How conservative is the PPO2 Limit

[Foxfish]

One of the other questions I've raised in the past and which still eludes me is why the MOD limit. Why not just have the computer monitor cumulative exposure time for the various depths you dive. The deeper you dive, the quicker your fraction of allowable exposure would accumulate.

A couple of possible reasons spring to mind. One is that at depths exceeding MOD the PPO2 of inspired air is so high that it becomes dangerous regardless of exposure time. That being the case I'm interested in a medical explanation in simple english.

The other possibility is that the limit is purely pragmatic. On a single dive it is hard to exceed the time limit even using the PPO2 of 1.6 b.

 

[Doppler]

Steve, are you saying that the CNS clock is indeed tracking CNS toxicity. In all of my training, including my admittedly limited technical training, I have been told that the term CNS clock is a misnomer and is actually tracking pulmonary o2 toxicity (as do OTU's). Granted this is a bit of an educational issue as opposed to a safety issue, if you are tracking both CNS % and OTU's and obeying the prescribed limits (both single dive and cumulative). If I read/understood you correctly, I would assume that this is one of those myths you are referring too.

Mark

Hi Mark: thanks for the question. Ironically, my editor and I are working through the gas toxicity chapter of my new book! I could do a quick cut and paste but that might make me unpopular. LOL

Anyhow, I would suggest some reading. This entry is not bad Oxygen toxicity - Wikipedia, the free encyclopedia and I believe I may have contributed a couple of paragraphs but you might also order TDI's Nitrox book. I co-authored that one and unless it's been radically changed, should cover much of what seems to be confusing you.

In simplest terms, there are TWO types of oxygen toxicity that we need to be aware of. The PAUL BERT effect which interacts with the central nervous system (CNS toxicity) and the Lorrain Smith effect which is a pulmonary condition and which is NOT historically of much concern for divers.

CNS toxicity is managed by tracing DOSE.... that's to say, oxygen depth over allowable time. I have posted links to and copied and pasted NOAAs CNS table in this thread already. To track CNS exposure, we must know the partial pressure of oxygen delivered by our mix at depth and the time we spend breathing it. The partial pressure of oxygen is determined by the total pressure at depth (for instance 30 metres is 4 bar) and we multiply that by the fraction of oxygen in our mix, let's say we have an EAN35. 4 X 0.35 = 1.4 bar... this is our oxygen pressure. The NOAA single dive limit for that partial pressure is 150 minutes. If our bottomtime were 50 minutes, we will have used about one-third of our allowable oxygen time and the CNS "clock" will be at about 34%. NOAA tables also contain 24-hour CNS limits. The potential outcome of being sloppy with CNS toxicity is convulsion and that usually results in death by embolism, drowning or heart attack.

Pulmonary toxicity is NOT THE SAME. Simply put, it is not a factor for this sort of dive, or really for any sort of recreational dive and it is not worth the bother of tracking except in extreme technical dives. Although severe cases of pulmonary toxicity cause true health concerns , the potential outcome of pulmonary toxicity for a diver is a sore throat and shortness of breath... SEVERAL ORDERS OF MAGNITUDE LESS OF A CONCERN.

OTUs are used to track pulmonary toxicity and have NOTHING TO DO WITH CNS.

Oxygen toxicity can also effect vision, but forget that for the time-being.

 

[markmantei]

Hi Mark: thanks for the question. Ironically, my editor and I are working through the gas toxicity chapter of my new book! I could do a quick cut and paste but that might make me unpopular. LOL

Anyhow, I would suggest some reading. This entry is not bad Oxygen toxicity - Wikipedia, the free encyclopedia and I believe I may have contributed a couple of paragraphs but you might also order TDI's Nitrox book. I co-authored that one and unless it's been radically changed, should cover much of what seems to be confusing you.

In simplest terms, there are TWO types of oxygen toxicity that we need to be aware of. The PAUL BERT effect which interacts with the central nervous system (CNS toxicity) and the Lorrain Smith effect which is a pulmonary condition and which is NOT historically of much concern for divers.

CNS toxicity is managed by tracing DOSE.... that's to say, oxygen depth over allowable time. I have posted links to and copied and pasted NOAAs CNS table in this thread already. To track CNS exposure, we must know the partial pressure of oxygen delivered by our mix at depth and the time we spend breathing it. The partial pressure of oxygen is determined by the total pressure at depth (for instance 30 metres is 4 bar) and we multiply that by the fraction of oxygen in our mix, let's say we have an EAN35. 4 X 0.35 = 1.4 bar... this is our oxygen pressure. The NOAA single dive limit for that partial pressure is 150 minutes. If our bottomtime were 50 minutes, we will have used about one-third of our allowable oxygen time and the CNS "clock" will be at about 34%. NOAA tables also contain 24-hour CNS limits. The potential outcome of being sloppy with CNS toxicity is convulsion and that usually results in death by embolism, drowning or heart attack.

