UTD Ratio deco discussion

[decompression]

Crap, I was hoping you weren't going to ask me that......lol. I can't give you gospel as I've been away from RD for quite awhile. In fact I did this profile to compare it to Buhlmann and to see if it was functional for both. Ill check my emails as I think I've asked AG about Altitude before....(thanks to you).

 

[boulderjohn]

you could supposedly use the same or similar profiles of RD at sea level or at altitude.

I still don't see why.

Let's just compare bubble growth. If a diver ascends from 136 FFW to the surface at sea level, we could calculate bubble growth according to Boyle's Law (P1V1=P2V2):

5*1=1*V2
5/1=V2
5=V2​

The bubble would grow to 5 times its size.

Here is that same bubble growth calculation at 6,000 feet (0.8 Atmospheres)

4.8*1=0.8*V2
4.8/0.8=V2
6=V2​

The bubble would grow to 6 times its size.

We would see the same kind of math in relation to the gradient between gas dissolved in the tissues and ambient pressure, meaning that it would take longer for dissolved gas to reach a safe pressure prior to final ascent.

​

 

[decompression]

As I'm a sea level kinda guy and I know altitude is a big deal where you are ....the occasional times I've dove altitude, it was a simple case of starting the dive with a time penalty. Dependant on elevation, your dive could start with X mins of bottom time at the beginning of the dive. That's for rec profile, what's the tech method you use?

 

[boulderjohn]

As I'm a sea level kinda guy and I know altitude is a big deal where you are ....the occasional times I've dove altitude, it was a simple case of starting the dive with a time penalty. Dependant on elevation, your dive could start with X mins of bottom time at the beginning of the dive. That's for rec profile, what's the tech method you use?

I use Buhlmann ZHL 16 C with the altitude correction built in. I usually plan the dive in Multi-deco. I enter the altitude in the settings, as I did in the plans I posted. (For me, it is usually 4,600 feet.) I can do the same plan at the same settings directly on my Shearwater computers. My computers adjust for altitude, both in their depth readings and in their decompression calculations during the dive.

As for the time at altitude prior to the dive, I am usually at that altitude long enough before the dive that it isn;t a factor, and I am usually getting DOWN to that altitude, so it really doesn't matter. If I were in an unusual situation, I would enter my time at that altitude into the settings, and that would become part of the computations.

 

[Kevrumbo]

I still don't see why.

Let's just compare bubble growth. If a diver ascends from 136 FFW to the surface at sea level, we could calculate bubble growth according to Boyle's Law (P1V1=P2V2):

5*1=1*V2
5/1=V2
5=V2​

The bubble would grow to 5 times its size.

Here is that same bubble growth calculation at 6,000 feet (0.8 Atmospheres)

4.8*1=0.8*V2
4.8/0.8=V2
6=V2​

The bubble would grow to 6 times its size.

We would see the same kind of math in relation to the gradient between gas dissolved in the tissues and ambient pressure, meaning that it would take longer for dissolved gas to reach a safe pressure prior to final ascent.

​

John, if you were at critical supersaturation with a leading Fast Tissue, then a Boyle Law bubble mechanics expansion pathology as above would be a vital issue whether at sea level or altitude. Classical RD with deepstops has you stop just as that leading fast tissue starts to desaturate, and not anywhere near a critical tension M-Value Buhlmann value where you encroach on tissue supersaturation. Bubble micro nuclei stays small, surface and internal tensions remain high and inert gas diffuses out from the bubble into the blood. Unfortunately as indicated by the NEDU Study, this comes at the expense of the Slow Tissues which are still on gassing while you're at the deepstops decompressing the Fast Tissues.

Again as implied by the NEDU Study, the trouble becomes the slow tissues at critical supersaturation bubbling over into VGE and possible DCI symptoms, a risk that's further exacerbated when surfacing at altitude and the lower ambient atmospheric pressure.

 

[boulderjohn]

John, if you were at critical supersaturation with a leading Fast Tissue, then a Boyle Law bubble mechanics expansion pathology as above would be a vital issue whether at sea level or altitude. Classical RD with deepstops has you stop just as that leading fast tissue starts to desaturate, and not anywhere near a critical tension M-Value Buhlmann value where you encroach on tissue supersaturation. Bubble micro nuclei stays small, surface and internal tensions remain high and inert gas diffuses out from the bubble into the blood. Unfortunately as indicated by the NEDU Study, this comes at the expense of the Slow Tissues which are still on gassing while you're at the deepstops decompressing the Fast Tissues.

Again as implied by the NEDU Study, the trouble becomes the slow tissues at critical supersaturation bubbling over into VGE and possible DCI symptoms, a risk that's further exacerbated when surfacing at altitude and the lower ambient atmospheric pressure.

I understand everything you said about the NEDU study, and I agree with it. I don't understand how it has any impact on what I wrote about the difference between ascending at sea level and ascending at altitude.

 

[decompression]

John, what does the altitude correction do to your profile? Any idea what it was based on? I know it's far more rare than sea level but were there studies done on buhlmann at altitude? Again, I have no real experience deco diving at altitude.

 

[Kevrumbo]

I understand everything you said about the NEDU study, and I agree with it. I don't understand how it has any impact on what I wrote about the difference between ascending at sea level and ascending at altitude.

