Views: 65 Author: Site Editor Publish Time: 2026-07-06 Origin: Site
Article Summary: In industrial temperature measurement, choosing the right instrument is critical for safety and efficiency. This comprehensive guide explores the bimetal thermometer working principle, explaining how its mechanical bimetallic strip operates without external power to provide reliable, on-site temperature readings. We compare the cost-effective and highly durable bimetal thermometer with the high-precision PT100 RTD sensor across various industrial applications. Whether you need an axial, radial, or every-angle installation, this article provides essential selection criteria, installation standards, and troubleshooting tips to help engineers optimize their process monitoring.
In modern process industries—particularly across energy, water treatment, and petrochemical sectors—selecting the right temperature instruments directly impacts the safety and stability of your plant. Among the complex family of temperature sensors, there is one "evergreen" device that requires no digital signals or external power, yet operates reliably in harsh outdoor environments and areas with high electromagnetic interference: the Industrial Bimetal Thermometer.
At Jiangsu Jiechuang Science And Technology Co., Ltd., our engineering team frequently helps clients navigate the choice between a purely mechanical bimetal thermometer and a high-precision PT100 resistance temperature detector (RTD). This article breaks down the internal mechanical physics of the bimetal thermometer and comprehensively compares it with the PT100, helping you make the most practical and cost-effective selection for your engineering projects.
The core of a bimetal thermometer comes down to a classic mechanical and thermodynamic phenomenon: the engineering application of thermal expansion coefficients.
The sensing element inside the stem is a composite material made of two or more thin layers of different metals bonded tightly together.
Active Layer (High Expansion): Typically made of brass or specific alloys. This metal is highly sensitive to temperature changes and expands significantly when heated.
Passive Layer (Low Expansion): Usually an alloy like Invar, which has an extremely low coefficient of thermal expansion and barely stretches when temperature changes.
When the process temperature rises, the active layer attempts to elongate rapidly while the passive layer remains relatively stationary. This forces the entire composite strip to bend toward the low-expansion side. The radius of this curvature has a strict linear relationship with the temperature change.
To increase sensitivity and reduce the sensor's physical footprint, the bimetallic strip is typically coiled into a compact helical (spring-like) shape for industrial use.
One end of the helical coil is firmly fixed to the bottom of the stainless steel stem.
The free end is connected to a long, slender central transmission spindle.
As temperature fluctuations cause the helical coil to twist, the central spindle rotates with it, driving the gear mechanism to amplify the movement and directly driving the pointer on the dial to indicate the real-time temperature. This purely mechanical design completely eliminates the risk of measurement failure due to power outages.
In practical engineering projects, design institutes and facility owners frequently compare these two instruments. The comparison below outlines their fundamental differences based on real-world industrial applications:
Power Supply: Bimetal Thermometer requires no power; PT100 requires constant current.
Signal Output: Bimetal Thermometer offers local pointer display only; PT100 provides a resistance signal that can be transmitted remotely.
Accuracy: Bimetal Thermometer is moderate (Typically ±1.0% to ±2.5% FS); PT100 is extremely high (±0.1°C to ±0.15°C).
Auto Control: Bimetal Thermometer is not suitable; PT100 is a perfect fit (core sensor for automated systems).
Response Time: Bimetal Thermometer is slower (10-30 seconds); PT100 is faster (1-5 seconds).
Overall Cost: Bimetal Thermometer is highly economical and maintenance-free; PT100 is higher (requires transmitters and secondary instruments).
| Comparison Metric | Bimetal Thermometer | PT100 RTD Sensor |
|---|---|---|
| Working Principle | Mechanical displacement from solid thermal expansion | Linear change in Platinum (Pt) resistance value |
| Power Supply | ❌ No power required | ⚡ Requires constant current |
| Signal Output | Local pointer display only | Resistance signal (convertible to 4-20mA / HART) |
| Accuracy | Moderate (Typically ±1.0% to ±2.5% FS) | Extremely High (Up to Class A, ±0.15°C) |
| Auto Control | Not suitable | Perfect fit (Core sensor for DCS/PLC) |
| Noise Immunity | Excellent (Immune to EMI/RFI) | Average (Susceptible to equipment noise) |
| Response Time | Slower (10-30 seconds) | Faster (Often under 1-5 seconds) |
| Overall Cost | Highly economical & maintenance-free | Higher (Requires transmitters & cabling) |
When to choose purely mechanical: Heat exchanger inlets/outlets, exterior storage tanks, utility pipelines, and blind spots in remote outdoor areas without power infrastructure.
