Helium Leak Testing of Hermetically Sealed Pyrotechnic Devices

Pyrotechnic devices used in aerospace systems are often expected to remain dormant for years and then function correctly within milliseconds when commanded. The unusual combination of long-term storage followed by extremely rapid operation makes environmental protection of the energetic material particularly important.

Initiators, pressure cartridges, boosters, detonators, and other pyrotechnic components contain energetic materials that can be affected by moisture or other environmental contaminants. For devices designed  with a hermetic enclosure, the integrity of that enclosure therefore becomes an important part of long-term reliability.

One of the most sensitive and widely used methods for verifying hermetic integrity is helium mass-spectrometer leak testing.

What Does “Hermetic” Really Mean?

In engineering terminology, a hermetic seal should not be interpreted as an absolutely impermeable barrier.

No practical enclosure is perfectly leak-free.

Instead, hermeticity is defined by an allowable leakage rate. A device is considered hermetically sealed when its measured or calculated leakage is below the limit established by the applicable specification.

Leakage paths can occur through several locations, including:

• welded or brazed joints,
• glass-to-metal seals,
• ceramic-to-metal interfaces,
• electrical feedthroughs,
• closures,
• weld defects,
• microscopic cracks, and
• imperfections introduced during manufacturing.

The purpose of leak testing is therefore not simply to determine whether a device “leaks,” but to determine whether the leakage is sufficiently small to satisfy the required environmental protection over the intended service life.

Why Helium?

Helium is an excellent tracer gas for fine-leak testing.

A helium atom is small and can migrate through extremely small leak paths. Helium is also chemically inert, present in normal air only at a very low concentration, and readily detected with high sensitivity using a mass spectrometer.

These characteristics allow helium leak detectors to identify leakage far below what could practically be detected using pressure decay, bubble testing, or similar gross-leak techniques.

ASTM E493/E493M specifically addresses mass-spectrometer leak testing of devices that are already sealed and explicitly includes pyrotechnic devices among its applications.

The Basic Helium Bombing Method

Many pyrotechnic devices are completely sealed when they arrive for final acceptance testing. There is therefore no convenient connection through which helium can simply be introduced into the internal cavity.

The solution is a technique commonly called helium bombing.

Helium Bombing → Depressurization → Dwell/Transfer → Vacuum Chamber → Mass Spectrometer Measurement

1. Helium Bombing

The sealed device is placed inside a pressure vessel, commonly called a helium bomb chamber.

The chamber is filled with helium at a specified pressure and the device remains exposed for a specified period.

If a microscopic leak path exists, helium is driven through that path and accumulates inside the device cavity.

The amount of helium entering the device depends on several parameters, including:

• leak size,
• bomb pressure,
• bombing duration,
• internal free volume, and
• gas-flow characteristics of the leak path.

The bombing conditions therefore cannot be selected arbitrarily. They form part of the test definition.

2. Removal from the Bomb Chamber

After the prescribed exposure period, the bomb chamber is depressurized and the device is removed.

At this point, a leaking device contains some quantity of helium.

Once atmospheric pressure is restored outside the device, that trapped helium begins escaping through the same leakage path through which it entered.

The clock is now important.

3. Transfer and Dwell Time

There is inevitably some time between removing the device from the helium environment and measuring it.

During this interval, helium is already escaping.

For a relatively large leak, a significant fraction of the helium may escape before measurement. For a very small leak, helium may remain inside much longer.

Consequently, bomb pressure, bomb time, internal volume, and post-bomb dwell time are all part of the leak measurement.

ASTM E493/E493M specifically warns that bombing pressure, bombing time, and dwell time must be controlled because variations can substantially affect the measured result.

4. Vacuum Measurement

The device is placed into a test chamber connected to a helium mass-spectrometer leak detector.

The chamber is evacuated.

Any helium remaining inside the device that escapes through a leak enters the vacuum system and is transported to the detector.

The mass spectrometer is tuned to helium and produces a signal proportional to the helium flow reaching the instrument.

The result is typically reported as an indicated helium leak rate.

Actual Leak Rate vs. Indicated Leak Rate

This distinction is particularly important when interpreting helium bombing tests.

The leak detector measures the helium escaping from the device at the time of measurement. That value is not necessarily identical to the standardized leak rate used as the acceptance criterion.

Consider two identical leak paths.

One device has just been removed from the helium bomb chamber.

