Draeger Flow Rate Question

[Diver Dude]

I have been diving with a Draeger now for about six weeks. I have the Uwatec Oxy2 and associated computer so I know my po2 and inspired o2 fraction at all times.
So far my deepest dive has only been to 75 feet.
My diving buddy has had the same set up I have for 10 years so I get a lot of info from him.
My observation has been that Draeger’s recommended orifices and gas mixtures are very conservative. Understandably so because many of these units are dove without oxygen sensors.
After I took the rebreather class my first dive was with my buddy to 40 feet. He suggested I use 50% mix with the 60% orifice. I did and my max po2 was .89. Inspired o2 was 41-43%.
The next weekend we dove to 75 feet. My buddy suggested the I use 50% mix with the 60% orifice and I will end up with an inspired o2 of just under 40%. I did and my inspired o2 was 38-40%. Po2 was 1.2
If I used Draegers recommendation I would have used 40% mix with the 40% orifice. This would have created more bubbles and wasted a lot more gas, something I am trying avoid.
My question is, does anyone have a formula to figure out a more realistic inspired o2 using different orifices with different gas mixtures?

 

[wedivebc]

I have been diving with a Draeger now for about six weeks. I have the Uwatec Oxy2 and associated computer so I know my po2 and inspired o2 fraction at all times.
So far my deepest dive has only been to 75 feet.
My diving buddy has had the same set up I have for 10 years so I get a lot of info from him.
My observation has been that Draeger’s recommended orifices and gas mixtures are very conservative. Understandably so because many of these units are dove without oxygen sensors.
After I took the rebreather class my first dive was with my buddy to 40 feet. He suggested I use 50% mix with the 60% orifice. I did and my max po2 was .89. Inspired o2 was 41-43%.
The next weekend we dove to 75 feet. My buddy suggested the I use 50% mix with the 60% orifice and I will end up with an inspired o2 of just under 40%. I did and my inspired o2 was 38-40%. Po2 was 1.2
If I used Draegers recommendation I would have used 40% mix with the 40% orifice. This would have created more bubbles and wasted a lot more gas, something I am trying avoid.
My question is, does anyone have a formula to figure out a more realistic inspired o2 using different orifices with different gas mixtures?

sure,
fi02 = (fo2X flow lpm) - o2 consumption lpm
------------------------------------
flow - o2 consumption

 

[Diver Dude]

Dave,
My question wasn't that clear..sorry.
Take my 75 feet dive as an example.
If it wasn't for my experienced buddy, Iwould have used 40% mix with the 40% orifice.
Instead I took my buddies advice and used a 50% mix with the 60% orifice and the results were great....fio2 was 38-40%, almost no bubbles and half the gas usage.
Is there a formula that will prove that I can use a 50% mix with the 60% orifice at 75 feet? I have to believe flow rate would be involved in the equation to compensate for depth.
Thanks for the reply!
Bryan

 

[wedivebc]

Dave,
My question wasn't that clear..sorry.
Take my 75 feet dive as an example.
If it wasn't for my experienced buddy, Iwould have used 40% mix with the 40% orifice.
Instead I took my buddies advice and used a 50% mix with the 60% orifice and the results were great....fio2 was 38-40%, almost no bubbles and half the gas usage.
Is there a formula that will prove that I can use a 50% mix with the 60% orifice at 75 feet? I have to believe flow rate would be involved in the equation to compensate for depth.
Thanks for the reply!
Bryan

Yes I think my formula is just what you want. Assume you consume 1l/m at low work load and 2 l/m at mild work and 3 l/m at high work
the flow rate for a 60% orifice is between 5.2 - 6.5 l/p so under low work load (assuming your unit flows 6 l/m and you're using 50%) would work like this:
fi02 (fraction of O2 in loop) = (.5 * 6) -1 / (6-1) = .4 (or 40%)
so your low work load dive with a 60% orifice using 50% nitrox would result in a loop fraction of O2 of 40%
Of course that is in theory only since it would be irresponsible of me to suggest you use your rebreather for anything other than what it was designed for

 

[Diver Dude]

OK ..that formula makes sense..but I'm still not seeing something here. Please bear with me ...I am a novice at this and I very much value your thoughts on this theoretical question
Where does depth come into the equation?
Say I want to go to 130 feet. The equation would be [(.38*6)-1]/6-1=.25.
I'm still using the 60% orifice. Will the 60% orifice be enough to fill the inhalation bag at 130 feet?
If it is, why do I have the 50%, 40%, and 32% orifices?
Thanks,
Bryan

 

[Camerone]

Right - that equation now tells you that you're on 25% fiO2 in the loop.

