The core working principle relies on filling fluid sealed inside the remote diaphragm, capillary and sensor chamber to transmit process pressure from the measuring diaphragm to the differential pressure sensing unit. Silicone oil is the most mainstream filling medium for dual-flange transmitters, while high-temperature silicone oil is specially developed for harsh high-temperature process conditions. The property of filling fluid directly determines measurement accuracy, temperature drift performance and service life of the whole instrument. Improper selection of filling fluid will lead to signal deviation, vaporization, viscosity surge and permanent instrument failure. This paper introduces the transmission mechanism of dual-flange transmitters, compares the physical properties and applicable scenarios of standard silicone oil and high-temperature silicone oil, analyzes common field failures caused by filling fluid mismatch, and summarizes key criteria for filling fluid selection. It provides practical reference for instrument engineers in process design, equipment tendering and on-site maintenance.
Introduction
Dual-flange level transmitters adopt double remote diaphragm seals connected by stainless steel capillary tubes. The two flanges are mounted on upper and lower measuring points of the tank respectively. When liquid level changes, hydrostatic pressure acts on the diaphragm, and the pressure signal is transferred through the internal filling fluid to the DP sensor. Without qualified filling liquid, pressure transmission cannot be realized.
Standard silicone oil is the general-purpose filling fluid for most dual-flange applications. It features stable chemical inertia, low volatility and proper viscosity. Its typical working temperature range is -40°C to 205°C. It can resist most neutral media and mild corrosive environments, and maintain stable fluidity under normal temperature fluctuations. It is widely used for measuring water, light oil, low-viscosity chemical solutions in conventional tank level monitoring. However, under high-temperature process conditions exceeding 205°C, ordinary silicone oil will gradually deteriorate. High temperature accelerates oxidation, increases thermal expansion, causes obvious measurement drift, and may generate bubbles inside the capillary. Once vapor bubbles form in the sealed filling circuit, pressure transmission is interrupted, resulting in unstable level readings.
High-temperature silicone oil is engineered to solve the above high-temperature pain points. Typical grades such as DC704 high-temperature silicone oil can operate stably up to 315°C. It has extremely low saturated vapor pressure, excellent thermal oxidation resistance and a lower thermal expansion coefficient compared with conventional silicone oil. Even under continuous high temperature and thermal cycling, it is less likely to vaporize or form bubbles. It is the preferred filling fluid for high-temperature reactors, hot oil storage tanks, thermal medium systems and petrochemical process vessels.
Key Performance Comparison and Application Boundary
Thermal expansion is one of the most critical indicators. All silicone oil expands when heated; volume change will create extra pressure on the sensor and bring temperature drift. High-temperature silicone oil has been optimized to reduce this effect. Meanwhile, viscosity variation is another factor. At low temperature, viscosity rises and slows down pressure response. At high temperature, low viscosity ensures fast signal transfer.
Chemical compatibility must be checked before selection. Silicone oil is inert to most non-solvent process media. If the measured medium can permeate or react with silicone oil through diaphragm defects, filling fluid contamination will occur. Vacuum conditions also need special attention. Under strong vacuum, ordinary silicone oil is prone to outgassing, while high-temperature silicone oil with ultra-low vapor pressure performs much better in vacuum and high-temperature combined working conditions.
Common field issues caused by wrong filling fluid selection include zero drift after temperature rise, slow response, intermittent jumping level signals, and permanent damage of diaphragm assembly. When users use standard silicone oil in high-temperature tanks, long-term operation will cause silicone oil aging and capillary blockage. Repair requires full re-filling and degassing of the diaphragm seal system, which brings high maintenance cost and production downtime.
Selection Guidelines
First, confirm the maximum and minimum process medium temperature, not only the normal operating temperature, but also transient peak temperature. If the peak temperature exceeds 205°C, high-temperature silicone oil shall be selected. Second, evaluate ambient temperature variation, especially for outdoor installation in cold regions. Even if the process side is high temperature, low ambient temperature may increase the viscosity of high-temperature silicone oil, and thermal insulation for capillaries may be needed. Third, consider working pressure and vacuum degree. Strong vacuum environments require high-temperature silicone oil with ultra-low vapor pressure. Fourth, review medium corrosiveness and diaphragm material matching.
Manufacturing process also affects reliability. During production, the diaphragm, capillary and transmitter cavity must be fully degassed before silicone oil filling. Remaining dissolved gas inside will separate into bubbles under temperature change, damaging measurement performance. The whole filling system is hermetically sealed after filling. Any leakage will lead to loss of silicone oil and complete loss of measurement function.
Conclusion
Filling fluid acts as the pressure transmission medium of dual-flange level transmitters, and silicone oil remains the dominant choice in industrial measurement. Standard silicone oil offers cost-effective and stable performance for conventional temperature scenarios below 205°C. High-temperature silicone oil, with superior thermal stability and low vapor pressure, is indispensable for high-temperature reactors, hot oil tanks and vacuum process systems. Instrument engineers need to match filling fluid according to process temperature, pressure, vacuum and medium characteristics. Correct selection of silicone oil type reduces temperature drift, avoids bubble generation and aging failure, and guarantees long-term reliable and accurate liquid level measurement. Reasonable filling fluid configuration is a simple but vital step to maximize the service performance of dual-flange level transmitters in industrial automation control.
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