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JonG
29-06-2018, 02:22 PM
As I have said elsewhere at the moment the recommended duration of 3 hours is not an issue based on profiles I am doing now, but if everything goes according to plan, eventually I would envisage getting to a point where 3 hours may not be enough.

I am interested to hear what others views are on the duration of the scrubber, I have read various approaches on RBW Scrubber duration on the KISS and Mark has used the following to give some kind of calculated stab at what the canister should at more realistic breathing rates than the tests specify:

KISS is prety good due to the long scrubber housing increasing dwell time. Conversly this affects WOB, but as I have said before the WOB issues on the KISS were mainly in the hoses and mouthpiece IMHO


Theres a mathematical calculation for theoretical scrubber time which can be simplified with a margin of error built in.

C02 = 85% of V02

V02 = 4% RMV

SO take an RMV of 20lpm and you get 0.8 of which 85% = 0.68 (call it .7)


SO the KISS scrubber tested at 40m at 4c and 1.6lpm co2 = 157min run


1.6 devided by .7 = 2.28


Multiply 157 by 2.28 and you get 359mins or 5.9hours

If you accept water temperature has a significant effect on scrubber efficiency then you will appreciate the same test done in 15c water would give better results


But depth is also a factor as the testing is all at 40m. So there needs to be a view on time spent at depth?

Personally I have done 6 hours on a KISS several times but the water temp was 20c and my max depth was 14m

OW diving deep, I never did more than 4 hours 15mins after about an hour around 60-65m


Again personally, I never got excited about pushing 5-6 on a single deep dive (though only did that on the Inspo) and I never got excited about 6 doing 20c+ 14m max multiple Mexico cave dives


However I'd never do three X two hour 40m dives (if that makes sense) Id do two no problem, but being at 40m for 45mins for the third dive was a no no in my book

TOTAL Chasey voodo but I would stay happy and confident in my own belief bubble and that's the point. If your belief bubble is 3 hours, stick with that as theres no need to push scrubbers and the consequences of them failing are dire.

Janos
29-06-2018, 03:41 PM
As I have said elsewhere at the moment the recommended duration of 3 hours is not an issue based on profiles I am doing now, but if everything goes according to plan, eventually I would envisage getting to a point where 3 hours may not be enough.

I am interested to hear what others views are on the duration of the scrubber, I have read various approaches on RBW Scrubber duration on the KISS and Mark has used the following to give some kind of calculated stab at what the canister should at more realistic breathing rates than the tests specify:

[/I]

I simply don’t think you calculate scrubber duration. I’ve got a degree in physics rattling around somewhere and it’s beyond my ability to come up with a sensible calculation that I would trust.

There are too many variables, including gas density and scrubber dimensions. All scrubbers break through with plenty of unused lime in them. A scrubber that is very long and thin will have less unused through than a short fat one. (By way of example, a scrubber that was one grain of sofnolime thick but very wide would break through almost instantly, even though most lime is unused).

Instead I used to stick roughly to the Inspo guidelines when I dived a Kiss, adding a little bit of a margin as there’s a bit more lime in the Kiss.

I was comfortable doing 3 hour dives on the kiss, and did a few that were a bit longer. Resting on the 6m stop I’m not concerned but I would be at depth.

Janos

gobfish1
29-06-2018, 05:29 PM
jon its up to you on how much scrubber use is safe ,



you first need to know your o2 use on your dives you can split it in to two parts if you like working and resting
or just use your avg on a dive ,

once your have o2 use/lpm, you can then work co2 Production , im sure you know all that already , what you do with them numbers is then up to you ,

after reading about the Plura cave dive survivors, and the depths and run time,s one of the dives put on his yellow box , im happy with my small numbers ,

OutOfTest
30-06-2018, 06:14 AM
Iíve had the KISS scrubber to 6 regularly. Normally in about 12 degree water.

On the trip Iím on currently Iíve got 5 hours on it in 7 degrees.

I try not to do more than this. Iíve done 8 before, I was nervous at that.