Depth is irrelevant to your fiO2 with the Dolphin [caveat: as long as you have the required pressure differential across the mass flow orifice]. Depth affects the partial pressure of O2 in the loop, however, and you can convert with the standard formulas

[[ Depth (fsw) / 33 (fsw) ] + 1 ] * fiO2 = ppO2

The bags will fill no matter what depth you are at and what orifice you're using. The system bags will fill completely and vent. 60% will vent less, though, as there will be less gas waste due to discharge, because of the slower flow rate.

Out here, I always use a 60% flow with a ~40% fill, which puts me right around 30% in the loop. I also always monitor the loop, as well...

 

[Diver Dude]

Oooohhh. I get it!
I still have that one question.
Why does Draeger supply the other three orifices?
The differance between the 60% and 32% orifices is almost 3x the gas flow. What would be the situation to use that much gas?
Camerone: What is the deepest you have gone to while using the 60% orifice?

 

[wedivebc]

Oooohhh. I get it!
I still have that one question.
Why does Draeger supply the other three orifices?
The differance between the 60% and 32% orifices is almost 3x the gas flow. What would be the situation to use that much gas?
Camerone: What is the deepest you have gone to while using the 60% orifice?

The leaner the mixture the deeper you can go because of MOD which is OK at depth. Try some caculation using 32% or even air with a 60% orifice. When you start getting negative numbers things are going wrong. Try some some calculation using a workload of 3l/m you will see how fast you become hypoxic. The danger zone is not the depth it is the lean mixes you need to get to depth at the surface and when you are working harder like swimming back to the boat.

I have to ask why wasn't your question about 3 orifices covered on your course? You did get training right?

 

[Diver Dude]

Yes, I took the TDI course.
What happened is in the classroom I had enough to deal with just trying to understand rebreather theory, all the formulas, tearing down and setting up the unit, ect., ect. I realy didn't take the time to question what was being taught to me.
Now that I have a little bit of time on the Draeger...I'm having questions.
And yes, I could have called up my instructor and asked him. I just feel that the Rebreather Forum @ ScubaBoard is full of very experienced and knowledgable divers that are very well informed on rebreather theory and I value more than one opinion.
Anyway, I am going to play with that equation with differant orifices to get a better understanding of what is going on.
Thanks the info, guys!!
If anyone else would like to add their two cents worth.. please do!

 

[Camerone]

Camerone: What is the deepest you have gone to while using the 60% orifice?

Around 130 feet. I've switched to CCR at this point (Evolution), as the dives I'm doing have been at the point where it's a pain to get the Dolphin set up. You need to be extra careful with the monitoring required when you're operating below the MOD of your cylinder gas, and it was getting to the point that I was really exceeding what the Dolphin was designed for. It's a recreational rebreather, and I was using it for decompression dives, or at a depth where the variations in fiO2 with workload were becomming close to problematic.

I also got tired of never quite having the right fill in the cylinder for where I was going. I guess you could say that my diving started to move more technical, so I outgrew it...

I really dislike Drager's labelling of the orifices as 40/50/60%. It's pretty clear that they did this so that if you match source gas to orifice, even without monitoring, you're "safe", assuming you don't get a clog while diving in one of the jets.

On the other hand, I think it encourages people, even if they're trained (and I am absolutely NOT picking on you) to dive the unit without understanding what's really going on with how it works. I'm very glad to see you ask the question that you did.

One of the exercises that you should have done in training was calculate your RMV three times - at rest, during a normal swim, and kicking like a madman for a while. If you haven't done this exercise, you should on your next dive. Go down about 30 feet, running a "safe" flow orifice, and just hang out for a few minutes, breathing normally, but not moving (pretend it's a deco stop or something.) Note your fiO2 on a slate. Swim for a few minutes as you normally do, and again note your fiO2. Lastly, hang on to a rock and kick like hell, or swim as hard as you can for a couple of minutes, and note your fiO2.

When you get back up, plug them into the same formula that you used to calculate fiO2 earlier (but, instead, solve for the RMV, using the known fiO2 and cylinder gas) and work out your range.

I built myself a nice Excel spreadsheet that I use when I fill the Dolphin to figure out what I want in the bottle, how long it's going to last me, and what the MODs and fiO2s will be with all three jets and at light, moderate, and heavy workloads. I carry it on my Pocket PC Phone when I go to the dive shop and compute everything at fill time when I analyze my gas.