Pressure decay and mass flow both use dry air, but they measure leakage differently. The right choice depends on volume, leak limit, cycle time, temperature and production requirements.
Pressure decay and mass flow are two of the most common dry-air methods used for production leak testing.
They are often discussed as if one is an old method and the other is a newer, better method.
That is not a useful way to choose.
The right method depends on the application: leak limit, part volume, test pressure, cycle time, temperature behaviour, part variation, fixture design and the measurement capability required.
InterTech itself makes the point directly: mass flow is not always the best method. When accuracy and cycle-time requirements are less demanding, pressure or differential-pressure decay may be the better economic fit. [1]
The useful question is therefore not “Which technology is better?” It is “Which measurement method best fits this part and production requirement?”
How pressure decay works
In a pressure-decay test, the part is pressurised and isolated from the pressure source. Pressure is measured, a defined period passes, and pressure is measured again.
If gas escapes, pressure falls. Leakage is inferred from the pressure change over time.
InterTech’s test-method guide describes pressure decay as an indirect measurement because the pressure change and test volume are used to calculate a leak rate. [2]
The instrumentation can be comparatively simple. For applications with moderate leak limits and less demanding cycle-time requirements, it can be a cost-effective solution.
Volume matters
For a given leak, a larger enclosed volume produces a smaller pressure change over the same period.
That means part and circuit volume affect how quickly a pressure-decay system can produce a useful signal.
InterTech notes that actual test-part and circuit volume must be known in order to calculate leak rate. Its Leak Testing 101 material also explains that larger part volume can require longer measurement time because the pressure change caused by a given leak becomes smaller. [2][3]
This is one reason a pressure-decay test that works well on a small casting may not scale neatly to a large tank or engine circuit.
Temperature can look like a leak
Gas pressure changes with temperature.
During pressurisation, air can heat through compression and then cool as it exchanges heat with the part and fixture. Part temperature, trapped ambient air and environmental change can all affect the pressure signal.
InterTech’s temperature-compensation guidance identifies adiabatic heating/cooling, trapped air and part temperature as contributors to the apparent leak signal. [4]
This matters particularly when parts arrive at the test station at different temperatures or when the test cycle begins before the air has stabilised.
Good leak testing is therefore not only about the sensor. Fill strategy, fixture volume, stabilisation time and thermal behaviour all matter.
How mass flow works
Mass-flow testing takes a different approach.
The test part is pressurised and the system measures the amount of air that must flow into the part to replace the gas escaping through leaks.
In other words, the leakage is measured as flow.
InterTech describes mass-flow leak testing as a one-step process and publishes instruments that measure leak rate directly in standard cubic centimetres per minute. [2][5]
This can reduce the dependence on calculating leakage from a pressure change between two points in time.
Why mass flow can shorten cycle time
Pressure decay requires a measurable pressure change to develop.
Mass flow can measure the replacement flow directly once the system has stabilised sufficiently for a reliable reading.
InterTech’s Leak Testing 101 Part 3 states that mass-flow methods can provide faster and accurate testing over a wider range of leak-to-volume ratios and conditions than pressure-decay approaches in appropriate applications. [6]
InterTech also markets its M1075 mass-flow platform as reducing test cycle time versus pressure decay in suitable applications. [5]
The phrase “in suitable applications” matters. A fast sensor cannot compensate for a poor fixture, unstable pressure or uncontrolled thermal effects.
Pressure stability matters in mass-flow systems too
Mass flow is not immune to the physics of the test.
Because the system is measuring small gas flows, pressure regulation, sensor range, temperature and stabilisation still matter.
A widely cited comparison from Sciemetric notes that mass-flow testing can be sensitive to supply-pressure fluctuations, while pressure-decay accuracy is tied strongly to volume and pressure/temperature stability. [7]
The practical lesson is that neither method removes the need for a properly engineered test circuit.
What about very small leaks?
At very low leak rates, dry-air methods may no longer be the best answer.
At very low leak rates, helium mass-spectrometer methods may become appropriate depending on the application, the required sensitivity and the production environment. InterTech’s current dry-air equipment is specified for leak rates as low as 0.005 sccm, so method selection should be based on the actual test requirement rather than a fixed threshold. [1][5]
That is why method selection should begin with the actual leak specification rather than a preferred instrument.
When pressure decay is a good fit
Pressure decay can be a strong choice when:
- the test volume is known and reasonably consistent
- the acceptable leak rate creates a measurable pressure change
- cycle time is not extremely tight
- part and ambient temperature can be controlled
- the application benefits from relatively simple instrumentation
- differential-pressure decay can remain a good fit in some higher-pressure applications
InterTech’s own guidance notes that differential pressure decay remains appropriate in many high-pressure applications because lower instrumentation cost can outweigh the longer test cycle. [8]
When mass flow is a good fit
Mass flow becomes attractive when:
- production cycle time is important
- a direct flow reading is valuable
- different part volumes are tested
- large volumes make pressure change slow to measure
- leak-to-volume ratios make pressure-decay measurement difficult
- the process needs strong repeatability and quantitative leak-rate data
InterTech also uses mass-flow technology in applications involving large engine circuits and different volumes, illustrating one of the method’s practical advantages when volume varies substantially. [9]
The overlooked part: fixture and test design
A good leak tester connected to a bad fixture will produce a bad production test.
Before choosing the method, define:
- What exactly is being sealed?
- Where can the fixture itself leak?
- What is the combined part-and-fixture volume?
- How quickly can the part be filled or evacuated?
- What temperature does the part arrive at?
- Does the part flex under pressure?
- Is there trapped volume?
- How repeatable is the seal?
- What cycle time is actually available?
- What GR&R is required?
These questions often matter more than whether the instrument says “pressure decay” or “mass flow” on the front.
Do not specify the instrument before the application
A useful leak-test RFQ should include:
- component description
- test medium
- test pressure or vacuum
- maximum acceptable leak rate
- internal volume
- required cycle time
- expected part temperature range
- production volume
- automation level
- PLC or communication requirements
- current test method, if one exists
That gives an applications engineer enough information to recommend a method instead of simply quoting a model.
The conclusion
Pressure decay remains useful because it is simple, familiar and cost-effective in the right operating window.
Mass flow is powerful because it can measure leakage directly and can offer advantages in cycle time, volume range and process capability.
Neither wins automatically.
The better test is the one that produces repeatable, meaningful pass/fail decisions at the required production rate for the actual component.
Sources and references
[1] InterTech Development Company – Leak Testing 101 Part 4 – Source
[2] InterTech Development Company – Leak & Flow Test Detection Methods – Source
[3] InterTech Development Company – Leak Testing 101 Part 1 – Source
[4] InterTech Development Company – Leak & Flow Test Temperature Compensation – Source
[5] InterTech Development Company – Leak Test Equipment – Source
[6] InterTech Development Company – Leak Testing 101 Part 3 – Source
[7] Sciemetric – The difference between a pressure decay leak test and a mass flow leak test – Source
[8] InterTech Development Company – Leak Testing 101 Part 2 – Source
[9] InterTech Development Company – Application for testing 4.5L, 6.8L and 9.0L engines – Source