Abstract: A process and apparatus for regulating the quantity of fresh air in high-speed trains by means of pressure-wave valves. A signal for adjusting a drive unit is determined from the passenger count and outside temperature. This drive unit causes a stop fork to move into a flap stop position of the valve in the "open" operating mode so that the required amount of fresh air can be supplied. The process according to the invention results in a considerable saving on energy, since only the absolutely necessary amount of fresh air need be heated or cooled and the period of time in which ventilation can be used rather than cooling is lengthened.

Abstract: Level control apparatus for cyrogenic liquids having a cryogenic liquid. Means is provided for introducing cryogenic liquid into the chamber and outlet means is provided to permit removal of the cryogenic liquid from the chamber. Level sensing means is mounted on the flask and has a probe extending into the chamber so that it will come in contact with the cryogenic liquid when the cryogenic liquid reaches a predetermined level within the chamber. The probe includes a thermocouple with a resistor secured to the thermocouple for heating the thermocouple. When the resistor is in contact with the cryogenic liquid, the thermocouple remains cold and provides a signal of one type. When the resistor is uncovered by the cryogenic liquid and the thermocouple becomes hot, a signal of a different type is provided. The signals from the thermocouples are utilized for controlling the entrance of the cryogenic liquid into the chamber and to maintain the liquid level at a predetermined level within the chamber.