Leak and functional testing: telling a leaking part from a leaking fixture

Choosing a method against what you must actually detect

The starting point is an engineering statement of what leakage is unacceptable in service, derived from the function rather than from what the equipment happens to measure. Pressure decay is simple, inexpensive and sensitive to everything, temperature included. Mass flow measurement stabilises faster on some geometries. Tracer gas methods reach far lower thresholds, cost more, need gas handling and recovery, and can locate a leak rather than merely reporting one. Immersion remains useful in development and is poor in production because it depends on somebody watching. Match sensitivity, cycle time and cost to the requirement instead of buying the most sensitive option available.

Temperature and volume make pressure tests lie

Filling a part with air warms it, the pressure then falls as it cools, and the instrument records that as a leak. The same physics works in reverse for a part arriving warm from an earlier process. Dead volume in the fixture magnifies the effect, ambient swings across a shift move the baseline, and a part with thin flexible walls creeps under pressure. Remedies are stabilisation time before measurement, controlled part handling where the tolerance is tight, minimising trapped volume in the test head, and differential arrangements that compare the part against a sealed reference of similar volume.

The fixture seals the part and creates leaks of its own

A test head has to seal against a real production surface carrying its own variation. Seals wear, collect debris and take a set. A part loaded slightly off centre seals imperfectly. The outcome is a reject with nothing to do with the part, and repeated rejects on sound product train operators to re-run everything, which quietly halves the value of the test. Run a fixture-only check with a sealed master at shift start and after any seal change, count seal cycles rather than replacing on failure, and record whether rejects cluster on a particular fixture, cavity or shift.

Masters and calibrated leaks: proving the test can still reject

A test unable to fail a known-bad part is worse than no test, because it manufactures confidence without evidence. Two masters are needed: a good master that must pass, and a reject master, usually a part fitted with a calibrated leak, that must fail. Run both at defined points, at minimum at shift start and after any intervention on the equipment or the fixture. Record the readings as data rather than as a tick, because drift in the reading from a good master gives early warning that something in the system is changing before any part gets judged wrongly.

What a failure means and what you do with it

A fail is information rather than merely a reject. Establish where the leak is: a tracer method with a probe, a bubble check on a sample, or sectioning where the failure repeats and defies explanation. Trend failures by cavity, fixture, shift and material lot, because a leak problem is nearly always a process problem somewhere upstream, in a seal, a weld, a moulding parameter or an assembly operation. Retain the records, since a leak result is often the evidence a customer asks for when a field failure is disputed, and a bare pass with no recorded conditions carries little weight.

Frequently asked questions

Why does the same part pass in the morning and fail in the afternoon?

Almost always temperature. The part, the fixture and the surrounding air all change through the day, and pressure-based measurement responds to all three. A part arriving warm from an earlier process behaves differently again. Look at stabilisation time, trapped volume in the test head, whether parts get a controlled cooling period before test, and whether ambient conditions near the station swing as doors open or shifts change. A differential arrangement against a reference volume removes much of the effect.

Is tracer gas testing worth the extra cost?

Where the acceptable leak is small enough that pressure methods cannot resolve it dependably, yes, because the alternative is a test that cannot see what matters. It also locates leaks, which shortens root cause work considerably. Against that it needs gas supply, recovery, containment and more maintenance, and cycles can run longer. Where a pressure method resolves the requirement comfortably with margin to spare, tracer gas is expensive precision you have no use for.

How do we prove our leak test is actually working?

Run a calibrated reject master through it regularly and record the outcome. A good master shows only that the test does not reject sound product; a known-bad part is the one that demonstrates it can still detect a fault. Add a fixture-only check to separate part leaks from head leaks, keep the calibration of the leak standard traceable, and trend the readings rather than logging pass or fail, since drift shows in the readings long before it shows in a verdict.