Sorry for the lag in posting, but I've been involved in a research project finishing up coating calibration gas cylinders to make them more stable. Knowing the problems that you all face in the field with reactive gases such as hydrogen sulfide and nitrogen dioxide, for the last twenty years I've been developing a solution to this tricky problem. As an emergency responder, I've had to deal with it, too.
When I order a replacement calgas of 10 ppm hydrogen sulfide in air, I expect that it will show up the same on my monitor as did the last cylinder of the same mix. It doesn't always happen that way. Usually, the new mix shows higher than the last calgas cylinder. What's the problem here?
Reactive mixtures can react with a variety of chemicals, such as residual lithium stearates used to extrude the cyinders when they're manufactured. In theory, this lubricant is totally cleaned from the cylinder. That doesn't always happen. Even a slight residual can affect the new mixture.
Also, there are impurities in the aluminum to boot. Iron and copper are the worst actors. Again, in theory, these active sites are all neutralized by the gas mixture manufacturer so that there is no fade. That doesn't always happen, either.
We've all had the experience. H2S mixes and NO2 mixes fade after the first few months, no matter what the calgas manufacturers tell us. I've got to much experieince in this field to doubt the responders. Nine time out of ten, t's the manufacturers fault.
So if I calibrate with mixes that are continually fading, what's the impact on the gas detector? It gets progressively worse! Its calibration gas points begin to creep away from the true calibration.
That's what my research is all about- improving calgas cylinder integrity to the point that there is NO interaction with either residual lubricants or copper/iron impurities in the cylinders.