The common sense procedure therefore seems to me to be:

1. Choose your filling station carefully, if you don't like what you see, walk away (do not use that filling station).
    
2. Make sure your cylinders are labelled in such a way as to make it difficult to mistake Air for Oxygen, or Nitrox or Trimix or any other gas combination.
    
3. Unless you have absolute trust in the gas blender always watch him fill the cylinders.
    
4. Watch how the filling station operates their mixing panel.
   I have seen some places where they have full mixing facilities but some of the operators are only qualified to pump air so mistakes can easily be made by them.
    
5. Do not get your gas from a filling station that does not have clean (double filtered or oil free air), or if you do, take your own in-line filter.
   I have seen stupid operators that rinse the filling whip in a bowl of dirty water and say it is then clean enough to pump an O2 clean cylinder.
    
6. If the filling station pumps anything other than Air then always analyse your cylinders (also the Air cylinders). If the analysis is anything other than what you expected, be careful. It could be that they have put something else in your cylinder.
   I have seen an incorrectly purged and incorrectly set panel put what could have been a slug of Argon, possibly Air and O2 into my Oxygen cylinder.
   Make them flush the cylinder if they get it wrong!
    
7. Analyse your cylinders before and after filling and if you use the filling station analyser calibrate it and operate it yourself, better still take your own with you. Analyse again before the dive.
    
8. Label everything yourself, even air.

 Rebreather-Index

http://Rebreather.de/rebreather/get_fills.htm © Stephen Bird, Okt. 1999