The shelf life of calibration gases are varied. Every mixture will have a particular shelf life which is determined specifically for that product.
What determines the Shelf Life of Calibration Gas?
The composition of a gas mixture filled in a sealed container can only change through the chemical reaction of one of more of the mixture’s components with another material. These other materials can come in several different forms, and the severity of the reaction can vary depending upon environmental conditions, or the application in which the gas is used.
Over time, these chemical reactions that occur inside the cylinder will change the mixture composition enough that it can no longer be used for its intended purpose. The gas originally inside the cylinder is not lost, but rather it has reacted to form new compounds. After this point, the gas mixture is no longer considered to be within the certified mixture tolerance, and, if used, can incorrectly calibrate an instrument, causing a potentially dangerous situation.
There are many factors to consider when determining the shelf life for a calibration gas. These three factors impact the shelf life of both non-refillable (disposable) and high pressure calibration gas mixtures.
1. Gas Type
We can primarily divide all gas mixtures in two types: reactive, and non-reactive.
Reactive gas mixtures will have shorter shelf lives than non-reactive mixtures, because they are unstable. They are typically filled in specially prepared aluminium cylinders with stainless steel valves.
Reactive calibration gases include:
H2S (hydrogen sulphide), SO2 (sulphur dioxide), NH3 (ammonia), CL2 (chlorine), NO (nitric oxide), & NO2 (nitrogen dioxide), among others.
Non-reactive gases are inherently stable, and can be certified for a longer period of time. Typically this is 3 years.
Non-reactive gases include:
Alkane and alkene hydrocarbons, like CH4 (methane), C5H12 (pentane), C4H10 (propane), or C6H14 (hexane), as well as other commonly used calibration gases like CO2 (carbon dioxide), hydrogen, and nitrogen.
It is worth noting that CO (carbon monoxide) and O2 (oxygen), are actually reactive gases, but do not typically react in normal gas calibration applications, and can be safely treated in the same way as the non-reactive gases.
These gases are normally filled in steel cylinders (with some exceptions).
2. Gas Concentration
The concentration of each component (reactive gases) can also affect the shelf life.
In some cases, a higher concentration of the reactive gas can be certified for a longer period than a lower concentration. In low concentrations, a few reactions can have a much larger effect on the overall composition of the mixture than the same reactions in a highly concentrated mixture (example: reactions reducing 10ppm chlorine to 5ppm over 12 months, compared with reactions reducing 2.0% chlorine to 1.998% over the same period).
3. Gas Cylinder Quality and Size
Gas Cylinder Quality:
On a microscopic level, the interior walls of the cylinder must be as smooth as possible, and this also requires special manufacturing. If the walls are “rough,” it allows the gas to come into contact with a larger surface area, where it may react with the cylinder material itself, or any contaminants hiding in the rough surface.
Improved cylinder design and materials has been a significant reason for gas mixtures in non-refillable cylinders to be extended to two years for some corrosive gases. The quality of the internal walls is a key to that extended life as is the material of the valve.
Gas Cylinder Size:
In addition to quality materials larger containers allows for longer shelf life in high pressure cylinders. Because the ratio of the internal wall surface area to gas volume is substantially less there is a less potential for reaction.
Continued research and trial will bring new materials and capabilities to extend gas mixture shelf life in the future.