The present invention provides, with reference to FIG. 1, an internal combustion engine (10) having a fuel injector which comprises a sprung piston (55) or a resilient diaphragm piston (8155) and an electrical coil (57, 8158) for displacing the piston (55, 8155). The piston draws fuel into and expels fuel from a pumping chamber (52, 8152). The number of operations of the injector per engine cycle is controlled by an electronic controller (23, 8159) to control the quality of fuel delivered per cycle to a combustion chamber. A voltage measured in the coil (52, 8158) by movement of the piston under action of the spring or due to its own resilience is used to give an indication of vapour pressure of the fuel. A device akin to the injector can be used to draw fuel from a pipeline to measure the vapour pressure of the fluid.