I analyze gas no less than 5 times before it ever makes it to the water. I keep a spreadsheet for each cylinder or set of doubles. I mix gas several days prior to the scheduled dive. I analyze immediately after final gas insertion and then at least once a day until I get in the water. Each time time I analyze I check and double check manifold valve positions. I alternate DIN ports to make sure I get the same reading on both sides. I use multiple O2 sensors. I consider this investment in time and the sensors cheap compared to the value of my life and the lives of my dive partners.

I analyze helium but I am never all that concerned about it. For most dives I try to get to within +/- 1% of my target helium. (For short excursion dives, most tables show very little difference within +/- 5% helium versus nitrogen). I just cut my tables (and dial in on the dive computer) once I know exactly what I have prior to the dive.

The analysis only confirms what my careful calculations and partial pressure blending has provided. Usually there are no surprises.

One last comment. I know many computer deco generators have mix calculators with them. Don't get lazy with these things. I always do calculations by hand first then check them on the computer. This serves as a check of my math AND a check of my input skills on the keyboard, both of which could have errors. This puts me more "in tune" with what is actually in my tanks throughout the process and not relying on numbers being spit out by the computer. This is very important to me since I rarely start the blending process with empty tanks. I teach college mathematics and my best students are always the ones who write it all down first and then check their results against those of the calculator. They already know what to expect for an answer before the calculator is utilized.

If you mix at home and top off at a shop do not leave your tanks unattneded. Stay with them and monitor every thing being done to them. Don't let your tanks be equalized with others getting air at the same time. I am very fortunate that no one comes in contact with my mix at any stage of the process. On occassion when I do mix away from home I watch the filling like a hawk. Get involved with the process. If the shop doesn't like you standing over them..... GO SOMEPLACE ELSE. I for one LIKE IT when I mix someones gas and they want to watch and understand what is going on. It is similar to the aviation industry. You are the captain of the aircraft. You are responsible for every aspect of the aircraft before you take off. You are at liberty to watch your aircraft be fueled and check the tanks once they have been filled.