Your experiences may differ.


Sent from my iPhone using Tapatalk

Mark Chase
30-06-2018, 08:46 AM
Jon be aware that I was diving with Janos when I was running 6ish on my KISS :D

Hes a super cautious guy, and theres nothing wrong with that, it all goes back to stick to the limits of what you can dive with confidence.



Physicists may need all the variables tied up to make their math look pretty, but I am an engineer and I am happy to go with logical assumptions and empirical data.


SO I have to pick up on the issue of "you cant calculate scrubber time"

Hes right and wrong, but mainly wrong.

Hes right, as theres a lot of variables in the filling of the scrubber

The KISS case 157mins scrubber time is an average across a range of (one assumes) pretty good factory fills. I mean if they cant fill a KISS scrubber, who the hell can?

So if we accept theres a good base line on the fill, and that we are filling the scrubber correctly, then the math begins to be relevant

However there are variables again with water temperature and depth.

BUT the 157min data is at an almost impossible 40RMV and a water temp of 4c Id frankly not bother to dive in.

More importantly its at a ludicrously high 1.6lpm C02 production rate, which calculates back to a 47lpm RMV???

So using the math purely on the C02 rate, leaves in the padding for temperature and the fact your unlikely to be at 40m for 157mins

I also factor in a 20 SAC and frankly my SAC is usually well below 15

Do the math on 15lpm and I am looking at 7.5 hours


IMHO KISS put the formula in there for the thinking diver to extrapolate from. They couldn't possibly suggest a range of run times, the liability would be insane, but by putting the formula up there they have left us to work it out for our selves.


In this nanny state we live in, people seem to want lines in the sand, black and white answers and the CCR manufacturers have said, OK its X hours, safe in the knowledge the 50% of CCR users who haven't got a clue about C02 and scrubber function will dive safely in side the say, 3hour limit.


This is a very sensible position. Personally i'd do 90% of my diving in the 60-75m range and id do 1 three hour dive and 1 fresh scrubber on 90% of my dives


However, when we went to Mexico, Egypt and when I did very deep / long dives, I had no qualms about running 6 hours to get around availability / cost of lime and obligations of the dive.



It makes me laugh that back in the day I suggested 6 hours on the inspo and was slagged left right and centre on the internet, but since the temp stick and RMS systems on the rEvo, we now have divers punching 6+ on multiple 30-40m dives I'd never push that far and I remember one fella claiming 9 hours regularly on the rEvo. And its all good now because its a little electronic box that says so.


I don't have nor do I want a temp stick. I am confident with 6 hours and I cant see the need / dive to push that further in the foreseeable future.


BUT a huge Caveat here.


Blindly pushing the scrubber is the act of a fool. You have to have a proper appreciation of the variables like work load, SAC depth temp etc etc and factor those in on the fly during the dive. In this respect the Temp stick mob have an edge but I personaly wouldn't trust the temp stick 100% anyway.

The big one is self belief.

If you are diving in doubt, then when your SAC goes a bit high, you will loose faith, panic and fall into the incident pit. Fact is, if you were diving in doubt that SAC increase as you passed hour 5 was probably self induced fear/ stress


I did the math to chose 6, but I also looked at what others were doing. I spoke to people like JJ and our cave instructor Mithais Andrius and cave divers like Dr Jerome Mear and Martyn Farr who took the KISS to something mad like 180m

Compared to what they were doing, 6 hours was a bit Girley (can we say that these days?)


Having taken all the above on board id finish off by saying, IMHO the average CCR well packed, can easy do 6+hours with a margin of safety, but that pushing it to 6+ hours at UK fill prices is totally frigging stupid.


