Capillary thermostats are used in a wide range of applications, from household appliances like ovens and refrigerators to industrial processes and HVAC systems. These thermostats are responsible for maintaining a desired temperature range, with the capillary tube being a critical component. In this article, we will explore the function of the capillary tube in a capillary thermostat and its importance in temperature control. For additional information on our capillary thermostats, please visit the product page.

Stemco 351-254371 Capillary ThermostatUnderstanding Capillary Thermostats

Capillary thermostats work on the principle of thermal expansion and consist of three main parts:

  1. Temperature-Sensitive Element (Bulb): This part is placed in the environment where temperature control is needed.
  2. Diaphragm or Bellows: Connected to the bulb, this component moves in response to temperature changes.
  3. Capillary Tube: A thin, flexible tube filled with a temperature-sensitive fluid that transfers movement to the switching mechanism.

The Function of the Capillary Tube

The capillary tube acts as a link between the temperature-sensitive element and the switching mechanism. Made from a material that reacts to temperature changes, the tube typically contains a gas or liquid with a low boiling point. As the temperature changes, the fluid inside the tube expands or contracts, causing the tube to elongate or shorten. This movement then transmits mechanical force to the switching mechanism.

368 Capillary Diaphragm Assembly

How Capillary Thermostats Operate

To illustrate, consider a capillary thermostat used in a household oven:

  • First, place the temperature-sensitive bulb inside the oven.
  • Then, extend the capillary tube from the bulb to the control mechanism outside the oven.
  • As the oven heats up, the fluid in the bulb expands, causing the capillary tube to elongate.
  • This elongation transmits force to the diaphragm, triggering the switch to turn off the heating element.
  • Conversely, when the temperature drops, the fluid contracts, the capillary tube shortens, and the switch turns the heating element back on.

Benefits of Capillary Thermostats

Precise Temperature Control: Capillary thermostats offer accurate temperature regulation due to the direct link between the temperature-sensitive element and the switching mechanism.

Remote Sensing: The capillary tube allows the placement of the sensing element away from the control mechanism, facilitating remote temperature monitoring and control.

Durability: The mechanical nature of capillary thermostats makes them robust and less susceptible to electronic failures common in digital systems.

Considerations for Optimal Performance

While capillary thermostats offer many advantages, factors like the length and diameter of the capillary tube can affect the thermostat’s responsiveness and accuracy. Therefore, proper installation and regular maintenance are crucial for ensuring optimal performance.

The capillary tube plays a significant role in translating temperature changes into mechanical motion, which then triggers the switching mechanism to maintain the desired temperature range. Whether in household appliances or industrial processes, capillary thermostats with their capillary tubes ensure consistent and controlled temperature regulation. Understanding their operation and benefits helps in selecting and maintaining these important devices for various applications.

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Bulb & Capillary Thermostats

Original equipment manufacturers worldwide use thermostat switches designed and manufactured by STEMCO, a Division of Senasys. No matter which style or model you choose, all of the capillary thermostat switches operate using the same physics. A fluid is encapsulated in a metal and when the temperature changes the fluid either expands or contracts. This fluid change moves a diaphragm which is in touch with a snap action switch and either opens or closes a circuit.

Ambient Compensation

Our capillary thermostats can be ambient compensated to offset the differential in temperature which keeps our thermostat controls more accurate as it’s environment changes.

Custom Options

Capillary thermostat switches built by Senasys can also be built to customer specifications. This insures you get the exact control you want without paying for features you don’t need.