Safety of radial vs axial scrubber

[emmbee]

To the rest... <sigh> ...

Read and despair

Gian,

I'm not sure you're correct in your understanding of CE testing. Technology evolves, knowledge advances. Standards get improved. That's fine. It doesn't the previous one were bad (although of course they may have been) or didn't work. It just means we can do better now.

14143 has tests for WOB and scrubber duration. The purpose of these is not to condense the behavior of a unit into a couple of numbers. They're so that dreadful designs with crap WOB don't get on the market, and manufacturers and instructors have reasonably conservative numbers that won't, sorry, shouldn't get people killed.

Now. Imagine a scrubber with a straw in it, letting some exhale pass through unfiltered. In effect, during duration testing, it will be as if the CO2 addition was set higher, so that's going to reduce the duration (and lower the WOB). Now compare that with another (smaller and longer) scrubber with the same duration and a higher WOB. Which one's safer? The one the keeps you with an elevated blood pCO2 for the whole dive?
Okay, that's stupid. What about a scrubber that as holes on the sides so that a fraction of the gas gets injected downstream directly. Won't change the duration much, if at all, may slightly decrease the WOB. "Safer", right? Except when you do get breakthrough, it's going to much be faster, because the gas will go through scrubber material that's already been partially used. I'm not sure that's safer at all.

The point of all this (and do correct me if I got the thinking wrong), is that those numbers do not tell the whole story about how a scrubber behaves, in fact they don't even tell the whole story about what they're measuring (I mean, what's with the .5kPa duration limit, how's that for arbitrary).

So you can't aggregate them into "safer". There's other factors to take into consideration.

While I'm talking ****, has anyone considered making scrubbers with rough walls, reduce channeling on the sides sort of thing? Or is it just a stupid idea?

As for the actual topic, thanks to Dave and Paul, again, and now Mark, Matthew and Paul H too.

Cheers,

Matthieu

Edit: if there's one thing that annoys me about CE standards, it's that the citizenry that paid for them with their taxes has to pay again to read them.

 

[alin]

Normally, I am not an avid poster, I preffer to just lurk around and red, I just post if I have something to say, otherwise I let others, more experienced than me, to write. I don't want to start an argur or any pissing contest with this but here we go:

First, thank to OP for this thread, it's (for me) one of the most interesting ccr threads that I have read in a long time.

To Gianaameri:
You are a very clever and very skill home builder, and you have some experience. But please, when Paul (both of them) or Dave Sutton gives you an advice or their oppinion, don't argue about. Those people (and a few others) have witnessed, or conducted or have access to much more tests that you or me or many will have / seen / read. You can talk and share oppinions / creeds and but don't be blind and say straight that radials are safer than axials since as you have said, you don't dive axials. Axial vs radial it's a longer debate and it's about detail / size / construction of the scrubber, as rEvo's daddy was trying to explain you in the beginning of this debate. If you don't understand this, stay away from this thread, some people (more competent) have give to the OP enough answers to make his own decision, in the first 2 pages already.

Have you tried axial for 10 hours? With stages and scooters and all s...t? And see if it's not safe? The majority of the manufacturers who are producing axials are knobs and they do it because axials are cheaper? Is the rebreather diving cheap? People have been diving axials to +200 (even on Inspo Classic as it comes, look on Safaga dive for example) longer than many of us were dreaming about rebreathers. The OP had a question, if you have no advice, better just read and maybe learn something. Don't go into details that do nothing but to confuse the poster. What was your first rebreather? How many of us are us are using the same rebreather as when we started? We all read, do researches, ask questions, before we buy our first unit and few years later we buy another one or we modify the original unit.

Back to the original poster, what types of dives you are planning to do? Deep, exploration dives? There are very few on the market, look into Boris or Aurora Blue (it looks very promising). Are you doing max 100 meters, a few hours of deco? Most of the units on the market will do. See what is easier for you to get, what are people around you using, how easy it's to service the unit, spare parts, etc..

I'll be in north of Norway at some point in this summer (in Mo i Rana), if you wanna try an axial, send me a pm.

All the best to all, safe diving and a Merry Christmas.

alin

 

[WFO]

Do you think the ISC Axial scrubber is better than the ISC 8 lbs. Radial scrubber, or maybe indifferent between one and the other, or maybe the other way around?

Which one of the two do you dive and why?

Not really fair to compare an 8lb radial with a 5.5lb axial for purposes of this thread.

Look at UT 240. All the budget in the world and they went for axial?

