Guide
How should a helium leak-rate requirement be written on a drawing for machined fittings and quick-disconnect parts?
A helium leak-rate requirement is a drawing note that fixes the largest tracer-gas leak a part may show, together with the test method, pressure, sample plan and the party that runs the test. Short answer: write the limit with its unit and gas, name the test method and the standard edition, give the test pressure and its direction, say whether every part or a sample is tested, and say who owns the tester; neither ASTM E499/E499M-11(2025) nor the OCP UQD Specification v2.0.0 (2026) sets an acceptance limit, so the number has to come from your own specification.
By Saola Manufacturing engineering team · Published · Updated
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Why is a helium leak rate not an acceptance limit by itself?
ASTM E499/E499M-11(2025) covers how to test and locate leaks and sets no acceptance limit, and the OCP UQD specification names helium test standards without giving a leak-rate number. The limit has to come from the user’s own specification, which means from your drawing.
The ASTM store lists E499/E499M, Standard Practice for Leaks Using the Mass Spectrometer Leak Detector in the Detector Probe Mode, with two methods: direct probing, which is for locating leaks and gives only a rough size, and accumulation, which collects the tracer gas in an enclosure so a leak can be measured against a calibrated leak. Its scope addresses leaks of 1 × 10-7 Pa·m³/s or greater; that figure is the lower end of what the practice covers, not a pass mark. The store shows the 2017 reapproval as historical and the 2025 reapproval as active, so cite the edition you mean.
The specification says in Section 9 that all helium leak tests shall be in accordance with ASTM E493, that the required sample size per verification test is at least five per ISO 18869:2017, and that instruments must meet that standard’s accuracy requirements. It uses helium only as an optional screening before and after shipping-temperature cycling, and its acceptance criteria are hydrostatic (no visible leakage) or a pressure-decay limit. The text of ASTM E493 was not opened for this guide, so check its current edition and the test mode it describes before copying the reference onto a drawing.
Danfoss’s UQD fact sheet (September 2026) says it guarantees 100% helium-leak testing for its couplings, and gives no rate. A “helium tested” claim without a stated rate, method and pressure is therefore not a requirement you can write onto a purchase order.
What unit and which gas does the limit refer to?
State the limit in one unit, for one named gas and one pressure difference, because the same leak gives a different rate for different gases and pressures. One mbar·l/s equals 0.1 Pa·m³/s, so the two common units differ by a factor of ten.
Leybold’s leak-measurement page uses mbar·l/s, explains that one such unit raises the pressure of a one-liter evacuated vessel by one mbar each second, and notes that the leak rate depends on the type of gas. It also says a helium standard leak rate is defined in DIN EN 1330-8. The unit conversion is derived: 1 mbar is 100 Pa and 1 liter is 0.001 m³, so 1 mbar·l/s is 0.1 Pa·m³/s. On that basis the ASTM scope figure of 1 × 10-7 Pa·m³/s is about 1 × 10-6 mbar·l/s.
The same Leybold page lists tightness classes, with water-tight below 1 × 10-2 mbar·l/s, while its basics page calls helium the only credible method below 1 × 10-6 mbar·l/s. None of the sources used here gives a conversion from a helium rate to a liquid leak for a particular seal. Treat the link between a helium limit and a coolant weep as a design-verification result, and do not let the supplier infer it from a table.
Which test mode should the drawing name?
Name the mode that matches the question: an integral test for a total pass or fail, a sniffer or spray test to find where a leak is, and accumulation where the rate must be measured against a calibrated leak. Cite the standard and edition for the mode chosen.
Leybold’s basics page describes the integral method as showing whether there is a leak, though not how many, and the local method as showing where. The sniffer page explains that a sniffer finds leaks above about 1 × 10-7 mbar·l/s, that background helium in air of about 5 ppm is the main limit, that a typical tracing speed is 1 cm/s, and that quantifying a leak with a sniffer is difficult unless the detector and probe are calibrated together. A sniffer test with a hand-held probe is therefore a locating tool that a drawing should not use for a numerical acceptance limit without a defined procedure.
For a production limit on a turned fitting or coupling body, an integral or accumulation test with a defined fixture is usually the better fit, and a sniffer is used to find the leak when a part fails. That is an engineering recommendation, not a statement from the standards. The table compares the modes.
| Test mode | Part and gas arrangement | What it tells you | Watch for |
|---|---|---|---|
| Integral (envelope or chamber) test | Part enclosed in a vessel, helium flowing in or out of the part is detected | Whether the part leaks and how much in total | Does not say where, or how many leaks |
| Local test with a sniffer probe | Part filled with helium under pressure, probe moved over joints and surfaces | Where a leak is (E499 Method A, direct probing) | Quantifying is hard; background helium limits sensitivity; detector and probe must be calibrated together |
| Local test with sprayed helium | Part evacuated and connected to the detector, helium sprayed on suspect points | Where a leak is | Needs a part and fixture that can be evacuated |
| Accumulation | Part inside an enclosure that collects escaping tracer gas | A measured rate when read against a calibrated leak (E499 Method B) | Enclosure volume, time and calibration drive the result |
What must the drawing note contain?
State the method and edition, the gas, the pressure and direction, the limit with its unit, the part state, the calibration, the test sequence, the sample plan and the report. A note that omits any of these leaves the supplier to guess.
The table lists each item with the reason for it. Pressure direction deserves a comment: the hydrostatic test in the specification applies pressure in the direction that tends to close the valve for a disconnected plug or socket, which shows how a coupling standard states direction. Part state matters just as much, since a plug alone, a socket alone and a mated pair leak through different paths, and the seals fitted at the time of test change the answer.
