A question about the Partial Pressure of Oxygen

[DevonDiver]

Is this ppO2 not going above 1.4 (or 1.6 for short periods) what's behind the Maximum Operating Depth (MOD) of enriched air nitrox? The MOD would then be the depth at which the ppO2 of your mixture reached or approached 1.4?

Yes. Pretty much every agency sets a working limit between 1.3 - 1.45ppO2 for nitrox diving. This results in Max Operating Depths (MOD) for a given mixture that are dictated by that PPO2 threshold. A contingency limit (emergencies only) is set at 1.6ppO2.

Technical divers use the same working limits, but also utilise a 1.6ppO2 for the purposes of decompression only. This allows the divers to utilize higher O2 mixtures slightly earlier in the ascent, for maximum deco efficiency - it also enable the use of 100% O2 for the shallow (6m>) stops. 1.6ppO2 is deemed acceptable for decompression only, as the divers are typically static/inactive during this phase of the dive. Exertion/increased RMV/elevated cardio-vascular metabolism is often regarded as having a potential impact on susceptibility to Ox-Tox.

Helium diffusion rates are 2.65 times faster than Nitrogen.

Described very simplistically - this has the direct effect of 'super-charging' the half-times of the tissue compartments. Fast compartments become very fast compartments. Slow compartments become fast compartments etc. The knock-on is that ascent rate becomes much more critical... the very-fast compartments require a much slower ascent speed etc etc etc. Ascent rate violations are similarly far less forgiving. Likewise, whilst the faster compartments will consequently off-gas faster... the slower compartments will be more saturated than they would with air... in essence a wider 'spread' of tissue compartments' need off-gassing.

Total gas tension in blood was measured over a century ago, and partial pressures of indivigual gasses more than fifty years ago. There reallly is a pressure difference due to metabolizing oxygen into carbon dioxde. That much is not speculation. The pressure difference certainly influces diffusion and impacts bubble diameter and diffusion out of bubbles. But how much and its impact on decompression schedule is speculation. So we are left with rules of thumb from divers who were human lab rats.

So, going back to the OP's original questions, the metabolism of oxygen within tissues decreases the pressure exerted by the oxygen, which consequently allows bubbles to form/grow. The cumulative force of varied pressures (gas pressure, pressure from body tissues, ambient atmospheric pressure, bubble surface tension) is what counter-acts the pressure of inert gasses coming out of saturation and attempting to 'grow' into bubbles. Thus, an increased ppO2 is positive in maintaining an 'over-pressure' upon the bubble nuclei that prevents them from growing. Where O2 is absorbed into bubbles, it subsequently serves to 'deflate' them afterwards. Inert gasses are ineffective in providing such over-pressure, as they would be absorbed into the growing bubbles, rather than limiting them and would then persist in that state.

(did I get that right? the challenge of translating bubble dynamics into 'plain speak' )

 

[FPDocMatt]

Now I have to ask, What is a technical diver? Is it anybody who dives to do a specific job underwater (such as maintaining a drilling rig), or is it anybody who does decompression diving? If the former, then what do you call a diver who does decompression diving but isn't a technical diver?

...use the same working limits, but also utilise a 1.6ppO2 for the purposes of decompression only. This allows the divers to utilize higher O2 mixtures slightly earlier in the ascent, for maximum deco efficiency - it also enable the use of 100% O2 for the shallow (6m>) stops. 1.6ppO2 is deemed acceptable for decompression only...

It seems to me that what raises your ppO2 is going deep. If you're in the process of ascending, how would you go from having a lower ppO2 to a higher ppO2? For example, let's say you go to a depth that raises your ppO2 to 1.4. You swim around for awhile. Then you decide to decompress. How does your ppO2 now increase to 1.6 for decompression purposes? If you ascend, your ppO2 will drop, not increase.

Fast compartments become very fast compartments. Slow compartments become fast compartments etc. The knock-on is that ascent rate becomes much more critical... the very-fast compartments require a much slower ascent speed etc etc etc. Ascent rate violations are similarly far less forgiving. Likewise, whilst the faster compartments will consequently off-gas faster... the slower compartments will be more saturated than they would with air... in essence a wider 'spread' of tissue compartments' need off-gassing.

My (limited) understanding is that a fast compartment will saturate more quickly than a slow compartment. Thus, if a fast compartment is now a super-fast compartment, you'll reach your NDL sooner. But a faster compartment will also off-gas more quickly, so you would not have to decompress for as long. So with Helium you would get more saturated more quickly (requiring longer decompression times), but be able to decompress more quickly (requiring shorter decompression times). The question is, what is the net--longer or shorter decompression for a dive with Helium?

