Have inaccurate temperature measurements ever caused your experiments to fail or products to be scrapped? While bimetallic thermometers are widely used, prolonged operation or improper handling can easily lead to measurement inaccuracies. This article thoroughly examines the causes of bimetallic thermometer inaccuracies and provides detailed calibration guidance to help you master calibration techniques and ensure reliable measurements.

A bimetallic thermometer measures temperature by utilizing the different thermal expansion coefficients of two bonded metals. Its core component consists of two metal strips (typically steel and copper) with different expansion rates. When temperature changes, the differential expansion causes the bimetallic strip to bend. This bending, proportional to temperature change, is converted into a temperature reading through mechanical or electronic mechanisms.

Known for their simple construction, durability, and affordability, bimetallic thermometers are extensively used in:

• Industrial processes: Monitoring temperatures in chemical reactors, food processing equipment, etc.
• HVAC systems: Measuring room temperatures, pipe temperatures
• Meteorological observations: Recording air and ground temperatures
• Food safety: Measuring core temperatures of food products

Several factors can compromise measurement accuracy during operation:

• Extended use: Metal fatigue reduces the strip's elasticity
• Severe vibration: May deform or loosen the bimetallic strip
• Thermal shock: Frequent rapid temperature changes create stress
• Environmental corrosion: Rust or chemical degradation affects performance
• Over-range use: Exceeding measurement limits causes permanent deformation
• Improper storage: High humidity or extreme temperatures degrade components

Uncalibrated thermometers can lead to:

• Defective products from inaccurate process temperatures
• Safety hazards in critical applications
• Energy waste in climate control systems
• Compromised experimental data

This technique uses ice-water's stable 0°C (32°F) reference point:

Materials: Crushed ice, distilled water, insulated container

Procedure:

1. Create an ice slurry with minimal water in the container
2. Submerge the thermometer's sensing area without touching container walls
3. After stabilization (typically 3-5 minutes), verify the 0°C reading
4. Adjust calibration screws if deviation exists
5. Repeat for verification

Key considerations:

• Use distilled water to prevent impurity effects
• Maintain sufficient ice volume
• Avoid thermal interference during calibration

This approach uses water's boiling point (varies with atmospheric pressure):

Materials: Distilled water, heating apparatus, barometer

Procedure:

1. Boil distilled water in a clean vessel
2. Submerge the thermometer's sensing area
3. Record local atmospheric pressure
4. Calculate adjusted boiling point: 100°C + [(Local Pressure - 101.325 kPa) × 0.037°C/kPa]
5. Compare reading to adjusted value and calibrate accordingly

Key considerations:

• Precise pressure measurement is essential
• Ensure complete sensor immersion
• Account for altitude effects on boiling point

For thermometers used within specific ranges, this method uses reference instruments:

Materials: Certified reference thermometer, temperature-controlled bath

Procedure:

1. Set bath to target temperature (e.g., 55°C for food safety applications)
2. Co-locate both thermometers in the bath
3. Compare readings after stabilization
4. Adjust calibration to match reference values

Key considerations:

• Use high-accuracy reference instruments
• Ensure identical immersion depths
• Select stable temperature sources

• Clean the thermometer thoroughly
• Inspect for physical damage
• Gather appropriate tools (calibration wrenches, etc.)
• Select method based on application requirements

Thermometers without adjustment mechanisms or those failing repeated calibration attempts require manufacturer servicing or replacement.

• Annual calibration for general applications
• Quarterly or monthly for precision-critical uses
• Immediately after mechanical shock or extreme exposure

New instruments should always be calibrated before initial use unless accompanied by current certification. Instruments stored for extended periods or showing questionable readings also require verification.