Scrapers
Rod seals
Piston Seals
Guide rings
Back-Up rings
O-Rings / X-Rings
Flat seals
Spring-loaded seals
Radial shaft seals
Rotary seals
High pressure seals
Sealing sets
O-ring measuring towers
Turned parts
Milled parts
Turned and milled parts
Bushes or bearings
Moulded parts
Large format 3D printing
Post-processing of 3D printed parts
Heat-resistant 3D printed components
Plastic Components with Flame Retardancy
Leak Rate
Definition and Significance of the Leak Rate
The leak rate (also called the leakage rate) describes how much fluid passes per unit time through a leak or across a sealing point. A fluid here is a gas or a liquid. The decisive point is that this value is always tied to defined test conditions. In particular, this includes pressure differential, temperature, test medium, and measurement duration. Without this information, a leak rate can hardly be compared or evaluated technically.
In sealing technology, the leak rate is used to clarify whether a sealing connection achieves the required tightness and how strong a leak is under realistic or near-standard conditions. Especially with gases, a measure is frequently used that describes the gas passage as a pressure-volume change per unit time.
A widespread unit is mbar·L/s. It can be interpreted intuitively: 1 mbar·L/s corresponds approximately to a situation in which the pressure in a volume of 1 liter changes by 1 mbar within 1 second (with a suitable measurement setup). As a result, it becomes clear that the leak rate is a time-related parameter and not simply “a loss”.
Leak Rate vs. Leakage Volume
The leak rate states how fast fluid escapes. The leakage volume, by contrast, describes how much in total is lost over a period of time.
A short example illustrates this: if a seal causes a leak rate that remains approximately constant over the day, then the leakage volume follows from “rate × time”. For this reason, the rate is central in specifications and test reports, while the volume often becomes relevant only in operating scenarios (e.g., emission balance, consumption).
| Term | What is described? | Time relation | Typical use |
|---|---|---|---|
| Leak rate | Passage per unit time | direct (per second, hour, etc.) | Test requirement, comparison of sealing concepts |
| Leakage volume | Total passage over a period | indirect (integrated over time) | Daily/annual loss, emissions, media consumption |
Units and Interpretation (Practice in Sealing Technology)
Which unit is sensible depends on which medium is being tested and which measurement method is used. In sealing technology, several representations are common:
- mbar·L/s: frequently used in vacuum and leak testing of gases; describes a pressure-volume flow per unit time.
- sccm (standard cubic centimeters per minute): normalized volume flow, referenced to standard conditions (reference pressure and temperature). This matters so that values from different test rigs become comparable.
- cm³/h or L/min: practice-oriented in applications, for example in flow or consumption considerations.
For interpretation, the following applies: a leak rate is only reliable when it is clear under which conditions it was measured. Even a different temperature or a different test gas can noticeably change the measured value. In specifications, therefore, alongside the number, it should always be stated at which pressure, which temperature, and which medium the value applies.
Types of Leakage: Bypass Leakage and Permeation
In sealing technology, it is technically important to distinguish where a measured leak rate originates. Two main mechanisms occur particularly often:
Bypass leakage means that the medium flows past the sealing element. This happens through gaps or defects. Common causes are assembly errors, wear, deformation, insufficient preload, or surface defects on the sealing faces.
Permeation means that molecules diffuse through the sealing material. This is particularly relevant for gases and occurs to some extent in many elastomers (rubber-elastic materials). Permeation is not a “gap leak” but a material-related passage that can vary strongly with temperature and material structure.
Why the Distinction Matters
The distinction influences which countermeasure works at all. With dominant bypass leakage, design and process levers often help — for example a better controlled sealing gap or more robust assembly. With dominant permeation, by contrast, the leak rate often remains similar even when the contact pressure is increased, because the medium continues to migrate through the material.
| Mechanism | “By what” does the leak rate arise? | Effective levers (typical) |
|---|---|---|
| Bypass leakage | Flow across a gap or defect | Geometry, surface quality, preload, assembly, wear control |
| Permeation | Diffusion through the sealing material | Material choice, temperature, wall thickness/diffusion path, optionally barrier layers |
Test Conditions and Measurement Methods (Brief Overview)
A leak-rate value only becomes technically reliable when the test conditions are documented. In practice, this includes at least: pressure differential across the sealing point, temperature, test medium, measurement duration, as well as the information whether internal leakage (e.g., between two media chambers) or external leakage (toward the outside) is being considered. The question whether a measured value is true leakage or partly permeation also belongs to the classification.
For measurement, different methods are used depending on the required detection limit. Common methods include:
| Measurement principle | Brief description | Suitability (typical) |
|---|---|---|
| Pressure decay / pressure rise | Leak rate is inferred from a pressure change in the test volume | Robust production tests, medium sensitivity |
| Sniffer method | Escaping test gas is detected externally | Localization of leak points, depending on test gas |
| Helium leak testing (mass spectrometry) | Helium is used as a tracer gas; detection with a mass spectrometer | Very small leak rates, low detection limits, near-standard testing |
Which method fits depends on how small the permissible leak rate is, which medium is relevant, and whether the goal is only “pass/fail” testing or locating leak points. For very small permissible leak rates, helium is frequently used, because it is well detectable and allows highly sensitive measurement chains.
In the end: a leak rate is a precise parameter when number, unit, test conditions, and measurement method are stated together. When limit values are tight or permeation plays a role, specialized consultation on specification and test concept can be sensible.
