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

[SentinelAce]

I am practicing. If I calculated this dive correctly for example. 90FT for 30 minutes, the best mix is 37%. I am using the IANTD dive tables but I obviously do not have a 37% table. How do you determine the first stop? I assume I need a table? or is there a calculation to determine your stops?

 

[doctormike]

Most people creating ascent plans for dives involving staged decompression use something like this...

 

[tc]

You’ll either need to convert to an equivalent air depth and use air tables or use deco software to make a set of tables.

Know the planned time will help.

If you have a computer that does nitrox you can plug in your mix and go to plan mode on the computer to check that way.

 

[Centrals]

36% table should be good enough.
Use "Equivalent air depth" for the conversion if you want to do some calculation. 65' is the answer.
You still need the "Air " table at the end.

 

[boulderjohn]

I personally am very confused by this. I am not involved with IANTD, but....

The equivalent air depth of 90 feet with 37% according to the DSAT table is 65 feet. With the PADI tables, you have a maximum bottom time of 40 minutes on air, and that includes rounding up to 70 feet. That means that with a 30 minute dive, you are very much within NDLs. You do not even have a mandatory safety stop, let alone a "first stop" to calculate.

You seem to be describing a no stop, recreational dive while asking a technical diving question about it.

 

[doctormike]

I wondered about that too - but I plugged the OP's dive into Multi-Deco with GF 30/70 and got 1 minute at 30 feet and 2 minutes at 20 feet. On the other hand, Multi-Deco will almost always give you a minimal stop, even for dives that seem to be within NDL.

 

[SentinelAce]

I made up the depth. Using the dive T formula, I get 37%. I'm trying to plan a dive based on telling the dive shop what mix I want. The formula is supposed to give me the best mix for the given depth and time. Lets assume I don't have my dive computer

 

[boulderjohn]

I made up the depth. Using the dive T formula, I get 37%. I'm trying to plan a dive based on telling the dive shop what mix I want. The formula is supposed to give me the best mix for the given depth and time

Okay, so you are looking for best mix using the T formula. Got it, although I still don't understand the first stop question.

Here is the T formula written out linearly in algebraic form: PG = FG X P

Your best mix is the fraction of gas (FG that will give you the partial pressure of Gas (PG) you want at that depth (pressure = P). To get that, you have to divide PG/P.

I assume by PG you want the standard 1.4. To find the pressure of 90 feet of sea water, you first divide by 33 (the depth of water equaling one atmosphere) and get 2.7. Then you add 1 to add the weight of the atmosphere and get 3.7. If you divide 1.4 by 3.7, you get .378, the best mix for that depth.

Now you need to figure out how long you can stay there. For that you need to use your tables, which I don't have.

 

[doctormike]

I made up the depth. Using the dive T formula, I get 37%. I'm trying to plan a dive based on telling the dive shop what mix I want. The formula is supposed to give me the best mix for the given depth and time. Lets assume I don't have my dive computer

I still don't understand the question. First you asked about stops. Then you said that you were trying to plan a dive based on telling the dive shop what mix you want? Wouldn't you plan the dive and then choose the mix based on that?

Best mix has nothing to do with time (at least, leaving aside oxygen clock issues that you probably aren't considering here). It's a term for the mix that gets you closest to a PPO2 of 1.4 at your maximum depth, minimizing inert gas (N2) loading while minimizing the risk of oxygen toxicity over a relatively short dive.

 

[SentinelAce]

I still don't understand the question. First you asked about stops. Then you said that you were trying to plan a dive based on telling the dive shop what mix you want? Wouldn't you plan the dive and then choose the mix based on that?

Best mix has nothing to do with time (at least, leaving aside oxygen clock issues that you probably aren't considering here). It's a term for the mix that gets you closest to a PPO2 of 1.4 at your maximum depth, minimizing inert gas (N2) loading while minimizing the risk of oxygen toxicity over a relatively short dive.

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