There are different complex ways to control an under-pressure. A variant often selected due to low procurement costs is a vacuum regulator with external leakage. This functions similar to a relief valve: as soon as the set under-pressure is reached, a spring-loaded valve opens and vents the vacuum side. Here energy loss is the result. An energy-saving, but more expensive solution is the use of special vacuum pressure regulators, which control the vacuum through a regulated valve without supply of surrounding air (similar to a pressure regulator for overpressure).