Pulmonary toxicity is NOT THE SAME. Simply put, it is not a factor for this sort of dive, or really for any sort of recreational dive and it is not worth the bother of tracking except in extreme technical dives. Although severe cases of pulmonary toxicity cause true health concerns , the potential outcome of pulmonary toxicity for a diver is a sore throat and shortness of breath... SEVERAL ORDERS OF MAGNITUDE LESS OF A CONCERN.

OTUs are used to track pulmonary toxicity and have NOTHING TO DO WITH CNS.

Oxygen toxicity can also effect vision, but forget that for the time-being.

Thanks for the detailed response Steve. I don't doubt you, but wow, someone should let PADI know...

From the DSAT tech deep manual, page 152:
"The "CNS clock" also manages pulmonary oxygen toxicity"

Mark

 

[Foxfish]

Ran some numbers to check CNS time limits based on the allowable oxygen exposures at the various depths of five hypothetical dives in one day. Results were as follows calculated using a simple Excel spreadsheet.

DIVE 1
DEPTH	PPO2	TIME	OTL	FOTL	CSDFOTL	CDFOTL
10	0.65	18	540	3.3	3.3	3.3
15	0.82	3	360	0.8	4.2	4.2
20	0.98	2	300	0.7	4.8	4.8
25	1.15	1	240	0.4	5.3	5.3
30	1.31	10	180	5.6	10.8	10.8
32	1.38	10	180	5.6	16.4	16.4
	TOTALS	44		16.4
	Surface interval 1 hour
DIVE 2
DEPTH	PPO2	TIME	OTL	FOTL	CSDFOTL	CDFOTL
10	0.65	10	540	1.9	1.9	18.2
15	0.82	6	360	1.7	3.5	19.9
20	0.98	2	300	0.7	4.2	20.5
25	1.15	3	240	1.3	5.4	21.8
30	1.31	6	180	3.3	8.8	25.1
32	1.38	6	180	3.3	12.1	28.5
	TOTALS	33		12.1
	Surface interval 1 hour
DIVE 3
DEPTH	PPO2	TIME	OTL	FOTL	CSDFOTL	CDFOTL
10	0.65	35	540	6.5	6.5	34.9
15	0.82	1	360	0.3	6.8	35.2
20	0.98	3	300	1.0	7.8	36.2
25	1.15	1	240	0.4	8.2	36.6
30	1.31	1	180	0.6	8.7	37.2
32	1.38	12	180	6.7	15.4	43.9
	TOTALS	53		15.4
	Surface interval 2 hours
DIVE 4
DEPTH	PPO2	TIME	OTL	FOTL	CSDFOTL	CDFOTL
10	0.65	20	540	3.7	3.7	47.6
15	0.82	6	360	1.7	5.4	49.2
20	0.98	2	300	0.7	6.0	49.9
25	1.15	2	240	0.8	6.9	50.7
30	1.31	1	180	0.6	7.4	51.3
32	1.38	16	180	8.9	16.3	60.2
	TOTALS	47		16.3
	Surface interval 2 hours
DIVE 5
DEPTH	PPO2	TIME	OTL	FOTL	CSDFOTL	CDFOTL
10	0.65	1	540	0.2	0.2	60.4
15	0.82	1	360	0.3	0.5	60.6
20	0.98	5	300	1.7	2.1	62.3
25	1.15	7	240	2.9	5.0	65.2
30	1.31	2	180	1.1	6.2	66.3
32	1.38	13	180	7.2	13.4	73.6
	TOTALS	29		13.4
DAILY TOTALS	206		73.6
NOTES
PPO2 - Partial pressure of oxygen for FO2 of 32 %
OTL - Oxygen Time Limit defined by as the allowable NOAA time at the given depth.
FOTL - Fraction of Oxygen Time Limit = TIME/OTL as percentage.
CSDFOTL - Cumulative Single Dive FOTL as percentage.
CDFOTL - Cumulative Daily FOTL Limit as percentage.
NDL's checked using Suunto Dive Manager 3.1
Assumes diving up to within 0 minutes of the NDL.
No reduction in CNS exposure during the surface interval.

 

[Foxfish]

As a comparison, the following is a printout of the Suunto Dive Manager results for the fifth dive of the day. Note the OTU's and the CNS exposure percentage values.

