J
jeremy
11 minutes
Proportional Band vs Proportional Gain
Proportional band is another way to set the P term. Some controllers ask for a gain, and some ask for a band. The band is the error span that moves the output from 0 to 100 percent. In percent of the sensor span, PB = 100 / Kc. Here PB is the band in percent, and Kc is the gain with no units. So a 10 percent band is a gain of 10. A 200 percent band is a gain of 0.5. A wide band is a low gain and a gentle loop. A narrow band is a high gain and a fast, jumpy loop. That means the two knobs turn in opposite ways. To calm a loop, you raise the band but lower the gain. Many temperature controllers give the band in degrees, not percent. A band of 20 degrees means the heater goes from off to full power across a 20 degree span near the setpoint. To turn degrees into percent, divide by the sensor span and multiply by 100. Before you copy numbers from a rule, a forum post, or another controller, check which one the screen shows. Copying a band as a gain can make the loop ten times too fast or too slow. Every controller shows this a bit differently, so check its manual. After this Concept you can convert a proportional band to a gain and back, and you know which way to turn each knob.