Abstract: A system for delivering fluid, e.g. liquefied petroleum gas or "LPG", through a delivery path (10) under pressure. A delivery control valve (16) is selectably operable to close and open the delivery path (10) and monitors a parameter of the fluid, e.g. dielectric constant, and to sense the presence of vapor or gas phase in the fluid. A control means (30) responsive to the indicating signal from the sensing means (20) causes the delivery control valve (16) to close the delivery path upon sensing the presence of a significant proportion of vapor or gas phase in the fluid. A gas introduction point (35) upstream of the sensing means (20) enables gas to be introduced so as to knowingly expose the sensing means (20) to fluid containing gas phase and thereby enable controlled testing of the correct functioning of the sensing means (20).