To convert pressure decay into a cable-entry leak rate, multiply the effective gas volume by the pressure drop, divide by the measurement time, then apply the reporting pressure and temperature. I first separate gas loss from cooling, volume change and fixture leakage. With those effects controlled, the result gives an average gas-loss rate for the assembly and conditions tested.
The worked example below converts a 0.2 kPa drop in a 0.500 L volume over 60 seconds into 1.0 mL/min at the stated reference conditions. A second calculation shows why cooling by only 0.3°C can produce a similar pressure fall.
Key takeaways
- Include the connected fixture and tubing in the effective gas volume.
- Establish gas-temperature stability during the measurement interval.
- Keep pressure decay, pressure-volume throughput and reference volume flow distinct.
- Report the gas, test conditions and reference conditions with the result.
- Apply the assembly’s acceptance limit and decision rule to that matching quantity.
Define the gas volume and pressure boundary
Leak rate is the amount of fluid crossing a defined boundary per unit time under stated conditions. I start by drawing that boundary around the connected gas space. A complete junction-box test includes the cover joint and every connected entry. A fixture that isolates one gland measures a narrower assembly. The leak-rate glossary explains the quantity and its reporting conditions.
Start by identifying the gas, the direction of the pressure difference and the interfaces included in the test. If the whole enclosure is under test, the cover gasket and other entries may contribute to the result. The junction-box seal integrity guide explains how to identify those boundaries.
Effective gas volume is the communicating gas space represented by the pressure measurement, including the specimen cavity, fixture and tubing inside the isolation boundary. Solid components displace gas, while connecting tubes add volume. Establish this volume by the method specified for the rig; use calibrated equivalent volume where the measurement method requires it. Cosmo Instruments’ leak-calibration explanation describes why equivalent internal volume is needed to convert a pressure response into leak-rate units.
Volume errors carry directly into this conversion. If the actual effective volume is 0.5 L but you enter 0.4 L, the calculated rate is 20% too low for the same recorded pressure drop. Establishing the volume correctly therefore comes before the conversion.
How much pressure decay can cooling produce?
I compare the pressure trace with the gas-temperature record before converting the drop. Cooling lowers pressure even when the gas quantity stays constant. NASA’s ideal-gas equation relates absolute pressure, volume, gas quantity and absolute temperature. For a fixed quantity of gas in a fixed volume, pressure changes in proportion to temperature in kelvin.
Consider a separate, leak-free example. Gas starts at 200 kPa absolute and 20°C, or 293.15 K. The calculation uses the kelvin scale; NIST’s temperature conversion adds 273.15 to a Celsius value. If it cools by just 0.3°C while the volume stays constant, the predicted pressure drop is:
That is slightly larger than the 0.2 kPa loss used in the leakage example below. Interpreting the whole drop as escaping gas would give a false leakage result.
For example, suppose a hypothetical measurement budget allows at most 0.02 kPa of apparent pressure loss from cooling at these starting conditions. The fixed-volume gas relationship gives an allowable temperature change of 293.15 × 0.02 ÷ 200 = 0.029315 K, about 0.029°C. I use this type of calculation to compare the required thermal control with the rig’s demonstrated capability. Set the actual allocation through the measurement uncertainty budget; the 0.02 kPa allowance here is illustrative.
The two hypothetical cases below start from the same conditions. One assumes gas loss at stable temperature; the other keeps the gas quantity fixed and allows it to cool.
Filling can leave the gas and specimen at different temperatures. I start the detection interval when the procedure’s stabilization criteria are satisfied, using the temperature measurements appropriate to that rig. Room temperature alone can miss continuing changes inside the specimen. A compliant enclosure or fixture can also expand after filling, increasing gas volume and reducing pressure. These effects are especially important when the measured drop is small.
NASA-STD-7012A, section 5.5, includes internal and barometric pressure, temperature, specimen volume and fixture volume in its pressure-decay method for NASA hardware. Those variables provide a useful measurement reference. For a cable entry, use the stabilization criteria, test pressure and acceptance limit established for the actual assembly and rig.
Calculate reference leak rate step by step
I calculate pressure-volume throughput first, then convert it to a volume flow at explicit reference conditions. The following model treats the pressure loss as gas loss from an ideal gas at constant effective volume and gas temperature. Fixture leakage and other background effects are controlled by the measurement procedure. Use this form for the stabilized interval to which those assumptions apply.
