I recieved several emails of people asking me what standard gases I like and why? So rather than repeat myself I wanted to answer all. This will also open it up for discussion and I can answer your questions.

Let start by looking at a PDF that outlines the standard gases and why that can be found on the 5thd-x.com website under learning center that outlines the standard mixes.

Download the Standard Mixes PDF File here

Here is a summary list of reasons of why I use what I use:

I use standardized mixes to keep it simple. Here are some parameters of the mixes:

Bottom mixes have an MOD PPO2 of 1.4 ata
Bottom mixes have an average PPO2 of 1.2 ata for “our” working depth
Bottom mixes have a buffer from “our” working depth and the MOD of 1.4?Bottom mixes are created by adding HE and then topping with Nitrox 32% (easy for banking 32% and doing trimix fills) or by quick formulas for “air top”
Bottom Mixes keep our END shallower than 100'/30m
Deco mixes have an MOD PPO2 of 1.6 ata
Deco mixes have an average PPO2 of 1.2 ata except for the O2 at 20’/6m (averaged over the range the deco mix is used
Deco mixes are used over an average of 5 - 10’/3m stops except for the O2
HE used is consider to be the higher the better but always enough to have a END of 100’/30m or less based on a conservative formula of END=(1-HE)*ATA’s. This formula assumes O2 to be narcotic.


I have always liked standard mixes and used the ones listed on the PDF. I normally use a nitrox 32 for most of the diving shallow to an average of 80'/24m and a max of 100'/30m. Then beyond that I use the standard mixes as listed. I like that the mixes listed have the ability to be mixed by adding the HE percentage to my tanks and then toping them with Nitrox 32% at the LDS. The mixes listed are very much inline with the standard GUE/DIR mixes except for the 25/25 which I prefer over the 30/30 as promoted by GUE. I never understood the GUE mix of 30/30 as it does not fit inline with any of the other standard DIR mixes. The GUE mix of 30/30 fits none of the logic that DIR uses.

It can not be mixed by just adding HE to the tanks and then toping with Nitrox 32 like all the others. 30/30 requires air top mixing only.
GUE reocmmendeds that you use 30/30 as a recreational mix although it has a max of depth of 120'/36m (PP02 of 1.4), which does not meet the Max depth of recreational diving of 130'/36m and has no buffer zone. So in fact the idea of GUE's recreational mix 30/30 does not extend to the limit of recreational diving as promoted by other agencies.
The gas mix of 30/30 is not used as a deco mix in other situations.
The gas mix 30/30 just seems odd to me. It just does not fit with the others such 21/35, 18/45, 15/55 and so on. All these seem to meet the above criteria.

A 25/25 mix on the other hand seems to fit much better. It meets the principlas listed above. 25/25 as a recreational mix can be:

Mixed by adding 25% HE to your tanks and then toping with nitrox 32%.
Used at a depth of 130'/39m (recommend recreational max depth) and has a PPO2 of 1.2 at 130'/39m
It has a 1.4 MOD to 150'/45m (A little buffer)
It has a END of 88'/26m when used at 130'/39m. Very clean.
And the decompression according to GAP (NAUI version) will give you very similar results to using it like 32% and min ascent profiles.

Anyways these are just some of my thoughts.

Andrew