Gradient Factor for Dummies

CharlieT · 18th December 2006, 16:24

It's a good thing the article was for dummies!

All those formulas had me cross-eyed

Thank you Kevin

Charliet

**UWSojourner** · 18th December 2006, 18:09

Quote: (Originally Posted by svendv)

Kevin,
Dear Forum members,

I was lucky to receive a copy of your article. It is really a good initiative to write such articles, helping us all understand basic decompression principles.

I have a question, so I registered on the Forum to post it.

Why is the Gradient considered T-p.abs (as also indicated on the pressure T/P.abs graph by E. Baker) and not T-pp.N2.alv. ?

In fact it is the partial pressure difference who makes the Gradient, not the absolute pressure.

I don't understand why the pressure graphs show an ambient pressure line, giving for P.abs=1 --> T=1, normally T should be P.N2.alv=(P.abs-0,063)x0,79 for air according Bühlmann. This line gives you the equilibrium situation, above you desaturate, unther you saturate.

Best regards,

Sven.

Hi Sven,

You're right that your rate of ongassing/offgassing is based on the difference between tissue tension and your inspired gas partial pressure for each inert gas. However, the M-values derived by Buhlmann relate to ambient pressure (depth) and not to the pressures of inspired gases. M-values are more concerned about the risk of, not the rate of, decompression.

Suppose you created gradient factors as you describe. Let's call them SGF (Sven's gradient factors).

SGF = ( TissuePP - PiInert ) / ( MValue - PiInert).

Now let's suppose you ascend on air to a stop at 20ft with SGF=0.6. Your inspired partial pressure N2 would be 40.6fsw. Let's say at that point you switch to breathing pure O2. What happens to PiInert? It goes to zero. So what happens to SGF? It increases.

You could implement such an algorithm. But you would lose some important characteristics of GFs. First, higher GFs imply (in Buhlmann's world of only dissolved gases) more aggressive decompression (not quicker, but riskier). The point of GFs is to control your ascent into Buhlmann's cutoff line of risk, M-values.

However, SGF's could not be interpreted that way. The interpretation of increasing SGF's would be quicker, not necessarily riskier, decompression. It is not riskier to breath O2 at 20ft (from a bends perspective), but it sure speeds your decompression. In that sense SGFs would not be useful in all cases as a decompression algorithm as deco schemes attempt to limit risk.

Hope that helps.

Kevin

saturation · 22nd December 2006, 14:25

I'd like to say how good a job you did on this article. I just saw it today.

Everyone who does decompression diving should read it, along with Baker's original paper.

Gene_Hobbs · 23rd December 2006, 15:42

Kevin,

Thank you so much for this! This will be a great aide for teaching. It does bring up another question though, how do you teach M-values?

We used to just have people read Workman's CALCULATION OF DECOMPRESSION SCHEDULES FOR NITROGEN-OXYGEN AND HELIUM-OXYGEN DIVES. 1965. RRR ID: 3367, NEDU: AD0620879

Then we would run profiles through DecoPlanner and talk about the graphical analysis there.

A couple of days ago, I ran across this one.

Systematic Guide to Decompression Schedule Calculations.
Braithwaite, 1972
RRR ID: 3945, NEDU: AD075102
This one was written to make the calculations in the Workman report easier.

Any other ideas?

**UWSojourner** · 24th December 2006, 00:27

Quote: (Originally Posted by Gene_Hobbs)

It does bring up another question though, how do you teach M-values?

"Understanding M-Values" by Erik Baker seems like a good reference also.

