A calibration cylinder may still contain plenty of gas when its stated stability period ends. That remaining inventory does not answer the essential measurement question: can the certified component values still be relied on for the intended task?
For procurement, stability belongs alongside concentration, uncertainty and packaging from the start. A cheaper cylinder with insufficient usable validity on arrival may be a poor purchase, especially when delivery time and laboratory consumption differ from the assumptions behind the order.
Separate the target, tolerance, value and uncertainty
Messer's characterization guidance distinguishes manufacturing tolerance from uncertainty: tolerance concerns permitted deviation from the requested concentration, while uncertainty accompanies the assigned value. The stability period describes how long the certificate's value applies, within the supplier's stated uncertainty and conditions. These terms answer different questions.
The following invented example uses amount fraction in µmol/mol. It illustrates the distinctions, not an available product or recommended specification.
| Item | Illustrative value | Meaning |
|---|---|---|
| Requested concentration | 100 µmol/mol | The buyer's target |
| Manufacturing tolerance | ±5% relative | Permitted production interval: 95 to 105 µmol/mol |
| Certified value | 98.6 µmol/mol | The value assigned to this mixture |
| Expanded uncertainty | ±1% relative, k = 2 | Approximately ±0.99 µmol/mol around the assigned value |
Calibration records should use the applicable certified value rather than substitute the ordered target. The uncertainty statement also needs its coverage information; manufacturing tolerance is not a replacement.
Why some compositions need particular attention
The mechanism depends on the mixture. Linde Germany's mixture guidance identifies possible reactions with the cylinder wall, adsorption onto internal surfaces and instability of certain components under pressure. These mechanisms must be assessed for the actual components, concentrations and package.
Phase behaviour is another consideration for mixtures containing condensable components. Airgas's hydrocarbon-analysis guidance discusses predicting mixture phase behaviour to reduce calibration errors from heavy-hydrocarbon condensation. That example does not mean every calibration mixture can condense under its specified conditions.
A generic description such as “inert balance gas” therefore cannot establish the stability of every minor component. Ask for evidence relevant to the complete mixture.
Check when the period starts and what conditions apply
The clock does not necessarily start on delivery. Messer describes stability from manufacture, while Linde Germany starts its assured period from certificate issue. Confirm the dates on the actual supplier's documents and specify the minimum remaining validity required on arrival.
Review storage and use conditions, including any minimum usable pressure. As a specific example, Measurement Canada's requirements for its natural-gas reference mixtures require minimum use pressure on the certificate because composition may deviate below it. This is an application-specific requirement, not a universal residual-pressure threshold for all calibration gases.
Treat expiry as a documented decision point
Plan replacement before validity ends. If continued use is proposed, obtain the supplier's written position and check the applicable measurement method or quality requirements. Revalidation is not automatic because the cylinder is unopened or an instrument still gives a plausible response.
The EPA's 2012 traceability protocol provides one formal example of recertification involving analytical comparison and documented acceptance criteria. Its rules apply to standards within that protocol; they should not be transferred to an unrelated mixture or used to promise that any cylinder can be recertified.
A practical procurement checklist
- List components, balance gas, concentrations, units and concentration basis.
- State manufacturing tolerance separately from required uncertainty and coverage information.
- Request the stability start date, expiry date and minimum validity on delivery.
- Confirm storage, withdrawal-equipment and minimum usable-pressure conditions where applicable.
- Match cylinder quantity to expected use and arrange replacement or an agreed review before expiry.
The calibration-gas enquiry page provides the wider specification checklist. Include certificate and traceability requirements in your request for quotation, with the relevant instrument or method.
Sources and scope
Primary sources linked above were reviewed on 24 October 2026. This procurement guide does not establish universal shelf lives, prescribe operating procedures or offer recertification services. Supplier conditions and applicable method requirements govern the actual mixture. See the quality documentation overview for related purchasing evidence.