Calculated % of gas needed - How do I get first stop with no dive table?

[doctormike]

Well I would have to plan the dive a head of time, so I can tell the shop what Mix I need, no?

Exactly. Maybe this is a language thing. You said that you were trying to plan a dive based on telling the shop which mix you wanted. That just confused me. I think that you are just asking how to calculate best mix for a given depth? That's the formula that boulderjohn gave you above.

 

[boulderjohn]

Exactly. Maybe this is a language thing. You said that you were trying to plan a dive based on telling the shop which mix you wanted. That just confused me. I think that you are just asking how to calculate best mix for a given depth? That's the formula that boulderjohn gave you above.

That is how I understand it. The only thing the shop needs to know is what mix to give you, and the only thing that matters is the depth. (That assumes you are talking about ocean diving. For freshwater diving, it is a little different, although most people do not take that into consideration.) Once you have the best mix for the depth, it is up to you to decide the rest of the dive plan.

 

[ajduplessis]

Use the closest "worst gas" available on the table and calculate you ascent profile. If you have EAN37 use EAN36 or less.

 

[dberry]

The OP asks about how to plan stops based on time, depth, and mix. That remains confusing to me. Ideal mix (37% EAN for the original hypothetical dive of 30min at 90ft) - that's just setting the mix to the desired MOD - a basic EAN class assignment. But in basic EAN course you're diving in recreational limits, so there are no stops (other than a non-mandatory Safety Stop). "Equivalent depth" calculations are used to let you adjust your NDL time with EAN mixes. No stops here.

OP: are you planning dives that require a mandatory deco stop on the way up? Are you going to use backgas (37%) for your deco? You're much better off simulating the deco using your dive computer or desktop deco packages to do the calculations.

I suppose the exception might be the Ratio Deco system some programs promote, but I doubt that's what the OP is talking about.

 

[decompression]

Wait a minute.....is the OP using 37% as Deco gas? I’m with John, no idea about a “first stop”........

 

[Diver-Drex]

As the others have said, you have already determined best mix for your planned 90fsw depth. If your planning using tables your next step is to either 1) calculate the equivalent air depth and use an air table or 2) use EAN36 instead of 37 and use a EAN36 table. You use the tables to determine NDL and stops just like you do with air.
All of this is or should be taught in a Nitrox course. Have you taken a course?