Pneumatic leak testing fixtures

What a leak-test fixture must decide

A production leak test answers a defined question: does this part leak more than an agreed rate, through a defined boundary, at a defined pressure? The fixture and the method must be chosen around that question, not around whichever instrument is available.

“Leak-free” is not a usable specification. Every measurement has a method, a resolution and an uncertainty. The acceptance requirement should be written in measurable terms, for example a maximum leak rate in standard cubic centimetres per minute (sccm) at a stated test pressure and temperature. The limit itself is set by the product designer or customer, not by the fixture builder or air-system supplier.

Be clear what defects the test must find:

• External leaks through walls, joints, welds or seals.
• Leakage between internal cavities.
• Gross leaks such as a missing O-ring or plug.
• Blocked passages that stop part of the product being tested at all.

A single pressure-decay reading may not separate these. A gross leak can empty the part so quickly that the instrument never reaches test pressure; a blocked port can make a faulty part look good because the leaking cavity was never pressurised.

Product leak testing is not a plant leak survey

Product leak testing qualifies manufactured parts such as valve bodies, castings, radiators, tanks, fuel components and housings. A plant leak survey finds air escaping from a compressed-air distribution system to cut energy waste. The tools overlap (ultrasonic detectors appear in both), but the decisions, limits and methods are quite different. For plant surveys, see our leak detection audit page.

Choosing the test method

Pressure containment and leak acceptance are separate requirements. A proof or burst test checks strength; a leak test checks tightness. Do not use one fixture setting to approve both unless the product specification says so. Pneumatic pressurisation stores energy, so test pressures, guarding and fixture design must follow your safety procedures and the product authority’s limits. Our page on pneumatic pressure testing boundaries covers the safety side.

Account for all the volume and the fixture’s own behaviour

In a pressure-decay test, the same leak gives a smaller pressure drop in a larger volume. The total test volume includes the part, the fixture cavities, the seals’ trapped space, the tubing, valves and the transducer. Leaving out fixture volume makes the conversion from pressure drop to leak rate wrong.

The basic conversion is:

Leak rate (volume per time at reference pressure) ≈ total test volume × pressure drop ÷ (test time × reference pressure).

For example, a 200 cm³ total volume that drops 50 Pa in 10 seconds at a reference of 101,325 Pa gives about 200 × 50 ÷ 101,325 ÷ 10 cm³ per second, which is about 0.0099 cm³/s, or roughly 0.59 sccm. If the fixture actually added another 50 cm³ that was ignored, the true leak would be about 0.74 sccm. These numbers are illustrative; your own volumes must be measured or calculated.

The fixture itself also moves. Elastomer seals compress and creep, thin-walled parts and fixture plates flex under pressure, and that volume change looks like a leak. A stiff fixture with stable seals and a defined clamping force gives a more repeatable result.

Temperature and stabilisation

When air is admitted quickly it heats by compression, and then cools towards the part temperature. Parts straight from washing, welding or machining are often warmer or cooler than the fixture. Both effects change pressure in a closed volume without any leak.

As a rough guide, for a fixed volume, absolute pressure changes in proportion to absolute temperature. At 3 bar gauge (about 4 bar absolute) and around 30 °C (about 303 K), a change of only 0.1 K moves pressure by about 4 bar × 0.1 ÷ 303, which is about 130 Pa. That can be larger than the pressure drop from the leak limit itself. This is why test sequences include fill, stabilise and measure stages, and why part temperature control matters.

1. Fill: bring the part to test pressure in a controlled way.
2. Stabilise: hold until the compression heat and seal settling have reduced to an acceptable level.
3. Test: isolate and measure pressure change or flow over a defined time.
4. Exhaust: vent safely, then release the part.

Stabilisation and test times are found by trials on known good parts, not copied from another product.

Seals, valves and replenishment paths

• Design seals for both integrity and repeatability: replaceable, protected from damage, and with a defined compression.
• Keep the isolation valve close to the part and confirm it is itself leak-tight.
• Check for paths that can top up pressure during the test, such as a leaking supply valve; these hide leaks.
• Make gross leaks and blocked paths visible with fill-pressure checks and, where needed, a flow check during filling.
• Keep fixture background leakage measured and recorded, using a sealed master part.

Calibration, validation and the decision rule

Instrument calibration checks the transducer and timer. Application validation checks that the whole station, including the fixture, can detect the leak limit reliably on the real product. Both are needed.

