【2026 Pressure Instruments】Key Points for Selecting Isolation Diaphragms for Pressure Transmitters
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The diaphragm of a pressure transmitter is the sensitive element that comes into direct contact with the medium being measured, and its selection directly determines the lifespan and measurement accuracy of the instrument. The key considerations in selection are corrosion resistance, wear tolerance, and thermal expansion matching.
I. Common diaphragm materials + applicable media 1. 316L stainless steel (most versatile) Applicable to: clean water, engine oil, air, weakly neutral media, ordinary wastewater Prohibited for: concentrated acids and bases, high chloride levels, highly corrosive environments, seawater Temperature tolerance: -40~120℃
2. Hastelloy HC-276: Suitable for hydrochloric acid, wet chlorine gas, sulfides, chlorinated chemical solutions, and highly acidic media. Advantages: Excellent resistance to pitting and crevice corrosion. Use with caution in the presence of concentrated strong alkalis
3. Monel: Suitable for strong alkalis, caustic soda solutions, hydrofluoric acid, seawater; Not suitable for strongly oxidizing acids
4. Titanium alloy Ti: Suitable for oxidizing acids, wet chlorine, saline water, seawater, and organic acids. Prohibited use: Absolutely avoid exposure to hydrogen-containing media or hydrogen-rich environments, as it is highly prone to hydrogen embrittlement
5. Tantalum diaphragm (for high-level severe corrosion): Suitable for concentrated sulfuric acid, concentrated nitric acid, aqua regia, and environments with high temperatures and severe corrosion. Advantages: Nearly resistant to all strong acids. Disadvantages: Completely ineffective in alkaline media, in the presence of hydrogen gas, or in high-temperature alkaline solutions
6. PTFE/fluorocarbon-coated diaphragms: Suitable for highly corrosive, viscous materials, slurries, and crystalline media. Disadvantages: Not resistant to high temperatures, prone to creep, moderate precision, and not suitable for high pressures
II. Key Points for Selection 1. Match precisely based on the corrosive medium: Acidic environments: C276 > titanium > 316L; Alkaline environments: Monel > 316L. In the presence of chloride ions, 316L should not be used; instead, choose HC276 or duplex steel. For slurries or materials that crystallize easily, use flat diaphragms with no recessed structures
3. Pressure and diaphragm thickness: Low pressure/low differential pressure – choose a diaphragm with high sensitivity; High pressure/impulsive pressure – opt for a thicker, stiffer diaphragm; Negative pressure/vacuum – a specialized vacuum-grade, flat diaphragm is necessary
4. Selection of filling fluid (oil inside the diaphragm): General use at normal temperatures: silicone oil; for low temperatures below freezing point: silicone oil designed for low temperatures; for food and pharmaceutical applications: food-grade mineral oil; for high-temperature conditions: high-temperature thermal conductivity filling fluid. Prohibition: The medium must not be miscible or permeable with the filling oil
5. Classification by structural type: Convex diaphragm: suitable for general liquid and gas applications; Flat diaphragm: appropriate for viscous substances, sludge, powders, and applications prone to clogging; Embedded diaphragm: used in high-temperature and high-pressure sealed pipelines; Flanged diaphragm: used for pressure measurement at long distances and to isolate environments with strong corrosivity