So, going back to the OP's original questions, the metabolism of oxygen within tissues decreases the pressure exerted by the oxygen, which consequently allows bubbles to form/grow. The cumulative force of varied pressures (gas pressure, pressure from body tissues, ambient atmospheric pressure, bubble surface tension) is what counter-acts the pressure of inert gasses coming out of saturation and attempting to 'grow' into bubbles. Thus, an increased ppO2 is positive in maintaining an 'over-pressure' upon the bubble nuclei that prevents them from growing.

This doesn't sound quite right to my neophyte mind. I doubt that oxygen absorbed into tissues is going to decrease the formation of nitrogen bubbles on ascent. For one thing, the oxygen is going to get metabolized quickly, so won't stay absorbed in tissues for long. For another, I don't think gases behave that way. One gas coming out of solution I don't believe is affected by the presence of another gas in the same solution.

 

[feitr]

Now I have to ask, What is a technical diver? Is it anybody who dives to do a specific job underwater (such as maintaining a drilling rig), or is it anybody who does decompression diving? If the former, then what do you call a diver who does decompression diving but isn't a technical diver?

Not sure there is an agreed upon definition for the boundary between rec and tech diving. I'd class commercial diving, diving mixed gases, decent penetration cave/wreck diving as technical and the rest (basically open water diving on air/nitrox) as rec. Doing open water decompression dives on air/nitrox is probably not considered tech diving (I certainly wouldn't consider it so). I guess the definition gets most blurry when you start to consider things like deep diving twin air tanks (to say 50m), diving air but using additional deco mixes, etc.

It seems to me that what raises your ppO2 is going deep. If you're in the process of ascending, how would you go from having a lower ppO2 to a higher ppO2? For example, let's say you go to a depth that raises your ppO2 to 1.4. You swim around for awhile. Then you decide to decompress. How does your ppO2 now increase to 1.6 for decompression purposes? If you ascend, your ppO2 will drop, not increase.

Deco/stage bottles. Tech divers will breath high O2 mixes (50% nitrox, pure O2, etc) to shorten deco times that are separate to the mixture breathed at depth.

 

[IanBrit]

Hi Matt,

These are all good questions.

On the first one, a tech diver is a diver who is PLANNING to not be able to escape directly to the surface should an emergency arise. This could be because they're inside a shipwreck, they could be deep in a cave system or may have a deco obligation (or combinations of these). Note that for every tech diver, you'll get a slightly different answer, but this will do for now. We often dive with different gas mixes for switching during dives or on a breather.

Your statement on the PPO2 dropping as you ascend is true but only if you're not on a breather or you're diving a single back gas mix. When performing decompression stops, it's standard practice to switch to a different gas that will spike the PPO2 up to 1.6 at the switch point. So, if you're coming up from, say, a 200fsw dive, you'd typically switch to EAN50 at 70fsw and then to EAN80 at 30fsw (or EAN100 at 20fsw). This has the effect of bumping the PPO2 to 1.6 and keeping it high until the next switch. That EAN50 will drop to around PPO2 0.62 before the switch to EAN80 and while it's clearly dropped, it's still better than what air would be at that point.

Why do this? As the other posters have commented, it's to do with controlling the nitrogen uptake and release. By switching to a higher PPO2, the body will be taking in less nitrogen on the deeper stops AND it's also encouraging the nitrogen in the body to leave quicker. Because the higher PPO2 means a lower PPN2, a sharper diffusion gradient between the cells and the blood and between the blood and the lungs will be created. Since the nitrogen will be trying to balance this mismatch, it's encouraged to leave the body quicker. The other postings cover the blood physics around bubble nuclei control and tensions etc and these, coupled with the fact your body is still under pressure, control the bubble formation while still encouraging the nitrogen to get out.

On the final bit about helium changing deco schedules, depending on the exact mix, it has a marginal affect and can speed up the deco process by a few minutes. The main reason for breathing it is to limit the narcosis at depth and not to extend bottom times dramatically. For example, a dive to 200fsw on an 18/35 mix with an EAN50 and EAN80 deco pair yields a total dive dive of 56 minutes for a BT of 20 minutes. If you ditch the helium and dive the same profile on air (with the two deco gases) you'll get a total dive time of 57 minutes. The main difference is that on the first dive your body will behave narcosis wise as if it's at 100 fsw. On the air dive, your narcosis level will match the dive depth AND you'll be breathing oxygen at a PPO2 of 1.48.

I hope that helps.

 

[DevonDiver]

Now I have to ask, What is a technical diver? Is it anybody who dives to do a specific job underwater (such as maintaining a drilling rig), or is it anybody who does decompression diving? If the former, then what do you call a diver who does decompression diving but isn't a technical diver?

