Figuring out the right mix / MOD with the fiO2 formula

[Johnnythediver]

I have recently started my SCR class and bought a Dräger Dolphin rebreather. During the class I figured by using the fiO2 formula that I could use for example a EAN 40 flow rate (10.4 lpm) with a 32 mix and still be on the save side, as long as I monitor my ppo2. Essentially extend my MOD and available dive time, obviously increasing the decompression time. Has anybody experience with that?

 

[Life-Is-Good-Diver]

I have recently started my SCR class and bought a Dräger Dolphin rebreather. During the class I figured by using the fiO2 formula that I could use for example a EAN 40 flow rate (10.4 lpm) with a 32 mix and still be on the save side, as long as I monitor my ppo2. Essentially extend my MOD and available dive time, obviously increasing the decompression time. Has anybody experience with that?

Welcome to the board Johnny, and welcome to the silent world of rebreather's !!

Once you get certifed on your Dolphin, you should drop us a PM, as we live in central FL and goto the Key's alot to dive.

I also just got certified on the Dolphin, and bought my first rebreather !!

I don;t really understand fully what you are asking.....maybe your instructour will help you out during your class.... im missing somthing here when you say " obviously increasing the decompression time " ???

 

[Dr. Doug Ebersole]

I have recently started my SCR class and bought a Dräger Dolphin rebreather. During the class I figured by using the fiO2 formula that I could use for example a EAN 40 flow rate (10.4 lpm) with a 32 mix and still be on the save side, as long as I monitor my ppo2. Essentially extend my MOD and available dive time, obviously increasing the decompression time. Has anybody experience with that?

The flow rates on the Drager orifices are set so that at high VO2 (i.e., high workloads) you will not be breathing a hypoxic mixture. Your cylinder duration is directly related to your flow rate so that your cylinder will last longer on the 50% orifice than it will on the 40% orifice as the flow rate for the 40% is higher. Though "off-label", a lot of SCR divers will dive a 40% mix on the 50% orifice. This will give you a longer cylinder duration because of the lower flow rate of the 50% orifice but will allow you a deeper MOD because of the EAN 40 in your tank. The tradeoff is your inspired FIO2 will be lower (work out the SCR rebreather equation from your class) so you will have shorter no-decompression times than if you were diving EAN 40 with the 40% orifice. That having been said, I would strongly advise you not to "play around" with the various orifices and mix combinations until you (1) have a great deal of experience with your unit and (2) have a PO2 monitor (Drager oxygauge, Air ZO2/Oxy 2, VR3, HS Explorer, etc) so that you know exactly what you are breathing. If you have any other questions, please PM me. I'm an SCR instructor for both the Drager and the Azimuth. I'd be happy to help in any way possible.

 

[wedivebc]

What debersole said just want to add... It is strongly recommended that you do not exceed the MOD of your breathing gas even if you calculate a lower fiO2 because it can creep up on you. What I do is use a 1.6 limit for my MOD since that is the very highest my body will ever see on a dive but please don't exceed that limit.

cheers,
Dave...

 

[Life-Is-Good-Diver]

Hey Douglas , how you been , it's Marco.....i had a fast Question, Im diving 70' with a 42% mix in my tank, which gives me a MOD of 77' at a 1.4 PPo2, and i went with the 60% orfice, with the 5.7 flow rate, to give me longer tank duration. Workload= 1.0 for a reef, drifting along, im hearing that with that orifce, it may not be enough gas at that depth, i maybe should have went with the 50 orfice which gives a 7.3 LPM.

When we dove i remember that being way to much gas for me at that depth, and i was venting alot, loosing alot of gas, i don;t see a problem with the a 60% orfice at that depth, and i will be only inspiring 30% at depth, ( In the Loop ) breathing mixture.

Im trying to lower the LPM to get longer tank duration, but i also don't want there not be enough 02 in the loop if my workload goes up either, and im metablizing more O2 then i can get supplied with a 5.7 LPM flow rate........should i have any concern's ?

 

[hornetplt]

I recommend that if a single ppO2 monitor is used for an SCR, that it be for a sanity check of what you are really breathing in the loop and possibly for calculation of inert gas loading. Plan MOD based upon actual mix in tank. Do not relay on a single gauge(with no redundancy) to push depth based upon ppO2 gauge is telling you. The larger fluctuations in loop gas content of an SCR combined with reliance on one gauge(which could be inaccurate) is a place you do not want to be.

 

[Life-Is-Good-Diver]

I recommend that if a single ppO2 monitor is used for an SCR, that it be for a sanity check of what you are really breathing in the loop and possibly for calculation of inert gas loading. Plan MOD based upon actual mix in tank. Do not relay on a single gauge(with no redundancy) to push depth based upon ppO2 gauge is telling you. The larger fluctuations in loop gas content of an SCR combined with reliance on one gauge(which could be inaccurate) is a place you do not want to be.

Thank's, great advice well taken.

I use the HS Explorer single cell right now in the inhale bag for PPo2, and im going to goto either the VR3 with the Drager P conector, for another, or just go with the 3 cell HS explorer.

What do you do for PPo2 montiering?

I was thinking of starting a thread just on PPo2, im hoping to go with 1 cell in the bag, and 1-2 cell's on the shoulder right in the loop on the inhale side as back up, even if i switch it to CCR, i would keep the PPo2 montering the same i think.

 

[hornetplt]

Thank's, great advice well taken.

I use the HS Explorer and VR3 for pp)2, both p-port'ed into my Azimuth.

 

[Dr. Doug Ebersole]

Thank's, great advice well taken.

I use the HS Explorer and VR3 for pp)2, both p-port'ed into my Azimuth.

I have been using an Air Z O2 and Oxy2 for my Azimuth but the placement of the Oxy 2 interferes with looking to the right. It is always hitting my face when I turn that direction. Where do you hook in your HS Explorer and VR3?

 

[hornetplt]

I have been using an Air Z O2 and Oxy2 for my Azimuth but the placement of the Oxy 2 interferes with looking to the right. It is always hitting my face when I turn that direction. Where do you hook in your HS Explorer and VR3?

My HS Explorer and VR3 are p-ported into inhalation side of counterlung(back mounted).