In order to create a fluid-tight seal relative to a seal seat edge, especially a cylinder head, in a sealing device for a fuel injector, particularly for a nozzle retaining nut of the fuel injector, the sealing device has a sealing area that is provided with a concave zone having a radially circumferential concave outer contour which can be made to sealingly rest against the seal seat edge. The first test regimen produces a first set of defect candidates. The projection is openable for receipt of the inlet tube and is arranged to be clamped into engagement with the inlet tube to form a fluid-tight interface by use of a clamping mechanism. Unmasked areas of the composite layer are irradiated with ultraviolet light. Standardized uptake from a functional image, alone or in combination with other features, may indicate the likelihood of side-effect. Also disclosed is an ink level detection system that includes a pressure transducer for sensing a pressure of the ink supply that is indicative of remaining ink.