CFD Online Discussion Forums - measured pressure in the discharge pipe jumps

I have a tank filled with some liquid for experimental purpose. I am emptying the tank from the bottom and in this bottom pipe measuring the pressure at the center point of the pipe. When the valve is close, I measure the static pressure at the point of pressure sensor. Now, I open the valve full. Due to the nature of the experiment, I have a high velocity in the flow, about 850 l/min. Now the pressure measured by the sensor also includes the kinetic force of the liquid coming from the velocity. But the pressure measured at this point is less than the pressure measured when the valve was fully closed. The tank is fast getting emptied and with time the pressure measured in the middle of the pipe is decreasing. Till now I understand why the pressure is decreasing.

After the pressure reaches a low value, I partly (about 70%) closed the discharge valve thereby reducing the flow rate to about 250 l/min. At this point, the pressure measured jumps up and then goes down again as the tank is getting emptied. I repeated this whole emptying process again by fully opening and partly closing the discharge valve, everytime I close the discharge valve partly, the pressure measured at the center of the discharge pipe makes a jump. I dont understand this jump. When I reduce the flow rate, the kinetic forces are low whereas the static forces are the same, therefore I assume it should not jump up but jump down. Can anyone clearify this ? Did anyone witnessed similar behaviour?

Thanks in advance.

Jan