The Best of Both Worlds: If your project requires the reliability of a local mechanical dial but you also need to transmit data to a central DCS, we strongly recommend our Remote Type Bimetal Thermometer with Transmitter Output. This hybrid solution combines mechanical safety with modern 4-20mA/PT100 automation.
To accommodate various piping layouts and spatial constraints, bimetal thermometers have evolved into structural types such as the Radial type (bottom connection where the dial is parallel to the stem) and the Axial type (back connection where the dial is perpendicular to the stem).
Immersion Depth: The temperature-sensing segment must be fully immersed. The effective insertion depth must reach the mainstream of the medium, typically requiring a depth of ≥ 1/2 of the pipe diameter.
Thermowell Protection: In high-pressure, high-velocity, or corrosive media, never expose the stem directly. You must use a robust protection tube, such as our WSS Series Bimetallic Thermometer with Flanged Connection Thermowell. This allows for online gauge replacement and calibration without shutting down your process.
Range Optimization: The normal operating temperature of your process should fall within the 1/3 to 2/3 zone of the dial's full scale.
While purely mechanical gauges are highly durable, they can fail under severe conditions. Here is a troubleshooting guide from frontline engineers:
Pointer gets stuck: This can be caused by dirt, rust jamming the internal mechanism, or the transmission parts becoming deformed. You may need to disassemble the gauge head for cleaning or replace the core.
Pointer jitter: Severe pipeline vibration or media flow impact causes the pointer to fluctuate rapidly. In high-vibration areas, it is recommended to reinforce the installation or use vibration-resistant liquid-filled models.
Condensation inside the dial: Fogging typically occurs when the instrument's sealing ring ages or is damaged, allowing moisture to enter. The gauge head must be opened, dried, and resealed.
Despite the high degree of modern automation, the bimetal thermometer continues to dominate local temperature displays in process industries due to its zero energy consumption, high reliability, and excellent cost-effectiveness. As a trusted supplier, Jiangsu Jiechuang Science And Technology Co., Ltd. recommends scientifically combining them with PT100s: use bimetal thermometers as your on-site "safety sentinels" and PT100s as the "smart brains" for remote monitoring and control. For expert guidance on your next project, the team at JIECHUANG is ready to help you configure the perfect monitoring network.
At Jiangsu Jiechuang Science And Technology Co., Ltd., we don't just sell instruments; we provide comprehensive monitoring solutions. Whether you need standard mechanical gauges, customized thermowell dimensions, or integrated transmitter outputs, our factory is equipped to deliver high-quality, ISO-certified products tailored to your specific application.
⏱️ Our Promise: Send us your specifications, and our engineering team will provide a technical evaluation and a preliminary quote within 24 hours.
Q1: Can I use a bimetal thermometer in high-vibration pipelines like pump discharges? A: Standard bimetal thermometers can suffer from pointer fluctuations and internal mechanical wear under heavy vibration. For these harsh conditions, it is highly recommended to reinforce the installation or use vibration-resistant models to stabilize the pointer and protect the internal transmission mechanism from vibration-induced damage.
Q2: What is the correct insertion depth for a bimetal thermometer stem? A: To ensure an accurate measurement, the temperature-sensing element must be fully immersed in the mainstream of the fluid. As a general engineering rule, the insertion depth should be at least half the diameter of the pipeline (≥ 1/2 pipe diameter). Inserting it too shallowly will result in inaccurate readings.
Q3: Why is there condensation or fog inside the dial glass of my thermometer? A: Fogging typically occurs when the instrument's sealing ring ages or fails, allowing moisture from the environment to enter the casing. To resolve this, the gauge head must be opened, thoroughly dried, and resealed properly with new sealing rings.
Q4: Why did my bimetal thermometer pointer stop returning to zero or become permanently inaccurate? A: This is usually caused by over-temperature exposure. If a bimetal thermometer is used beyond its maximum scale limit for a prolonged period, the internal bimetallic strip will suffer permanent deformation. This mechanical damage is irreversible and results in pointer failure, meaning the instrument core must be replaced. Always ensure your operating temperature falls between 1/3 and 2/3 of the gauge's full scale.