 

[gianaameri]

Read and despair

Gian,

I'm not sure you're correct in your understanding of CE testing. Technology evolves, knowledge advances. Standards get improved. That's fine. It doesn't the previous one were bad (although of course they may have been) or didn't work. It just means we can do better now.

14143 has tests for WOB and scrubber duration. The purpose of these is not to condense the behavior of a unit into a couple of numbers. They're so that dreadful designs with crap WOB don't get on the market, and manufacturers and instructors have reasonably conservative numbers that won't, sorry, shouldn't get people killed.

Now. Imagine a scrubber with a straw in it, letting some exhale pass through unfiltered. In effect, during duration testing, it will be as if the CO2 addition was set higher, so that's going to reduce the duration (and lower the WOB). Now compare that with another (smaller and longer) scrubber with the same duration and a higher WOB. Which one's safer? The one the keeps you with an elevated blood pCO2 for the whole dive?
Okay, that's stupid. What about a scrubber that as holes on the sides so that a fraction of the gas gets injected downstream directly. Won't change the duration much, if at all, may slightly decrease the WOB. "Safer", right? Except when you do get breakthrough, it's going to much be faster, because the gas will go through scrubber material that's already been partially used. I'm not sure that's safer at all.

The point of all this (and do correct me if I got the thinking wrong), is that those numbers do not tell the whole story about how a scrubber behaves, in fact they don't even tell the whole story about what they're measuring (I mean, what's with the .5kPa duration limit, how's that for arbitrary).

So you can't aggregate them into "safer". There's other factors to take into consideration.

While I'm talking ****, has anyone considered making scrubbers with rough walls, reduce channeling on the sides sort of thing? Or is it just a stupid idea?

As for the actual topic, thanks to Dave and Paul, again, and now Mark, Matthew and Paul H too.

Cheers,

Matthieu

I understand numbers.

EN14143 and NEDU have set some specific limits for WOB...

You are either IN the limits, or you are OUT.

Further within the limits is safer than further towards or outside the limits.

There is very little to understand beyond that.

Now, based on the available standards and testing methodology, I tried to answer the question raised:

"Safety of radial vs axial scrubber" - which is safer?

Is axial scrubber safer than radial scrubber? - Gerhard

The available numbers to date show that Radial is safer than Axial.

That is the only true and professional answer - the rest is unsubstantiated "theories."

 

[anarchista]

there is so much rubbish published on the internet. Where are your test results? Show me that for the same volume of sorb, the wide entry surface (slow flow) canister has longer duration than the smaller entry surface (faster flow) canister (within practical limits as scrubbers are used at this moment)

You showed that you have the right stuff?
As for the equation of continuity stream agree, that elementary physics.

rc greet

 

[matt]

If they worked fine, we would not have so many unexplained fatalities to date amongst experienced divers.

Which are attributed to axial scrubber?

In the case of this thread, you'd want to buy the rebreather with the lowest WOB and the highest scrubber Duration, and not the one with the highest WOB and lowest scrubber duration (despite its popularity amongst the uninformed).

It's one consideration. It's not the key one for me.

Matt.

 

[matt]

However this might be taken up by the units springs, or you might lose a few cm of scrubber bed due to channeling but not the whole thing.

Minor point: AFAIK the springs in the Inspiration (other may vary) are not for taking up the slack from bad packing they are to maintain pressure on the "o-ring of death" at the top of the stack. I appreciate your comment may not have been aimed at this unit specifically.

Matt.

 

[gianaameri]

Which are attributed to axial scrubber?



It's one consideration. It's not the key one for me.

Matt.

The only way to improve the situation which is not optimal (to say the least) is to reduce risk overall, precisely because we do not know the cause of the fatalities.

This means reducing the risk of the equipment.

This also means reducing the risk of human error.

Radial scrubbers reduce risk vs. Axial scrubbers, hence that is why I dive one.

BUT, I dive it because I have numbers for it, or I would not dive it (i.e. we need testing and numbers to reduce risk).

I am a risk manager, and this is my risk management assessment, based on available data.

You are free to make a different assessment.

 

[matt]

You are free to make a different assessment.

Thanks, I will.

This thread is in the Noobs forum, it's a bit OT in places too.

Matt.

 

[gianaameri]

Thanks, I will.

This thread is in the Noobs forum, it's a bit OT in places too.

Matt.

The Noob asking the question specifically asked about "safety."

Good customer service and spare parts availability are nice to have, but do not make a safer rebreather.

 

[matt]

The Noob asking the question specifically asked about "safety."

