Bulb and capillary thermostats are advanced devices that leverage the principles of liquid expansion to control temperatures accurately. Whether you’re using them in HVAC systems, food equipment, or pool and spa controls, these thermostats rely on the same underlying physics to function. This guide dives into the science of how bulb and capillary thermostats work and explores their various applications and benefits. As always, if you have any questions, please do not hesitate to contact us.

Liquid expansion capillary thermostatsThe Science of Liquid Expansion in Capillary Thermostats

At the heart of bulb and capillary thermostats lies a fundamental physical principle: liquid expansion. Here’s a detailed look at how this principle operates within the device:

Components and Mechanism

  1. Bulb: The bulb is a larger diameter tube typically mounted within the material flow being monitored. This bulb contains a liquid that responds to temperature changes.
  2. Capillary: Connected to the bulb is a slender tube known as the capillary. The capillary transports the liquid from the bulb to the thermostat’s head.
  3. Head: The head of the thermostat contains a diaphragm or bellow. This component moves in response to changes in the liquid volume caused by temperature fluctuations.

When the temperature of the surrounding environment changes, the liquid inside the bulb expands or contracts. For example:

  • Temperature Increase: The liquid heats up and expands. This expansion causes the liquid to move through the capillary tube and push against the diaphragm or bellow in the head.
  • Temperature Decrease: The liquid cools down and contracts. This contraction pulls the liquid back from the diaphragm or bellow, allowing it to return to its original position.

This movement of the diaphragm or bellow, in turn, activates a snap-action switch within the thermostat. Depending on the design, this switch can either open or close an electrical circuit, thereby controlling the connected device.

Stemco 358E-144 Capillary ThermostatKey Features and Advancements

Bulb and capillary thermostats are not just simple on-off devices. They incorporate several advanced features to enhance performance and reliability:

  • Ambient Temperature Compensation: This feature offsets the differential in temperature, ensuring that the thermostat maintains accuracy despite fluctuations in the surrounding environment. This compensation is crucial for maintaining stable temperature control in dynamic conditions.
  • Snap-Action Switch: The use of a snap-action switch ensures a rapid response to temperature changes. This quick action minimizes the lag between detecting a temperature change and triggering the corresponding control action.

Applications and Practical Uses

Due to their precise control and reliability, bulb and capillary thermostats find applications in a variety of fields. Here are some common uses:

  • Food Equipment: These thermostats serve as limit or control devices in ovens, refrigerators, and other food-processing equipment. They help maintain safe and consistent temperatures crucial for food safety and quality.
  • HVAC Systems: In heating, ventilation, and air conditioning systems, bulb and capillary thermostats regulate the temperature of the air being conditioned. They ensure that indoor environments remain comfortable and energy-efficient.
  • Pool and Spa Controls: These thermostats maintain the desired water temperature in pools and spas, providing a consistent and enjoyable experience for users.
  • Industrial Equipment: In industrial settings, they control temperatures in machinery and processes, ensuring operational safety and efficiency.

Quality and Reliability

The design and manufacturing process of these thermostats are critical to their performance. High-quality materials and rigorous testing protocols ensure that each device operates reliably under various conditions. At Stemco, a division of Senasys, we prioritize quality control, with each thermostat undergoing 100% individual testing before leaving our facility.

Understanding the science behind bulb and capillary thermostats reveals why they are so effective in various temperature control applications. The principle of liquid expansion, coupled with advanced features like ambient temperature compensation and snap-action switches, makes these devices indispensable in many industries. By ensuring precise and reliable temperature control, they contribute significantly to safety, efficiency, and comfort in numerous applications.

Send us your application details today!

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.