Airtightness Testing of Four-Core Through-Bulkhead Penetration Assemblies

Airtightness testing of four-core through-bulkhead penetration assemblies verifies that the hermetic seals around four electrical conductors remain leak-tight under the pressure, thermal, and mechanical conditions of marine, subsea, and aerospace service. This article explains the construction and leak paths of four-core penetrators, the applicable test samples, test items, and proven methods such as helium mass spectrometry, pressure decay, and bubble emission testing, with reference to IEC 60068-2-17, GB/T 2423.23, ISO 20485, EN 1593, and EN 13184.

Technical Overview

A four-core through-bulkhead penetration assembly is a hermetic feedthrough that carries four independent electrical circuits through a pressure boundary such as a ship hull, watertight bulkhead, deck, pressure housing, or containment wall. The assembly typically consists of four conductor pins sealed into a metal housing—commonly stainless steel, titanium, or nickel-iron alloy—using glass-to-metal seals, ceramic-to-metal seals, or epoxy potting compounds, and it is installed by welding, brazing, threaded mounting, or flanged bolting with gaskets or O-rings.

The purpose of airtightness (hermeticity) testing is to confirm that no measurable gas or water leakage occurs along the potential leak paths of the assembly. These leak paths include the pin-to-insulator interface, the insulator-to-housing joint, the housing-to-bulkhead weld or mounting thread, and any soldered or brazed joints inside the connector. A leaking penetrator can admit seawater into electronic compartments, destroy the watertight subdivision of a vessel, corrupt sealed instrumentation housings, or cause corrosion and pressure loss in sealed systems. For this reason, hermeticity verification is performed on 100% of production units and repeated during qualification testing after environmental and mechanical stress exposures.

Testing is generally carried out in accordance with recognized standards published by IEC, ISO, EN, ASTM, and GB, either in the manufacturer's in-house laboratory or by accredited third-party laboratories with calibrated reference leaks and traceable instrumentation.

Testing Samples

Series-Production Assemblies

Newly manufactured four-core penetrators are subjected to 100% screening before shipment. Samples must be clean, dry, fully cured (in the case of potted designs), and thermally stabilized—typically at 23 ± 5 °C—for at least one hour before testing. Connector interfaces are mated or capped so that leakage can only be attributed to the sealed body of the assembly rather than to open test ports.

Qualification and Type-Test Specimens

First-article units, samples representing new tooling, design changes, or alternative sealing materials undergo a full qualification sequence. Hermeticity is measured in the as-manufactured condition and again after thermal cycling, thermal shock, vibration, and hydraulic overpressure exposures, because fine cracks in glass-to-metal seals and potting compounds frequently develop only after mechanical or thermal stress.

Repaired, Reworked, and In-Service Units

Assemblies that have been re-soldered, re-potted, re-welded, or refurbished after service must be re-tested before return to operation. For units already installed on a hull or pressure housing, in-situ verification is possible using sniffer-probe tracer gas methods or localized bubble testing applied to the exposed side of the penetration.

Testing Items

The scope of airtightness verification for four-core through-bulkhead assemblies typically includes the following items:

• Fine leak testing by helium mass spectrometry (tracer gas method)
• Gross leak testing by immersion bubble emission
• Internal pressure decay or pressure change testing of hollow-body designs
• Back-pressurization (helium bombing) leak testing of fully sealed units
• Weld seam and mounting interface tightness verification
• Seal integrity re-verification after thermal cycling, thermal shock, and vibration
• Seal integrity re-verification after hydraulic proof pressure testing
• Post-test electrical verification, including insulation resistance and dielectric withstand, to confirm that pressure testing has not degraded the four conductor paths

Testing Methods

Helium Mass Spectrometry Fine Leak Testing

Helium leak testing per ISO 20485, GB/T 15823, and ASTM E493 is the preferred method for verifying the fine-leak tightness of hermetic four-core penetrators. Two techniques are used. In the vacuum chamber method, the assembly is exposed to pressurized helium (back-pressurization or bombing) at a defined pressure and duration, then placed in an evacuated test chamber connected to a mass spectrometer; helium escaping through a leak path is collected and quantified. In the sniffer method, the assembly is pressurized internally with helium or a helium-air mixture and a probe is traversed slowly along the seals and welds to locate leaks.

For glass-to-metal sealed penetrators, a typical acceptance criterion is a measured helium leak rate not exceeding 1 × 10⁻⁹ Pa·m³/s, roughly equivalent to 1 × 10⁻⁸ atm·cc/s. Instruments are calibrated before each test session using calibrated reference leaks traceable per ISO 20486, and the ambient helium background is measured and subtracted from the result. The bombing pressure and exposure time must be selected so that the measured signal remains above the detectable limit for the specified maximum acceptable leak rate, taking into account the allowed dwell time between pressurization and measurement.

Pressure Decay Testing

Pressure change testing per EN 13184 is applied to assemblies with an accessible internal cavity. The component is pressurized with dry air or nitrogen—commonly to 1.1 to 1.5 times the design pressure or to the value stated in the product specification—isolated from the supply, and held while the internal pressure is monitored over a defined stabilization and measurement period. The leak rate is calculated as the volume multiplied by the corrected pressure drop divided by the measurement time, with compensation for temperature drift and atmospheric variation. The method is well suited to production screening of larger flanged or threaded penetrator bodies where the internal volume is known.