The second has been sitting for a considerably longer time before testing.

Even though their physical leak paths are identical, the second device may contain less helium and therefore produce a lower indicated signal.

The quantitative relationship between these variables is given in the leak-rate equation below, using Sₜ for indicated helium leak rate, L for actual helium leak rate, Pₑ for absolute bombing pressure, P₀ for reference atmospheric pressure, V for internal free volume, T for bombing time, and t for post-bomb elapsed time.

Thus, an acceptance procedure normally establishes an allowable indicated leak rate corresponding to the specified actual leak-rate requirement under the defined laboratory conditions.

This is one reason that copying a leak-detector acceptance number from one device to another can be inappropriate.

The internal volume matters.

Calculating the Indicated Helium Leak Rate

For a sealed device tested by helium bombing, the expected indicated helium leak rate can be calculated from the actual helium leak-rate requirement and the defined bombing and measurement conditions. For the helium-to-helium case used here, the relationship may be written as:

where:

The equation describes two competing processes. During bombing, the term [1 − exp(−(L/V)T)] represents helium entering and accumulating within the device. After the bomb pressure is released, exp(−(L/V)t) represents helium being lost through the same leak path before the measurement is completed. The pressure ratio Pₑ/P₀ accounts for the absolute helium bombing pressure.

This form is convenient because the time variables are explicitly expressed in seconds. In a practical calculator, bombing time and wait time may be entered in minutes or hours and converted internally to seconds before the equation is evaluated.

Connection to AIAA S-113 Leak-Rate Tables

AIAA S-113, Method 103, provides helium bombing parameters and maximum allowable measured helium leak rates as a function of device internal free volume. The tabulated values are applications of this same helium bombing relationship under specified bomb pressure, exposure time, and maximum post-bomb test time. The standard also permits calculation of bomb conditions and corresponding measured leak-rate limits for volumes not directly covered by the table.

This is useful when a pyrotechnic device has an internal free volume or test condition that does not correspond exactly to a tabulated case. Rather than treating the table as a collection of unrelated acceptance numbers, the engineer can calculate the expected indicated leak rate from the governing actual leak-rate requirement and the specific test parameters.

The calculator should be used as an engineering aid. The governing program, component drawing, test procedure, and applicable specification remain the controlling requirements for acceptance testing.

Internal Volume Has a Major Effect

Consider a very small initiator and a comparatively large pressure cartridge.

Suppose both have exactly the same physical leakage conductance.

During identical helium bombing conditions, the concentration and pressure history of helium inside the two devices will not necessarily be the same because their internal free volumes differ.

A small cavity can charge with helium relatively rapidly.

A larger cavity may require considerably more bombing time to achieve the same internal helium concentration.

The subsequent helium decay after removal from the bomb chamber also depends on cavity volume.

For this reason, the helium bombing parameters and acceptance threshold should be established with knowledge of the internal free volume of the particular device configuration.

The Problem of Very Large Leaks

At first glance it seems obvious that the larger the leak, the larger the helium signal should be.

Unfortunately, helium bombing does not always behave that way.

Imagine a device with a relatively large opening.

During helium bombing, helium enters very rapidly.

But when the device is removed from the chamber, the helium also escapes very rapidly.

By the time the component reaches the mass-spectrometer chamber, much of the helium may already be gone.

The detector can therefore indicate a surprisingly low helium signal even though the device has a serious seal defect.

This phenomenon creates what is sometimes called the gross-leak problem.

It is why fine-leak testing should not automatically be regarded as a replacement for gross-leak inspection or testing.

The two methods address different leakage regimes.

Gross Leak and Fine Leak Are Complementary

A robust hermeticity verification approach may therefore involve two stages:

Gross Leak Test → Helium Fine Leak Test

The gross-leak method identifies relatively large openings that could lose their tracer gas before fine-leak measurement. Typically a gross leak path can be identified with microscobic inspection. 10X magnification is usually sufficient to observe relatively large defects.

The helium mass-spectrometer test then identifies microscopic leakage paths that gross-leak methods cannot detect.

NASA, military and industry specifications usually require gross-leak testing or inspection of loaded hermetically sealed pyrotechnic devices before helium leak testing.

This sequencing is technically significant rather than merely procedural.

A Representative Aerospace Pyrotechnic Requirement

Most pyrotechnics specifications usually state that the actual leak rate for loaded, hermetically sealed pyrotechnic devices shall not exceed 5 × 10⁻⁶ std cc/s of helium at one atmosphere differential pressure and laboratory ambient temperature.

