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Production Leak Testing by the Numbers
Leak rates and units, flow through leaks, pressure decay, temperature, tracer gases, the helium detector, reject limits, capability and cost
How to specify, test and control leak-tightness in production, from leak rate units and pressure decay to helium tracer testing, reject limits, capability and cost, worked with numbers.
Start readingAbout this book
Nothing is perfectly leak-tight. Every joint, casting and seal passes some gas, and the question a manufacturer must answer is not whether a part leaks but whether it leaks more than its function can tolerate. A hydraulic oil cooler that passes a few cubic centimetres of air a day will never weep oil; one with a crack a fraction of a millimetre long will leave a puddle under the tractor. Leak testing turns that functional question into a number, measures each part against it, and does so fast enough to keep up with production.
This book shows how to do that with numbers. It starts with what a leak rate is and the units it is stated in, and how gas flows through the tiny channels that make up real leaks. It then works through the pressure decay test, its great enemy, temperature, and the flow and differential methods that improve on it. It explains tracer gas methods and the helium mass spectrometer detector, whose heart is an ion bent into a circle by a magnetic field. It finishes with the decisions: setting a reject limit with calibrated leaks and uncertainty, keeping the station capable over time, and choosing a method on cycle time and cost.
It is for manufacturing, quality and test engineers, product designers, buyers and production supervisors. It assumes engineering or trade training and comfort with the gas laws and basic statistics; the physics of flow and of the detector is explained as it is used. Each chapter works the running case with real numbers, includes a plausible mistake and how it was found, links the BIZOMAT tools that do the chapter’s calculations, and ends with an exercise and its worked answer.
The running case
Every chapter follows one product line. This is an illustration. A manufacturer in Ballarat, Victoria, brazes aluminium bar-and-plate oil coolers for agricultural machinery. Its OC-40 cooler carries hydraulic oil at up to 10 bar and must not weep oil over the tractor’s life.
- The part: a vacuum-brazed aluminium core with two cast end tanks; oil-side internal volume 0.85 L; about 600 made per shift.
- The specification: the customer’s drawing requires the oil side to leak no more than 1 × 10⁻³ mbar·L/s of air at 10 bar gauge.
- The test line: every cooler passes a pressure decay test with air at 6 bar gauge to catch gross leaks, then a helium test in an evacuated chamber, with the oil side filled with helium.
- The people: a test engineer who owns the two stations, production operators who run them, and a quality engineer who sets and audits the limits.
Volumes, pressures, leak rates, temperatures, cycle times, costs and results are the manufacturer’s working figures or are calculated from stated assumptions. They do not describe BIZOMAT work or any real manufacturer, customer, equipment maker or gas supplier. Prices are in Australian dollars excluding GST.
How to use the book
Read Chapters 1 and 2 first: they define leak rates and explain the flow through leaks that every method depends on. Chapters 3 to 5 cover pressure-based methods and should be read in order. Chapters 6 and 7 cover tracer gases and the helium detector. Chapters 8 to 10 bring the decisions together: limits, capability and choosing a method.
Every calculation in the book was checked by running it, and the BIZOMAT tools linked from each chapter were checked against the calculations they perform. Where a tool was built for a different purpose but does one step of a chapter’s calculation, such as the closed gas vessel tool’s pressure change with temperature, the chapter says which output is used.
Source limits and professional responsibility
This book is educational. Leak testing of pressure equipment, gas-carrying and refrigeration components, medical devices and safety-related parts is governed by product standards, regulations and customer specifications that set their own methods, limits and records, and by the competent people responsible for them. Tracer gases are used under pressure and in enclosed spaces, and hydrogen mixtures need their own safety assessment. The book names the standards it refers to by number and scope only; they must be read in full before use.
Who this book is for
Manufacturing, quality and test engineers, product designers, buyers and production supervisors who specify leak-tightness, choose leak test methods, set reject limits or run leak test stations.
What you will learn
- State a leak specification in consistent units, relate it to the product's function, and convert between pressure–volume, volume and mass units.
- Explain how gas flows through real leaks, why different gases and pressures change the rate, and what that means for converting a test result to service conditions.
- Design a pressure decay or flow test, predict its signal and its temperature error, and decide when it cannot reach the specification.
- Explain how tracer gas methods and the helium mass spectrometer work, and choose between sniffing, accumulation and vacuum chamber testing.
- Set reject limits with calibrated leaks and uncertainty, monitor the station's capability, and compare methods on cycle time and cost.
Inside this book
Chapters
- 01Leak rates, units and specificationsWhat a leak rate measures, the pressure–volume, volume and mass units it is stated in and how to convert them, how big real leaks are, and how to turn a product's function into a leak specification.
- 02How gas flows through a leakMean free path and Knudsen number, viscous, molecular and choked flow through leaks, how leak rate changes with pressure and gas, and converting a helium result to air correctly.
- 03The pressure decay testHow a pressure decay test turns a pressure drop into a leak rate, test volume and time, measuring the volume, what the test can resolve, and limits that do not transfer between parts.
- 04Temperature, the hidden leakWhy hundredths of a kelvin swamp a pressure decay test, fill heating and its time constant, warm parts, compensation and its limits, and a line move that caused 18% false rejects.
- 05Differential tests, flow tests and reference leaksHow differential decay and mass flow testers improve on direct decay, what a master part cancels, reference leaks for setting and proving a tester, and a master that leaked.
