 The specimen transfer chamber (tr-ch) shown in the diagram of ESEM gas pressure stages contains a small water reservoir so that the initial ambient air can be quickly pumped out and practically instantaneously replaced with water vapor without going through a limited conductance tube and valve.The main specimen chamber can be maintained at 100% relative humidity, if the only leak of vapor is through the small PLA1, but not during violent pumping with every specimen change.Once the wet specimen is in equilibrium with 100% relative humidity in the transfer chamber, within seconds, a gate valve opens and the specimen is transferred in the main specimen chamber maintained at the same pressure.