A modular fuel nozzle configuration is defined which permits lower-cost manufacturing operations such as injection moulding to be employed. The invention is also directed to a process for the preparation of an aqueous solution of a salt of a thiocarboxylic acid which comprises reacting an aqueous solution of a sulfide andor hydrosulfide with an acid halide in the presence of a said solid catalyst. A plurality of individual anodic chambers are thus provided, at least one of which is evacuated below atmospheric pressure. In one implementation, data corresponding to the amplitude of the output signal is analyzed to generate metric values that are used to iteratively adjust the tuning of the error loop. In addition, an image apparatus having the image data test unit and a method of testing image data using the image data test unit are also provided.