A fluid injection device integrating a piezoelectric sensor, a fluid injection apparatus and a method for analyzing fluid content in a fluid injection device. The reaction takes place within a reaction zone of the dynamic reactor during a mean residence time less than what is generally known in the art in order to produce a product mixture containing primarily lower alkyl esters. A photosensitive cell operates a switch to permit the light means to illuminate when a pre-determined threshold has been exceeded relevant to the amount of ambient light present around the mailbox. A sectional area reducing part is provided in the mounting piece between the shell body and a distal end part of the mounting piece.