How does a diaphragm fusible bulb operate?

A diaphragm fusible bulb is a type of temperature-sensitive device used for safety and control applications.

It operates as follows:

1. Construction: The diaphragm fusible bulb contains a volatile liquid or gas sealed within a thin-walled glass or metal bulb. This bulb is connected to a flexible diaphragm.
2. Normal operation: At normal temperatures, the volatile substance inside the bulb remains in a liquid or gaseous state, exerting a certain pressure on the diaphragm. This pressure keeps the diaphragm in a neutral position.
3. Temperature rise: When the surrounding temperature increases, the volatile substance inside the bulb expands, increasing the pressure on the diaphragm.
4. Diaphragm movement: At a specific temperature, the increased pressure causes the diaphragm to flex or move, actuating a mechanism connected to it.
5. Circuit interruption: This movement of the diaphragm can be used to interrupt an electrical circuit, activate an alarm, or trigger a safety shutdown. For example, in a fusible link application, the diaphragm movement would melt a solder link, breaking the circuit.
6. Reset: Once the temperature drops and the volatile substance contracts, the diaphragm returns to its neutral position, allowing the device to be reset and reused.

The specific temperature at which the diaphragm actuates is determined by the choice of volatile substance and the bulb design. Diaphragm fusible bulbs are commonly used in applications such as thermostats, fire alarms, and overload protection devices to provide temperature-based control and safety functions.