Question

why to measure small pressure difference we use low density liquids like oil
Please answer the above question fast

Answer 1

if we are measuring difference of 50 KN/M^2 in the pipe where water flows, manometer with mercury will show the level difference of 37.47 cm {[50/(13.6*9.81)]=.3747m = 37.47m}. 

Where as if you use a liquid of specific gravity 1.5 as manometric liquid it will show the level difference of 3.4 m. So, you manometer height should  be more than 3.4 m (You will be claimbing on ladder every time to take measurements :-P )

Becase of above reasons, we provide different and heavier liquid as manometer fluid...