An airtight chamber that separates areas of different air pressure or environment on a spacecraft (or submarine?), and through which crew members enter or leave, once the pressure or environment in the chamber has been equalized with the area toward which they are headed.

This is what a standard brewer's airlock looks like. It is used in brewing to produce a closed environment for fermentation. It can be made of plastic or glass. The plastic ones cost about a dollar and the glass ones around ten. The glass ones usually on have one ripple on each side, while the plastic one i drew below has three. You fill it with a small amount of water and as the yeast ferments, the CO2 it produces can bubble up through the airlock and escape. This way CO2 can leave the bottle but nothing can get back in. It keeps the yeast's environment sterile.
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1. An isolated chamber on vessels with their own air supply (including submarines and space craft), which has two airtight doors. One connects with the body of the vessel, and the other with outside.

The inside of the vessel is at atmospheric pressure. The only way for a person to leave the vessel is through the air lock. The chamber is pressurised (or filled with air) to atmospheric pressure. The person enters through the inner door, dons a space suit (or diving suit). The inner door is closed and the air is let out of the chamber; the person then exits via the outer door.


2. A piece of equipment used in home wine making that allows ventilation without contamination.

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   / ____ \    \   /
  / /    \ \    | |
 _| |_   | |   _| |_   
/     \  | |  /     \  
|     |  | |  |-----|  
\_   _/  | |  \_   _/  
  |-|    | |    | |    
  | |    | |    | |    
  \ \____|_|____/ /
   \_____________/
         | |
        _| |_
       / | | \
       \_____/

The air lock is placed in a bung in the top of the demijohn containing fermenting wine. A small quantity of water is poured into the section which connects the two chambers.

Excess CO2 from fermentation forces the water into the right hand chamber, until the water level is high enough for a bubble to pass through, returning the water to its original state. This is accompanied by a characteristic "bloop" sound.


3. An air bubble in the fuel intake to a diesel engine. Air locks are a problem to the engine, as they result in a loss of power, compression, and potentially damage to the engine. Modern diesel engines are self bleeding, hence air bubbles do not succeed in reaching the intake valves.


4. Drawing an analogy from 1. an air lock is also the name given to an anteroom with a pair of security doors which cannot be open at the same time. These are used in military establishments and some banks. A similar arrangement is used with gates in safari parks to allow vehicles to pass through without the animals escaping.

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