Question Deco gas as diluent for deco

[LFMarm]

Thanks all for the answers. It makes perfect sense as I had not considered that in deco diluent is not used except for flushes.

For those wondering where the hell the question was coming from, it's from this table I built to show what percentage of O2 you need depending on your setpoint as a function of dept. I use it to figure out when the eCCR will struggle to maintain the setpoint (red cells) and hence I would move to the low setpoint (and fly manually). My (wrong) idea was to switch to richer gas when getting shallower to be able to maintain higher pO2 without need for a lot of O2 addition. As many have explained, this doesn't make much sense because you are not adding a lot of diluent in those phases, unless you want to run SCR.

 

[Wibble]

Need to run a lower PPO2 than "is possible" as high PPO2 will increase the oxygen toxicity, both CNS (central nervous system) and Pulmonary Oxygen Toxicity (sore lungs!).

Hence lower PPO2s are used on deeper dives which by definition means more time at deco (= greater CNS and lungs)

 

[LFMarm]

Limit of diving hours over a 24h period based on CNS is the first row of the table — I use that as a starting point when planning dives for the week

 

[justinthedeeps]

You can tell when your deco's pretty much over... you stop offgassing.

At the start of your deco, you will be exhaling a lot of inert gas (N2, He) you've absorbed at depth. This isn't measured by the PPO2-measuring cells, so your loop 'fills'. Occasional nose exhales drops the loop back down to minimum loop, knocking off the slight buoyancy increase.

As you get to the end of your decompression, less inert gas is offgassed, so your buoyancy doesn't change as much.

Tangent time: what is the actual volume of gas absorbed and released from the body during a long (or saturation) dive?

I have heard from reputable source that it is "mLs," or actually a quite surprisingly small gas volume relative to loop volume. But I don't have real measurements in front of me. Certainly a research or clinical study somewhere has measured this?

 

[Merida Cave Diver]

I for one am glad you asked your question. I’m still working up to this level so all of this is good data. Thank you for asking and bringing up this topic.

 

[L13]

Tangent time: what is the actual volume of gas absorbed and released from the body during a long (or saturation) dive?

I have heard from reputable source that it is "mLs," or actually a quite surprisingly small gas volume relative to loop volume. But I don't have real measurements in front of me. Certainly a research or clinical study somewhere has measured this?

Back of the envelope calculation:

• human body can be approximated by 100kg of water
• solubility of N2 in water is~ 20mg/Kg
• density of N2 at 1 bar is ~1mg/mL
• saturation at ~40m --> 5bar
• at saturation 5x20mg/Kg = 100mg/kg
• at surface 80mg/Kg released --> 80ml/Kg
• 100Kg x 80ml/Kg = 8000ml

• ==> ~8 liters of N2 released ~= 0.3 cuft

Solubility in fat is ~3x solubility in water, so ~24 L ~= 1 cuft if we approximate the body with 100kg of fat. I believe fat is the body tissue with the highest solubility for N2 (and He).

Solubility of He is < 0.1 x solubility of N2

 

[FF / EMT]

Back of the envelope calculation:

• human body can be approximated by 100kg of water
• solubility of N2 in water is~ 20mg/Kg
• density of N2 at 1 bar is ~1mg/mL
• saturation at ~40m --> 5bar
• at saturation 5x20mg/Kg = 100mg/kg
• at surface 80mg/Kg released --> 80ml/Kg
• 100Kg x 80ml/Kg = 8000ml

• ==> ~8 liters of N2 released ~= 0.3 cuft

Solubility in fat is ~3x solubility in water, so ~24 L ~= 1 cuft if we approximate the body with 100kg of fat.

Solubility of He is < 0.1 x solubility of N2

On the front of the envelope can you do it in imperial

 

[L13]

On the front of the envelope can you do it in imperial

I converted the final answer to imperial but here it is again:
~0.3 cuft in water or ~1 cuft in fat. for a 130ft dive (220 lb diver).

 

[L13]

On the front of the envelope can you do it in imperial

Only masochists do the BOE calculations themselves in imperial.

 

[justinthedeeps]

Back of the envelope calculation:

• human body can be approximated by 100kg of water
• solubility of N2 in water is~ 20mg/Kg
• density of N2 at 1 bar is ~1mg/mL
• saturation at ~40m --> 5bar
• at saturation 5x20mg/Kg = 100mg/kg
• at surface 80mg/Kg released --> 80ml/Kg
• 100Kg x 80ml/Kg = 8000ml

• ==> ~8 liters of N2 released ~= 0.3 cuft

Solubility in fat is ~3x solubility in water, so ~24 L ~= 1 cuft if we approximate the body with 100kg of fat. I believe fat is the body tissue with the highest solubility for N2 (and He).

Solubility of He is < 0.1 x solubility of N2

Yay helium

Excellent upper bound calculation. Though it also predicts about 2 to 6 litres of standing nitrogen gas dissolved in our bodies at sea level, which seems like a lot!
Presumably actual amounts have been measured somehow during lab decompressions (gonna look for it..)