Bubble-Tube Depth Gage

The sketch show a bubble-tube apparatus. An open tube extends to very near the bottom of a tank of oil. In operation, compressed nitrogen is supplied to the tubing at low pressure. The nitrogen bubbles from the bottom of the tube into the oil so slowly that the pressure in the gas at depth equals the oil pressure adjacent to it (at B). Use the information given. Calculate the depth from seal level to the surface of the oil.