Choosing a Bimetal Thermometer: 5 Factors to Consider

In many industrial processes, maintaining the correct temperature is essential for product quality, process efficiency and safe operation. From food processing and chemical production to HVAC systems and general manufacturing, operators rely on temperature instruments to monitor conditions and identify potential issues before they affect the process.

Mechanical temperature instruments such as bimetal thermometers are commonly used because they provide a durable and easy-to-read local temperature indication without requiring external power. Their dial display also allows operators to check process conditions from a distance, helping keep personnel away from hot equipment or hazardous areas. 

However, selecting the right instrument involves more than choosing a temperature range. Dial size, mounting configuration, materials of construction and environmental conditions can all influence how well the thermometer performs once installed in the process.

In this article, we will explain how bimetal thermometers work and review five factors to consider when selecting a bimetal thermometer for your application.

When should you use a bimetal thermometer?

Bimetal thermometers should be used when you need a reliable local indication of temperature for air, gas or liquid in an industrial environment. These instruments are commonly installed in a thermowell, which is placed into the process so operators can quickly verify temperature conditions.

Because bimetal thermometers operate mechanically, they provide dependable readings in applications where electrical power is unavailable or where a simple visual reference is preferred.

They are often used in industries such as:

• Manufacturing

• Food and beverage processing

• Chemical and petrochemical production

• HVAC systems

• Water and wastewater treatment

In many facilities, a bimetal thermometer is installed alongside electronic sensors to provide a secondary visual confirmation of temperature conditions.

How do bimetal thermometers work?

Bimetal thermometers work by using two bonded metals that expand at different rates when exposed to temperature changes. The bonded metals are formed into a coil or helix. As temperature rises or falls, each metal expands or contracts at a different rate, causing the coil to wind or unwind.

This motion is mechanically linked to the pointer inside the thermometer. As the coil moves, the pointer rotates across a calibrated dial to indicate the measured temperature. Because this mechanism is entirely mechanical, bimetal thermometers can provide direct temperature readings without requiring an external power source, making them well suited for many industrial environments. 

Figure 1. How bimetal thermometers work

What factors should you consider when choosing a bimetal thermometer?

As mentioned above, choosing the right bimetal thermometer requires evaluating several application and design factors that affect readability, durability and long-term performance.

While these instruments use a simple mechanical sensing element, their effectiveness in a process environment depends on selecting the correct dial size, mounting configuration, materials of construction and features designed to handle conditions such as vibration or operator access.

The following five considerations can help you determine which bimetal thermometer is best suited for your application.

1. What dial size do you need?

The correct dial size depends on how easily operators need to read the instrument in its installed location.

A larger dial is easier to read from a distance and allows for larger, clearer scale markings. This can help reduce the risk of misreading the temperature, particularly in environments where operators cannot always stand close to the instrument.

Common dial sizes include:

• 2-inch dial – suitable for compact equipment or close viewing distances

• 3-inch dial – commonly used for general industrial applications

• 5-inch dial – preferred when readings must be visible from farther away

Selecting the appropriate dial size helps ensure the thermometer remains easy to read under normal operating conditions.

2. What mounting or connection type is required?

The mounting configuration determines how the thermometer connects to the process and how easily the dial can be viewed.

Common mounting options include:

• Back mount (rear connection) where the stem extends from the back of the case

• Bottom mount (lower connection) where the stem connects from the bottom of the instrument

• Adjustable or “every-angle” mount that allows the dial to rotate for improved visibility

Adjustable mounting designs allow operators to position the dial for optimal readability regardless of how the instrument is installed. Selecting the appropriate connection type ensures the thermometer can be installed securely while still providing a clear temperature indication.

Figure 2. Bimetal thermometer mounting options

3. Which materials are best for your application?

The materials used in a bimetal thermometer affect its durability, corrosion resistance and suitability for the process environment.

For many industrial applications, 304 stainless steel is commonly used because it offers good strength and corrosion resistance. In harsher environments—such as chemical processing—316 stainless steel may be required to provide improved resistance to corrosive media. 

Other material considerations include the dial window, which may be made from:

• Plastic for lighter-duty environments

• Shatter-resistant safety glass for improved durability and protection

Choosing materials that are compatible with the application helps ensure the instrument maintains reliable performance over time.

4. Should you use a liquid-filled thermometer?

A liquid-filled thermometer is often recommended when vibration or mechanical movement is present.

Choosing a silicone liquid-filled thermometer like the Ashcroft® EL Bimetal Thermometer can significantly extend the instrument's lifespan. The liquid fill dampens the effects of vibration, reducing wear and tear on the internal components. These thermometers are also capable of operating within a wide temperature range of -40 °F to 550  °F (-40 to 300 °C), making them an ideal choice for processes that are subject to constant movement or shaking, including: 

• Pumps and compressors

• Rotating equipment

• Mobile or vibrating machinery

5. Do you need an adjustable or tamperproof design?

Bimetal thermometers are available in externally adjustable and tamperproof designs, and the right choice depends on how the instrument will be used.

• Externally adjustable thermometers like the Ashcroft® EI Bimetal Thermometer allow operators to make minor calibration adjustments in the field to maintain accuracy.

• Tamperproof thermometers, like the Ashcroft® CI Bimetal Thermometer on the other hand, are designed without adjustment mechanisms. These instruments are typically hermetically sealed to prevent moisture ingress, coil freeze, corrosion and unauthorized changes to the temperature reading. 

Applications that require controlled calibration access may benefit from adjustable models, while regulated or safety-critical environments often prefer tamperproof designs.

Ready to learn more?

Selecting the right bimetal thermometer helps ensure accurate temperature monitoring and reliable process performance. Note: in some applications, especially those involving high pressure, corrosive media or flowing process fluids, a thermowell should be used to protect the thermometer from damage and allow the instrument to be removed without shutting down the process.

If you would like to learn more about temperature measurement and instrumentation, check out the additional resources below. If you have questions, you can always contact us to speak to a product expert. 

In the meantime, download our guide to learn about other temperature measurement solutions offered by Ashcroft.