hey guys,  
I need your help. I'm working on a project - level control in a tank with water. 
I suppose to control the level of the water , for example - the user can choose the height he wants the water to fill the tank , and a pump starts to pour water into the tank. 
I work with the PID VI. The user can choose how to control the water with the controllers - P , Pi , Pd or the combined PID. 
For example , the P control works like the equation m=P*e 
m - stands for the strengh of the water pumping 
e - stands for the error - difference between the height I want , and the height found in the tank. 
p - is a constant 
As bigger the difference between the wanted height and the found height , as stronger the strengh of the pumping. 
While the tank is being filled , the pumping suppose to get wicker until it stops. 
This algorithm that I wrote shows :  
The user chooses the set point - the height he wants. 
The DAQ measures the height - (height * 100) / wanted height --> gives us the error. 
But , while the tank is being filled , and the error is getting smaller , the pumping strengh stays the same. 
P.S 
I didn't use the entrance that called Process Variable in the PID VI. That's where I suppose to attach the "error". But when I do that the pump stops working. 
Please help I don't know what to do... : ( 
It's my final project and I'm desperate.