minimum gas on the fly

[007sascha]

Hello,
I started thinking about minimum gas while diving and found a very good function on hwos. Of cource it`s not a replacement for planing, it`s only an addition.
I would like to know the minimum pressure at current deep to get up with a buddy who lot/emptied his gas. (assumption 3x normal SAC(AMV))

Is the following correct?
Only one Gas:
-Set the AMV "bottom gas" to 60 l/min (e.g. 3x normal)
-Set only one Gas as active
-Set Future TTS to "problem solving time" e.g. 3min
-Set Tank size
Now i should see while diving at the custom view "Gas Needs" how much gas i must have to solve the problem in 3min and get up to surface form current depth.

One travel Gas and Deco gas:
(Minimum gas to get safe to the first gas-change-depth)
-Set the AMV "bottom gas" to 60 l/min (3x normal)
-Set the AMV "deco gas" to 20 l/min (normal)
-Set Future TTS to "problem solving time" e.g. 3min
-Set Tank size
Now i should see at the custom view "Gas Needs" how much gas i must have to solve the problem in 3min and get up to the first gas-change.

This methode has the disadvantage that the gas-calculation at the simulator is wrong. Future Deco and "problem solving time" may differ but you have only one parameter.

Did I made some mistake? Only one thing is right: you can`t have enough gas

[Ralph]

Hi Sascha,

thanks for your question and for describing a possible & valuable use of the "gas needs" function. Initially, this function was developed to compute the gas needs for a bailout ascent when diving on a rebreather. But soon after start of coding, it was understud that this function will as well work in pure OC diving. Here's how it works:

First, the deco schedule is calculated on basis of the GF-low and GF-high values, the last stop depth and (lately added: ) the set ascent speed. Then the gas needs are calculated using the bottom SAC for the fTTS "Problem solving time" and the initial ascent up to the first stop. After reaching the first stop, the deco SAC will be used for all stops, the intermediate ascents from stop to stop and the final ascent to surface, whereby the final ascent is calculated with a fixed rate of 1 meter per minute. For the gas switches an additional time can be set, default is 1 minute, it will be added to the stop time. During this gas switch time, gas needs are added up on both gases, the old one and the new one. So calculation is on the safe side, regardless if you switch early or late.

After having computed the gas needs by times, depths and respective SACs as volumes, they are converted in required pressures (bar) by division with the tank sizes. Currently, ideal gas laws are applied, so in case you run into pressures above 200 bar, be carefull that the real pressure needed will be a bit higher than the indicated one.

Finally, attentions and warnings are generated by comparing the pressure needs with the set pressure budgets aka turn pressures per tank.

So to figure in the combined need of two divers, you can either manipulate the SACs (multiply by 3 or whatever factor is thought to be appropriate) or you can reduce the tank pressure budgets (divide by above factor) or manipulate the tank sizes (again by telling the OSTC that they are smaller than they actually are). In the later cases, the attentions and warnings come at the correct time, but you do not have the alter the SACs. With the tank size solution, even the indicated pressure needs match with your SPG's readings because the supply for the 2nd diver is carried in the extra capacity of the tanks.

You can go all ways, owing you know what you are doing, what you have configured on the OSTC and understand what the numbers it is showing do mean.

happy & save diving to you!
Ralph