Record the inputs
These hypothetical air-test values describe one stabilized measurement interval. They let us follow the units and calculation; select the actual test pressure and interval through the assembly’s test procedure:
| Input | Symbol | Assumed value |
|---|---|---|
| Effective connected gas volume | 0.500 L | |
| Start absolute pressure | 200.0 kPa absolute | |
| End absolute pressure | 199.8 kPa absolute | |
| Detection interval | 60 s | |
| Constant test gas temperature | 20°C = 293.15 K | |
| Reference absolute pressure | 100 kPa absolute | |
| Reference temperature | 20°C = 293.15 K |
The measured pressure loss is , giving 0.2 kPa. The 100 kPa reference pressure defines the volume used for reporting; it is not the test pressure or the pressure difference across the seal. Record the external pressure separately to describe that test difference.
Calculate pressure-volume throughput
Pressure-volume throughput is the pressure-volume quantity lost per unit time, expressed here at the test gas temperature. For constant volume and temperature, its average magnitude is:
With litres and kilopascals, the numerical conversion is convenient: 1 kPa·L equals 1 Pa·m³. This follows from 1 kPa = 1,000 Pa and 1 L = 0.001 m³; the litre conversion is listed in NIST’s SI conversion tables.
The gas temperature belongs with this quantity. It represents pressure multiplied by volume per unit time, not a volume flow in L/min.
Convert to a reference volume flow
Reference volume flow is the gas quantity lost per unit time expressed as the volume it would occupy at a specified pressure and temperature. Applying the ideal-gas relationship gives:
For this equation, use volume in L, both pressure terms in kPa, temperatures in K and the interval in seconds. The factor 60 converts the output to L/min. The temperature ratio uses absolute temperatures, even when an instrument or calculator asks you to enter °C.
For the stated example:
I report this as 1.0 mL/min of air at 100 kPa absolute and 20°C reference conditions, averaged over 60 seconds. The test started at 200.0 kPa absolute with gas at 20°C. Keep the external pressure with that record as well, because it determines the pressure difference driving gas across the boundary. Measurement uncertainty and the applicable limit complete the acceptance decision.
The tool also returns 0.2 kPa/min as the average pressure-decay rate. That value describes the pressure trace; 1.0 mL/min describes an equivalent gas volume loss at the chosen reference conditions.
Keep the quantity, unit and reference conditions together
A comparison of two leak-rate numbers starts with the quantity, unit and reference conditions side by side. The numbers are comparable only when they describe the same quantity under matching conditions.
| Reported quantity | Example unit | What must accompany it |
|---|---|---|
| Pressure-decay rate | kPa/min | Effective volume, gas temperature, pressure conditions and interval |
| Reference volume flow | mL/min or L/min | Reference absolute pressure and temperature, gas identity and test conditions |
| Pressure-volume throughput | Pa·m³/s | Temperature at which the throughput is expressed, gas identity and test conditions |
| Mass flow | kg/s | Gas identity and test conditions; a gas-specific conversion if starting from volume flow |
For the same gas loss in the worked example, changing only the reporting reference pressure from 100 to 101.325 kPa produces about 0.987 mL/min at 20°C. Nothing about the specimen has improved. The equivalent reporting volume has changed.
I put the reference pressure and temperature beside any value described as “standard” or “normal” volume flow. NIST’s Leak Calibration Service, Appendix A, explains the different gas-flow quantities and the multiple temperatures used for standard conditions. Changing the reporting reference rescales the volume associated with a given gas loss. Changing the actual pressure difference across a seal can change the gas loss itself; that requires a corresponding test or a validated flow model.
Use absolute pressures for the calculator’s start and end inputs and for the reference pressure. A difference between two gauge readings equals the corresponding absolute-pressure difference only if their atmospheric reference has not changed. The absolute pressure also matters when checking the thermal effect shown earlier.
How do I separate joint leakage from other effects?
Pressure decay measures the net response of the connected gas space. I use the time trace, temperature record and fixture controls together to identify the most useful next check.
A connected gap at the cable seal, panel seal or fixture can allow gas to escape. Permeation carries gas through intact material; NIST’s study of permeation leak standards distinguishes flow through a physical channel from diffusion through a material barrier. NIST’s Leak Calibration Service also identifies outgassing as a pressure-measurement contribution. By gas balance, uptake into materials can lower pressure, while release into the measured space can partly mask an outward loss. The guide to gas and vapor permeation through gland seals develops these material and interface distinctions for cable entries.
