Liquid-in-Glass vs. Bimetallic Dial Thermometers

Merit Instruments
3 days ago
10 min read

How They Work, Their Advantages, and How to Select the Right Technology

Temperature measurement is one of the most common requirements in industrial, commercial, HVAC, process, and mechanical applications. While modern electronic temperature instruments are widely available, two mechanical technologies remain extremely useful because of their simplicity, reliability, and ability to operate without an external power source:

Both technologies provide direct temperature indication, but they operate according to fundamentally different physical principles. Understanding those differences is important when selecting a thermometer for an application.

A liquid-in-glass thermometer measures temperature by observing the thermal expansion of a liquid, while a bimetallic thermometer uses the different rates of thermal expansion of two bonded metals to produce mechanical movement.

Neither technology is universally "better." The correct choice depends on factors such as required accuracy, temperature range, response time, vibration, installation configuration, readability, durability, and application environment.

1. How Liquid-in-Glass Thermometers Work

A liquid-in-glass thermometer is one of the simplest temperature-measuring devices.

The basic construction consists of:

A glass bulb or sensing reservoir
A narrow capillary tube
A temperature-sensitive liquid
A graduated temperature scale
A protective housing or mounting arrangement, depending on the design

The sensing bulb contains a liquid that expands when its temperature increases and contracts when its temperature decreases.

As the liquid expands, it moves upward through the very small internal diameter of the capillary. The position of the liquid column corresponds directly to the temperature indicated on the scale.

The Measurement Principle

The operating principle is based on thermal expansion.

When temperature increases:

Temperature ↑ → Liquid expands → Column rises → Temperature indication increases

When temperature decreases:

Temperature ↓ → Liquid contracts → Column falls → Temperature indication decreases

The capillary is deliberately manufactured with a very small internal diameter. This allows a relatively small change in liquid volume to produce a readily visible movement along the temperature scale.

The relationship between temperature and liquid-column height is established during manufacture by calibrating the scale to the characteristics of the sensing liquid and thermometer construction.

2. The Sensing Liquid

Different liquids have different thermal expansion characteristics. The liquid selected must remain suitable over the intended temperature range and provide predictable expansion.

Historically, mercury was widely used in glass thermometers because of its relatively predictable expansion and useful temperature range. However, environmental and regulatory considerations have resulted in mercury being restricted or eliminated from many modern applications.

Modern industrial thermometers may use alternative temperature-sensitive liquids designed for the intended operating range.

The choice of liquid affects:

Temperature range
Expansion characteristics
Readability
Response time
Environmental considerations
Overall thermometer design

This is one reason that a liquid-in-glass thermometer should be selected as a complete calibrated instrument rather than simply viewed as a glass tube containing a generic liquid.

3. Advantages of Liquid-in-Glass Thermometers

Liquid-in-glass thermometers have several important advantages.

Simple and Reliable

There are very few moving components. The measurement principle is based directly on thermal expansion, making the instrument inherently simple.

No External Power Required

A liquid-in-glass thermometer is completely passive. It does not require:

Batteries
Electrical power
Wiring
Signal conditioning
Electronic components

This makes it particularly useful in locations where electrical power is unavailable or undesirable.

Direct Temperature Indication

The liquid column provides a direct visual indication of temperature. There is no electronic display or electrical signal that must be interpreted.

Good Stability

When properly manufactured and installed, liquid-in-glass thermometers can provide stable temperature indication over long periods.

Useful for Local Temperature Indication

They are particularly well suited to applications where an operator needs to walk up to the equipment and visually determine temperature.

4. Limitations of Liquid-in-Glass Thermometers

The same characteristics that make liquid-in-glass thermometers simple also create some limitations.

Glass Construction

Glass can be vulnerable to mechanical damage, particularly in applications involving:

Vibration
Impact
Rough handling
Transportation
Industrial machinery

Protective designs can reduce these risks, but the fundamental sensing element remains glass.

