How to measure pressure of oxygen only if oxygen and air is present in glass bulb?

I think it is a technique. My technique is like this,: Connect a T- openings on the opening end of the manometer.Connect the closed Valve outlet pipe of the Oxygen cylinder to the top end of the T- connection.And also the end should be one way valve. Now give a pressure on the bulb, the mercury column will come up to the top end replacing air.The Valve will close after the air is exhausted (Taking care that mercury should not spilled out).; Open the Valve of the oxygen taking for the instant with the care that the pressure does not exceed to break the tube.