and oxygen exhaust through the blue pressure relief valve at the centre of the chest; this valve also regulates the pressure of the suit. Effectively, the suit uses an open-circuit life support system that somewhat resembles scuba equipment. This has the advantage of simplicity; the disadvantage of a high rate of oxygen consumption is considered acceptable given that it is only intended for emergency use. The suits weigh around and are described by those who have used them as a considerable encumbrance when worn on the ground. Despite this, they are intended to be worn for up to 30 hours in a pressurised environment or two hours in a vacuum. They can also float and have a neck dam that allows the visor to be raised in water without the risk of flooding the suit. However, Soyuz crews are provided with buoyancy aids and cold-water survival suits which would preferably be used if the Soyuz accidentally landed in water. , a total of 309 flight suits had been made along with 135 training and testing suits. Operational use Each Soyuz crew member is provided with a made-to-measure suit for flight although, from the numbers made, it appears that the suits provided for ground training are re-used. It is considered vital that the flight suit fits correctly and the wearer will spend two hours sitting in a launch couch with the suit inflated to make sure of this. Straps on the arms, legs and chest allow the fit to be adjusted slightly. To don the suit, the two zips that make a 'V' on the chest are opened. Underneath, there is a large tubular opening in the inner pressure layer known as the appendix. Legs go in first, followed by the arms into the sleeves and head into the helmet. When the suit is on, an airtight seal is made by tightly rolling up the appendix and securing it with strong elastic bands. The large bulge of the rolled-up appendix is secured under the V shaped flap in the suit's outer layer. When worn on the ground, the suit is attached to a portable ventilation unit —a hand-held device that supplies air to the suit, cooling it first with an ice filled heat exchanger. Grey leather outer boots are also worn on the ground; they protect the feet of the suit from damage and are removed before entering the spacecraft to