Reading Angle

Because the temperature is determined by viewing the liquid column against a scale, readability can depend on viewing angle, scale size, and lighting.

Installation Considerations

The sensing bulb must be positioned appropriately in the process or environment being measured. Incorrect immersion depth or poor thermal contact can result in inaccurate readings.

Slower Response in Some Applications

The sensing bulb and surrounding medium have thermal mass. Depending on the design and installation, the thermometer may respond more slowly to rapid temperature changes than some alternative technologies.

5. How Bimetallic Dial Thermometers Work

A bimetallic thermometer uses a completely different measurement principle.

Instead of measuring the expansion of a liquid, it relies on the fact that different metals expand at different rates when heated.

A bimetallic element is manufactured by bonding two dissimilar metals together.

When the temperature changes, one metal attempts to expand more than the other. Because the two metals are bonded together, they cannot expand independently. The resulting mechanical stress causes the element to bend or twist.

That mechanical movement is then converted into rotation of a pointer on a dial.

6. The Bimetallic Coil

Industrial dial thermometers commonly use a helical or spiral bimetallic element.

The bimetal is formed into a coil so that a relatively small amount of thermal expansion can produce a useful amount of rotational movement.

As temperature increases:

Temperature ↑ → Differential metal expansion ↑ → Coil twists → Pointer rotates

As temperature decreases:

Temperature ↓ → Differential expansion decreases → Coil returns → Pointer rotates back

The pointer movement is mechanically connected to the sensing element and displayed on a calibrated dial.

This allows a relatively small temperature change to produce a clearly visible movement of the pointer.

7. Advantages of Bimetallic Dial Thermometers

Bimetallic thermometers have become extremely common in industrial and commercial applications because they offer an excellent combination of durability, simplicity, and cost.

Excellent Mechanical Durability

Unlike a glass thermometer, the sensing mechanism is contained within a protective metal case.

This makes bimetallic thermometers well suited to many industrial environments where vibration, handling, and mechanical exposure are concerns.

Easy-to-Read Dial

The large pointer and dial provide an immediate visual indication.

Dial thermometers are particularly useful where operators need to monitor temperature from several feet away.

No External Power

Like liquid-in-glass thermometers, bimetallic thermometers are mechanical instruments and require no electrical power.

Flexible Installation

Bimetallic thermometers can be manufactured in numerous configurations, including:

Back connection
Bottom connection
Adjustable-angle designs
Remote-reading configurations
Various stem lengths
Different dial sizes

This makes them adaptable to a wide variety of equipment and piping arrangements.

Rugged Industrial Construction

A typical industrial bimetallic thermometer may incorporate a stainless steel case and stem, making it suitable for demanding environments.

Cost Effective

For many general-purpose temperature applications, a bimetallic thermometer provides an economical solution without the complexity of electronic instrumentation.

8. Limitations of Bimetallic Thermometers

Bimetallic thermometers also have limitations that should be considered during selection.

Mechanical Movement

Because the instrument relies on mechanical movement, accuracy can be affected by:

Shock
Vibration
Mechanical wear
Improper installation
Excessive temperature cycling

High-quality industrial designs can minimize these effects, but mechanical instruments are not completely immune to them.

Response Time

The sensing stem and bimetallic element have thermal mass. Consequently, the thermometer does not respond instantaneously to changes in temperature.

The actual response time depends heavily on:

Stem diameter
Stem length
Material construction
Thermowell use
Process fluid
Fluid velocity
Temperature differential

Position Sensitivity

Some mechanical thermometers can be affected by orientation or mechanical installation conditions. Manufacturers therefore specify installation requirements for particular designs.

Calibration

Like any mechanical measuring instrument, bimetallic thermometers can require calibration verification if high accuracy is critical. Most bimetallic dial thermometers have an external recalibrator screw that allows the user to reset the temperature to a known standard.

9. Liquid-in-Glass vs. Bimetallic Thermometers

The most important differences between the two technologies can be summarized as follows.

