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| General purpose temperature switch with bottom connection for capillary and bulb specific for each application. Image courtesy United Electric Controls |
One method of changing the state of a switch in response to a process temperature change is a bulb and capillary temperature switch. The switch operation produces a state change in the mechanical switch when the temperature of a process control operation crosses a certain threshold. Bulb and capillary switches have the advantage of operating without electricity, simplifying their application.
The physical operating principle behind the capillary thermostat relies on the use of a fluid. The fluid inside the thermostat expands or contracts in response to the temperature at the sensing bulb. The change in fluid volume produces a force upon a diaphragm or other mechanical transfer device. The diaphragm is connected to, and changes the status of, an adjoining circuit using a snap action switch. For example, a main use of the operating principle in action is when a commercial food company relies on the capillary switch to control temperature related to processing and distribution. Each individual use of a bulb and capillary thermostat is specifically designed based on manufacturer and industry specifications, all of which apply the same physical principle of fluid based physics.
Because of their simplicity and comparatively modest cost, commercial versions of bulb and capillary switches find application throughout residential and commercial settings. Some common applications include warming ovens, deep fat fryers, and water heaters. The HVAC industry uses capillary and bulb switches because the rate of temperature change found in their applications fits the adjoining range offered by the bulb and capillary type switches. Operation of the temperature switches is subject to a few limitations. The switching point is often fixed, so the application must be without a requirement for an adjustable setpoint. The temperature range over which the switches are suitable is comparatively limited, with a matching of the bulb and capillary fluid system to the application temperature range a necessary task in product selection. Within its proper sphere of use, though, bulb and capillary temperature switches offer simple, reliable operation, with little requirement for maintenance.
Bulb and capillary switches are typically used to evaluate average temperature and are especially useful for applications where the temperature is to be maintained at a well-known, consistent value. The bulb portion can be configured to accommodate mounting within the media to be controlled. The devices can be applied effectively to liquid and gaseous media when the proper bulb is used.
Industrial versions of bulb and capillary switches are fitted with appropriate housings for the installation environment. Hazardous location installation can be accommodated, as well as high current ratings and auxiliary functions. There are almost countless variants of bulb and capillary temperature switches available. Don’t overlook these simple mechanical devices as candidates for application in any temperature control process. Share your application requirements and challenges with product specialists for useful recommendations.
