GUE Fundamentals

[ginti]

BTW what Gradient Factors are used for GUE decompression dive planning? Am assuming it's Buhlmann + GF nowadays?

Are computers used for CCR dives or is it still Ratio Deco using a bottom timer and a good memory?

These topics have been debated so often here on SB.

I am not going to answer about the GF, since at my level they do not have so much importance. But I can say that Ratio Deco is NOT the primary method to calculate decompression, at least in OC. It is the last resort (after proper planning, the computer...).

 

[ginti]

Interesting I thought all thing GUE were better MAYBE I'll have to think a bit about it..

Wow, the thread is degenerating a bit here What would you do better?

 

[Heat Miser]

Wow, the thread is degenerating a bit here What would you do better?

I won't be tempted down that rabbit hole

 

[Wibble]

When I dive 45 minutes at 30 meter with sp 1.2 I’m breathing 30% oxygen. When I’m diving ean32 my PO2 is 1.3. I have more deco with my JJ then I would have when dive oc. (Most of the dives the rebreather is more effective then oc, but sometimes oc is more effective.)

Why SP 1.2 for 'shallow' dives? Can you manually run it higher to avoid additional decompression obligation?

There's a slight irony in CCR that it often results in slightly longer decompression times than the equivalent open circuit dive. But the flexibility and ease of use of CCR... lovely.

 

[ginti]

I won't be tempted down that rabbit hole

ahahaha true, and this thread is also degenerating a bit too much

However, as a matter of fact, GUE course material says "All GUE standard gases have a desirable density lower than air at 30 m/100 ft"

Now, air density at 30m and 15°C should be 4.9 kg/m3, therefore I assume the gas density of the standard gases should always be lower than 4.9. However, that desirable makes things a bit more complicated. For instance, Nx32, having more oxygen (denser than the nitrogen), is slightly denser than air at 30m. But that is the order of magnitude.

I made again the calculation that you did, and I have fairly different results. ut I did it very quickly, so maybe I made some mistakes...

Out of curiosity, how did you calculate the density? I am really curious, so we can continue even in private if you prefer...

 

[Wibble]

Don't forget to add the additional "suck" required on a rebreather -- Work of Breathing. Many depend upon the angle of the diver.

 

[barth]

I haven't seen that before but its conservative, compared to the Gas densities, DI pO2, and ENDS I would target on the revo and reflect your 1.2 setpoint and higher He content. *Not that I have ever dived deeper than 60m.

I was taught to target 5.2 gr/l gas density, dil ppO2 1.1, END of 20-25m off a higher in loop setpoint 1.3

These are yours based off standard gases
View attachment 701715
But I can see why you were mentioning carrying Deco stages on shallow CCR dives, because of the onboard bailout in the D7 at a dil pO2 o 1,0 you start racking up up deco penalty a bit faster than someone with offboard bailout and higher O2 content

Interesting I thought all thing GUE were better MAYBE I'll have to think a bit about it..

I do like the gue configuration. It’s still possible to donate a longhose. Because of that it’s easy to dive with buddies who dive oc. When I do a tech dive in open water I only have decostage(s), there is no bottomstage. I also like the backup regulator with necklace. There is a lot of redundancy with the D7, Lola valves and regulators. When I do a shore dive I prefer to carry my deep bailout at my back.

But the set is also heavy and big, there are caves which I can dive with doubles but not with my gue ccr configuration because it’s to big. Same dilluent and bailout is not good for deco especially shallow dives (and cave dives). It’s possible to use the gue configuration in caves. But if I would only dive caves I would have choose an other configuration and/or rebreather.

I think the JJ especially in gue configuration is an allround rebreather.

 

[barth]

Why SP 1.2 for 'shallow' dives? Can you manually run it higher to avoid additional decompression obligation?

There's a slight irony in CCR that it often results in slightly longer decompression times than the equivalent open circuit dive. But the flexibility and ease of use of CCR... lovely.

The setpoint is always at 1.2. Only during deco I use my mav to add more oxygen. I know it’s also possible to change the setpoint in the controller during the dive, because I did try to change it during a deco stop at 6 meter.

I always use 1.2 during the dive, other teammembers are also using 1.2. Are your teammembers using always 1.3 or does it depends on the dive?

 

[BlueTrin]

ahahaha true, and this thread is also degenerating a bit too much

However, as a matter of fact, GUE course material says "All GUE standard gases have a desirable density lower than air at 30 m/100 ft"

Now, air density at 30m and 15°C should be 4.9 kg/m3, therefore I assume the gas density of the standard gases should always be lower than 4.9. However, that desirable makes things a bit more complicated. For instance, Nx32, having more oxygen (denser than the nitrogen), is slightly denser than air at 30m. But that is the order of magnitude.

I made again the calculation that you did, and I have fairly different results. ut I did it very quickly, so maybe I made some mistakes...
View attachment 701716

Out of curiosity, how did you calculate the density? I am really curious, so we can continue even in private if you prefer...

Is that the correct way to compute this?

Gases - Densities

Densities and molecular weights of common gases like acetylene, air, methane, nitrogen, oxygen and others.

www.engineeringtoolbox.com

Density at 1 bar for 32% O2 68% N2 at 0C:
1.25062 x 0.68 + 1.4290 x 0.32 = 1.3077

At 30m -> 4 bar (without saltwater coefficient)

That gives 5.23081?

There are densities for 20C as well.


Just re-read your post and it mentioned 15C … couldn’t find the factors on my phone

 

[ginti]

Is that the correct way to compute this?

Gases - Densities

Densities and molecular weights of common gases like acetylene, air, methane, nitrogen, oxygen and others.

www.engineeringtoolbox.com

Density at 1 bar for 32% O2 68% N2 at 0C:
1.25062 x 0.68 + 1.4290 x 0.32 = 1.3077

At 30m -> 4 bar (without saltwater coefficient)

That gives 5.23081?

There are densities for 20C as well.


Just re-read your post and it mentioned 15C … couldn’t find the factors on my phone

So, for the 15°C factor, I computed the density using the formula pV=nRT. Since p is constant, the volume is proportional to the temperature (in Kelvin, that is 15°C=388K, 0°C=373K), and the density is inversely proportional to it. Therefore, it is sufficient to divide the density by a factor =373/388.

Anyway, it doesn't matter what temperature you use for the calculation since the reference is the air at 30m. The important thing is that you compare densities relative to the same temperature.

So it's ok to use 5.2308, but you need to compare it with other densities calculated at the same temperature (0°C). I believe that the 5.2 that @Heat Miser mentioned comes exactly from here (nx32 density at 0°C and 30m deep), and nx32 is always higher than the other GUE CCR standard gases at their MOD (see my table). This is why I believe that @Heat Miser made a mistake, but maybe I am missing something or I myself made some mistakes in my calculation.

OC gases are a bit different; I did the calculation during the tec1, but I don't really remember it (shame on me!), and I am too lazy to do it again now

EDIT: I modified the post, you calculated it at 0°C, not at 20°C... at higher degree, the densities are even lower (compare it with my table)