These specifications also recognize that laboratory testing produces an indicated leak rate dependent on parameters including bomb pressure, exposure time, and wait time.

This distinction between the standardized actual leak-rate requirement and the laboratory indicated leak-rate criterion is fundamental to properly specifying helium leak tests for pyrotechnic hardware.

It should also be emphasized that 5 × 10⁻⁶ std cc/s is an example but not universal limit for every pyrotechnic device. The governing program and component specification must establish the applicable acceptance requirement. Some specifications may require a smaller leak limit, such as 1 × 10⁻⁶ std cc/s.

Where Do Hermetic Leaks Occur in Pyrotechnic Devices?

For many pyrotechnic components, the energetic material itself is not the difficult part of achieving hermeticity.

Interfaces are.

A simple bridgewire initiator, for example, might contain:

Electrical Pins → Glass-to-Metal Seal → Header → Energetic Charge → Metal Housing → Welded Closure

Potential leakage paths can therefore include the glass-to-metal seal surrounding an electrical pin, the interface between the header and housing, a welded closure, or a microscopic crack generated during manufacturing.

Thermal cycling can be particularly important.

Metals, glasses, ceramics, and sealing alloys have different coefficients of thermal expansion. Repeated thermal excursions can impose stresses on interfaces that were initially leak-tight.

For this reason, leak testing is sometimes performed not only during initial manufacturing acceptance but also after environmental exposure such as thermal cycling, vibration, or mechanical shock.

A seal that survives environmental testing provides considerably more useful engineering information than one demonstrated only in the pristine manufacturing condition.

Why Hermeticity Matters to Energetic Materials

Moisture intrusion can affect energetic devices through several mechanisms.

Depending on the energetic composition and device architecture, contamination may influence:

• ignition sensitivity,
• ignition delay,
• combustion behavior,
• electrical insulation,
• bridgewire or initiator interfaces,
• corrosion of metallic components,
• energetic-material stability, and
• long-term functional reliability.

The important engineering question is therefore not simply:

“Does the device leak?”

A more meaningful question is:

“Is the leakage sufficiently low that the internal environment remains acceptable throughout the required storage and service life?”

That connects hermeticity directly to reliability engineering.

Leak Rate Is Not an Indicator of Service Life

Another common misconception is to interpret a helium leak-rate value directly as a prediction of device life.

It is not.

Leak rate is one input to the problem.

Long-term environmental ingress depends on factors including:

• cavity volume,
• dynamics environments,
• pressure history,
• temperature,
• humidity,
• contaminant partial pressure,
• material permeability,
• energetic material hygroscopic properties, and more.

The engineering purpose of the allowable leak-rate specification is therefore to establish a measurable manufacturing requirement that supports the device’s environmental and reliability objectives.

Test Equipment and Calibration

A typical helium fine-leak test system consists of:

Helium Pressure Vessel, Vacuum Chamber and Helium Mass Spectrometer Leak Detector

The leak detector must itself be calibrated using a traceable calibrated helium leak.

Background helium is also important.

Because helium can accumulate in a laboratory where bombing operations are repeatedly performed, excessive background concentration can reduce measurement sensitivity or create unstable readings.

Proper test-fixture design, calibration, vacuum integrity, cleanliness, helium management, and controlled timing are therefore essential to obtaining meaningful data.

A sophisticated leak detector cannot compensate for a poorly controlled test process.

Engineering Perspective

Pyrotechnic systems present an unusual reliability problem.

Most mechanical systems reveal deterioration gradually. A bearing becomes noisy. A pump loses efficiency. A valve begins leaking.

A pyrotechnic device may provide no such warning.

It can remain completely dormant for many years and still be expected to perform its function almost instantaneously when commanded.

That is why apparently mundane manufacturing characteristics such as weld integrity, glass-to-metal sealing, contamination control, and hermeticity become important elements of pyrotechnic system reliability.

Helium leak testing partially provides a way to verify one of those characteristics without firing or otherwise functionally consuming the device.

The test does not prove that a pyrotechnic device will function correctly.

It proves something narrower but extremely valuable: 

The integrity of the environmental barrier protecting the device is consistent with its specified hermeticity requirement. For long-life aerospace pyrotechnic systems, that is an important piece of the overall reliability argument.