The following patterns help direct an investigation. Each pattern suggests a check; the paired control or temperature measurement determines whether that explanation fits.
| Observed pattern | Contribution to examine | Useful next check |
|---|---|---|
| Pressure falls most quickly just after filling, then the slope decreases | Gas cooling or fixture/specimen settling | Compare the temperature trace and repeat using the procedure’s stabilization criteria |
| A blanked fixture shows a similar falling trace | Fixture seals, valves, tubing or instrument background | Resolve the fixture response before attributing it to the cable entry |
| Pressure continues falling after temperature and volume have stabilized | Gas escape through joints or material permeation | Use the specified localization or control test to separate the paths |
| Pressure rises or an expected fall becomes smaller | Warming or gas released into the measured space | Compare temperature and conditioning history, then check the background response |
I use measured, repeatable controls when a procedure calls for a background correction. A change in the blanked-fixture response can identify a rig problem before any cable seal is replaced. If several paths remain combined in the measurement, keep the result attached to that complete tested boundary. This makes the result useful for comparing assemblies while preserving the distinction between total gas loss and a specific leaking joint.
Compare the result with a matching acceptance limit
To judge the calculated 1.0 mL/min, pair it with the applicable assembly limit at matching conditions. The comparison needs the same gas, pressure direction, test pressure, temperature, conditioning and reporting reference. I check the method and included interfaces too. A whole-box result includes paths that a single-entry fixture may exclude, so its limit and boundary need to describe the same assembly.
A decision rule is the agreed method for accounting for measurement uncertainty when judging conformity with a limit. ILAC-G8:09/2019 explains this relationship and the use of acceptance boundaries. Apply the rule specified for the job and agree any required selection before measurement. This gives a borderline result a consistent interpretation.
Keep these details together so the calculation remains traceable:
- The specimen and included interfaces, gas identity, pressure direction, and pressure on both sides.
- The effective volume and how it was established, temperature record, stabilization criterion, and detection interval.
- The measured pressures, calculated quantity and unit, reference conditions, and relevant background checks.
- The method’s detection capability, measurement uncertainty, applicable limit, agreed decision rule, and resulting decision.
For a zero display, check the method’s resolution and established detection capability. A loss below that capability may remain unresolved, and offsetting background effects can reduce the apparent decay. Report the resolved result or applicable detection bound using the procedure’s reporting convention.
IEC 60529 classifies enclosure ingress protection using its applicable tests. Keep the gas-leak result and the IP assessment associated with their respective test methods. A production gas-leak test can support inspection when its relationship to the required performance has been validated for that assembly.
Conclusion
I turn a pressure-decay trace into a leak rate by first defining the connected volume and checking temperature, volume stability and fixture background. Then I calculate the gas loss and state the reference conditions. For the example, the result is 1.0 mL/min at 100 kPa absolute and 20°C; the separate cooling case shows why temperature belongs in the interpretation.
Use the same boundary, gas, pressure conditions and reporting basis when comparing the result with an assembly limit. Apply the agreed uncertainty decision rule, and record the result with the tested configuration so it remains useful for later inspection or troubleshooting.
FAQ
Can a pressure-decay result prove an IP rating?
An IP rating is established through the applicable ingress tests. Pressure decay measures gas behavior under its own test conditions. It can support production inspection when a correlation has been validated for the assembly, but a value in mL/min has no universal conversion to an IP code.
Can I enter gauge pressure into the calculator?
Use absolute start and end pressures as requested by the calculator. Convert gauge readings using the applicable atmospheric pressure. Two gauge readings give the same pressure difference as absolute readings when their atmospheric reference stays constant. Reference pressure and the pressure used for thermal calculations must also be absolute.
Does a zero result mean the cable entry is gas-tight?
A zero display may represent a change below the method’s resolution or detection capability. Temperature or background effects can also offset part of a loss. I report the resolved result or detection bound with the test conditions, then apply the specified limit and decision rule.
Why must the test volume include connected tubing?
The pressure reading represents all communicating gas spaces inside the isolation boundary. Connected tubing and fixture cavities contribute to the gas quantity that corresponds to a given pressure drop. Use the effective or calibrated equivalent volume required by the method, including those connected spaces.
Sources and technical references
- Cosmo Instruments — Leak calibration, accessed 2026-10-28.
- NASA Glenn Research Center — Equation of State, accessed 2026-10-28.
- NASA — NASA-STD-7012A, Leak Test Requirements, 2023, section 5.5, accessed 2026-10-28.
- NIST — Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically, accessed 2026-10-28.
- NIST — SP 250-38, NIST Leak Calibration Service, 1992, accessed 2026-10-28.
- NIST — Commercial helium permeation leak standards: Their properties and reliability, 1996, accessed 2026-10-28.
- ILAC — G8:09/2019, Guidelines on Decision Rules and Statements of Conformity, accessed 2026-10-28.
- IEC — IEC 60529:1989+AMD1:1999+AMD2:2013 CSV, Degrees of protection provided by enclosures (IP Code), accessed 2026-10-28.