• Use a calibrated leak standard (a calibrated orifice or leak master) to challenge the station at or near the limit.
• Run repeatability and reproducibility studies across parts, operators and shifts.
• Check drift over time with daily master checks: a known good part and a known leaking master.
• Set the pass, fail and retest rules before production results are seen, including how measurement uncertainty is treated near the limit.

Air supply for leak-test stations

Leak-test stations use little compressed air on average, but they are demanding about air quality and pressure stability. Water and oil can contaminate parts and seals and upset the test; pressure fluctuations disturb fill and mass-flow readings.

Average air demand is small: total volume times pressure ratio times cycles per hour. Fill flow at the start of each cycle is higher, so local storage and regulator size matter more than compressor size.

Diagnosing a station before blaming the product

1. Run the known good master: if it fails, the station is at fault.
2. Check supply pressure stability and air quality at the station.
3. Inspect seals for damage, contamination and wear.
4. Check part temperature against the validated range.
5. Check the isolation valve and fittings with the master part.
6. Review recent recipe or fixture changes.
7. Only then investigate the product.

How Reddy Compressors helps

We supply and service the compressed-air side of leak-test stations: compressors, dryers, filters, regulators and receivers that give clean, dry, stable air. We are an independent multi-brand business at Tarnaka, Secunderabad, serving industry since 2002, with over 20 service engineers and technicians covering Telangana and Andhra Pradesh and supply of equipment and spares across India.

• Air-quality and dew-point checks at the point of use.
• New and used/refurbished compressors, dryers and filtration across makes such as ELGi, Atlas Copco, Ingersoll Rand, KAESER and Chicago Pneumatic; we are not an authorised dealer of any brand. We have tie-ups with all major and reputed brands.
• Service, AMC and spares for compressors and dryers.
• Rental compressors from our own fleet to keep test lines running during breakdowns.

We do not design leak-test instruments, set product leak limits or certify test stations. Those belong to your instrument supplier and product authority.

What to send us

• Number of test stations, test pressure and cycle rate.
• Air quality required: dew point, oil content, particle class.
• Current compressor, dryer and filter make, model and nameplate photos.
• Any problems seen: drifting results, water in fixtures, pressure fluctuation.
• Pressure readings at the station supply, if available.

Frequently asked questions

How does a pressure decay leak test work?

The part is filled with air to a set pressure, allowed to stabilise, then isolated. The instrument measures how much pressure falls over a fixed test time. Using the total test volume and the reference pressure, the pressure drop is converted to a leak rate, typically in sccm. Temperature changes and fixture movement also change pressure, so stabilisation and a stiff fixture are essential.

Why do my leak test results drift during the shift?

Common causes are part temperature changes after washing or machining, seal wear, supply pressure fluctuation, moisture in the air and ambient temperature swings. Run a known good master and a known leaking master at regular intervals. If the masters drift, the station or air supply is at fault; check seals, regulator stability and dryer performance before changing product limits.

What air quality is needed for leak testing?

Clean, dry air at a stable pressure. A refrigerated dryer with a typical +3 °C pressure dew point and good filtration suit many stations; moisture-sensitive products may need a desiccant dryer at a typical −40 °C dew point and oil-free air. A dedicated precision regulator and a small local receiver help keep the fill pressure stable from cycle to cycle.

How do I choose between pressure decay and mass flow leak testing?

Pressure decay suits small to medium volumes and moderate limits, and is simple and widely used. Mass flow measures the air needed to hold pressure and often suits larger volumes or higher leak rates, where pressure decay would be slow. Tight limits may need differential pressure decay or tracer gas. The product’s limit, volume and cycle time decide the method.

Does fixture volume affect leak test results?

Yes. In a pressure-decay test, the same leak produces a smaller pressure drop in a larger volume, so all volume, including fixture cavities, tubing and valves, must be included in the conversion. Flexing plates and compressing seals also change volume during the test and can look like a leak. A stiff fixture with minimal, well-defined volume gives better sensitivity.

How much compressed air does a leak test station use?

Very little on average. Each cycle uses roughly the total test volume multiplied by the absolute pressure ratio, then vents it. The important figure is the fill flow at the start of each cycle, which affects regulator and local receiver sizing. Supply stability and air quality matter far more than compressor capacity for most leak-test stations.

Related pages

• Pneumatic application engineering centre
• Pneumatic pressure testing planning boundaries
• Compressed-air quality measurement
• Dew-point measurement
• Refrigerated compressed-air dryers
• Leak detection