🔍 Key Concepts

• Proportional control vs reset (integral) control in mechanical controllers • How a pilot-operated temperature or pressure controller behaves as the process variable moves away from setpoint • Visual clues in the illustration that indicate throttling (modulating) action versus simple on-off action

💭 Think About

• Look closely at the moving parts connected between the sensing element and the main valve plug. Do they allow the valve to move gradually to many positions, or just snap between fully open and fully closed? • Ask yourself whether there is any mechanism that would slowly "wind up" or continue to change the valve position over time to eliminate a steady offset between setpoint and actual value. • Consider how feedback from valve position or controlled pressure/temperature might be fed back into the bellows or springs. Does the design show a second feedback element that would provide a reset (integral) effect?

✅ Before You Answer

• Identify whether the main valve plug is continuously positionable (modulating) or simply open/closed. • Check if there is a single measuring element and spring (typical of proportional-only control) or an additional feedback bellows/chamber that would create reset action. • Make sure you can explain in your own words whether this device would hold a fixed offset from setpoint under steady load, or gradually drive that offset to zero over time. That distinction is key to choosing between proportional only and proportional plus reset.