Did you know the US Air Force evaluated commercially available smoke hoods for passenger protection and selected the Essex unit as the best. Sep 18 -20, 2001. The main difference between the SCU and VRU is the VRU has semi-automatic activation of the oxygen. This means the hood is design in such a way that you automatically activate the oxygen as part of the steps of putting the VRU on. The VRU is a closed circuit rebreather system.
Oxygen flows into the hood and is breathed and rebreathed by the user.
The oxygen flow is begun by the manual or semi-automatic activation. Oxygen was selected as the breathable atmosphere because of weight and volume considerations. During respiration, oxygen is removed from the air and converted to carbon dioxide, which is then exhaled back into the air. If compressed air was used instead of oxygen, the air inside the hood would need to be constantly replenished to maintain the oxygen content at a life-supporting level. Because the VRU uses pure, aviator's grade oxygen, the exhaled air can be rebreathed once the carbon dioxide is removed.