Characteristic	Liquid-in-Glass	Bimetallic Dial
Measurement principle	Liquid thermal expansion	Differential expansion of two metals
Display	Liquid column	Mechanical pointer and dial
External power	Not required	Not required
Moving mechanical components	Very few	Yes
Mechanical durability	Moderate	Generally high
Vibration resistance	Application dependent	Generally better
Readability	Good at close range	Excellent for local visual monitoring
Installation flexibility	More limited	Highly flexible
Response time	Application dependent	Application dependent
Accuracy	Can be very good	Good for general industrial applications
Remote indication	Limited	Available with suitable configurations
Glass exposure	Yes	Generally no
Typical use	Laboratory, HVAC, process, general temperature measurement	Industrial, HVAC, piping, machinery and process applications
Cost	Generally economical	Generally economical

The table illustrates an important point: the choice should be based on the application rather than on the thermometer technology alone.

10. Accuracy vs. Resolution

Accuracy and resolution are sometimes confused when selecting a thermometer.

Accuracy describes how close the indicated temperature is to the actual temperature.

Resolution describes the smallest temperature increment that can be meaningfully read from the scale.

For example, a large dial with closely spaced graduations may provide excellent readability, but that does not automatically mean the thermometer is more accurate.

Similarly, a liquid column may provide very fine visual resolution without necessarily providing greater overall system accuracy.

The thermometer should therefore be evaluated according to its specified accuracy rather than simply the appearance of its scale.

11. Response Time and Thermal Mass

Response time is particularly important in applications where temperature changes rapidly.

A thermometer does not measure the temperature of the process instantaneously. The sensing element must first reach thermal equilibrium with the medium being measured.

Several factors influence response time:

Sensing Element Size

A larger sensing element generally contains more thermal mass and may respond more slowly.

Process Medium

Liquids, gases, and solids transfer heat to the sensing element at different rates.

Flow

Moving fluids generally transfer heat more effectively than stagnant fluids.

Thermowells

A thermowell provides valuable protection for the thermometer, but it introduces additional material between the process and sensing element. This can sometimes marginally increase response time but protects against other considerations such as flow rates and allows for easy instrument removal without the need to drain or shutdown a system.

Installation

Poor immersion, inadequate insertion depth, or poor thermal contact can significantly affect the speed and accuracy of measurement.

This is an important engineering consideration: the thermometer's published response time is only part of the measurement system's response.

12. When Should You Choose a Liquid-in-Glass Thermometer?

A liquid-in-glass thermometer can be an excellent choice when:

A simple, passive temperature indicator is required
Electrical power is unavailable
Direct visual temperature indication is preferred
Mechanical complexity should be minimized
The installation environment is relatively protected
The application benefits from the simplicity of a liquid-column indication

They are particularly useful in applications where the thermometer is primarily being used as a straightforward local temperature reference.

13. When Should You Choose a Bimetallic Dial Thermometer?

A bimetallic thermometer is generally the better choice when:

The installation is industrial
Mechanical durability is important
Vibration or handling is expected
A large, easily visible dial is desired
Flexible mounting configurations are required
A rugged metal construction is preferred
No electrical power is available
General-purpose industrial temperature measurement is required

Bimetallic thermometers are especially common in:

HVAC systems
Boilers
Pumps
Compressors
Piping systems
Water treatment equipment
Process equipment
Machinery
Industrial skids
Mechanical equipment

14. The Importance of the Thermowell

In many industrial installations, the thermometer is installed inside a thermowell.

A thermowell is a protective barrier between the process and the temperature sensor.

It can provide several important benefits:

Protects the sensing element from corrosion
Protects the thermometer from process pressure
Allows the thermometer to be removed without opening the process
Protects against mechanical damage
Simplifies maintenance and replacement

However, thermowells must be correctly selected for the application.

Important considerations include:

Process pressure
Process temperature
Process fluid
Corrosion resistance
Insertion length
Connection type
Thermowell material
Flow velocity
Mechanical loading

A thermometer and thermowell should therefore be considered as one temperature-measurement system, rather than two completely independent components.

