objects, destroying the bacteria and spores’ protein structure.


Figure 1: Illustration of Autoclave

Principle of Autoclave

The use of moist heat facilitates the destruction of all microorganisms, including heat-resistant endospores. This is accomplished by heating the device’s contents above the boiling point of water. Autoclave principle and working based in accordance with the principle of gas laws, this can be accomplished by increasing the device’s internal pressure (Figure 2).

Normal procedure is to heat at a steam pressure of 1.1 kilograms per square centimeter (kg/cm2) [15 pounds per square inch (lb/in2)], which results in a temperature of 121 degrees Celsius. Depending on the volume of the load, the autoclaving time to achieve sterilisation at 121°C is generally considered to be between 15 and 20 minutes.

To ensure the success of sterilisation, one must ensure: The air should be evacuated so that steam can fill the chamber. The autoclave should be loaded with items so that steam can easily permeate them.


Figure 2: Autoclave principle and working mechanism. 

Components of Autoclave

Autoclave instrument is used to sterilize equipment and maintain it free of microorganisms.  They do this by sterilizing the equipment placed inside them with steam and high pressure. Autoclave instrument is used in laboratories and other industrial conditions where equipment cleanliness is critical, such as food processing.

1. The chamber

This is the main compartment of the autoclave instrument, where the things to be sterilised are placed. They come in a variety of sizes, ranging from 100 to 3000 litres, and have both an inner and an outside chamber. This outer chamber is called as a ‘jacket,’ and it is filled with steam to speed up the sterilising process.

2. The Door

The autoclave instrument front door is designed to keep the outside atmosphere out. This ensures that the items inside the chamber are adequately sterilised. The contents are secured and leaking is prevented with a lock and a door seal.

3. Safety Valve

Steam is fed into the chamber at a higher ambient temperature through a conduit at the bottom of the cylinder. The temperature rises by 20 degrees Celsius above the typical boiling point of water as a result of the increased pressure. As a result, a safety valve is installed to prevent the pressure inside the chamber from rising above a predetermined maximum temperature.

4. Regulator

A thermostat starts a timer once the target temperature is attained. Depending on the contamination level of the materials inside the autoclave, the timer is normally set at 15-20 minutes. The way in which they were loaded into the chamber will have an impact on the timing as well.

5. Pressure Unit

A whistle is present on the autoclave’s lid, which is identical to that of a pressure cooker. By elevating itself, the whistle adjusts the pressure inside the chamber by releasing a set amount of vapour.

6. Cooler

Many autoclave instrument will have a system in place to chill any wastewater before it is discharged into the sewer. This is frequently done to prevent damage to the laboratory’s drain pipes.

Types of Autoclaves


Figure 3: Types of Autoclave

Despite the fact that autoclave instrument come in a variety of shapes and sizes, they all work in the same way. The type of autoclave instrument (Figure 3) to employ is determined on the application.

  1. Laboratory Bench: This type of autoclave is widely utilised all over the world. It has an air and steam discharge tap, as well as a pressure gauge and a safety valve. At the bottom of the chamber, there is also an electric immersion heater.
  2. Gravity Displacement: In laboratories, these are commonly used. This autoclave uses a heating unit to generate steam inside the chamber, which can travel around the chamber for efficient sterilisation. In comparison to other autoclaves, it is also quite inexpensive.
  3. Positive Displacement: This type of autoclave uses a separate steam generator unit to generate steam, which is then transferred into the autoclave. Steam may be generated in seconds; hence it is known to be speedier.
  4. Negative Displacement: A steam generator and a vacuum generator are both present in negative pressure displacement autoclaves. The vacuum generator draws all of the air out of the autoclave, whereas the steam generator, like the positive pressure displacement autoclave, generates heat and passes it into it. This is the most popular autoclave, but it is also the most expensive.

Procedure for Autoclave

  1. Before using the autoclave, it must be inspected for any remnants of the previous cycle. Then, a sufficient amount of water is added to the chamber.
  2. The sterilizing materials are now placed inside the chamber. The lid is then closed, the screws tightened to ensure airtightness, and the electric heater is turned on.
  3. Once the water inside the chamber boils, the air-water mixture is allowed to escape through the discharge tube to let all the air inside to be displaced. The complete displacement can be ensured once the water bubbles cease to come out from the pipe.
  4. The drainage pipe is then closed, and the steam inside is allowed to reach the desired levels (15 lbs in most cases). Once the pressure is reached, the whistle blows to remove excess pressure from the chamber.
  5. After the whistle, the autoclave is run for a holding period, which is 15 minutes in most cases. Now, the electric heater is switched off, and the autoclave is allowed to cool until the pressure gauge indicates the pressure inside has lowered down to that of the atmospheric pressure.
  6. The discharge pipe is then opened to allow the entry of air from the outside into the autoclave. Finally, the lid is opened, and the sterilized materials are taken out of the chamber.

Uses of Autoclaves

Because dry heat sterilisation cannot be used to sterilise materials containing water, autoclaves are essential devices. Autoclaves are also used for a variety of other purposes.

  1. They are put to use to decontaminate certain types of biological waste as well as sterilise media, instruments, and other types of laboratory equipment.
  2. Before being disposed of, it is recommended that regulated medical waste that has the potential to contain bacteria, viruses, and other biological materials be inactivated through the use of an autoclave.
  3. Autoclaves are frequently utilised in medical research facilities for the purposes of disinfecting and sanitising glassware, medical equipment, and surgical instruments.
  4. In a similar manner, autoclaves are utilised in the process of sterile preparation of pipette tips, autoclavable containers, plastic tubes, and culture media.

Precautions of Autoclave

  1. When unloading the autoclave instrument, wear appropriate Personal Protective Equipment (PPE) such as a lab coat, heat resistant gloves, and eye protection.
  2. If there is water coming out the bottom of the autoclave, never open the door. A build-up of scalding water can be caused by clogged steam lines, equipment failure, or clogged drains.
  3. To avoid steam burns and shattered glassware, wait until the pressure drops to zero before opening the door at the end of a cycle. Standing right in front of the door is not a good idea.
  4. Never overheat liquids. When liquids reach temperatures above their normal boiling point but do not appear to be boiling, this is referred to as superheating. Any movement of the liquid could cause it to violently flash to steam and spray.
  5. Superheated liquids may boil out of their containers or explode in situations where personnel are rushing to remove flasks or bottles from the autoclave.