Corrugated Diaphragm Leak Tightness Testing and Hermeticity Evaluation

Corrugated diaphragm leak tightness testing verifies the hermetic integrity of thin-walled welded metal and polymer diaphragms used in pressure instruments, valves, compressors, and vacuum systems. Accredited third-party laboratories apply helium mass spectrometry, pressure decay, and bubble emission techniques referenced to ISO, ASTM, IEC, and GB standards to quantify leak rates and confirm compliance with application-specific acceptance limits. This article explains the principle, sample types, test items, methods, applications, and common questions related to corrugated diaphragm seal testing.

Technical Overview

A corrugated diaphragm is a thin, flexible membrane—typically manufactured from 316L stainless steel, Inconel, Monel, Hastelloy, titanium, Elgiloy, or engineered polymers such as PTFE—formed with concentric corrugations that provide axial flexibility while maintaining media separation. Wall thicknesses commonly range from 0.02 mm to 0.5 mm, and the diaphragm is usually joined to a flange, capsule, or housing by TIG, laser, or electron beam welding. Because the membrane is so thin, even microscopic weld porosity, inclusions, or microcracks introduced during corrugation forming can create leak paths that compromise the entire assembly.

Leak tightness testing, also referred to as hermeticity or seal integrity testing, quantifies the amount of fluid (gas or liquid) that passes through or across the diaphragm assembly per unit time. The measured leak rate, expressed in units such as Pa·m³/s, mbar·L/s, or standard cubic centimeters per second (scc/s), is compared against acceptance criteria defined by the product specification or applicable standard. For high-vacuum and hazardous-media applications, typical requirements extend to 1 × 10⁻⁷ Pa·m³/s or stricter, which demands highly sensitive tracer-gas methods rather than simple pressure checks.

Testing is generally performed by accredited third-party laboratories using calibrated reference leaks traceable to national metrology institutes, in accordance with standards such as ISO 20485, ASTM E432, ASTM E499, ASTM E493, ASTM E515, IEC 60068-2-17 (Test Q: Sealing), and GB/T 15823.

Testing Samples

Welded Metal Diaphragm Capsules

Two corrugated diaphragms welded at their periphery to form a sealed capsule, widely used in pressure transmitters, absolute pressure sensors, and filled-system thermometers. The welded seam is the primary leak path of interest.

Diaphragm Seal Assemblies

Complete chemical seal units consisting of a corrugated diaphragm welded or clamped to an upper housing with a flush flange, lower connection, and capillary fill fluid. These assemblies isolate corrosive, viscous, or hot process media from measuring instruments.

Corrugated Diaphragms for Valves, Pumps, and Compressors

Standalone metallic or polymer diaphragms used as sealing and motion-transmitting elements in diaphragm valves, metering pumps, and diaphragm compressor heads handling high-purity or toxic gases.

Hermetic Bellows and Diaphragm Seals

Corrugated metal bellows and diaphragm stacks serving as hermetic shaft seals, vacuum feedthroughs, and expansion elements, where weld-ring integrity directly determines system leak performance.

Testing Items

• Helium leak rate and hermeticity classification of the welded diaphragm assembly
• Leak detection of peripheral weld seams, port welds, and clamp joints
• Pressure decay (pressure change) leak testing at specified test pressures
• Bubble emission leak testing under water or surfactant application
• Proof pressure and hydrostatic strength verification without leakage or permanent deformation
• Burst pressure determination (destructive) to establish the ultimate margin
• Sealing performance per IEC 60068-2-17 Test Q for component-level qualification
• Permeation assessment for polymer diaphragms and gasket materials
• Leak re-verification after pressure cycling, thermal cycling, or fatigue testing

Testing Methods

Helium Mass Spectrometer Leak Detection

Helium tracer gas testing is the most sensitive and widely used method for corrugated diaphragms. In the vacuum (outside-in) configuration, the assembly is evacuated and connected to a mass spectrometer while helium is sprayed on the exterior; any gas penetrating a defect is detected at sensitivities down to approximately 1 × 10⁻¹⁰ Pa·m³/s, allowing individual leak localization. In the sniffer (inside-out) configuration, the diaphragm is pressurized with helium and a probe scans the weld seam. Procedures follow ASTM E499, ASTM E493, ISO 20485, and GB/T 15823, with system calibration using certified reference leaks in accordance with ISO 20486.

