Abstract: Apparatus as described for indicating, for alarm and/or safeguard control purposes, when the value of a second parameter lies beyond (i.e. is above or below) a predetermined limit. The apparatus includes two or more sensors each arranged to provide a respective warning signal when the parameter lies beyond a respective specified value, the value for at least one of the sensors being equal to or beyond the predetermined limit; and logic means including a validation circuit connected to the two sensors and arranged to provide a "parameter-beyond-limit" signal, which may be used, for example, to operate an alarm and/or an emergency trip, only when both sensors provide a warning signal. Various forms of apparatus are described in which the parameter sensed is the level of water in various items of steam plant as used in power stations, e.g. boilers, vacuum condensers, bled steam lines and direct contact feed heaters, to provide an indication of high and/or low water level.