A spring-loaded spherical ground joint acts as pressure valve, with tightness guaranteed by a buffer liquid of higher viscosity. The spring pressure is adjustable, but the valve can also be pre-set and lead-sealed to order. Maximum pressure 0,1 bar.
Use as a non-return valve under vacuum is possible. An inserted riser tube prevents contamination by buffer liquid.
Where aggressive media are involved, the spring can be covered with inert fluid.