A couple of 'deep' questions

[David Pye]

Hi all,

I've got a couple of questions about technique/scrubbers. I know these questions would certainly be answered in MOD3 (if not before), etc. But at present, I don't have any rebreather training (or even a rebreather) and would like to know the answers!

If anyone could suggest any answers, I'd be interested to hear.

1) Using hypoxic diluents at the surface. How do people do this? The inspiration manual implies that it's not a great idea. So, do people:

Have multiple diluents, and switch between them during descent?
Rely on the controllers to bring the ppO2 up to a safe level
Manually inject O2 to bring the PPO2 up to a safe level

2) Scrubber efficiency at depth. Now, I've heard it said often that the scrubber is less efficient at depth. The logic that was used to explain it to me was that as the diver's CO2 production is unrelated to depth, that the scrubber has less 'contact' with the CO2 molecules it's supposed to remove as there are many times more molecules of inert gas flowing through it with each breath. But the thing I'm wondering is this: Assuming there is an acceptable ppCO2 the diver can tolerate, the number of CO2 molecules that are acceptable in the loop increase with depth. Therefore, while the number of molecules the scrubber needs to capture to keep the ppCO2 constant doesn't change, the total number of CO2 molecules 'allowed' in the loop will increase, thus increasing the probability that a CO2 molecule will impact the scrubber and be absorbed. So, I'm curious to know if this should cancel out the effects mentioned above. I realise I haven't done a very good job of explaining this, but hopefully someone might realise what I'm trying to say!

David

[NotDeadYet]

Originally Posted by David Pye

1) Using hypoxic diluents at the surface. How do people do this? The inspiration manual implies that it's not a great idea. So, do people:

I dive a KISS so this may not be applicable. Also I haven't done any training (it's all, er, been on the job) so I may be talking out my arse. I flush with O2 on the surface while the DSV is in my gob so I don't have to worry while swimming, descend, and manually manage the transition on to a weaker loop gradually which will probably include a substantial flush of dil at some point on the way down. I don't know if the controller on the desperation allows it.

So, I'm curious to know if this should cancel out the effects mentioned above. I realise I haven't done a very good job of explaining this, but hopefully someone might realise what I'm trying to say!

I'm not entirely sure I understand the question. In short, it may well be an urban myth that the scrubber doesn't work as well deep but for me I'm happy to believe it. It's one of the few situations that I blindly and happily accept the diving dogma. I don't want to take the chance on the KISS scrubber.

Cheers,

Stuart

[Phoenix]

Originally Posted by David Pye

<snip>
If anyone could suggest any answers, I'd be interested to hear.

1) Using hypoxic diluents at the surface. How do people do this? The inspiration manual implies that it's not a great idea. So, do people:

Have multiple diluents, and switch between them during descent?
Rely on the controllers to bring the ppO2 up to a safe level
Manually inject O2 to bring the PPO2 up to a safe level

My unit maintains a PO2 of .4 on the surface so not an issue, if i havent turned the O2 on then my HUD would be alarming.

As regards the CO2 issue have a look at:
http://www.ccrb.co.uk/downloads.html
the rebreather fundamentals paper. I notice you are in Brackhell, if you go to the BSAC club you can always discuss this with Nick Jewison, he'll be more than happy to help.

Ian

[Depth-junkie]

I use 10/52 heliair in my unit, hypoxic on the surface if breathed OC but as soon as i switch my unit on it maintains a low setpoint of 0.70, so thats giving me 70% O2 at the surface in the loop.

During decent the fresh gas pulled through the ADV or Manual dil button mixes with the current loop gas of that 70% O2 and dilutes it down, but it only drops the PP down to around 0.60 and within a second the oxygen controller has bought it back up to the setpoint of 0.70.

I have never had a low O2 alarm (set to go of at 0.40) whilst using a low 02 mix dil on decent.

The only downside i have found to using 10/52 over say 15/50 is that it cant be breathed OC all the way to the surface. (i use same dil for all dives regardless of depth) So even on shallow dives of say upto 30-35m i have to take a small bailout (3ltr of air) with me. where as 15/50 i could bailout on all the way to the surface just off the inboard gas, so i would not need the 3ltr of air.

No big deal really because i find 10/52 gives me better flexability, ease of mixing and remixing (accurate helium % with no analyser) qiuck dil flushes and ease of calculating PP02 at depth. So for me the upside out ways the down.

ATB
Gareth

[schford]

ppCO2 the diver can tolerate, the number of CO2 molecules that are acceptable in the loop increase with depth

No it doesn't it stays the same eg your measuring Partial Pressure. The fraction of CO2 that it is acceptable in the mix will decrease but the partial pressure that is acceptable stays the same.

Think of it in terms of O2 to get your head round it - your max PPO2 is 1.6 regardless of depth. It's the same with CO2 - although thats not quite true as when you couple nitrogen in with it the theory does go that the gases interact to contrive to screw you over so the deeper you go the less CO2 you can tolerate (maybe!)

[Drmike]

Originally Posted by David Pye

1) Using hypoxic diluents at the surface. How do people do this? The inspiration manual implies that it's not a great idea. So, do people:

Have multiple diluents, and switch between them during descent?
Rely on the controllers to bring the ppO2 up to a safe level
Manually inject O2 to bring the PPO2 up to a safe level

The manual is right it isnt a good idea - it adds risk for a newbie rb diver or even experienced divers during cluster f_cks. People let the controller maintain setpoint at the surface and/or manually inject O2. If they are doing that its fine - the problem happens if they forget or are prevented form doing it.