 

[Foxfish]

Similar tables again but this time using PPO2 36%

DIVE 1
DEPTH	PPO2	TIME	OTL	FOTL	CSDFOTL	CDFOTL
10	0.65	19	540	3.5	3.5	3.5
15	0.82	3	360	0.8	4.4	4.4
20	0.98	2	300	0.7	5.0	5.0
25	1.15	1	240	0.4	5.4	5.4
27	1.21	21	210	10.0	15.4	15.4
	TOTALS	46		15.4
	Surface interval 1 hour
DIVE 2
DEPTH	PPO2	TIME	OTL	FOTL	CSDFOTL	CDFOTL
10	0.65	18	540	3.3	3.3	18.8
15	0.82	10	360	2.8	6.1	21.5
20	0.98	2	300	0.7	6.8	22.2
25	1.15	3	240	1.3	8.0	23.5
27	1.21	15	210	7.1	15.2	30.6
	TOTALS	48		15.2
	Surface interval 1 hour
DIVE 3
DEPTH	PPO2	TIME	OTL	FOTL	CSDFOTL	CDFOTL
10	0.65	33	540	6.1	6.1	36.7
15	0.82	1	360	0.3	6.4	37.0
20	0.98	6	300	2.0	8.4	39.0
25	1.15	1	240	0.4	8.8	39.4
27	1.21	13	210	6.2	15.0	45.6
	TOTALS	54		15.0
	Surface interval 2 hours
DIVE 4
DEPTH	PPO2	TIME	OTL	FOTL	CSDFOTL	CDFOTL
10	0.65	20	540	3.7	3.7	49.3
15	0.82	6	360	1.7	5.4	51.0
20	0.98	2	300	0.7	6.0	51.6
25	1.15	2	240	0.8	6.9	52.5
27	1.21	1	210	0.5	7.3	52.9
	TOTALS	31		7.3
	Surface interval 2 hours
DIVE 5
DEPTH	PPO2	TIME	OTL	FOTL	CSDFOTL	CDFOTL
10	0.65	1	540	0.2	0.2	53.1
15	0.82	1	360	0.3	0.5	53.4
20	0.98	1	300	0.3	0.8	53.7
25	1.15	1	240	0.4	1.2	54.2
27	1.21	33	210	15.7	16.9	69.9
	TOTALS	37		16.9
DAILY TOTALS	216		69.9
NOTES
PPO2 - Partial pressure of oxygen for FO2 of 36 %
OTL - Oxygen Time Limit defined by as the allowable NOAA time at the given depth.
FOTL - Fraction of Oxygen Time Limit = TIME/OTL as percentage.
CSDFOTL - Cumulative Single Dive FOTL as percentage.
CDFOTL - Cumulative Daily FOTL Limit as percentage.
NDL's checked using Suunto Dive Manager 3.1
Assumes diving up to within 0 minutes of the NDL.
No reduction in CNS exposure during the surface interval.

 

[Foxfish]

The following is a printout of the Suunto Dive Manager results for the fifth dive of the day for PPO2 36%. Note the OTU's and the CNS exposure percentage values.

 

[Doppler]

Thanks for the detailed response Steve. I don't doubt you, but wow, someone should let PADI know...

From the DSAT tech deep manual, page 152:
"The "CNS clock" also manages pulmonary oxygen toxicity"

Mark

No worries... and yes, I know about that entry in PADI's books. In effect, that point is true: in that if you watch out for CNS loading, the likelihood of a pulmonary complication is very, very slim. (For the record, my personal brush with pulmonary toxicity was following a six-hour decompression and our CNS clocks were very much over the limit.. so hardly a normal dive.) And in practice, most technical divers only BOTHER TO TRACK CNS since if it is managed correctly, WHOLE BODY or PULMONARY can be ignored.

HOWEVER, PADI's materials have begat a whole raft of instructors who believe that OTUs have something DIRECTLY to do WITH CNS.... and that is NOT the case. And believe it or not, I, and many others, have spoken with PADI on the topic.

---------- Post added September 19th, 2013 at 08:18 AM ----------

Ran some numbers to check CNS time limits based on the allowable oxygen exposures at the various depths of five hypothetical dives in one day. Results were as follows calculated using a simple Excel spreadsheet.

And your conclusions?

 

[Foxfish]

And your conclusions?

1. A diver who uses PPO2=1.4 b would be very unlikely to exceed to allowable daily NOAA limits even when doing 5 dives a day near the MOD.
2. I suspect that if you used PPO2=1.6 b you could quickly approach the NOAA daily limit.
3. If you set the PPO2=1.6 b in the SDMP (Suunto Dive Manager Planner) and go a few metres over the MOD the CNS exposure (I think this is what I call CSDFOTL - Cumulative Single Dive FOTL as percentage on my spreadsheet) quickly exceeds 100%.
4. There was a big difference between the results from my spreadsheet and the SDMP. Some of that would be due to rounding and average depths assumed for a time interval. But it is also obvious the way the SDMP calculates the CNS and OTU limits is more complicated than a simple integration of what I've called the FOTL's (Fraction of Oxygen Time Limit = TIME/OTL as percentage).
5. The CNS exposure in the SDMP is affected by the dives you've done in previous dives on that day.
6. I suspect that part of the reason for the confusion on this topic is all the acronyms that are used. Clearly I've done my bit to add to the confusion. I suspect for example that OTU's are defined differently on different computers. Edit *On the Suunto Cobra, the OTU's are a measure of exposure to pulmonary toxicity and the OLF (Oxygen Limit Fraction) is used to report both the CNS and pulmonary exposure as mentioned previously.*