Halftime table computing????

[Vessper76]

I am working on my DM certification and working through the Diving Knowledge workbook and Encyclopedia of Rec Diving, but I am struggling with how to compute at what depth a particular halftime will start off gassing. I understand how the halftimes work. Here is the question I am working on.... In meters/feet of pressure, how much nitrogen would the 5, 10, 20, and 60 minute halftime compartments each have after 60 minutes at 18meters/60 feet?

I can go through and figure out the percentage of each halftime and whether or not the compartment is saturated. However, the I am struggling with part of figuring out the meters/feet of pressure.

If someone could explain how this is done, I would be greatly appreciative. I don't want someone to answer the problem I am working. I would like to figure that part out on my own. thank you in advance for any help that is provided.

 

[captndale]

You need only calculate the partial pressure of the compartment you are working with and then calculate the equivalent depth, remembering to subtract atmospheric pressure.

 

[Charlie99]

I vaguely remember a thread with a question that had similar values. It turns out that the "correct" answer per the certifying agency was a weird one that ignored details such as what gas you are breathing.

In the real world, you do as Captndales says. More specifically, you calculate the partial pressure of N2 you are breathing, the partial pressure/tissue tension N2 pressure that the tissues start out at, and then calculate away.

After an extended period of time at sea level, all compartments will be saturated at 0.79atm of N2. (actually, more like 0.75atm, because of the water vapor pressure in your lungs, but let's ignore that). 0.79atm of N2 is more commonly described in decompression literature as 0.79 x 33 = 26 fsw (this is absolute pressure, in fsw).

At 60'/18meter/2.8ata the inspired partial pressure, if you are breathing air, is 2.8x0.79= 2.2ata. 2.2ata is also commonly described as 72.6 fsw.

So to summarize, all the compartments start at 26 fsw of N2 at sea level, and after 1 halftime of sitting at 60'/18m depth, the pressure in the compartments will be halfway between the 26fsw start point and the inspired pressure of 72.6fsw (assuming air). After 60 minutes, the 60 minute ht compartment would be halfway between 26 and 72.6fsw or 49fsw. The 5 and 10 minute compartments would be very close to the saturation value of 72.6fsw of N2. The 20 minute compartment will be at 7/8 of the way from 26fsw to 72.6fsw, or about 68fsw or 2.06ata of N2.

If you are shown a different method of calculating nitrogen loading that totally ignores what you are breathing, then you are being taught an oversimplified calculation that has little relationship with the real world.

 

[Vessper76]

I believe the reason we are having to work through these exercises is to show us how the original tables were made and how they were used. These tables are based on John Scott Haldane. I do not believe we will be actually using the these tables to figure actual dives or anything like that. Just another lesson in dive theroy. Thank you both for your help.

 

[Charlie99]

I believe the reason we are having to work through these exercises is to show us how the original tables were made and how they were used. These tables are based on John Scott Haldane.

Haldane ignored N2 fraction since he only worked with air. He DID, however, pay attention to the difference between depth (pressure below sea level) and absolute pressure (pressure difference from a vacuum). In other words, he used PSI absolute rather than PSI gauge when he generated tables that maintained less than 2 to 1 pressure differential.