John, if you were at critical supersaturation with a leading Fast Tissue, then a Boyle Law bubble mechanics expansion pathology as above would be a vital issue whether at sea level or altitude. Classical RD with deepstops has you stop just as that leading fast tissue starts to desaturate, and not anywhere near a critical tension M-Value Buhlmann value where you encroach on tissue supersaturation. Bubble micro nuclei stays small, surface and internal tensions remain high and inert gas diffuses out from the bubble into the blood. Unfortunately as indicated by the NEDU Study, this comes at the expense of the Slow Tissues which are still on gassing while you're at the deepstops decompressing the Fast Tissues.

Again as implied by the NEDU Study, the trouble becomes the slow tissues at critical supersaturation bubbling over into VGE and possible DCI symptoms, a risk that's further exacerbated when surfacing at altitude and the lower ambient atmospheric pressure.

Conversely, If you're using a pure Buhlmann Model and ascending to the leading tissue compartment's M-value with the highest allowable gradient between dissolved inert gas tensions and ambient pressure at a particular deco stop depth -and maximizing your offgas rate ideally without bubbling- then you would have to compensate a sea level dive table for a deeper theoretical ocean depth to convert an actual depth at altitude (as well as depth in ffw versus fsw if indicated) because of the greater decompression stress brought on by driving such a steeper pressure gradient, and surfacing at a lesser ambient pressure at altitude.

Remember that decompression models are based on pressure ratios, rather than on absolute pressures. In determining how your body rids itself of excess inert gas, decompression models rely upon the ratios of the pressures you experience at depth to the atmospheric pressure you experience after the dive. The key to not forming inert gas bubbles in your body -and thereby avoiding DCI- is to keep those pressure ratios within tolerable limits. Ratio Deco with deepstops unfortunately could not keep the slow tissues within tolerable limits for susceptible divers without significantly extending out an O2 shallow stop -whether at sea level or altitude.

 

[decompression]

I thought this was interesting.....

This research was done in 2007, pretty recent considering Buhlmann was circa 1960's....

In order to make any sea level dive table usable during high altitude diving, a new conversion factor is created. We introduce the standardized equivalent sea depth (SESD), which allows conversion of the actual lake diving depth (ALDD) to an equivalent sea dive depth. SESD is defined as the sea depth in meters or feet for a standardized sea dive, equivalent to a mountain lake dive at any altitude, such that [image omitted] [image omitted] [image omitted] Mountain lakes contain fresh water with a relative density that can be standardized to 1,000 kg m(-3), and sea water can likewise be standardized to a relative density of 1,033 kg m(-3), at the general gravity of 9.80665 m s(-2). The water density ratio (1,000/1,033) refers to the fresh lake water and the standardized sea water densities. Following calculation of the SESD factor, we recommend the use of our simplified diving table or any acceptable sea level dive table with two fundamental guidelines: 1. The classical decompression stages (30, 20, and 10 feet or 9, 6, and 3 m) are corrected to the altitude lake level, dividing the stage depth by the SESD factor. 2. Likewise, the lake ascent rate during diving is equal to the sea ascent rate divided by the SESD factor.

Further, after some research (me and a Google) essentially between 1000-4000 feet the addition is about 20' to sea depth, so alt depth 150 is sea depth 170.

So, using RD 2.0, you could simply add 20' to max depth, putting the dive at 200' and adding back the 10 minutes of deco.

So, comparing that profile to the 6000' Buhlmann dive, RD is considerably more conservative.

 

[Kevrumbo]

I thought this was interesting.....

This research was done in 2007, pretty recent considering Buhlmann was circa 1960's....

In order to make any sea level dive table usable during high altitude diving, a new conversion factor is created. We introduce the standardized equivalent sea depth (SESD), which allows conversion of the actual lake diving depth (ALDD) to an equivalent sea dive depth. SESD is defined as the sea depth in meters or feet for a standardized sea dive, equivalent to a mountain lake dive at any altitude, such that [image omitted] [image omitted] [image omitted] Mountain lakes contain fresh water with a relative density that can be standardized to 1,000 kg m(-3), and sea water can likewise be standardized to a relative density of 1,033 kg m(-3), at the general gravity of 9.80665 m s(-2). The water density ratio (1,000/1,033) refers to the fresh lake water and the standardized sea water densities. Following calculation of the SESD factor, we recommend the use of our simplified diving table or any acceptable sea level dive table with two fundamental guidelines: 1. The classical decompression stages (30, 20, and 10 feet or 9, 6, and 3 m) are corrected to the altitude lake level, dividing the stage depth by the SESD factor. 2. Likewise, the lake ascent rate during diving is equal to the sea ascent rate divided by the SESD factor.

Further, after some research (me and a Google) essentially between 1000-4000 feet the addition is about 20' to sea depth, so alt depth 150 is sea depth 170.

So, using RD 2.0, you could simply add 20' to max depth, putting the dive at 200' and adding back the 10 minutes of deco.

So, comparing that profile to the 6000' Buhlmann dive, RD is considerably more conservative.

For a given altitude A, the atmospheric pressure Pa (in atm) at that altitude is

Pa = (1 atm) * exp(5.255876 * ln(1 – (C * A))).

where C = 0. 0000068756 / 1 foot = 0. 000022558 / 1 meter,
depending on whether the altitude is given in feet above sea level or in meters above sea level.

With a calculated Pa (Pressure at Altitude determined from the above equation) and given Da (actual depth at Altitude in ffw), we have the general equation below giving Theoretical Ocean Depth (TOD), and with it we can use the dive tables that are based upon diving in the ocean, such as the PADI RDP:

TOD = Da * (1 atm / Pa) * (33 fsw / 34 ffw).