General definitions:

Recreational Diver: A 'sport' diver whose qualifications/agency recommended limitations limit them to no greater than 40m/130ft total linear distance from the surface, no-decompression, no overhead environment beyond the light-zone, no gas-switching during a dive, no nitrox >40%.

PADI, SSI, NAUI etc... all provide 'recreational diver' programs. They clearly differentiate these from more advanced, 'technical' syllabus.

Technical Diver: A 'sport' diver whose qualifications/capability exceed recreational diving limits, particularly in regard to conducting decompression dives, including the use of multiple/mixed gasses for accelerated decompression and extended depth beyond the PPO2 or narcotic limits of air or enriched air. Depth limits exceed recreational diving maximums, tending to be limited by the characteristics of the mixed gasses used. Not limited to <40% O2. May dive in overhead environments beyond the light zone.

Technical diving is often considered (as an over-arching term) to include accelerated deco, cave and advanced wreck diving...either individually, or in combination. Many divers also consider CCR (closed circuit rebreather) diving to be 'technical' - although it does not strictly necessitate the use of gasses, procedures or limitations described above.

Technical diving is also dependent on the correct application of the skills, procedures and preparations that are taught as effective risk mitigation factors for the scope of diving undertaken. Doing a dive beyond recreational limits, but using recreational diving equipment and procedures does not make it technical diving. Thus, there is also a 'mindset' or psychological differentiation between technical divers and 'aggressive/extreme' recreational divers - one group seek to mitigate anticipated risks... the other group is happy to accept those anticipated risks.... or is just ignorant of them. The only logical term for such practices would be Stupid Diving, although the divers who engage in it would love to have you believe otherwise...

Commercial Diver: A 'working' diver who is employed to conduct various forms of construction, maintenance and salvage underwater. Training/function/role extends from shallow SCUBA dives, surface supplied air sources, surface (chamber) decompression... through to extended saturation diving from a working 'bell' at great depths underwater.

Sport Diving: Scuba diving that is not commercial, scientific (biology, archaeology etc) or military in application. Generally viewed as 'fun' diving, but also includes professional work, such as recreational/technical scuba instructors, underwater photographers and videographers (not otherwise acting in a scientific role).


In some cases the exact differentiation between 'rec' and 'tec' may be confused... some agencies allow 'lite' decompression within the recreational training system (i.e. CMAS and BSAC). Others allow the use of normoxic He mixes ('recreational trimix') for narcosis reduction within 'recreational' diving ranges (typically 30m/100' - 40m/130'). CCR diving is being increasingly marketed to the 'recreational' market.

Note: these general definitions are based upon training syllabus and the related agency recommended limitations amongst the 'bigger' scuba-training agencies. It does not reflect the 'actual diving' necessarily done by divers - some people view recommended limits as entirely optional. Divers who were trained prior to the 1980's were limited by modern training syllabus and, likewise, may have a greater scope (more blurring between rec and tec) than modern generations.

It seems to me that what raises your ppO2 is going deep. If you're in the process of ascending, how would you go from having a lower ppO2 to a higher ppO2? For example, let's say you go to a depth that raises your ppO2 to 1.4. You swim around for awhile. Then you decide to decompress. How does your ppO2 now increase to 1.6 for decompression purposes? If you ascend, your ppO2 will drop, not increase.

You change gas!

Technical divers... doing 'accelerated' decompression will move onto richer O2 mixes during ascent/deco schedule.

My (limited) understanding is that a fast compartment will saturate more quickly than a slow compartment. Thus, if a fast compartment is now a super-fast compartment, you'll reach your NDL sooner. But a faster compartment will also off-gas more quickly, so you would not have to decompress for as long. So with Helium you would get more saturated more quickly (requiring longer decompression times), but be able to decompress more quickly (requiring shorter decompression times). The question is, what is the net--longer or shorter decompression for a dive with Helium?

The difference lies in the range of tissue compartments that are saturated. Not only speed of saturation, but (due to small molecular size) the degree to which He can saturate into the body. Whilst this is dictated by the exact depth/profile of the dive, along with the exact mixture used - the end result is that He gasses don't permit drastically shorter decompression. In some cases, the deco might be longer.

This doesn't sound quite right to my neophyte mind. I doubt that oxygen absorbed into tissues is going to decrease the formation of nitrogen bubbles on ascent. For one thing, the oxygen is going to get metabolized quickly, so won't stay absorbed in tissues for long. For another, I don't think gases behave that way. One gas coming out of solution I don't believe is affected by the presence of another gas in the same solution.