18th December 2006, 16:24	#21 (permalink)
CharlieT Custom Title Allowed! Current Rebreather/s: Inspiration Vision Other Rebreather/s: Join Date: Nov 2005 Location: London Posts: 449	Re: Gradient Factor for Dummies It's a good thing the article was for dummies! All those formulas had me cross-eyed Thank you Kevin Charliet
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18th December 2006, 18:09	#22 (permalink)
UWSojourner Pacific Northwest Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Feb 2005 Location: Portland Oregon Posts: 556	Re: Gradient Factor for Dummies Quote: (Originally Posted by svendv) Kevin, Dear Forum members, I was lucky to receive a copy of your article. It is really a good initiative to write such articles, helping us all understand basic decompression principles. I have a question, so I registered on the Forum to post it. Why is the Gradient considered T-p.abs (as also indicated on the pressure T/P.abs graph by E. Baker) and not T-pp.N2.alv. ? In fact it is the partial pressure difference who makes the Gradient, not the absolute pressure. I don't understand why the pressure graphs show an ambient pressure line, giving for P.abs=1 --> T=1, normally T should be P.N2.alv=(P.abs-0,063)x0,79 for air according Bühlmann. This line gives you the equilibrium situation, above you desaturate, unther you saturate. Best regards, Sven. Hi Sven, You're right that your rate of ongassing/offgassing is based on the difference between tissue tension and your inspired gas partial pressure for each inert gas. However, the M-values derived by Buhlmann relate to ambient pressure (depth) and not to the pressures of inspired gases. M-values are more concerned about the risk of, not the rate of, decompression. Suppose you created gradient factors as you describe. Let's call them SGF (Sven's gradient factors). SGF = ( TissuePP - PiInert ) / ( MValue - PiInert). Now let's suppose you ascend on air to a stop at 20ft with SGF=0.6. Your inspired partial pressure N2 would be 40.6fsw. Let's say at that point you switch to breathing pure O2. What happens to PiInert? It goes to zero. So what happens to SGF? It increases. You could implement such an algorithm. But you would lose some important characteristics of GFs. First, higher GFs imply (in Buhlmann's world of only dissolved gases) more aggressive decompression (not quicker, but riskier). The point of GFs is to control your ascent into Buhlmann's cutoff line of risk, M-values. However, SGF's could not be interpreted that way. The interpretation of increasing SGF's would be quicker, not necessarily riskier, decompression. It is not riskier to breath O2 at 20ft (from a bends perspective), but it sure speeds your decompression. In that sense SGFs would not be useful in all cases as a decompression algorithm as deco schemes attempt to limit risk. Hope that helps. Kevin
(Offline)

22nd December 2006, 14:25	#23 (permalink)
saturation New Member Current Rebreather/s: Not Bought Yet Other Rebreather/s: Not Bought Yet Join Date: Sep 2005 Location: Philadelphia, PA, USA Posts: 53	Re: Gradient Factor for Dummies I'd like to say how good a job you did on this article. I just saw it today. Everyone who does decompression diving should read it, along with Baker's original paper. __________________ Safe Diving and Best Wishes, I.G. Saturation, MD { Comments are informational only and not meant to be medical advice applicable to a particular case. Consult your physician when considering information posted here }
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23rd December 2006, 15:42	#24 (permalink)
Gene_Hobbs Custom Title Allowed! Current Rebreather/s: Not Bought Yet Other Rebreather/s: Not Bought Yet Join Date: Sep 2006 Location: Durham, NC, USA Posts: 118	Re: Gradient Factor for Dummies Kevin, Thank you so much for this! This will be a great aide for teaching. It does bring up another question though, how do you teach M-values? We used to just have people read Workman's CALCULATION OF DECOMPRESSION SCHEDULES FOR NITROGEN-OXYGEN AND HELIUM-OXYGEN DIVES. 1965. RRR ID: 3367, NEDU: AD0620879 Then we would run profiles through DecoPlanner and talk about the graphical analysis there. A couple of days ago, I ran across this one. Systematic Guide to Decompression Schedule Calculations. Braithwaite, 1972 RRR ID: 3945, NEDU: AD075102 This one was written to make the calculations in the Workman report easier. Any other ideas? __________________ http://rubicon-foundation.org/ Home of the Rubicon Research Repository. For help getting started with the Repository, please visit our FAQ page. PLEASE support our work. "Oxygen is addictive and deadly. Everyone who uses it will eventually die" --RW Hamilton, PhD 1991
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24th December 2006, 00:27	#25 (permalink)
UWSojourner Pacific Northwest Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Feb 2005 Location: Portland Oregon Posts: 556	Re: Gradient Factor for Dummies Quote: (Originally Posted by Gene_Hobbs) It does bring up another question though, how do you teach M-values? "Understanding M-Values" by Erik Baker seems like a good reference also. Last edited by UWSojourner : 25th December 2006 at 00:22.
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