Liquid-filled thermal systems (capsules or bulb-and-capillary systems) outperform bimetallic strips in high-pressure tank applications due to several key design advantages addressing the harsh environment:
1-Inherent Pressure Resistance & Isolation:
Liquid System: The sensing bulb and capillary are a hermetically sealed, robust metal vessel filled with an incompressible fluid. External tank pressure acts equally on the outside of the bulb. Since the internal fluid is essentially incompressible, this external pressure does not distort the bulb significantly or alter the fluid volume inside. The pressure signal transmitted to the diaphragm/bellows solely reflects the temperature of the fluid inside the bulb.
Bimetallic Strip: Exposed directly to the tank pressure. High pressure can physically distort, warp, or compress the strip assembly itself. This mechanical stress directly alters its curvature vs. temperature characteristic, causing significant calibration drift, hysteresis, or even mechanical failure.
2-Elimination of Pressure-Induced Stress & Fatigue:
Liquid System: The critical sensing element (diaphragm/bellows) is located outside the high-pressure zone, typically in a control head at ambient pressure. It only experiences the pressure generated by the thermal expansion of the fluid, not the tank pressure itself.
Bimetallic Strip: Must be physically strong enough to withstand the full tank pressure while still flexing accurately with temperature. Constant high-pressure cycling subjects the strip and its mounting points to significant mechanical stress, leading to material fatigue, creep, and eventual failure or permanent deformation.
3-Superior Accuracy & Stability:
Liquid System: Compensation techniques (like dual capillaries or specific bulb designs) can effectively neutralize the minor influence of ambient temperature changes on the capillary tube. The primary sensing mechanism (fluid expansion) is highly linear and stable. Calibration drift due to pressure is minimal.
Bimetallic Strip: Pressure-induced distortion directly changes its thermal response curve. Achieving and maintaining high accuracy (±1°C) under significant and varying pressure loads is extremely difficult. Calibration drifts significantly with pressure changes and over time due to stress.
4-Robustness & Sealed Construction:
Liquid System: The sensing bulb is a single, welded, pressure-tight unit. There are no internal joints or moving parts exposed to the process fluid. This makes it highly resistant to corrosion and leakage. Only the robust bulb needs to withstand the tank environment.
Bimetallic Strip: Requires a complex mechanical linkage to translate strip movement into a usable signal (switch, potentiometer). This linkage, its seals, and the strip itself are exposed to pressure, temperature, and potentially corrosive media, creating multiple potential failure points (binding, seal leaks, corrosion).
5-Reduced Sensitivity to Vibration/Shock:
Liquid System: The heavy, robust bulb is less susceptible to vibration. The pressure signal transmitted via the capillary is inherently damped. The external mechanism can be designed with damping.
Bimetallic Strip: Being a thin, spring-like element directly exposed, it can be highly sensitive to vibration and mechanical shock, leading to chatter (rapid on/off cycling) or inaccurate readings.
6-Remote Sensing Capability:
Liquid System: The capillary tube allows the critical control mechanism (diaphragm, switch, adjustment) to be located meters away from the high-pressure/high-temperature sensing point, protecting it from the harsh environment and simplifying access for adjustment/maintenance.
Bimetallic Strip: Must be directly inserted into the tank or high-pressure pipe, placing the entire sensing and switching mechanism in the harsh environment. Adjustment often requires accessing the high-pressure zone.