But inert gas contained in a bubble isn't in the same solution as the gasses saturated into a liquid surrounding that bubble.

The key to off-gassing is the differential between gas partial pressures in varying states of the cardio-vascular system. The pressure differential between gasses contained in the lungs and those in solution in the blood passing the lung (blood-lung barrier). The differential between gasses saturated in tissues and those in the blood that flush the tissues. The differential between gas trying to form in a bubble and those in the liquid surrounding it.

 

[FPDocMatt]

So, if you're coming up from, say, a 200fsw dive, you'd typically switch to EAN50 at 70fsw and then to EAN80 at 30fsw (or EAN100 at 20fsw). This has the effect of bumping the PPO2 to 1.6 and keeping it high until the next switch. That EAN50 will drop to around PPO2 0.62 before the switch to EAN80 and while it's clearly dropped, it's still better than what air would be at that point.

Wow. Now I'm wondering how you keep track of the 12 compartments. Do you have some sort of super dive computer, and tell it what you're doing at each point?

Because the higher PPO2 means a lower PPN2, a sharper diffusion gradient between the cells and the blood and between the blood and the lungs will be created. Since the nitrogen will be trying to balance this mismatch, it's encouraged to leave the body quicker.

Now this makes sense to me.

Thank-you for the excellent explanations.

 

[FPDocMatt]

Technical Diver: A 'sport' diver whose qualifications/capability exceed recreational diving limits.

Okay, so what I thought was a technical diver is a commercial diver. Thank-you for the definitions.

But inert gas contained in a bubble isn't in the same solution as the gasses saturated into a liquid surrounding that bubble.

The key to off-gassing is the differential between gas partial pressures in varying states of the cardio-vascular system. The pressure differential between gasses contained in the lungs and those in solution in the blood passing the lung (blood-lung barrier). The differential between gasses saturated in tissues and those in the blood that flush the tissues. The differential between gas trying to form in a bubble and those in the liquid surrounding it.

So breathing in a higher ppO2 results in a higher pressure inside the lungs and blood vessels, and so the nitrogen in the surrounding tissues will feel the higher pressure from the surroundings, and therefore come out of solution more slowly? That makes a certain sense, actually.

 

[DevonDiver]

A picture is worth a thousand words...

I'm just mystified what those 1000 words would be!!!

Article here: Bubble Decompression Strategies

 

[Mr Carcharodon]

So breathing in a higher ppO2 results in a higher pressure inside the lungs and blood vessels, and so the nitrogen in the surrounding tissues will feel the higher pressure from the surroundings, and therefore come out of solution more slowly? That makes a certain sense, actually.

Breathing a higher fraction of oxygen just reduces the fraction of inspired inert gasses. Nearly all oxygen is transported by hemoglobin which is nearly saturated even at .21 ATA, so breathing a higher FO2 only slightly increases the pressure of oxygen, but it does significantly decrease the pressure of inert gasses. So if the inerts are eliminated in the inspired gas the pressure gradient in supersaturated tissues goes up and the diffusion rate goes up proportionally. Diffusion also occurs across bubble membranes but they change in size in response to total pressure as well above a threshold size. I am mixing apples in oranges here a bit. For dissolved gas models you can just track partial pressures because it is all diffusion, but for bubbles models total gas pressure, and surface tension matter as well.

 

[TSandM]

Maybe to put it a bit more simply . . . As you ascend, the partial pressures of the gases you are breathing drop -- ALL of them. This is what creates the gradient that causes gases to diffuse OUT of the blood/tissues and into the lungs, where they are expelled. This is why a pure Buhlmann (dissolved gas) model will send you shallow and have you do all your decompression time there.

On the other hand, bubbles expand as the ambient pressure decreases, and bigger bubbles are thought to be more problematic. So you would like to slow your ascent and do some decompression deeper, to control bubble growth, but then you aren't diffusing very effectively, because the partial pressures of inert gases are still too high in what you are breathing.

So what you do is switch to a gas that has less inert gases in it, and the only kind of gas that does that is a higher oxygen mix. It would be great to be able to switch to pure O2 for decompression . . . but that's where you run into the oxygen toxicity thing, that limits how much oxygen you can have in your deco gas, depending on the depth at which you want to start using it. Rich oxygen mixes for decompression are chosen for a kind of pragmatic utility, in that they produce a ppO2 of 1.6 (which is generally considered safe for the low exertion, relatively low time conditions of deco) at a depth where you are definitely offgassing, but bubble size is still being controlled.

Your questions about helium are good ones, and are the reason that helium's effect on decompression is still somewhat controversial. Some models will give you longer deco times on helium mixes, and others won't change or will even give you shorter ones.