Good customer service and spare parts availability are nice to have, but do not make a safer rebreather.

My best guess: diving with broken unserviced rebreather is more risky than diving a unit where WOB difference is marginal.

The Noob asked nothing about CE which I see if filling the pages up again.

Matt.

 

[Janos]

The only way to improve the situation which is not optimal (to say the least) is to reduce risk overall, precisely because we do not know the cause of the fatalities.

Ah. You're going to sit under a tree.

If not - Gian - what's the acceptable risk of dying on a dive to you? 1 in 1000? 1 in 10000? 1 in 100,000? 1 in 1 million?

Janos

 

[emmbee]

Show me that for the same volume of sorb, the wide entry surface (slow flow) canister has longer duration than the smaller entry surface (faster flow) canister (within practical limits as scrubbers are used at this moment)

You showed that you have the right stuff?
As for the equation of continuity stream agree, that elementary physics.

Let's see.

We'll assume an ideal gas (reasonable at such low pressures), an incompressible fluid (reasonable at such low speed), an homogenous flow (reasonable again, if the scrubber is correctly packed)(more likely for an axial), and ignore the sink term.

The continuity equation simplifies to div v>=0.

For a cylindrical axial scrubber, that just means v is constant along the axis.
The volume is V=pi*r^2*h.
The flow speed v=u/s=u/(pi*r^2) u is the volumic flow, s is the section.
The path length is h.
The dwell time is t=h/v=pi*r^2*h/u
dwell time/volume is t/V=1/u.

For a double cylindrical radial scrubber, it's just a fancy way of saying v decreases linearly with r. At each r, we have the same volumic flow, so 2*pi*r*h*v=u, so v=u/(2*pi*r*h)
The volume is pi*(r2^2-r1^2)*h
The path length is r2-r1.
The dwell time is the integral between r1 and r2 of dr/v, which yields pi*h*(r2^2-r1^2)/u
dwell time/volume is t/V=1/u.

Same thing.

So for the same volume, the dwell time is the same.

QED. Unless you got something to say about the sink term, which is going to vanish as you go deeper anyway, get lost.

Cheers,

Matthieu

 

[anarchista]

Let's see.

We'll assume an ideal gas (reasonable at such low pressures), an incompressible fluid (reasonable at such low speed), an homogenous flow (reasonable again, if the scrubber is correctly packed)(more likely for an axial), and ignore the sink term.

The continuity equation simplifies to div v>=0.

For a cylindrical axial scrubber, that just means v is constant along the axis.
The volume is V=pi*r^2*h.
The flow speed v=u/s=u/(pi*r^2) u is the volumic flow, s is the section.
The path length is h.
The dwell time is t=h/v=pi*r^2*h/u
dwell time/volume is t/V=1/u.

For a double cylindrical radial scrubber, it's just a fancy way of saying v decreases linearly with r. At each r, we have the same volumic flow, so 2*pi*r*h*v=u, so v=u/(2*pi*r*h)
The volume is pi*(r2^2-r1^2)*h
The path length is r2-r1.
The dwell time is the integral between r1 and r2 of dr/v, which yields pi*h*(r2^2-r1^2)/u
dwell time/volume is t/V=1/u.

Same thing.

So for the same volume, the dwell time is the same.

QED. Unless you got something to say about the sink term, which is going to vanish as you go deeper anyway, get lost.

Cheers,

Matthieu

Paul wrote many posts ago, is not subject to discussion.
The problem arises when the graph soda lime binding capacity.

http://rebreathers.pl/forum/download.php?id=39

You not proved, that the binding is not a function of gas flow velocity.

Martin Parker and Kewin Gurr said something about CO2 scrubber in diving.
Espacenet - Bibliographic data
Espacenet - Bibliographic data

greet rc

 

[gianaameri]

My best guess: diving with broken unserviced rebreather is more risky than diving a unit where WOB difference is marginal.

The Noob asked nothing about CE which I see if filling the pages up again.

Matt.

Broken/Unserviced rebreathers should not be dived at all, ever (no matter what the WOB is).

What we are discussing here is which is safer, Axial or Radial.

EN14143 and NEDU define some parameters and I brought these up as a means to answer the question, based on actual numbers, relating them to the parameters.

I know of no other reasonable approach to answer the question.

It is a very good question though, which I posed myself when buying a Meg, and my conclusion was "I'll buy the scrubber which is safer" - so I bought the ISC 8lbs. Radial.

 

[gianaameri]

Ah. You're going to sit under a tree.

