Derivative (RATE):  In seconds ... the amount of time that it
advances the output.  This can used to adjust processes that
have lag, or are sluggish, or processes that are very sensitive
to output changes.

Integral (RESET):  In seconds, also called (repeats per minute)
... how many times it resets the error and re-evaluates the
values.  This removes the offset that occurs between the PV and
SP. When you make changes to the PID variables, cause an upset by
changing the setpoint (SP).  This will show you, via the graphs,
how well you are "tuning" the control loop.

When the program starts, the system is already placed into
automatic mode with a somewhat stable setting for the PID
algorithm.  The operator can alter the inlet valve position and
see how the computer adjusts the output valve to maintain the
level setpoint.

The water tank is constructed of a tank with a manual input valve
that the machine operator can control, a water tank with a level
sensor, and an output valve that can either be controlled
manually, or be controlled by the computer using the PID control
algorithm.

Because there are so many variations to PID control loops between
manufacturers of controllers, a basic algorithm has been selected 
that will allow experimentation with each of the 3 PID 
variables.

The objective of this program is to examine a simplified PID 
control loop used in an industrial setting, in this case, a water
tank with level control.
