Question on Vance's O2 Hacker Book

[Otter]

Ok, maybe I am not the sharpest knife in the drawer, so what am I missing here.....

On pg 37 of his 4th edition, he does a bunch of calculations to determine a gas mix for the Doria.

While he states the operation incorrectly, he computes total pressure at 240 fsw as 8.27 ata and then divides 1.4 ata (target Po2) by 8.27 ata x 100% for 17% (which I am assuming is the FO2)

He then computes target N2 for a 100 fsw END by

100 fsw
------- + 1 = 4.03 ata
33 fsw

PN2= .78 x 4.03 ata = 3.14 ATA

3.14 ata
-------- = .38 or 38%
8.27 ata

So, I think at this point, the FO2 is 17%, the FN2 is 38 percent. However, in the paragraph after all of the calculations, he states

"If the PO2 is 17%, and the FO2 is 38%, then the balance of the mix, 45%, will be He. So the answer is a 19/45/38 trimix (or a 19/45 as such mixes are usually referred to, omitting the FN2)."

Should it not have been...

"If the FO2 is 17%, and the FN2 is 38%, then the balance of the mix, 45%, will be He. So the answer is 17/45/38 trimix...."

If there weren't so many differences in what I thought it should be versus what it states, I would have just assumed they were typos.....am I missing something?

 

[Otter]

I didn't think this was that tough of a question.....but 40 reads and 0 replies? [sniff...sniff] Maybe I should have taken a shower.

 

[lhpdiver]

well 17 + 45 + 38 does equal 100. but you knew that right ?

 

[Otter]

well 17 + 45 + 38 does equal 100. but you knew that right ?

I got an answer to TDS, what I really wanted to make sure was that since the use of fractional and partial was seemingly being used incorrectly, that I wasn't missing some conversion. With a single difference in what I thought it should be, I would have assumed a typo. But with multiple errors in a short amount of text, I thought maybe I was missing a concept.

Thanks

 

[pwfletcher]

I got an answer to TDS, what I really wanted to make sure was that since the use of fractional and partial was seemingly being used incorrectly, that I wasn't missing some conversion. With a single difference in what I thought it should be, I would have assumed a typo. But with multiple errors in a short amount of text, I thought maybe I was missing a concept.

Thanks

You are correct ... it should be 17.

 

[BigJetDriver]

"If the PO2 is 17%, and the FO2 is 38%, then the balance of the mix, 45%, will be He. So the answer is a 19/45/38 trimix (or a 19/45 as such mixes are usually referred to, omitting the FN2)."

Should it not have been...

"If the FO2 is 17%, and the FN2 is 38%, then the balance of the mix, 45%, will be He. So the answer is 17/45/38 trimix...."

If there weren't so many differences in what I thought it should be versus what it states, I would have just assumed they were typos.....am I missing something?

Otter,

Your re-statement is the correct one. His previous statement is a case in which apples and oranges are mixed in the same basket, so to speak.

17 % plus 38 % plus 45 % is correct. The sum of the fractions of the gases in the mix equals 100 %.

Partial pressure of any gas is a function of the fraction of that gas and pressure.

Despite what is done in recreational scuba, a lot of commercial and research operations have taken to listing and identifying each fraction to remove any errors.

Example: 17% O2/45% He/38% N2

BJD :anakinpod