15. Material Selection Matters

The sensing element is only one part of a thermometer.

Industrial temperature instruments may be exposed to:

Water
Steam
Glycol
Chemicals
Oils
Refrigerants
Corrosive fluids
Outdoor environments

The materials used for the stem, case, connection, thermowell, and other wetted components must be compatible with the application.

For example, stainless steel construction can provide substantially better corrosion resistance than lower-cost materials in many industrial environments.

The correct material selection can significantly extend instrument life.

16. Selecting the Correct Temperature Range

The thermometer's temperature range should be selected carefully.

A common mistake is to select a thermometer simply because its maximum temperature exceeds the expected operating temperature.

A better approach is to select a range that provides adequate operating margin while maintaining useful scale resolution.

For example, if an application normally operates around 150°F, selecting an excessively large temperature range may make the normal operating temperature occupy only a small portion of the dial.

A properly selected range allows the operator to easily identify:

Normal operating temperature
Expected operating limits
High-temperature conditions
Low-temperature conditions

The goal is not simply to survive the temperature—it is to provide useful information about it.

17. Liquid-in-Glass or Bimetallic: Which Is Better?

There is no universal winner.

The two technologies solve the same basic measurement problem using different methods.

Choose Liquid-in-Glass When:

Simplicity and direct temperature indication are the priority.

A liquid-column thermometer can provide an elegant and straightforward measurement solution where the environment is suitable for glass construction.

Choose Bimetallic When:

Durability, visibility, and industrial flexibility are the priority.

A bimetallic dial thermometer is often the preferred solution for industrial equipment, piping, HVAC systems, and machinery where a rugged, easy-to-read instrument is required.

18. The Merit Instruments Approach

At Merit Instruments, thermometer selection begins with the application, not simply the instrument.

The correct temperature instrument depends on understanding what the customer is actually trying to measure and the environment in which the instrument must operate.

Key questions include:

What temperature range must be measured?
What is the normal operating temperature?
What is the process medium?
Is the thermometer exposed directly to the process?
Is a thermowell required?
What connection size and type are required?
What stem length is required?
Is vibration present?
How important is response time?
How large does the dial need to be?
What accuracy is required?
Is the instrument installed indoors or outdoors?
What materials are appropriate for the application?
Does the customer require local indication only, or remote/electronic measurement?

These questions often reveal that the most important decision is not simply liquid-in-glass vs. bimetallic.

The real question is:

Which temperature measurement technology provides the best combination of accuracy, durability, readability, response, installation flexibility, and cost for the application?

Conclusion

Liquid-in-glass and bimetallic thermometers have been used for generations because their operating principles are simple, dependable, and easily understood.

Liquid-in-glass thermometers use the thermal expansion of a liquid to produce a direct temperature indication. They offer simplicity and a straightforward measurement method, making them useful where the installation environment is relatively protected.

Bimetallic dial thermometers use the differential expansion of two bonded metals to create mechanical movement that is translated into pointer rotation. Their rugged construction, excellent readability, and flexible installation options make them particularly well suited to industrial and commercial applications.

Neither technology should be selected solely on price or appearance.

The best thermometer is the one correctly matched to the application.

By considering temperature range, process conditions, materials, installation, response time, accuracy, readability, vibration, and maintenance requirements, engineers and users can select a temperature instrument that provides reliable information throughout the life of the equipment.

At Merit Instruments, this application-first approach is central to how we help customers select pressure and temperature instrumentation. The objective is not simply to supply an instrument—it is to provide the right instrument for the job.

Technical Reference Summary

Liquid-in-Glass Thermometer

Operating principle: Thermal expansion of a temperature-sensitive liquid

Primary indication: Liquid column

Power requirement: None

Primary strengths: Simplicity, direct indication, passive operation

Primary considerations: Glass construction, readability, installation, mechanical protection.

Bimetallic Dial Thermometer

Operating principle: Differential thermal expansion of two bonded metals

Primary indication: Mechanical pointer and dial