GearHead once bubbled...
Whether a course is worthwhile or not is completely dependent on the quality of instruction materials, and the experience/skill of the instructor. In my case, I came her to get an answer to what you call a "basic question" because I received conflicting information from my coursebook and my LDS. I know of no other place that has better information from a number of reliable sources on this matter than Scubaboard. I trust Pug, Roakey, Giddins, et al far more than some of the youngsters at my LDS.
And to address your guess as to whether I "took an advanced course after basic": I earned my open water certification in 1996, and my EAN in 2002. Is that enough time?
Part of the reason for apparently conflicting answers is that not all the agencies agree with each other..
All are pretty much in agreement that regs that will be used over 40% need cleaning, some require it for any enriched mixture..., unless the tank is really bad its unlikely to contaminate your regs in a short period of time (contamination is cumulative).
All agencies state for partial pressure blended tanks where a high oxygen content will be pumped into the tank require oxygen cleaning...
Alot of the confusion come is where banked gasses are used... since no oxygen comes in contact with the tanks many feel its ok to fill a non cleaned cylinder with a banked gas..
I don't really agree with this myself, I believe all nitrox cylinders should be clean regardless of banked or not and should be gotten from a source that the gas is free of hydrocarbons (many membrane systems don't produce hydrocarbn free gas).. at lower concentrations an "oxygen" fire is unlikely but you can still get partial combustion which would add both co and co2 into the mix.. not very good at depth.. this can be prevented by slow fills, but how many tank monkeys do you see filling at 100-200 psi a minute (with air I'd go up to 600 psi a minute as a max, with oxygen I fill around 60 psi a minute)... I'll bet they fill your 80 in a minute or 2 instead of 20-30 minutes.. Fast filling generates enormous heat and the initial opening of the valve quickly is also a source of potential problems.
Even a contaminated tank could be filled with oxygen if there was very little heat and filled very slowly(assuming there are no other sources for ignition, like rust particles and the like)... You are just rolling the dice, the idea is to be as safe as possible..
Any place I'd fill my taks I'd want to see their quarterly gas analysis.. If they don't care enough to get their gas tested I would look elsewhere..
Grade "E" gas is not oxygen compatible... grade "E" allows 5 mg/m3 of hydrocarbons... Some call modified grade "E" as oxygen compatible, but there is no such PUBLISHED standard, I would suggest only using air that is <.1mg/m3. The cleanest published standard is CGA grade J which in reality is not attainable in scuba compressors some claim modified grade "j", but its so modified it looks more like grade "E" than grade "J"..
Undersea Breathing systems ( a membrane manufacturer) allows 5mg/m3 (same as grade E) and 10ppm co and 1000 PPM co2 (same as new grade "E") as far as I am aware ANDI is the only agency still requiring 500ppm Co2 everyone else is allowing the higher CO2 limit that was adopted for AIR as a breathing gas (not nitrox). The standard used by ANDI, IANTD and NAUI only allow 2 ppm of CO which is unlikely to be attained by a membrane system without lots of filtration. It is funny in that the membrane systems used to create nitrox are actually using the "waste" gas of the system.. These generators were actually designed to create NITROGEN gas! the gas with a high FO2 is actually the waste!
The high CO2 concerns me since one of the primary factors in Otox is carbon dioxide, remove the CO2 and otox is not a reality..