An example template, with the values left for the customer to supply, reads as follows.
HELIUM LEAK TEST: [STANDARD AND EDITION], [MODE], [GAS], [TEST PRESSURE AND DIRECTION]. MAXIMUM LEAK RATE [VALUE AND UNIT] AT [PRESSURE DIFFERENCE]. TEST DRY PARTS AFTER FINAL CLEAN, [SEALS FITTED OR NOT]. REPORT LOT, RATE, CALIBRATED LEAK VALUE AND DATE. [EVERY PART OR SAMPLE PLAN].
| Item | What to write | Why it matters |
|---|---|---|
| Method and edition | Standard number and edition, and the mode (integral, sniffer, accumulation) | Modes differ in what they can measure |
| Gas | Tracer gas and concentration, such as pure helium or a helium mix | The leak rate depends on the gas |
| Pressure | Test pressure, and the direction in which it is applied | A seal can leak in one direction only |
| Limit | Maximum leak rate with unit, for one stated gas and pressure difference | The standards give no limit |
| Part state | Plug alone, socket alone or mated pair; seals fitted or not; fixture description | The leak path changes with the state |
| Calibration | Calibrated reference leak, its value and calibration date, on the report | Without it the reading has no scale |
| Sequence | Before or after cleaning and surface treatment; dry parts only | Films and wet surfaces can hide leaks |
| Sample plan | Every part, or a stated sample per lot | Sets cost and risk |
| Report | Serial or lot, rate in the stated unit, date, tester, pass or fail | Evidence for the file |
Where does the machining supplier fit in helium testing?
A CNC shop controls what makes a machined part leak (sealing-face finish, thread and seal-groove dimensions, burrs and cleanliness) but the method, limit and sample plan stay on the buyer’s drawing.
Where a drawing requires a helium test, Saola confirms at the quote what it can support; any such test is arranged through a partner Saola manages, and the quote names where it is done. Three ownership models are common, and the drawing should pick one: the buyer tests at receiving; the supplier has a test laboratory run the test and ships the report; or the coupling maker tests after assembly, which is how the specification frames its verification tests, run on couplings with sample sizes set by ISO 18869 (summary page). Machined bodies without seals and valves cannot be tested the way an assembled coupling can.
What the shop can improve is the part itself: a roughness and visual limit on the sealing face, an O-ring groove cut to the seal supplier’s dimensions (the O-ring groove guide covers this), clean threads and burr-free edges. The sources of leakage in castings are different, and the cast housing leak testing guide covers porosity separately. CNC turning and milling run at the lead plant (CNC turning, CNC milling).
What should the RFQ and first-article request say about leak testing?
Ask the supplier to confirm in writing the test mode, equipment, calibration and who performs the test, to quote testing separately from machining, and to provide a sample report with the first articles. Decide at the RFQ whether the test is on every part or on a sample.
- Separate line: quote machining and leak testing as separate lines, so the cost of the test is visible and can be changed without reopening the machining price.
- Every part or a sample: the specification’s minimum of five samples applies to its qualification tests, not to production, and Danfoss states that every coupling in its UQD line is helium tested. For a volume of ten to twenty thousand parts a year, the choice is a risk and cost decision for the buyer to write down, not something the standards settle.
- First-article evidence: a report showing the rate in the stated unit, the calibrated reference leak and its date, and the part state during the test.
- Failures: how a rejected part is handled, whether it is re-cleaned and retested, and whether the lot is held.
- Change control: no change of test method, equipment or test site without notice.
Handled this way, the leak criterion is a measurable requirement in the purchase order and not a phrase on the drawing that two parties read differently.
Frequently asked
Does the OCP UQD specification give a helium leak-rate limit?
No. It names ASTM E493 for helium tests, sets at least five samples per test under ISO 18869:2017, and uses helium only as optional screening around temperature cycling. Its acceptance criteria are hydrostatic or pressure-decay based.
Which ASTM standard should the drawing cite?
Cite the one that matches the mode you choose and give its edition. ASTM E499/E499M covers probe and accumulation modes, and the OCP specification names ASTM E493 for its helium tests, so check the current status of whichever you cite.
Is a helium leak rate the same as a water leak rate?
No. The leak rate depends on the gas, and the sources used here give no conversion from a helium rate to a liquid leak for a given seal. Treat the correlation as a verification result.
Is helium testing of every part normal for couplings?
Danfoss states that it guarantees helium-leak testing of every coupling in its UQD line. Whether a machined part needs it depends on the application and risk, and the buyer should state the sample plan in the purchase order.
Can a sniffer test give a pass or fail number?
Only with care. The sniffer method is mainly for finding where a leak is, and quantifying it requires the detector and probe to be calibrated together and a controlled distance and speed, so a drawing should define the procedure.
Sources
- ASTM E499/E499M-11(2025), Standard Practice for Leaks Using the Mass Spectrometer Leak Detector in the Detector Probe Mode, ASTM International, 2025
- Universal Quick Disconnect (UQD) Specification, Version 2.0.0, Open Compute Project, 2026
- ISO 18869:2017, Hydraulic fluid power: Test methods for couplings actuated with or without tools (summary page), Genorma, ISO publication date July 2017
- Danfoss Hansen Universal Quick Disconnect (UQD) fact sheet, Danfoss, September 2026
- Basics of leak detection, Leybold knowledge pages, accessed October 2026
- How do sniffer leak detectors work, Leybold knowledge pages, accessed October 2026
- Definition and measurement of vacuum leaks, Leybold knowledge pages, accessed October 2026