Lori McPherson, Regional Sales Manager for Iwaki America, was published in the Industrial Waterworld Jan/Feb 2013 edition. Her article titled 'Understanding Time Proportional Control' details the following.
An underused control strategy that offers significant benefits is Time-Proportional Control (TPC). Unlike traditional proportional or even PID control that require a varying output to a modulating control device, time-proportional control can achieve a proportional control response to process variation using an on/off device by varying on and off times in a defined control period. The on/off device is generally a simpler, less expensive control device.