Combined water meters
Combined water meters and automatically acting switch valves which serve for switching between a main meter and a secondary meter were used in the nineteenth century. They include a valve disk including a surface which is larger than the surface of the sealing seat of the main water stream. As a result, the differential water pressure acts on a smaller surface in a blocked passage and on a larger surface in an open passage in order to abruptly open the valve disk. In order to counterbalance for the differential water pressure when the valve disk is closed, suitable means are employed, for instance, gravity, spring force, etc. It is desired that the valve disk open and close in each case as suddenly as possible so that no errors in measurement will occur.
All switch valves having a closure spring have one problem in common. The restoring force of the spring increases linearly with an increasing path of opening of the valve disk. The ideal, however, would be a constant or even a decreasing restoring force, as is present in the above mentioned weight loaded flap valves. Spring loaded switch valves must therefore be provided with a spring which is as long as possible in order to keep the increase in force slight. This, however, runs counter to the so-called short construction length which is preferred today.