P-only loop output is linear, and is calculated as follows:
Output = (KP x ES) + bias (if action = direct) or
Output = - ((KP x ES) + bias) (if action = reverse)
where ES = [PV - setpt]
If you are using proportional-only loop control, follow these guidelines:
Maximum Output and Minimum Output properties for the loop output, noting that the maximum value must be greater than the minimum.[output range (maxOutput - minOutput)] / throttling range
where throttling range is the corresponding result in the process variable.
For example, for a temperature loop where a 0-to-100% loop output results in a 20 degree swing in the process variable, a starting point KP is:
[(100% - 0%)/ 20deg.] = [(100% / 20deg.] = 5
When tuning the loop, you can try increasing this value (effectively using only a portion of the throttling range) to eliminate the amount of setpoint error. However, if you increase the KP too much, this typically results in a constant oscillation of the process variable (above and below the setpoint).