The process is repeated and the pressure in the recipient decreases progressively, but it cannot go below a certain point, which is reached when the air still in the recipient can no longer open the valve communicating with the pump body and the air in the clearance space can no longer open the valve providing communication between the pump body and the outside.
Even with the substitution, actually made, of the valve communicating between the pump body and the recipient in which the vacuum is to be created with one activated directly by the piston, the limit imposed by the clearance volume remains.
If we then take into consideration the fact that the seal ensured by the greased leather gaskets is very poor and that they emit vapours that enter the pump body, it is clear that with such piston pumps it is impossible to reach pressures lower than a few tenths of a millimetre of mercury.
With the piston pumps having two cylinders working alternatively, one of which creating a vacuum in the other cylinder and the latter in the recipient in which the vacuum is to be created, and with the use of automatic valves, it is possible to reach minimum pressures on the order of a millimetre of mercury.
Significant progress in piston pumps was reached with the Guericke pump by eliminating the clearance volume with the inversion of the two phases and ensuring an excellent seal of the piston and the valves by means of a coat of oil having an extremely low vapour tension. ...continues