how to control the pressure in a tank with a mass flow controller

[kenny1993]

hey there, 

We want to use a mass flow controller to control the pressure in a box. 

Now we have connected the mass flow controller(MFC) to the box. Also there is a differential pressure meter connected. Now we get in labview a signal of the pressure meter between 4 and 20mA also the MFC gives a signal between 4 and 20mA. 

But now when we get a differential pressure of 50Pa, we need to send a signal to the MFC sow he can adjust the incoming air so the pressure difference stays the same. 

We use a PID but the value off the pressure jumps between 30Pa and 70Pa but we want it so closely possible to 50Pa

Some one who can help? 

In the picture you can see our labview to test the system.

[LV_Pro]

You say you used a PID to control this, but how have you set the PID's variables? A PID's stable operation depends almost entirely on the correct selection of the control variable values, otherwise it can provide very little control or too much.

[kenny1993]

do you mean the PID gains or other variables? 

 now i did put the Kc = 1 ; the Ti = 0.010 min and the Td = 0 min