I think water vapour is the most important gas for understanding atmospheric processes.
correct me if i am wrong....... :)
The standard weights and meters scale for every two years because they may raise some errors due to regular use
The measuring device on regular and continous use may loose their standards and results in some error
The national standards body National Bureau of Standards & Metrology (NBSM) verifies the standard weights and meter scales for every two years
There are also some other factors which can contribute to bring error in the measurement of instruments