Below is my self beleife chart:


1 X 6 hour v deep dive 90% of time on deco 10c+ water YES


3 X 2 hour shallow dives warm water (15c+) 30m max YES

2 X 60m 3 hour dive No

3 X 2 hour dive to 40m No

JonG
30-06-2018, 01:12 PM
jon its up to you on how much scrubber use is safe ,



you first need to know your o2 use on your dives you can split it in to two parts if you like working and resting
or just use your avg on a dive ,

once your have o2 use/lpm, you can then work co2 Production , im sure you know all that already , what you do with them numbers is then up to you ,

after reading about the Plura cave dive survivors, and the depths and run time,s one of the dives put on his yellow box , im happy with my small numbers ,

I have read about using 02 consumption to track scrubber duration, but don't know if have all the figures correct to calculate it.

I have been recording 02 and dil consumption and from the o2 consumption i know I am using 35b on average per dive and each dive is on average 52 mins so my 02 consumption is 2lpm. I expect this will change/drop over time as I improve my control of the ppo2, do less fannying about in quarries, do more stationery dive time on deco, but it is what is for now.

I think[I] that Co2 is produced at a rate of 0.9 litres per litre of o2 consumed, which would mean that I am producing 1.8 litres of c02 per minute of dive time.

I also [I]think that sodalime absorbs 23l of co2 per 100g, so on a 2.7kg KISS fill, it is capable of absorbing 621 litres.

621/1.8=345mins

345/60=5.75 hours

However maths was never my forte.

There are also huge caveats here in terms of changes day to day physiologically and also the 23l/100g will be variable based on depth, water temp, breathing rate etc.

Using Mark's formula with a slightly higher than measured RMV of 15lpm I get:

15x4%=0.6

85%x0.6=0.51

1.6/0.51=3.13

157x3.13=491

491/60=8.2 hours

Arguably though I am consciously pushing a lot more gas around the loop which would increase the RMV.

Do these figures look right, obviously I am not going to take either figure as gospel, but it serves to provide some insight for me when making longer dives?

gobfish1
30-06-2018, 04:32 PM
jon your numbers seem ok .

on my o2 use the numbers tell me i should be good for 6.5h
i tend to try use half the recommended , so after 90mins i want to be in deco mode 21m or less after that im happy to do 2xs the last 50% that give me all i need for the diving i was doing , so not what id call pushing the boat out ,

30min at 90m puts you back above 21m in less than 90mis all the 6h 8h is/was Irrelevant to my diving ,

if i ever got to a point where 6h came in to play , id just have a plan b at hand ,

JonG
30-06-2018, 05:07 PM
Cheers Steve, a possible plan b being oc deco, providing cns is managed?

gobfish1
30-06-2018, 05:24 PM
Cheers Steve, a possible plan b being oc deco, providing cns is managed?

running at set point of 1.3 works well with the above , ie 50cns runs about same as 50% bucket. ie 90mins

well it should be as long as you managed your po2 on the dive ,lol
and work load ,

Mark Chase
30-06-2018, 10:39 PM
Using 02 consumption to gauge C02 production is scientifically accurate but practically virtually impossible.


I do mulriple 02 flushes on a given dive and then theres temperature drop [resure loss on the tank and venting to take into consideration.

The C02 / RMV relationship is much much easier to calculate.

Just ask yourself if a Co2 production rate of 1.8 makes any sense when calculated back to RMV


Exhaled C02 is a pretty good constant. Yes I am told super fit athletes have the ability to exhale more co2 than the average person, but ignoring that and accepting we are not up for the Olimpics any time soon, Id stick with C02 / rMV

gobfish1
30-06-2018, 10:50 PM
Using 02 consumption to gauge C02 production is scientifically accurate but practically virtually impossible.


I do mulriple 02 flushes on a given dive and then theres temperature drop [resure loss on the tank and venting to take into consideration.

The C02 / RMV relationship is much much easier to calculate.

