Determining Gas Supply - Question

[impulse]

Hello,

I just started to take PADI Tec Deep Diver course. I am glad that I took my first step to becoming a technical diver.

I have been working on the "determining gas supply and reserve requirements for multiple depths and decompressions stops" calculations.

On the DSAT Tec Deep Diver manual, page 95, there an example saying;

You plan to make a dive with air by following a standard air table. You plan to make the decompression more convervative by using EANx50 at 9 and 6 meters, and pure oxygen at 3 meters. Your planned is 45 meters for 40 minutes, with 5 minutes at 9 meters, 19 at 6 meters and 33 at 3 meters. Your SAC rate is 24 liters per minute during the working part of the dive and 18 liters per minute when decompressiong. Your ascent rate is 10 meters per minute. What are your total gas requirements for each gas, including a one third reserve?

EANx50 = 1077 liters
Oxygen = 1194 liters
Air = 8452 liters

Everything is ok but, in the book, air consuption during the descent is not calculated. Why is that?

I wanted to ask this to you before asking my instructor.

When calculating the gas reserve, gas consumption during the descent is ommited from the calculation or do I miss something?

Thanks.

 

[rongoodman]

Isn't the descent counted as part of the bottom time?

 

[Rhone Man]

I think for the purposes of gas consumption calculations, descent time is included in bottom time. So you are treated (for gas purposes) as sitting on the bottom once your head goes underwater.

Some computer models actually calculate gas consumption on descent based upon projected time to descend, but it is a bit too fiddly to do manually. And, given the safety margins you are building, there wouldn't be much purpose.

 

[impulse]

I do not know?

The book says that, when calculating the ascent from the bottom, say to the first deco stop, at a givven ascent rate, use the half depth.

For example,

When a diver is ascending from 50 meters to the first deco stop which would be 12 meters at a rate of 10 meters per minute SAC is 20 liters gas needed to ascent is :

50+12=62/2=31 meters (half way)
4,1 ATA X 3,8 X 20 = 311 liters. (could anyone check this if it's right?)

So if ascent is calculated using the half way ata but the total depth difference from the bottom to the first deco depth, I assume the descent must be calculated in a similar way but I am not sure about that?

 

[ucfdiver]

I'd guess one of two things

-Descent is calculated into bottom time.
-They cut corners for the academic exercise so you can focus on ascent.

 

[Rhone Man]

That is a little odd, but at least you know your ascent rate (so you can calculate the time of ascent), whereas descent rate is much more variable. I cannot remotely imagine that they completely discount descent in gas calculations, so the only answer must be that they factor it is as part of bottom time. After all, that's the same rule for decompression tables.

 

[PfcAJ]

I think you're trying to make an imprecise thing very precise.

Don't worry about your decent gas, calculate it as if you were spending that time at depth. This will give you a little buffer in your gas supply (which is a good thing).

A better question is why are you using air at that depth, 50% at 9 meters, and 100% at 3m... I hope thats just a math problem and not a real dive profile.

 

[Rainer]

A better question is why are you using air at that depth, 50% at 9 meters, and 100% at 3m... I hope thats just a math problem and not a real dive profile.

Nevermind sitting that long on O2 for such a dive...

 

[impulse]

Nevermind sitting that long on O2 for such a dive...

Trying to understand the calculation. All the data given was made up for the purpose of understandin th calculation...

 

[tflaris]

Some Agencies recommend averaging the deepest part of the dive and your first deep stop and using this averaged depth and the time from depth (in ATA) to the first deco stop times your sac rate would be an approximate gas consumption during travel from depth to first deco stop.


Probably confused more than I helped