trimix on scr

fragstone

Hi Guys,

i am interest to learn more about theoretical scr diving with trimix.

i search an formal to calculate the new flow rate on my dolphin orifice when i dive with any trimix instead with nitrox.

i hope you can help me?

thx for help and nice dives in 2008.
Thomas

evosale

http://f1.grp.yahoofs.com/v1/APN5R1h...TqLH6/flow.XLS

Gas flow through an orifice

How Dolphins work

Freef

__________________
David.

Currently owner of two differently sized ankles.

bletso

Been there done that. See http://www.airheadsscuba.com/flowcalc.html
of course I converted to MCCR back in the bad old days.

Dale

fragstone

thanks anybody for your quote.

@Dale: yes we are back in the good old time

Guust

Hope this program is helpfull

I used it years ago to dive the Dolphin on trimix to 60 meters.
I used the 32% nozzle with a tx25/25, inhalation mix was then tx20/25-30, perfect normoxic mix.

Note: this is on own risk

Attached Files

SCR trimixcalculator.zip (139.5 KB, 47 views)

Humuhumu

Hi,

Put a 28 to 30% Nitrox with a 40% injection and you're good for 60 meters :-) You don't need Trimix for that... (take care to have enough bail out with you: I use a 80 cft with air and limit my dive to 20 minutes botom time)

I know one diver who has tested it to 85 meters with Trimix... But I would say: go m-CCR if you want to do that !

Cheers

Bertrand

P.S.: take care, love you all :-)

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Humuhumu Nukunuku Apua'Ha

Freef

I would really advise against using nitrox below 40m in the Dolphin for two reasons.

1] Narcosis is a real problem.

2] Doing a loop flush [or even clearing a mask] with 30% in the tank at 60m will raise the pO2 of the mix up to 2.1 bar. What's more because of the amount of O2 in the loop in surface equivalent litres the pO2 will not drop that quickly-even if the supply is turned off.

Using a large [10L] tank and a fast flow rate will give a loop fO2 close to that of the tank fO2. The divers use of oxygen will also vary, so the supply fO2 has to be carefully considered. The supply fO2 also has to give a loop fO2 that can be breathed at all the depths the diver is considering using the rebreather at.

From my studies I think that the Dolphin can be used to 50m with a reasonable safety factor, and beyond this depth the diver's risk level increases considerably.

It must also be considered that the Dolphin scrubber was not designed for extremely deep diving, and the CO2 absorbsion abilities of the unit may not be adequate

__________________
David.

Currently owner of two differently sized ankles.

Freef

Getting back to the original question:

The flow rate of pure helium compared to pure nitrogen is only 5% different-not really enough to make a difference when calculating the fHe fO2 and fN2 of the loop compared to the cylinder.

__________________
David.

Currently owner of two differently sized ankles.

Humuhumu

Hi,

narcosis is a real problem, no discussion about that, everybody should know what is reasonable for him, I agree totaly.

Personaly I can live with a PpO2 of 2.1, it will not be a problem if the situation is not lasting: this means if this happens you will have to go on the bail out, that's it ! That's why I have a 80 cft AIR, with me.

What is also certain is that an original DOLPHIN is not made for that. The jacket is by far too small (or you dive with a dry suit), you cannot take pure O2 with you (difficult to attach it to the shell): I use pure O2 in the rebreather from -6 M to the surface, etc. The scrubber canister: never heard of any problem with the DOLPHIN canister, very robust and easy. In fact everything becomes more complicated from 50 to 55 M and over 55 M you must be very carefull.

For me the biggest problem is the mix: if you put 28% O2 and with a metabolism of 0.7 L/Min and a 40% injection (what is realistic, it's what I have, in some case even 0.6 L/Min) you will breath (loop) 22,8% O2 at 60 M and that means a PpO2 of 1,596, perfect to my point of view (you can stay 45 minutes...). Furthermore, if your metabolism is increasing (you get current and must use your fins, you have a problem, the water is cold, or whatever), this mix will be perfect up to 0.9 liters/Min (21,2% O2). and normoxic (>16%) and breathable up to 1,3 - 1,4 L/Min. even if at 1,4 L/Min. you will only have 16% O2,what is critical... (but in this case, you can flush or bail out).

With a 32% injection it's a little better, even with a 1.5 L/Min. metabolism you have 20% O2 (loop), but you will have a PpO2 of 1,7 at 60 M with a 0.7 metabolism and a 28% O2 mix.

If you put Trimix, you will solve the narcosis problem and reduce the nitrogen decompression problem (but you have a new Helium decompression issue...) not the O2 issue... and 25% O2 in the mix is poor, very poor...

So: go for a m-CCR version and fly the rebreather manualy.


Cheers

Bertrand

__________________
Humuhumu Nukunuku Apua'Ha