In some rebreathers , one - way valves direct the gas through a '' loop '' , while in others the inhaled and exhaled gas goes back and forth along a single tube : this is called the pendulum system .
In some rebreathers , one - way valves direct the gas through a '' loop '' . 
 In other rebreathers , the inhaled and exhaled gas goes back and forth along a single tube : this is called the pendulum system .