Just ask yourself if a Co2 production rate of 1.8 makes any sense when calculated back to RMV


Exhaled C02 is a pretty good constant. Yes I am told super fit athletes have the ability to exhale more co2 than the average person, but ignoring that and accepting we are not up for the Olimpics any time soon, Id stick with C02 / rMV

i also do a few o2 flush,s but only at 6m lol
on a 3h dive i use 150/200l depending on how keen i am on the green button comming up and the number of times i flush the bag at 6m,

jon did say his pissing about at the mo and hopes his numbers well come down . his numbers say his knocking out about 2.4 x the co2 pre min that i do ,
so id think he has room to Improve
hes only been doing 50min dives so wont be able to get a good avg o2 use as yet.
yes i know them leaky valves piss away more o2 than my eccr unit ,but its all a guestimate
when all said and done

ps
he do,s that Iron Man shit so maybe he has got a set off lung on him , unlike us 40 a day divers lol

Mark Chase
30-06-2018, 11:03 PM
I agereee on a 3hour dive ill use between 50 bar and 100 bar 02 from a 3ltr tank. But doing C02 calculation on a variable of 50 -100 bar is pointless.


I didn't use more 02 on the 100 bar 3 hour dive by breathing it, I just wasted it / flushed it


Typicly ill triple flush pre dive, inject gob loads of 02 on the surface on getting in, do multiple triple flushes on 6m and 3m stop and inject gob loads on the surface getting back to the boat and possibly spend 5mins on pure 02 once back on board


For a meeningfull calculation I should take a bar pressure reading once the tank pressure / temperature has normalised, start the run clock then and stop it + take tank pressure before I start the 02 flush on the 6m stop


A: My SPG isn't accurate enough and B I cant be arsed.

gobfish1
30-06-2018, 11:32 PM
I agereee on a 3hour dive ill use between 50 bar and 100 bar 02 from a 3ltr tank. But doing C02 calculation on a variable of 50 -100 bar is pointless.


I didn't use more 02 on the 100 bar 3 hour dive by breathing it, I just wasted it / flushed it


Typicly ill triple flush pre dive, inject gob loads of 02 on the surface on getting in, do multiple triple flushes on 6m and 3m stop and inject gob loads on the surface getting back to the boat and possibly spend 5mins on pure 02 once back on board


For a meeningfull calculation I should take a bar pressure reading once the tank pressure / temperature has normalised, start the run clock then and stop it + take tank pressure before I start the 02 flush on the 6m stop


A: My SPG isn't accurate enough and B I cant be arsed.

im sure thats all true , but hes not pushing to the end of the line hes just looking for a avg for now ,
you can work out what you piss away for the most part , and once back at 6m or top side a flush OR a vent is feek all , the best iv ever done on a 3h plus dive 70bar of o2 , and yes pissed away a good bit of that but so what ,its just numbers and along way from the end of the co2 scrubber life,
when his only looking to push 90min past the CE guestimate,

i could say i only use half the 150l and id get 13h on the box , point less to me as i only need 4.5h even with the pissing away of gas in the cals im still good for 6,4h

the bigger point for me is where and when to push it / when and where not to push it.
all the variable,s need taking on board ,
think you have all ready posted some thoughts on that in this thread ,

JonG
01-07-2018, 11:10 AM
Thanks gents there is obviously no totally quantitative solution but this has been useful for my understanding.

Also found this blog from SW that is worth a read if you haven't seen it before.

https://www.shearwater.com/monthly-blog-posts/co2-scrubber-divers-rebreather/

JonG
01-07-2018, 01:07 PM
i also do a few o2 flush,s but only at 6m lol
on a 3h dive i use 150/200l depending on how keen i am on the green button comming up and the number of times i flush the bag at 6m,

jon did say his pissing about at the mo and hopes his numbers well come down . his numbers say his knocking out about 2.4 x the co2 pre min that i do ,
so id think he has room to Improve
hes only been doing 50min dives so wont be able to get a good avg o2 use as yet.
yes i know them leaky valves piss away more o2 than my eccr unit ,but its all a guestimate
when all said and done

ps
he do,s that Iron Man shit so maybe he has got a set off lung on him , unlike us 40 a day divers lol

Pah! I laugh in the face of iron man distances, my speciality was 24 hours plus, multi discipline, non-stop up to 5 days.