Immersion Bubble Emission Testing

Bubble emission testing per EN 1593 and ASTM E515 is a robust gross-leak screen. The pressurized assembly is submerged in a transparent bath of water, often with a wetting agent, at a controlled temperature and pressure—frequently 0.1 to 0.5 MPa gauge above atmospheric, or as specified by the design pressure. The operator observes the seals, welds, and conductor exits for a defined observation period, typically one to five minutes; a continuous, steadily growing stream of bubbles indicates a rejectable leak, while isolated bubbles from trapped air are not counted. Parts should be rotated or pre-immersed to release entrained air before evaluation begins.

Differential Pressure and Mass Flow Methods

For higher-sensitivity pneumatic screening, differential pressure testing compares the test part against a matched reference volume at identical pressure and temperature. Because both sides are affected equally by thermal drift, small pressure differences caused by leakage can be resolved with greater repeatability than with simple pressure decay. Alternatively, mass-flow methods measure the flow required to maintain the test pressure, which directly indicates the leak rate. These techniques are commonly integrated into automated production test benches.

Test Sequencing, Acceptance Criteria, and Reporting

Because a large leak can mask a fine leak during bombing-based helium testing, fine and gross methods are applied in combination, following the combined-seal philosophy of IEC 60068-2-17 Test Q and its GB counterpart GB/T 2423.23. A typical production sequence is helium fine leak testing, followed by bubble emission or pressure decay testing as a gross leak check, followed by electrical verification of all four cores. Representative acceptance criteria are listed below:

• Hermetic glass-to-metal or ceramic-to-metal sealed four-core penetrators: helium leak rate ≤ 1 × 10⁻⁹ Pa·m³/s
• General watertight or potted assemblies: helium leak rate ≤ 1 × 10⁻³ Pa·m³/s, or as defined by the procurement specification
• Pressure decay testing: no pressure drop greater than the instrument measurement uncertainty over the specified hold period
• Bubble emission testing: no continuous bubble stream from any seal, weld, or conductor interface during the observation period
• Post-test electrical checks: insulation resistance and dielectric withstand within specification for all four conductors

The test report should identify the assembly part number and serial number, the method and standard applied, test pressures, temperatures, hold and observation times, calibrated instrument and reference leak data, measured leak rates, and a clear pass or fail statement for each tested item.

Applications

Airtightness testing of four-core through-bulkhead assemblies supports quality assurance across a wide range of industries:

• Shipbuilding: watertight cable penetrations through hull plating, bulkheads, and decks in commercial vessels and naval craft
• Submarine and submersible engineering: pressure hull electrical penetrators rated for deep submergence
• Underwater robotics: ROV and AUV housings, thruster feedthroughs, and sensor penetrations
• Offshore and subsea oil and gas: control module penetrators and umbilical terminations
• Aerospace: launcher and spacecraft feedthroughs separating pressurized and vacuum zones
• Nuclear and conventional power: electrical penetrations through containment structures and pressure vessels
• Vacuum technology and analytical instrumentation: four-conductor vacuum feedthroughs for signal and power transfer
• Rail and tunnel engineering: gas-tight wall penetrations in tunnels and underground stations

FAQ

What does four-core mean in a penetration assembly?

Four-core means the assembly carries four independent electrical conductors through the pressure boundary, each individually isolated and hermetically sealed, allowing up to four separate circuits for power, signal, or sensor lines to cross the bulkhead within a single housing.

Which standards govern airtightness testing of these assemblies?

Commonly applied standards include IEC 60068-2-17 (Test Q: sealing) and its Chinese counterpart GB/T 2423.23 for combined fine and gross leak testing, ISO 20485 for tracer gas leak testing, EN 1593 and ASTM E515 for bubble emission methods, EN 13184 for pressure change testing, and GB/T 15823 and ASTM E493 for helium leak testing. ASTM E432 provides guidance on selecting the appropriate leak testing method.

What helium leak rate is typically acceptable?

For hermetic penetrators with glass-to-metal or ceramic-to-metal seals, an acceptance limit of 1 × 10⁻⁹ Pa·m³/s helium is widely specified. Potted or general watertight designs commonly use limits in the range of 1 × 10⁻⁶ to 1 × 10⁻³ Pa·m³/s. The governing value must always be taken from the product or procurement specification.

When should helium testing be chosen over pressure decay or bubble testing?

Helium mass spectrometry offers the highest sensitivity, down to 10⁻¹² Pa·m³/s, and is essential for hermetically sealed designs with very small internal volumes. Pressure decay and differential pressure methods are faster and well suited to larger cavities in automated production. Bubble emission is a simple, low-cost gross leak screen and a valuable complement because it can detect the large leaks that invalidate fine-leak bombing measurements.

Can airtightness be verified after the assembly is installed?

Yes. Sniffer-probe helium testing can be performed on the accessible side of an installed penetrator, and localized bubble testing or enclosure pressure decay tests can verify the mounting interface. Where in-situ verification of critical boundaries is required, accredited third-party laboratories can provide mobile tracer gas equipment and calibrated instrumentation.

What are the most common causes of leakage in four-core penetrators?

Typical failure modes include micro-cracks or voids in glass-to-metal seals caused by thermal shock, incomplete wetting or porosity in potting compounds, pinholes in weld seams, damaged O-rings or gaskets at the mounting interface, and insufficient thread sealant. Cracks frequently appear only after thermal cycling, vibration, or hydraulic overpressure, which is why hermeticity is re-verified after environmental qualification exposures.

Does helium leak testing leave any residue on the assembly?

No. Helium is an inert, non-flammable gas that disperses rapidly and does not contaminate conductors, insulators, or electronics. However, extended helium exposure can temporarily raise the background level in subsequent tests, so adequate flushing or dwell time should be allowed between repeated measurements.