Cert1967
Contributor
I enjoy dive related reading and I found Steve Lewis's books and blogs to be good reads Staying Alive particularly so. I think he has combined basic science with interesting "war stories" to make it seem you are attending one of his lectures. The only thing missing is the opportunity for a Q/A session at the end of the book.
However I dont seem to understand his "Vacant Partial Pressure Method my results are at odds with the example in the book. This seems the cornerstone of the gas tox section and relates directly to gas supply, training, and really all 8 points. It would seem that of the 8 bullet points none work if the gas mix isn't usable.
As I understand the book's VPPM you are going to set limits for the acceptable Pp of N and O2. Lets say they are (choosing the levels for example only and matching those in the book);
1. Max PpN = 3.16 ATA
2. Max PpO2 = 1.40 ATA
Lets say we are making a dive to 40 meters of sea water with a hard level bottom 5.0 total ATA. An example only.
1. The Pp of N would exceed our max of 3.16 ATA with air (21/79)
2. The Pp of O2 would allow for a surface O2 fraction of .28 and yield a Pp of 1.40 ATA at 40m. Maxing the O2 ultimately reduces the He fraction?
3. We have used 4.56 ATA of the total 5.0 ATA at 40m maxing the allowable N and O2 fractions (1.40 + 3.16).
4. Now for the VPPM, we make up the missing 0.44 ATA with something else He which would yield a surface fraction of 0.088. The mix would be 0.280/0.088/0.632, right?
5. I suppose we could continue with this logic until the O2 fraction dropped to a severe hypoxic level lets say less than .18 and then its >1 gas mix exclusive of deco gases?
Ok, perhaps I dont get it the VPPM - or what makes up the missing pressure?
On to the Staying Alive example.
The books example was a dive to 30m starting with 21/0/79 - with a max allowable PpN of 3.16 and a max allowable O2 of 1.4, using only one gas. Here at best Steves example seem tortuous to me - and perhaps his proof reading complacent. Are we to assume that there is "head room" in his calculations as the 30m PpO2 = .84, below the 1.4 Pp targeted max?
That said, his math seems wrong to me - from the first quote excerpted, 1.40 + 3.16 does not equal 4.46 as he writes. My addition says 4.56.
The math or reasoning also seems wrong to me.
In the context of his VPPM - where he applies the max values of; PpO2 of 1.40ATA and PpN of 3.16ATA to a dive of 45m or 5.5 ATA, I am confused.
He has previously stated a max PpN of 30m/3.16ATA and a max PpO2 of 1.4 are the max values for his VPPM. These max Pp add to a total of 4.56, not 4.46 as he writes - 5.50ATA - 4.56ATA = 0.94ATA or the "vacant" partial pressure to be filled with He. Did I miss something?
Now perhaps my reasoning is in error. After all I am a salesman and not a scientist. But if we look to the last quote and add the Pp of O2+He+N (1.40+1.04+3.16), my math says 5.60ATA equaling a dive to 46msw not 45msw.
Perhaps I lack the basic knowledge - lack of scientific/math education - or I do not possess the wisdom to apply his VPPM correctly, maybe both. But in these examples the math calculations elude me.
How about a little help here - please straighten me out thanks.
*****************************
Lewis, Steve (2014-01-07). Staying Alive: (Kindle Locations 1376-1384). techdiver Publishing. Kindle Edition.
However I dont seem to understand his "Vacant Partial Pressure Method my results are at odds with the example in the book. This seems the cornerstone of the gas tox section and relates directly to gas supply, training, and really all 8 points. It would seem that of the 8 bullet points none work if the gas mix isn't usable.
As I understand the book's VPPM you are going to set limits for the acceptable Pp of N and O2. Lets say they are (choosing the levels for example only and matching those in the book);
1. Max PpN = 3.16 ATA
2. Max PpO2 = 1.40 ATA
Lets say we are making a dive to 40 meters of sea water with a hard level bottom 5.0 total ATA. An example only.
1. The Pp of N would exceed our max of 3.16 ATA with air (21/79)
2. The Pp of O2 would allow for a surface O2 fraction of .28 and yield a Pp of 1.40 ATA at 40m. Maxing the O2 ultimately reduces the He fraction?
3. We have used 4.56 ATA of the total 5.0 ATA at 40m maxing the allowable N and O2 fractions (1.40 + 3.16).
4. Now for the VPPM, we make up the missing 0.44 ATA with something else He which would yield a surface fraction of 0.088. The mix would be 0.280/0.088/0.632, right?
5. I suppose we could continue with this logic until the O2 fraction dropped to a severe hypoxic level lets say less than .18 and then its >1 gas mix exclusive of deco gases?
Ok, perhaps I dont get it the VPPM - or what makes up the missing pressure?
On to the Staying Alive example.
The books example was a dive to 30m starting with 21/0/79 - with a max allowable PpN of 3.16 and a max allowable O2 of 1.4, using only one gas. Here at best Steves example seem tortuous to me - and perhaps his proof reading complacent. Are we to assume that there is "head room" in his calculations as the 30m PpO2 = .84, below the 1.4 Pp targeted max?
That said, his math seems wrong to me - from the first quote excerpted, 1.40 + 3.16 does not equal 4.46 as he writes. My addition says 4.56.
The math or reasoning also seems wrong to me.
In the context of his VPPM - where he applies the max values of; PpO2 of 1.40ATA and PpN of 3.16ATA to a dive of 45m or 5.5 ATA, I am confused.
He has previously stated a max PpN of 30m/3.16ATA and a max PpO2 of 1.4 are the max values for his VPPM. These max Pp add to a total of 4.56, not 4.46 as he writes - 5.50ATA - 4.56ATA = 0.94ATA or the "vacant" partial pressure to be filled with He. Did I miss something?
Now perhaps my reasoning is in error. After all I am a salesman and not a scientist. But if we look to the last quote and add the Pp of O2+He+N (1.40+1.04+3.16), my math says 5.60ATA equaling a dive to 46msw not 45msw.
Perhaps I lack the basic knowledge - lack of scientific/math education - or I do not possess the wisdom to apply his VPPM correctly, maybe both. But in these examples the math calculations elude me.
How about a little help here - please straighten me out thanks.
*****************************
Lewis, Steve (2014-01-07). Staying Alive: (Kindle Locations 1376-1384). techdiver Publishing. Kindle Edition.
- Next step, we add the oxygen depth to the nitrogen depth (1.4 + 3.16) and arrive at 4.46 bar.
- This then represents the acceptable total pressure, ambient pressure, depth (all three are the same thing).
- Heres how simple the calculations become. Say we wanted to dive to 45 metres / 150 feet.
- The ambient pressure down there is 5.5 bar. Step one, how much vacant partial pressure would there be.
- Pretty simple to work out: Subtract 4.46 bar (our combined oxygen and nitrogen depths) from the 5.5 ambient pressure / depth at 45 metres or 150 feet. The result: 5.5 4.46 = 1.04 bar.
- Since the outcome of running down to our local dive shop and asking for a gas mix capable of delivering 1.4 bar of oxygen, 1.04 bar of helium, and 3.16 bar of nitrogen is likely to be hit and miss, we have one step left.
