A simple U-tube manometer is used to measure the pressure of oil (sp. Gr. 0.8) flowing in a pipe line. The pressure in the pipe is vacuum. Find the vacuum pressure in pipe. Its right limb is open to atmosphere while left limb is connected to pipe. The difference in the level of Hg (sp. Gr. 13.6) in two limbs is 20cm. the height of oil in left limb from the centre of pipe is 15 cm below.