I need help about ( negative weighing system ) it means i want to discharge liquid from (10 Kg ) tank , (1) kg after (1) kg after (1) kg?

I want to use weighing controller . but the problem how to detect the negative weight .

EX : My tank liquid is10 Kg . if i refill small gallon1 kg it means the10 is9 kg . so the load cell give9 kg meaning1 kg transfered alredy . after that the valve will cloes  , and pump will off