A dosing apparatus comprises a substantially rigid housing induding a void, a disposable element including a first bladdet located in the housing and having an inlet and an outlet and a one way valve associated with each of said inlet and outlet, and means for increasing and decreasing the volume of the first bladder increasing and decreasing the volume of the first bladder causes fluid to be dtawn into the first bladder through the inlet and to be expelled from the first bladder through the outlet. A substantially constant vacuum is exerted between inner surface of the void and the outer surface of the first bladder.