Conditioning the HPLC system prior experiment
It is important to condition the column performing any analysis. This is done by turning on the pump and selecting flow rate and temperature. Then, you only need to wait until the pressure in the system is constant. This means that the mobile phase has reached every pore of the column, activating its beads, and the system is ready for the injection of the sample. It will take some time for the mobile phase to activate all the beads, in this time the flow rate and pressure is unstable.
It is very important to have exactly the same condition when you are running several experiments and would like to compare them. This is why conditioning the system is an important step. You will need to repeat this step when you are measuring using another mobile or stationary phase. When you are changing the mobile phase, you also need to flush the tubing (connecting the mobile phase reservoir to the column) and make sure that there is no remaining old mobile phase left.