On the surface your PPO2 drops faster, if your exerting (climbing ladders, fighting surface current etc) and you havent noticed your solenoid isnt firing O2 into the loop, and your ADV is firing in hypoxic mix it doesnt take long to go hypoxic. ..which can kill you. Personally I turn off the ADV at the start of my descent from the bottom anyway.

If your dil is normoxic you have reduced one risk as a flush on the surface means you will be breathing a breathable mix.

I almost always dived hypoxic dils on even shallow dives. Now after a recent CF [ SmugMug - Drmike : Low PPO2 ] try to always dive normoxic (where depth permits) because you just never know what series of events can come together and leave you with only a hypoxic dil to breathe! Had I been using a normoxic dil during this CF it would have been a different story.


When the depths force you to use hypoxic dil then you rely on monitoring your gauges and the controller to mainatin setpoint and/or manually inject O2 on the surface (my prefference). Usually when ascending from a deco dive I have 100% in the loop at 6m and above anyway.

2) Scrubber efficiency at depth. Now, I've heard it said often that the scrubber is less efficient at depth.

It is. Different units drop efficency at different rates. Some popular units have huge reductions in efficiences as depth increases.

[turbanator]

These points, and a lot of others are covered in Bozanic's book, 'Mastering Rebreathers'.

Although, having said that, it's mainly based on air dils. but its worth getting if you're researching RBs and answers a lot of the questions that will occur to you at odd moments.

Usual disclaimer that it's no substitute for proper training etc. etc.

r
P

[Janos]

I think you are right to be worried about hypoxic dils. As Mike says, they might drop the loop ppO2 quicker than you'd expect on the surface. Also on the KISS my dil is plumbed into the DSV. I have developed a reflex to switch to OC, which is not a good idea on the surface!

And of course you don't have the electronics to watch over you on the KISS. The plus is that there are no electronics to f'up and kill you if you forget to monitor them!

That said, you're not going to need hypoxic dils above 60m. And there's an awful lot of diving above 60m.

Janos

[Padowan]

I only use the diluent only for the rebreather (no OC option on my inboard) and for most of my stuff that is beyond air depths (~30m for me) I use 10/52 heliair cos it's easy to mix, has a good PPO2 and END for the dives I do most of the time (50-80m) and it makes dil-flush PPO2 validations a breeze as 10 is an easy number to multiply things by.

My first OC source, my Bailout Valve (BOV), is plumbed directly into my deep bailout which currently is normoxic for dives up to about 80m (this means it's a slightly hot bottom mix bailout when diving 80m but as I'm not gonna be there for long and I want the same tin of gas to be good for me to use for all dives 50-80m, I consider it an acceptable compromise). I also do like DrMike and close off my ADV before I ascent, so any hypoxic diluent that gets during the ascent is my own doing, and once I get to 6m and am clear on deco, my loop is running 100% O2 and I can manually inject more O2 as I metabolise the loop down with the ADV shut off, and feel the loop going tight.

For dives deeper than 80m, the dil will be very hypoxic (6/72 is my next Heliair of choice, although I'm gonna start experimenting with Heliox 10/90 for my really deep stuff this year) and also your first OC source will be hypoxic - there's no real way to avoid this, so you need to accepth theis risk. However, to be honest, if you're doing dives that truly need a hypoxic diluent, you should know enough about what you're doing, and be suitably drilled, skilled and diligent that you can manage this safely.

My other caveat to this diotribe is that taking one breath of 10/52 on the surface is not gonna make you pass out immediately - but puffing away on it like a train swimming in a surface current will.

On your scrubber comments - CO2 absorbtion efficiency is a function of the total number of molecules that pass through the bed (flow rate and gas pressure), the shape and size of the bed, dwell time etc etc. There are lots of factors and modelling it is not an easy thing. The other thing is that the deeper you go and the denser the gasses you're breathing get, the Work of Breathing (WOB) increases which in turn increases the amount of CO2 production for a fixed external pyhsical work (ie, swimming at the same speed with the same drag in the same current)

Ok, that's enough of a ramble - hope theres something useful in there...

[David Pye]

Hi all, and thanks for your replies!

Phoenix - Thanks for that link, that PDF was most interesting. (And nice to meet you at Wraysbury too).

schford - I thought I had a reasonable grasp of how partial pressures 'worked' but I realise now that my understanding is a bit shakier than I thought. I understood the way things worked dealing with fractions of the loop, but not quantities. So, I think that gives me a reasonable idea of why the scrubbers are less effective at depth. Thanks!

Drmike - read your story re the near miss, most interesting.

Turbanator - I'll see if I can pick up that book. I'm not likely to get a 'breather in the near future, but want to make sure that when I finally do make the jump, I have as good a grounding as possible in the way things work as far as the theory goes.

Janos: I see what you mean about the diluent/BOV issue - noted!

Padowan - another thing I'd missed. The raised WOB at depth causing a greater production of CO2 also presumably contributes to the shortened scrubber life span.

In general, thanks also everyone for the info about how you manage hypoxic diluents. It's useful to know that there are plenty more than one answer for these things!

Thanks again,

David