1. A device for supplying gas for breathing to a closed ventilating circuit having a volume driver for circulating gas in the closed circuit, comprising: a metering unit controllable to supply a gas for breathing to the closed circuit; a controller connected to said metering unit for applying a supply amount signal to said metering unit to control the supply of gas to the closed circuit; a pressure sensor adapted for connection to the closed circuit for generating an actual pressure signal corresponding to an actual pressure in the closed circuit; pressure switch means switchable to supply at least two different present pressure values for gas in the closed circuit; a differential amplifier having a first input connected to said pressure sensor for receiving the actual pressure signal and a second input connected to said pressure switch means for receiving one of said different preset pressure values, said difference amplifier having an output connected to said controller for applying an error signal value to said controller corresponding to a difference between said actual pressure value and said one of said preset pressure values; and a computer connected to said controller for receiving said supply amount signal, to said differential amplifier output for receiving said error signal value and to said pressure sensor for receiving said actual pressure signal, said computer connected to said pressure switch means for switching between said at least two different preset pressure values and adapted for connection to the volume driver for stopping and starting operation of the volume driver, said computer including means for calculating gas consumption for a patient connected to the closed circuit and for calculating a leakage value representing leakage from the closed circuit as a function of said error signal value and said supply amount signal wherein said computer is programmed to execute the equations EQU E.sub.1 =V+L(P.sub.1 -P) EQU E.sub.2 =V+L(P.sub.2 -P) wherein, P.sub.1 and P.sub.2 are the at least two different present pressure valves, P is the valve of the actual pressure signal, E.sub.1 and E.sub.2 are valves of supply amount signals for the at least two preset pressure valves, V is the consumption amount and L is the leakage amount.