scr's ppo2 vs f02

[max101]

on a scr would it not be more usefull knowing your f02 rather than your ppo2, i would have thought this is more important because, if you have a 40% mix in your cylinder, what would you set your computer to ? isnt it better through out the dive to know how much this has varied, i know your not suposed to do deco on drager type units but people must do ??????

[schford]

You should use a computer that is capable of monitoring and working out your deco in real time depending on your PPO2 eg VR3 HS Explorer etc.

Rule one of rebreather diving is know your PPO2 and it applies just as much to SCR as CCR in fact I think more people have probably been killed going hypoxic at shallow depth in SCR.

With an SCR you are never diving just one Fo2 it depends on your work rate and the flow rate of fresh mix into the loop. EG you swim hard into the current and you burn up more O2 but the same amount of gas is being injected into the Mix so your % of O2 in the loop is going to drop.

Stuart

[Ron Mahoney]

Originally Posted by max101

on a scr would it not be more usefull knowing your f02 rather than your ppo2, i would have thought this is more important because, if you have a 40% mix in your cylinder, what would you set your computer to ? isnt it better through out the dive to know how much this has varied, i know your not suposed to do deco on drager type units but people must do ??????

As stated it's vital to know your PPO2 - this will also be an indication of the fraction of O2 in the loop.

We have been diving the Drager down to 60m+ using a VR3 for decompression and a Drager Oxygauge to monitor the loop. According to the spreadsheet we developed, using a trimix on a 15li/min flow-rate we should have a PPO2 of between 1.17 & 1.38bar, having monitored the PPO2 I know that I metabolise between 1 & 1.5 li/O2/min under varying conditions. We also carry our deco gas (usually 70%) and run that through the loop also. It's not what Drager or Apeks reccomend nor sanction but if you know what you are doing then it is no more complicated than O/C trimix.

[wreckweasel]

tis indeed FiO2 you use for deco calcs. You should always monitor the PO2 though, although with some units the FiO2 is less dependant upon workload which would allow you to run the lump without a PO2 meter.

[narked at 90]

i belive if your diving a scr such as a CMF you are better knowing your fio2 not your po2 your po2 fultuates to wildly to give you a clear indication on what is happening with your unit,where watching your fio2 which does not swing wildly will give a good indication of the condition of your jets "are they still flowing"

this is the reason the vr3 shows fio2 in scr mode and also how it works out if it shows you a warning- if your fio2 changes to much from what you have told the vr3 it will give you a warning
as has been said always know your po2 but you should already know your MOD against you supply gas running it to deepth because your computer tells you your loop has a loop po2 that is ok wont help you when you spike the loop because you cleared you mask or flushed your loop!

kind regards john routley

[jknights]

Originally Posted by narked at 90

i belive if your diving a scr such as a CMF you are better knowing your fio2 not your po2 your po2 fultuates to wildly to give you a clear indication on what is happening with your unit,where watching your fio2 which does not swing wildly will give a good indication of the condition of your jets "are they still flowing"

As a newbie to CCR I can say that this seems to ring true to the Gas Laws that I know pretty well..

So it is pO2 for CCR and FiO2 for SCR when trying to monitor you pO2 and also to work out you deco.
Knowing your expected FiO2 and your real FiO2 and MOD gives you safer diving because the planning will have been done with a pO2/FiO2 in mind.

[narked at 90]

thats about it imho,

i also thought that you should always know your po2 when i first started diving rebreathers but it can be very misleading on scr where your fio2 is a much better indication of where your at and what the breather doing.
ccr you have to know your po2
scr for me beter to know your breathing what is comming out of your nitrox tank!
i did ring delta p about there scr mode on the vr3 as at the time it did'nt make much sense to any one but with a talk and a lot of thinking it all started to make sense!
regards john routley

Originally Posted by jknights

As a newbie to CCR I can say that this seems to ring true to the Gas Laws that I know pretty well..

So it is pO2 for CCR and FiO2 for SCR when trying to monitor you pO2 and also to work out you deco.
Knowing your expected FiO2 and your real FiO2 and MOD gives you safer diving because the planning will have been done with a pO2/FiO2 in mind.

[londonsean69]

Originally Posted by wreckweasel

tis indeed FiO2 you use for deco calcs. You should always monitor the PO2 though, although with some units the FiO2 is less dependant upon workload which would allow you to run the lump without a PO2 meter.

I am currently running my EDO (RB80 copy) without monitors. Before you all flame me I will explain why - my soldering is complete crap

I keep burning out circuit boards and am on the verge of actually buying an Oxyguage.

However, I have a pretty good idea of what is in my loop, as I do a full flush about every 10 mins, thus bringing the FiO2 back up to the FO2 level. Also, I am a crap rebreather diver because I am constantly leaking gas from my nose, and every time I clear my mask I use loads of gas. When I clear my mask I can feel and hear fresh gas being injected.

I dive fairly rich mixes and am staying shallow with it, although I did have 30/30 in it on the Hydrobox a couple of weeks ago. However, I will not buy a VR3 as I gave my buddy merciless ribbing when he bought his, especially as he bent it twice on the trot

[jknights]

Sean,

Drager OxyGuage with new O2 sensor (sensor purchased March 2005, still sealed in shipping container, so the 2year life is still to start) Unit is in mint condition new £450
ono £300 ?

email or PM if interested.

Jonathan