- 06Tracer gas methodsWhy a tracer gas beats temperature, helium and hydrogen as tracers, filling the part, sniffing, accumulation and vacuum chamber techniques, background, and a vent that put helium into the test cell.
- 07The helium mass spectrometerHow a helium detector ionises gas and bends ions in a magnetic field, why it needs a vacuum, its calibration and response time in a chamber, and a shortened cycle that read leaks low.
- 08Setting the reject limitTurning a leak specification into a station threshold: gas conversion, an uncertainty budget, a guard band and its false-reject cost, and why retesting until a part passes defeats the limit.
- 09Capability and control of a leak testWhy leak data are log-normal and mixed, why a Cpk misleads, p-charts of reject rates, charting the reference leak, what a customer audit can show, and a capability claim corrected.
- 10Choosing a method, cycle time and costMatching leak test methods to limits and line rates, cycle time against takt, helium use and cost, appraising a recovery system, and a gas budget that counted the part instead of its fill.
Apply This in Practice
Use these BIZOMAT tools to try the method from this reading with your own numbers.
Explained in this book
Measurement Uncertainty Budget (Combined, Expanded, Test Ratio)
Shows the root-sum-square uncertainty budget and guard-banded acceptance for a dimensional gauge; the chapter applies the same method in percentages to set a helium station's reject threshold.
Used in Setting the reject limit
Explained in this book
Process Capability Check (σ from R̄, Cp, Cpk, Rejects, Limits)
Computes Cpk from subgroup data; on log leak readings it gives 2.83 against the threshold, which the chapter uses to show why a capability index misleads for a mixture of sound and defective parts.
Applies this method
Acceptance Sampling Probability (OC Curve)
Gives the probability that a customer's 50-part audit with no failures allowed accepts a delivery at a given defective share, showing what such an audit can and cannot verify after 100% testing.
Applies this method
Closed Gas Vessel Temperature Check (Pressure Rise, Contents, Fill Limit)
Gives the pressure change of a sealed volume of gas with temperature: 0.2 K in a cooler at 6 bar gauge moves the pressure by 0.068%, about 4.8 mbar, ten times a pressure decay reject limit.
Used in Temperature, the hidden leak
Applies this method
Compressed Air Leak Cost
Its choked-flow leak rate from a hole's diameter and line pressure gives the gross leak a pressure decay screen sees: a 0.1 mm hole at 6 bar passes about 7 mbar·L/s of air.
Used in How gas flows through a leak
Applies this method
Equipment Investment Appraisal (NPV, IRR, Payback, B/C, EUAC)
Appraises a helium recovery system from its cost, yearly gas saving, running cost and life, giving NPV, IRR and payback at two helium prices to test the purchase.
Applies this method
Gas Mixture Properties (Dalton: Molar Mass, Gas Constant, Specific Heats, Density, Partial Pressures, Mixing Entropy)
Works the partial pressures of a helium fill left mixed with the air in an unevacuated part by Dalton's law, showing the 90.8% helium fraction that would make every reading 9% low.
Used in Tracer gas methods
Applies this method
Gas Volume at Pressure and Temperature
Converts the test volume filled at 6 bar gauge to free air, giving the air used per pressure decay test, and the helium fill at 10 bar gauge to the 9.24 L of free gas each part consumes.
Used in The pressure decay test
Applies this method
p-Chart Check (Fraction Defective, Control Limits, Samples Out of Control)
Charts the share of parts rejected per shift with binomial control limits, flagging the shift with 11 helium rejects in 600 that traced to a brazing furnace door seal.
Applies this method
Pressure and Stress Converter
Converts the pressure part of leak rate units between pascals, millibar, bar, atmospheres and torr, the step behind converting Pa·m³/s, mbar·L/s and atm·cm³/s for a leak specification.
References and source notes
The manuscript and worked examples are original BIZOMAT writing. References inform particular concepts; their authors do not endorse this book. Web references were consulted on 3 November 2026.
- R. Fitzpatrick, University of Texas at Austin, lecture notes. Introduction to Plasma Physics: A Graduate Level Course (2006). Copyright work, used only as a map of ideas: the gyration of charged particles in a magnetic field and its radius, ionisation and collisions in low-density gases. No text, equations as set out there, figures or problems are reproduced, and all cases and calculations are BIZOMAT originals.
- C. N. Jackson Jr., C. N. Sherlock and P. O. Moore (technical editors), American Society for Nondestructive Testing. Leak Testing, Nondestructive Testing Handbook, third edition, volume 1 (1998). Background for leak rate units, gas flow through leaks, pressure change and flow methods, tracer gas methods and helium mass spectrometer leak detectors used throughout.
- International Organization for Standardization. ISO 20484, Non-destructive testing — Leak testing — Vocabulary (2017). Named by number and scope only: terms and definitions for leak testing. Its definitions are not reproduced.
- International Organization for Standardization. ISO 20485 and ISO 20486, Non-destructive testing — Leak testing — Tracer gas method; Calibration of reference leaks for gases (2017). Named by number and scope only: tracer gas test techniques and the calibration of reference leaks. Their requirements are not reproduced and must be read in full.
- National Institute of Standards and Technology. NIST/SEMATECH e-Handbook of Statistical Methods (2012). Background for the gauge study, uncertainty, capability, control chart and sampling methods used in Chapters 8 and 9 and in the BIZOMAT tools linked from them.