Pressure Decay Testing

The diaphragm assembly is pressurized with dry air or nitrogen to a specified level, isolated, and monitored for a defined dwell time. A pressure drop beyond the calculated threshold, compensated for temperature and volume effects, indicates leakage. This economical whole-part method suits production-line screening of diaphragm seals and valves at typical sensitivities in the range of 10⁻² to 10⁻⁴ Pa·m³/s, guided by the method-selection principles of ASTM E432.

Bubble Emission Testing

The pressurized sample is immersed in water or coated with a surfactant solution, and escaping gas forms visible bubbles at leak sites. Conducted per ASTM E515, this method provides simple visual leak localization and is often used as a preliminary or verification screen for larger defects before high-sensitivity helium testing.

Helium Bombing for Sealed Capsules

For hermetically sealed diaphragm capsules with no accessible internal volume, samples are exposed to elevated helium pressure (bombing) and then placed in a vacuum chamber. Helium that penetrated a defect escapes during evacuation and is detected by the mass spectrometer, providing a quantitative whole-part leak rate.

Proof Pressure and Burst Testing

Hydraulic or pneumatic pressure is applied at 1.3 to 1.5 times the design pressure (proof) or increased until rupture (burst) while monitoring for leakage, deformation, or failure. These destructive and semi-destructive tests validate structural margins alongside leak integrity.

Environmental and Post-Cycling Verification

Seal performance is re-measured after thermal cycling, pressure cycling, or mechanical fatigue to confirm that corrugation deformation and weld aging do not degrade leak tightness over the service life, consistent with IEC 60068-2-17 and product-specific qualification sequences.

Applications

• Process instrumentation: diaphragm seals protecting pressure transmitters, gauges, and switches from corrosive or viscous media
• Oil, gas, and petrochemical plants: isolation of sour gas, hydrocarbon, and catalyst service
• Chemical and pharmaceutical processing: hygienic diaphragm valves and seal assemblies
• Semiconductor and high-purity gas systems: diaphragm compressors, valves, and regulators
• Vacuum technology: corrugated bellows feedthroughs and vacuum valve diaphragms
• Aerospace and defense: fuel, hydraulic, and pneumatic diaphragm components
• Medical devices: infusion pumps, drug delivery systems, and sterilizable instrumentation
• Nuclear and power generation: containment-related sealing elements

FAQ

What leak rate is considered acceptable for a corrugated diaphragm?

Acceptance limits depend on the application. Industrial diaphragm seals often require better than 1 × 10⁻⁶ Pa·m³/s, while vacuum, semiconductor, and toxic-gas applications may specify 1 × 10⁻⁸ to 1 × 10⁻⁹ Pa·m³/s or stricter. The limit should always be defined by the product specification or end-user requirement.

How do I choose between helium testing and pressure decay testing?

Pressure decay is fast and economical for screening moderate leak thresholds in production. Helium mass spectrometry is selected when high sensitivity, leak localization, or very low leak-rate quantification is required. Many test plans combine both: pressure decay for 100 percent screening and helium testing for qualification and failure analysis.

Is corrugated diaphragm leak testing destructive?

Helium, pressure decay, and bubble emission tests are non-destructive when performed at or below proof pressure. Burst testing and certain fatigue-to-failure evaluations are destructive and are conducted on sacrificial samples.

How are polymer diaphragms evaluated differently from metal ones?

Polymer diaphragms exhibit inherent gas permeation, so testing distinguishes true defect leakage from material permeation using longer stabilization times and method-specific thresholds. Pressure decay and immersion bubble methods are commonly applied, with helium testing reserved for defect localization.

What sample preparation is required before testing?

Samples must be cleaned to remove oils, residues, and moisture, dried, and visually inspected for gross damage. Ports are connected with appropriate fixtures, and the internal volume is measured or calibrated to establish the correct test parameters.

What is the typical turnaround time?

Routine helium leak or pressure decay verification of prepared samples is usually completed within a few working days, while full qualification programs including proof, burst, and post-cycling leak verification may require several weeks depending on sample count and test sequence.