Choose span gas near 70 to 90 % of the range in a balance gas that matches the sample, with a certificate stating concentration, uncertainty and expiry. Use clean, suitable regulators and tubing, store cylinders safely, and never use gas past its expiry. Moisture cannot be calibrated from a cylinder; dew point needs a reference method.
- Choose concentration and balance gas for a span cylinder
- Read a calibration gas certificate
- Handle regulators, tubing and storage correctly
- Know which gases cannot be calibrated from a cylinder
Choosing the gas
- Concentration: 70 to 90 % of the analyser range, or at the process set point if it matters most.
- Balance gas: the gas that makes up the rest should resemble your sample, for example nitrogen for a nitrogen process. A mismatch in background can shift some sensors.
- Zero gas: pure nitrogen for oxygen, CO2 and similar; for trace oxygen choose a zero gas with oxygen well below the analyser's lowest range.
- Accuracy grade: a higher-grade mixture costs more and states a tighter uncertainty. Match it to the accuracy you need: What accuracy do you really need from a gas analyser.
The certificate
A calibration gas certificate states the concentration of each component, its uncertainty, the date of analysis, the expiry and the traceability of the reference used. Check these against the order. A cylinder with no certificate is not a calibration gas. Record the cylinder number in the calibration record so you can trace an error to a lot.
Regulators, tubing and storage
- Use a regulator made for the gas, in stainless steel for trace and reactive gases, and purge it before use so it does not carry air into the line.
- Use stainless steel tubing for trace oxygen and similar; plastic can add oxygen or moisture: Trace oxygen measurement and air ingress: ppm done right.
- Store cylinders upright and restrained in a ventilated, secure place, away from heat.
- Close the cylinder valve after use and keep the connection capped.
Shelf life and stability
Some mixtures are less stable than others, especially at low concentrations or with reactive components, because gas can stick to cylinder walls. The certificate gives an expiry. Do not use an expired cylinder, and do not use the last bit of a cylinder, where concentration can shift.
What cannot come from a cylinder
- Moisture. Water vapour at low dew points cannot be held in a cylinder at a known level. Dew point is calibrated against a chilled mirror or a controlled humidity source; field calibration is not possible on ACCURRA-S M015, so ask us before ordering.
- Some reactive or toxic gases are impractical to store at low concentration.
Sarvesh does not claim field calibration of dew point. Discuss the approach, such as periodic checks against a reference, when specifying a moisture analyser.
- Span gas at 70 to 90 % of range, in a background like the sample.
- Check the certificate: concentration, uncertainty, expiry, traceability.
- Use suitable regulators and stainless tubing; purge before use.
- Moisture cannot be calibrated from a cylinder.
Check your understanding
Answer in your head or on paper first, then open the answer. Recalling an answer teaches more than rereading.
What three items on a certificate matter most?
The concentration with its uncertainty, the expiry date, and the traceability of the reference.
Why use a balance gas that resembles the sample?
A different background can shift some sensors, so the calibration would not represent the process.
Why can't dew point be calibrated with a cylinder?
Water vapour at low dew points cannot be held at a known concentration in a cylinder.
Key terms
Frequently asked questions
What calibration gas do I need for ACCURRA-S 3015?
A zero gas with very low oxygen and a span gas in nitrogen near the top of your range, with a certificate.
How long does a calibration cylinder last?
Until the expiry on the certificate, and until pressure falls to the cylinder maker's minimum.
Can I make my own calibration gas?
Not for a measurement that matters. Certified gas gives a traceable result.
Do I need different gas for each analyser?
Only if the ranges, gases or backgrounds differ.
