I often get asked how do you know what mixes of bottom gas to take on a dive? There are two schools of thinking here, Best Mix for the dive or Standard Mixes for a variety of dives.
Essentially Best Mix means you calculate the mix based on the planned depth of the dive. So for a dive to 130′ fsw, at a PPO2 of 1.4, not getting into the narcotic gases and deep air arguments you will undoubtedly hear, the best mix is 28%. For 120 fsw, it’s 30%. You use Dalton’s Diamond to figure out the mix for the depth you want and you mix it.
When I started technical diving, I followed the best mix philosophy. While I still think for a given well planned dive, it works very well and can shave off some deco time… logistically it’s not as easy to be prepared to do a variety of dives. You need to plan the dive and get the fill for that specific dive. With standard gases , you can have a few sets of tanks available to do a wide variety of dives. This is even more true when you get into rebreather diving and bailout tanks, but that’s another article.
The standard gases philosophy is promoted by some of the largest Doing it Right / Dive it Right (DIR) agencies. While they still have a few practical differences between their standards, and if you search the web historically they’ve had even more changes over time… the basic gases are very similar, if not the same.
Below I’ve listed the UTD Standard gases. You can also check out the WKPP Approved Gases page for another nice view of this data (albeit slightly different as it’s GUE, not UTD).
Bottom Gases (Average PPO2 of 1.2)
Gas -> Working Range
32% -> 0′-100′
25/25 -> 90′ – 130
21/35 -> 100′ – 160′
18/45 -> 160′ – 200′
15/55 -> 200′ – 250′
12/60 -> 250′ – 300′
10/70 -> 300′ – 400′
Gas -> Working Range
100% -> 20′ – 0′
50% -> 70′ – 30′
35/25 -> 120′ – 80′
21/35 -> 190′ – 130′
18/45 -> 240′ – 200′
Interesting Rationale on selecting these mixes as Standard Gases from a thread with AndrewG (head of UTD, previous training head of GUE):
Look at the original criteria for standard mixes in order of importance:
Must be able to partial pressure blend by adding helium and then topping with Nitrox 32% at a local dive shop
Must have a low PPO2 at it’s max operational depth. 1.2 or less
Must have a buffer zone for safety. If you need to rescue someone or go deeper then there is a buffer.
Must have a narcotic equivalent depth of 100′ or shallower based on the conservative formula of (1-he)*ata”