The sealing principle of vacuum pressure switches

The sealing principle of vacuum pressure switches is based on the multi-dimensional coordination of material selection, structural design and process control, aiming to prevent medium leakage, ensure the accuracy of pressure measurement and the safe operation of equipment. Its core mechanism can be analyzed from the following aspects:

First, sealing requirements and failure risks

Functional requirements

Pressure isolation: Absolute isolation must be formed on both sides of the diaphragm to prevent the measured medium (such as corrosive gas, high-purity vacuum) from being mixed with the reference pressure chamber (usually atmosphere or vacuum).

Environmental protection: Prevent external contaminants (such as dust and water vapor) from seeping into the interior of the switch, which could cause electrical short circuits or mechanical jamming.

Data support: In semiconductor equipment, the leakage rate of the vacuum chamber should be ≤1×10⁻¹¹ Pa·m³/s, and the sealing performance of the switch should match it.

Failure consequence

Measurement inaccuracy: Leakage leads to a lower pressure reading, which may trigger false alarms or process interruptions.

Equipment damage: Corrosive media seep into the interior of the switch, corroding the contacts or circuit board.

Safety risk: Leakage of flammable and explosive media may cause an explosion (such as in hydrogen vacuum systems).

Second, sealing structure and materials

Static sealing design

O-ring seal

Material selection: According to the characteristics of the medium, nitrile rubber (NBR, oil-resistant), fluorine rubber (FKM, chemically resistant) or perfluoroether rubber (FFKM, high-temperature resistant/strongly corrosion-resistant) should be selected.

Installation method: Radial or axial compression is adopted, with a compression ratio typically ranging from 15% to 25% to balance sealing performance and friction.

Metal seal

Hard sealing: It achieves line contact sealing through precise metal-to-metal processing (such as V-rings, wedge-shaped rings), and is suitable for ultra-high vacuum (UHV) scenarios.

Soft metal seals: such as indium wire and copper gaskets, maintain plastic deformation ability at low or high temperatures.

Dynamic sealing design

Diaphragm seal

Welding process: Atomic-level bonding of the diaphragm and the cavity is achieved by laser welding or electron beam welding, with a leakage rate as low as 1×10⁻¹² Pa·m³/s.

Bonding and sealing: After curing with special adhesives (such as epoxy resin + metal fillers), a chemical bond is formed, and its reliability needs to be verified through thermal cycling tests.

Piston/Valve stem seal

Spring-Energized Seal: The spring-loaded polytetrafluoroethylene (PTFE) lip can form a seal at a low pressure drop and is suitable for frequent start-stop conditions.

Third, key technologies for achieving sealing performance

Surface treatment

Roughness control: The roughness of the sealing surface should be below Ra 0.1μm to avoid microscopic leakage channels.

Coating protection: Electrolytic polishing or nickel plating treatment is carried out on the stainless steel sealing surface to reduce the surface adsorption energy and prevent medium penetration.

Preload control

Bolt tightening: Apply a preload using a torque wrench or a hydraulic tensioner to ensure uniform compression of the O-ring.

Self-sealing design: For instance, the ferrule type joint achieves self-locking sealing through conical surface extrusion, and the preload force increases as the pressure rises.

Leakage detection

Helium mass spectrometry leak detection: Fill the switch with helium gas (at a pressure of 1bar), and detect the external helium concentration through a mass spectrometer. The sensitivity can reach 1×10⁻¹² Pa·m³/s.

Pressure decay test: Fill with nitrogen to 1.5 times the working pressure, hold the pressure for 24 hours, and if the pressure drop is ≤0.5%, it is qualified.

Fourth, sealing solutions for typical application scenarios

High vacuum equipment (such as electron microscopes

Sealing scheme:

The diaphragm and the cavity are welded by laser, and the welding depth is ≥0.5mm.

The electrical interface uses ceramic-metal sealing (such as Kovar alloy and alumina), with a temperature resistance up to 500℃.

Verification standard: Passed the ISO 3530 vacuum baking test (150℃/24h), and the leakage rate remained unchanged.

Corrosive media in chemical industry

Sealing scheme:

The O-ring is made of fluororubber (FKM), with A hardness of 70 Shore A and an acid and alkali resistance range of pH 0 to 14.

The dynamic seal is filled with polytetrafluoroethylene (PTFE+15% graphite), with a friction coefficient as low as 0.05.

Protective measures: The exterior of the switch is coated with a polyimide (PI) coating with a thickness of ≥50μm, which is resistant to hydrofluoric acid corrosion.

Food and pharmaceutical industry

Sealing scheme:

The seal complies with the FDA 21 CFR 177.2600 standard. The material is silicone rubber (VMQ) and there are no precipitates.

It adopts CIP/SIP online cleaning design, and the sealing surface is resistant to 134℃ high-temperature steam sterilization.

Verification method: Pass the EHEDG-certified droplet impact test to ensure no medium residue.

Fifth, sealing failure modes and prevention

The O-ring has failed

Failure causes: Compression permanent deformation (for example, the deformation rate of fluororubber reaches 25% after 24 hours at 200℃), medium swelling (nitrile rubber expands by 30% in volume after contact with hydraulic oil).

Preventive measures: Select materials based on the medium temperature/chemical properties and replace them regularly (the recommended cycle is ≤2 years).

Welding defect

Failure cause: Insufficient laser welding energy leads to incomplete fusion, or excessive energy causes cracks.

Inspection methods: X-ray flaw detection is used to inspect internal defects in welds, and penetrant testing (PT) is employed to check surface cracks.

Dynamic seal wear

Failure cause: Frequent start-stop leads to wear of the pan-plug sealing lip, and the leakage rate increases linearly.

Improvement plan: Adopt double-lip flap sealing, and automatically compensate with the lip when worn.

Sixth, the trend of optimizing sealing performance

Composite sealing technology

For example: The combination of metal seal and elastomer seal, the metal ring withstands high pressure, and the elastomer compensates for surface roughness.

Intelligent sealing monitoring

Integrate optical fiber sensors or piezoelectric films to monitor the stress/deformation of the sealing surface in real time and warn of leakage risks.

Green sealing material

Develop biodegradable bio-based rubbers (such as polylactic acid ester elastomers) to reduce environmental impact.