The present invention is related to sensor arrangements and particularly to sensor arrangements for symmetric response in a x-, y- and z-coordinate system. Liquid crystal in the light modulation layer 17 is oriented so as to incline, in association with the application of voltage by the driving circuit 2, within a plane that is parallel to a normal plane which includes the optical axes of both of the incident, read light and the output, modulated light. Upon refueling, the float rises and seals the membrane on a vent port valve seat. The main body can be generally rectangular in shape, with a top wall, a pair of side walls extending from the top wall and a pair of bottom walls, which extend inwardly from respective side walls in an overlapping relationship. The invention also relates to a central heating boiler provided with such a heat exchange element.