Capillary thermostats play a crucial role in temperature regulation across various industries and applications. These devices rely on a temperature-sensitive fluid, commonly referred to as the “fill,” within a capillary tube to control temperature. The selection of the appropriate capillary thermostat tube fill is essential as it directly impacts the thermostat’s performance and reliability. To purchase a capillary thermostat, click here. 
- Gas Fills: Gas fills are a popular choice for capillary thermostat tubes due to their ability to expand and contract with temperature changes. The most common gas fill used is a mixture of nitrogen and mercury. Nitrogen provides a stable and non-reactive base, while the addition of mercury creates a highly responsive fill. This combination ensures precise temperature control and quick response times. Gas fills are often preferred for applications that require high accuracy and fast thermal cycling.
- Liquid Fills: Liquid fills utilize a temperature-sensitive liquid to control the thermostat’s operation. Among the liquid fills, the most widely used is a blend of alcohol and water. This combination provides a suitable range of temperature control, typically from -30°C to 150°C. Alcohol-based fills offer excellent thermal expansion properties and are particularly useful in applications where precision is not critical but reliable temperature control is still required.
- Wax Fills: Wax fills consist of a paraffin-based substance that exhibits a significant change in volume with temperature variations. They are commonly used in applications where slow and gradual temperature adjustments are required. Wax fills are often found in household appliances such as ovens and refrigerators, where maintaining a steady temperature is crucial. The advantage of wax fills is their ability to provide a smooth and continuous response to temperature changes.
- Bimetallic Fills: Bimetallic fills employ the principle of differential thermal expansion of two dissimilar metals bonded together. As the temperature changes, the metals expand or contract at different rates, causing the bimetallic strip to bend. This mechanical movement controls the thermostat’s operation. Bimetallic fills are known for their durability, cost-effectiveness, and wide temperature range capability. They are commonly used in heating and cooling systems, industrial equipment, and household appliances.
- Electrically Conductive Fills: Electrically conductive fills, such as the ones containing silicone oil or glycol, rely on changes in electrical conductivity to measure temperature variations. The fill is in direct contact with a temperature sensor, and as the temperature changes, the electrical conductivity of the fill alters. This change in conductivity is detected and used to control the thermostat’s operation. Electrically conductive fills offer precise and responsive temperature control and are commonly found in electronic devices and HVAC systems.
STEMCO Capillary Thermostat Tube Fills
Our Stemco thermostats utilize a liquid fill consisting of either Xylene, Dowsil 550, Therminol 66, or HB40. Which fluid is used is based on a variety of different factors. We have a created cheat sheet for your benefit that you can view below for more information.
| Fluid | Min. Temp | Max. Temp | Required amount for 5/8 Diaphragm Diameter | Required amount for 3/4″ Diaphragm Diameter | Required amount in 1″ Diaphragm Diameter | Leak Test Time Required | Additional Info |
| Xylene | -30°F | 300°F | 0.3955 | 0.2494 | 0.1422 | 8 hours | To be used if the capillary temp is below 35°F. |
| Dowsil 550 | -60°F | 720°F | 0.283 | 0.1786 | 0.1019 | 24 hours | To be used if the capillary switch requires a food grade. |
| Therminol 66 | -13°F | 650/660°F | 0.2591 | 0.1642 | 0.0938 | 24 hours | Consistent with HB40. A minimum capillary temp 35°F is required. |
| HB40 | -13°F | 600°F | 0.2591 | 0.1642 | 0.0938 | 24 hours | Can go up to 625°F if needed. A minimum capillary temp 35°F is required. |
The selection of the appropriate capillary thermostat tube fill is vital to ensure accurate and reliable temperature control in various applications. Gas fills provide high accuracy and fast response times, liquid fills offer a wide temperature range, wax fills provide smooth and gradual adjustments, bimetallic fills are durable and cost-effective, while electrically conductive fills provide precise control in electronic applications. Understanding the different types of capillary thermostat tube fills allows engineers and designers to choose the most suitable option for their specific requirements, ensuring optimal performance and efficiency.
