device for checking, before start-up and after shut-down, that all or part of the measuring system is either filled completely with liquid (full hose measuring systems) or completely empty of liquid (, type of level sensor, a transparent tube through which the operator of a tank or boiler can observe the level of liquid contained within, läbipaistev toru, mille kaudu paagi või katla käitaja saab jälgida selles sisalduva vee taset