Bimetallic thermometers are widely used in industrial environments due to their simple structure, reliability, and independence from external power sources. However, they can suffer from various types of damage over time, which may lead to inaccurate readings or complete failure. This article outlines five common categories of damage and provides detailed explanations and preventive measures for each.
1. Mechanical Damage: Vibration and Impact
1.1 Vibration-Induced Component Loosening
In high-vibration environments (e.g., near large rotating equipment), the internal screws, gears, and pointer shafts can gradually loosen or wear out.
Typical issues:
Loosening of fasteners and internal misalignment
Shaft fatigue and eventual breakage
Pointer sticking or erratic movement
Example: A thermometer installed without a vibration-dampening pad showed pointer failure after several months of operation next to a compressor.
1.2 Impact Damage During Handling
Rough handling during installation or maintenance can cause:
Cracked glass covers
Bent or detached pointers
Broken weld joints on the bimetallic coil
These failures compromise both the mechanical integrity and the accuracy of the instrument.
2. Environmental Factors: Corrosion and Temperature Extremes
2.1 Corrosion-Induced Failure
In coastal or chemical plant environments, salt or chemical vapors can corrode poorly protected housings.
If the enclosure is not fully sealed, moisture can penetrate, leading to:
Rusted internal gears
Oxidation of bimetallic strips
Sticky or frozen pointer mechanisms
2.2 Over-Temperature or Under-Temperature Stress
Over-range exposure (e.g., using a -50~150°C thermometer in a 300°C environment) can permanently deform the bimetal strip, causing zero-offset errors or total failure.
Low temperatures may embrittle the bimetal, making it prone to cracking or delamination, especially during sudden temperature changes.
3. Process Medium Issues: Scaling, Abrasion, and Thermal Shock
3.1 Scaling and Erosion
When measuring viscous media like asphalt or slurry:
Deposits form an insulating layer on the probe, impeding heat transfer.
Abrasive particles wear down the protective sheath, deforming it and jamming internal components.
3.2 Sudden Temperature Changes
Rapid alternation between hot and cold fluids (e.g., steam-water mixing) can cause:
Thermal cycling fatigue of the bimetal strip
Layer separation in laminated bimetal materials
This often leads to pointer drift or full detachment from the calibration mechanism.
4. Poor Maintenance Practices: Lubrication and Calibration
4.1 Lack of Lubrication and Cleaning
Dust and grime accumulation on moving parts lead to gear wear and pointer jitter.
Dirt lodged between the bimetal and case wall restricts thermal expansion.
4.2 Improper Manual Adjustments
Manually twisting the pointer to “calibrate” without proper tools or reference can damage the shaft.
Over time, this causes bore enlargement and pointer dropout, requiring full replacement.
5. Installation Errors: Immersion Depth and Orientation
5.1 Insufficient Insertion Depth
If the sensing probe is not immersed deeply enough in the medium:
The bimetal strip fails to reach thermal equilibrium with the process fluid.
This causes delayed or inaccurate readings, especially in flowing or high-thermal-gradient applications.
5.2 Incorrect Mounting Orientation
Installing a horizontal thermometer upside-down in a vertical pipe:
Introduces gravitational torque on the pointer mechanism
Leads to gradual axial wear and eventual jamming
Preventive Measures and Best Practices
| Scenario | Recommended Practice |
|---|---|
| High-vibration environments | Install with vibration isolators or brackets |
| Corrosive environments | Use stainless steel or corrosion-resistant enclosures, and verify IP rating |
| Viscous/abrasive media | Periodic cleaning and replaceable protection sheaths |
| Calibration | Perform using reference bath and standard methods, avoid manual pointer bending |
| Installation | Ensure correct immersion depth (≥2.5 times the sensor diameter) and upright mounting |
Conclusion
By understanding the common failure modes of bimetallic thermometers and implementing appropriate preventive measures, users can significantly extend the lifespan and maintain the accuracy of these instruments in challenging industrial environments.