Variable Under Proportional Mode:
There are three terms generally used to describe the proportional mode of control are proportional gain, band, and offset.
Proportional band, (also called throttling range), is the modification in value of the controlled variable which causes full travel of the last control element. Above figure shows the relationship among valve position and temperature band for two variant proportional bands.
The proportional band of a particular instrument is expressed as a percent of full range. For instance, if full range of an instrument is 200oF and it takes a 50oF change in temperature to cause full valve travel, the % proportional band is 50oF in 200oF, or 25%. Proportional bands might range from less than 1% to well over 200%. Therefore, proportional bands over 100% cannot cause full valve travel even for full range change of the controlled variable.
Gain, also known as sensitivity, compares the ratio of amount of change in the last control parts to amount of change in the controlled variable. Mathematically, sensitivity and gain are reciprocal to proportional band.
Offset, also known as droop, is deviation in which remains after a procedure has stabilized. Offset is an inherent feature of the proportional mode of control. Instead of, the proportional mode of control will not necessarily return a controlled variable to its setpoint.