There’s an unspoken principle contained within the EPA’s “EPA Traceability Protocol for Assay and Certification of Gaseous Calibration Standards,” that is worth reading:
Standards that are certified under this protocol must remain in the cylinders in which they were originally assayed. Unassayed standards in cylinders that have been transfilled from cylinders assayed under this protocol cannot be certified as being EPA Protocol Gases.”
You’re not likely to see an EPA Protocol gas mixture in a disposable cylinder, but the EPA’s comment brings out a key element in preparing a gaseous calibration standard- the’re not batch prepared and transfilled. You can’t just assign a gas mixture the same analytical data that its source cylinder had. In other words, you can’t just carry over lab numbers.
And yet the vast majority of calibration gas standards prepared in disposable cylinders are done that way. Whatever the source cylinder is, that’s what numbers the disposable cylinder into which it was transfilled gets.
It can’t change during the transfill, right?
The disposable cylinder has exactly the same integrity and has the same inertness that the disposable cylinder has, isn’t that also true?
History might make you think otherwise.
You’re all familiar with the 17 liter cylinder, aren’t you? That’s exactly the same cylinder use for industrial propane for hand held torches.
Your 58 liter cylinder? Do you think it was specifically designed to be a high quality interface for your gas mixtures?
It was a fire extinguisher cylinder.
Pretty embarrassing isn’t it?
These cylinders and the other disposable cylinders weren’t designed to be great interfaces for your calibration gases. They were chosen because they were the right size to be portable and because they were cheap.
The surface to volume ratio in these small cylinders is horrendous. Wouldn’t you think someone would have put a little thought into the container before they started the disposable calibration standard market?