A leak-rate limit is meaningful only when its units, gas, reference conditions and test method are clear. Two reports can show different numbers for the same pressure-volume flow because their units differ. They can also show the same number for measurements that are not directly comparable because the gas or test conditions differ.
For a laser-welded hermetic package, the drawing and test procedure should connect the required performance to a reproducible measurement. This guide explains the units and the information needed to make a defensible acceptance decision. It does not prescribe a universal leak-rate limit for JMLASER equipment or for every package type.
Read pressure, volume and time together
Common leak-rate units include Pa·m³/s, mbar·L/s and atm·cm³/s. They express pressure multiplied by volume per unit time. Pa·m³/s is an SI expression. The uppercase L denotes a liter, and cm³ denotes a cubic centimeter, sometimes written as cc.
Pfeiffer's leak-detection guidance distinguishes these expressions from volumetric flow units such as mL/min. A volume-flow number needs a reference pressure and temperature to describe a comparable amount of gas flow. A bare cm³/s value should therefore not be silently treated as atm·cm³/s.
Pressure-volume flow is also different from pressure alone. A reading in mbar describes pressure; a reading in mbar·L/s describes a flow-related quantity. Neither is a direct measurement of a physical hole diameter.
Convert units without changing the measured quantity
The following relationships are dimensional conversions for the same pressure-volume flow. They do not convert helium leakage into air leakage or change one test method into another.
| Starting unit | Equivalent in Pa·m³/s | Equivalent in mbar·L/s |
|---|---|---|
| 1 Pa·m³/s | 1 | 10 |
| 1 mbar·L/s | 0.1 | 1 |
| 1 atm·cm³/s | 0.101325 | 1.01325 |
The arithmetic follows from 1 mbar = 100 Pa, 1 L = 0.001 m³, 1 cm³ = 0.000001 m³ and 1 standard atmosphere = 101325 Pa. NIST documents the standard-atmosphere definition. Use the full conversion factor when making a decision close to a limit; do not treat atm·cm³/s and mbar·L/s as exactly equal.
Worked example: normalizing a report
Suppose a hypothetical specification gives a limit of 1.0 × 10^-8 atm·cm³/s, and the measurement system reports 8.0 × 10^-9 mbar·L/s. Assume that the gas, method and all required reference conditions already match.
- Convert the limit: 1.0 × 10^-8 × 1.01325 = 1.01325 × 10^-8 mbar·L/s.
- Compare the reported value, 8.0 × 10^-9 mbar·L/s, with that converted limit.
- The reported value is numerically below the limit, but release still depends on measurement validity, the required decision rule and other specified tests.
This example demonstrates unit conversion only. Its numbers are not recommended package limits or JMLASER performance claims.
Distinguish helium readings from equivalent leak rates
A helium detector measures a tracer-related signal under its operating conditions. Some package procedures use that reading and additional inputs to calculate another reported quantity, such as an equivalent standard leak rate. The conversion may depend on the prescribed model and inputs including gas exposure, package free volume and timing.
Changing the display unit does not perform that physical interpretation. Likewise, a dimensional conversion cannot account for changing the test gas or pressure conditions. Gas-dependent conversions require an applicable procedure and assumptions about the flow, rather than a unit table alone.
Before comparing two reports, check what each column actually represents: the raw detector reading, a corrected helium result or a calculated equivalent rate. Keep both the raw result and the calculation inputs where required. The helium leak-testing guide explains the package and measurement conditions that need to accompany the number.
Treat standard volume-flow labels carefully
Labels such as standard cubic centimeters per minute refer to a volume flow at stated reference conditions. The word standard does not identify those conditions unless the supplier or procedure defines them.
To compare a volume-flow result with a pressure-volume-flow result, establish the reference pressure, temperature and reporting convention first. If temperature bases differ, dimensional arithmetic alone is insufficient to compare the corresponding gas quantity. Request clarification rather than replacing an ambiguous label with an assumed standard condition.
Separate detector capability from product acceptance
The smallest signal a detector can display or detect is not automatically a suitable production acceptance limit. The complete setup includes fixtures, background, response time, handling and calibration, all of which can affect the usable measurement.
NIST describes calibrated helium leak artifacts as tools for detector calibration and notes that permeation-type artifact flow depends on temperature. Use the reference artifact's certificate and applicable stabilization requirements. A detector check supports measurement validity; it does not prove that a specific package test has adequate coverage of every relevant failure mode.
Define how uncertainty and borderline results are handled in the approved procedure. Do not invent a guard band during inspection or repeatedly test a part until one reading falls below the limit.
Write a complete acceptance requirement
| Field | What the specification should identify | Why it matters |
|---|---|---|
| Product and condition | Package revision, assembled state and required conditioning | Results may depend on the tested construction and history |
| Method | Procedure identifier, revision and selected test condition | A limit alone does not define how to measure |
| Reported quantity | Measured gas rate or defined equivalent rate | Prevents comparison of different quantities |
| Limit and units | Numerical limit with an unambiguous unit | Prevents scale and conversion errors |
| Gas and conditions | Gas concentration, pressure and temperature as applicable | Establishes the physical meaning of the result |
| Package inputs | Free volume and timing where required by the method | Supports valid calculation and interpretation |
| Decision rule | Required treatment of uncertainty, invalid tests and retests | Makes disposition consistent |
| Complementary checks | Gross-leak coverage and other required inspections | Prevents a fine-leak number from becoming the whole release decision |
Use this checklist when discussing leak-test supporting equipment. For electronic packages, confirm which contractual or product requirements govern the assembly instead of assuming that a commonly cited standard applies automatically.
Review the result before releasing the package
First confirm that the instrument, fixture and test cycle were valid. Then normalize the units and apply any method-specific calculation. Compare the appropriate reported quantity with the limit using the defined decision rule. Finally, check the rest of the acceptance record.
In particular, a small fine-leak reading does not always exclude a large leak in a tracer-tested sealed package. The gross-leak versus fine-leak guide explains this coverage issue. Seam appearance, structural quality and dimensional requirements also remain separate checks.
JMLASER can use a clear leakage specification alongside drawings, materials and production requirements when discussing equipment and process solutions. A reproducible requirement is a more useful input than an isolated number copied from a detector brochure.