If not - Gian - what's the acceptable risk of dying on a dive to you? 1 in 1000? 1 in 10000? 1 in 100,000? 1 in 1 million?

Janos

ZERO RISK is the target.

It is not achievable, so wherever possible, I try to reduce the risk.

If I have reduced the risk to as much as reasonably practicable, I am satisfied.

If there is two ways to do something, I'll chose the safer of the two ways, even though this may add cost or inconvenience.

To make that decision, to decide amongst the safer of the two ways (or whether to undertake the activity at all), I do need to have accurate information.

The worst thing is if I make the decision based on the wrong information. This means I am not happy when I am given wrong information because it throws off all the risk management system.

So, it is all about managing the risk carefully, which I think is part of the fun in diving.

If it is done properly and carefully, I think the risk can be a lot better than the numbers you are citing.

 

[Janos]

ZERO RISK is the target.

It is not achievable, so wherever possible, I try to reduce the risk.

If I have reduced the risk to as much as reasonably practicable, I am satisfied.

If there is two ways to do something, I'll chose the safer of the two ways, even though this may add cost or inconvenience.

To make that decision, to decide amongst the safer of the two ways (or whether to undertake the activity at all), I do need to have accurate information.

The worst thing is if I make the decision based on the wrong information. This means I am not happy when I am given wrong information because it throws off all the risk management system.

So, it is all about managing the risk carefully, which I think is part of the fun in diving.

If it is done properly and carefully, I think the risk can be a lot better than the numbers you are citing.

That's a cop out and you know it.

So. A number please. What's an acceptable level of risk? Or if you're unable to provide a number, I'll accept another activity (eg 1000 miles of motorbike racing).

To say "I don't know" (or "as much as is practical / possible") invalidates your whole argument.

Janos

PS: "managing the risk carefully, which I think is part of the fun in diving." Really? Blimey.

 

[gianaameri]

That's a cop out and you know it.

So. A number please. What's an acceptable level of risk? Or if you're unable to provide a number, I'll accept another activity (eg 1000 miles of motorbike racing).

To say "I don't know" (or "as much as is practical / possible") invalidates your whole argument.

Janos

PS: "managing the risk carefully, which I think is part of the fun in diving." Really? Blimey.

The probability of the cave roof collapsing on me while I am diving - that is the number I am prepared to accept.

I got no clue what that number is, or how to calculate it (maybe RC has a Patent).

My argument is predicated on reducing risk as much as practicable, and in so stating, there is nothing which invalidates my argument.

If replacing an Axial scrubber with a Radial scrubber reduces risk, that I do, or I quit diving.

At the end of the day, if you can't pay, don't play... and I think the choice of an Axial scrubber is very much driven commercially to make rebreather diving more affordable, rather than safer (as much the numbers tell me).

Personal choice and life-style.

I am not a risk taker, where the risk can be reduced by paying USD 590 (I checked the number) for a safer scrubber, than the standard one, I pay-up (and yes, of course, it costs more to fill it with absorbent as it holds more absorbent than the standard Axial).

To me, it is a no brainer to pay USD 590 more for the added benefits.

 

[Paul Haynes]

Read and despair

While I'm talking ****, has anyone considered making scrubbers with rough walls, reduce channeling on the sides sort of thing? Or is it just a stupid idea?

Not a stupid idea at all. Gas will always take the pass of least resistance. A smooth internal wall aids the flow down the inside wall of a canister, which can result in an uneven use of the scrubber bed. One of the features of a well designed canister is rough internal walls, so yes it has and is being done, though not as far as I recall in the current range of civilian rebreather canisters.

Keep on thinking.

Rgds Paul

 

[paulraymaekers]

Not a stupid idea at all. Gas will always take the pass of least resistance. A smooth internal wall aids the flow down the inside wall of a canister, which can result in an uneven use of the scrubber bed. One of the features of a well designed canister is rough internal walls, so yes it has and is being done, though not as far as I recall in the current range of civilian rebreather canisters.

Keep on thinking.

Rgds Paul

nice idea, but probably no measurable difference in duration when using small grain sorb...

and for general info: i have both radial and axial scrubbers, I have always at least 5 units I can pick for diving, from mCCR with radial to full rms with axial, from micro over mini to standard

the best WOB is the rEvo standard/mini configuration, with radial scrubbers, and cooper hoses

the unit I dive is a micro FT with axial scrubber and rms, and drager hoses

small compared to big also has influence on safety: less dragg in the water when diving, better 'human touch' with the unit when diving, less weight, less extra weight needed to get the unit negative..

so it is clear that nothing is just black and white!