Gas flow lag/restrictions when using BOV under heavy OC conditions

Thanks Sven,

Will give this a try and see what the results are like

So a quick experiement last night under scientifically dubious conditions gave the following results.

Method:
I basically purged the 2nd stage at the end of a growing list of attachments to see the resulting ip drop.

Considerations:
Apart from the original stage config (ie just a reg on a tank) I've ignored the effect of hoses and just considered the attachments.
I also had quite a low pressure in the onboard dil tank (60bar) vs about 200bar in the offboard tank so this may have affected results slightly.

Results:
1st stage / hose / 2nd stage (20psi = 1.4bar)
1st stage / hose / 4way manifold / 2nd stage (25psi = 1.7bar)
1st stage / hose / switchblock / 4way manifold / 2nd stage (30 psi = 2bar)
1st stage / hose / QC6 connector / switchblock / 4way manifold / 2nd stage (45 psi = 3bar )

Conclusions:
It's all still breathable but it was interesting to see the increase with each new bit added.
The QC6 was the biggest culprit
Have a real reason to have to connect all this stuff (I know it's cool) but there is definitely a performance trade-off for the convenience of a single mouthpiece.

I looked at this so I could have a system with a breathable gas in the BOV at all stages of the dive. Having a rule like "switch your dil at 15m" seems like a much easier muscle memory to learn than other models like "don't use the BOV above depth = x". Thoughts?
 
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I looked at this so I could have a system with a breathable gas in the BOV at all stages of the dive. Having a rule like "switch your dil at 15m" seems like a much easier muscle memory to learn than other models like "don't use the BOV above depth = x". Thoughts?

Switchblocks with a BOV are a recipe for disaster. They have no place outside of commercial work with hardhats, comms, dive supervisors and all the other backup systems to make sure you don't screw up and if you do screw up you aren't as likely to lose a mouthpiece and drown. Here's a sober reminder about how easy and deceptive having the wrong gas plugged into a switchblock can be.
http://www.rebreatherworld.com/showthread.php?11311-WKPP-Diving-Incident-Analysis
They have a place with the RB80, but even those protocols can break down and lead to disaster. Using one "just because" and without integrated into rigorous plug & switch protocols? Bad idea.

How often are you diving below 80m or with less than 15/55? The MinOD for 15/55 is somewhere between the surface and 3ft and the MOD at 1.4 is 83m. Keeping your first deco gas handy on the surface for the rare 80m+ dive is a whole lot simpler and safer than messing with a switch block and developing some sort of team verification from scratch.
 
Guys you are overthinking it. All those connections are a disaster waiting to happen and everybody knows for years that bovs are so crappy at breathing from that are good only for a few breaths before you switch to a proper second stage. Do you really need pressure drop and flow tests to come to the same obvious conclussion like everybody else?
 
So a quick experiement last night under scientifically dubious conditions gave the following results.

Method:
I basically purged the 2nd stage at the end of a growing list of attachments to see the resulting ip drop.

Considerations:
Apart from the original stage config (ie just a reg on a tank) I've ignored the effect of hoses and just considered the attachments.
I also had quite a low pressure in the onboard dil tank (60bar) vs about 200bar in the offboard tank so this may have affected results slightly.

Results:
1st stage / hose / 2nd stage (20psi = 1.4bar)
1st stage / hose / 4way manifold / 2nd stage (25psi = 1.7bar)
1st stage / hose / switchblock / 4way manifold / 2nd stage (30 psi = 2bar)
1st stage / hose / QC6 connector / switchblock / 4way manifold / 2nd stage (45 psi = 3bar )

Conclusions:
It's all still breathable but it was interesting to see the increase with each new bit added.
The QC6 was the biggest culprit
Have a real reason to have to connect all this stuff (I know it's cool) but there is definitely a performance trade-off for the convenience of a single mouthpiece.

I looked at this so I could have a system with a breathable gas in the BOV at all stages of the dive. Having a rule like "switch your dil at 15m" seems like a much easier muscle memory to learn than other models like "don't use the BOV above depth = x". Thoughts?


Hello jwd,
Have been busy diving!

I have refined your scientific experiments by adding a flowmeter at the very end.
I think anyone can afford it. It is a plastic garbage sack of 60 L.
Purging my Golem Shrimp in the sack gave an amazing flow result in spite of all the "fancy stuff" in between.

I also tested 3 APEKS 2nd stages traditionally connected to my 80 cuf air bailout system.
One of them (XTX50) is fairly new and is still factory adjusted.

Golem Shrimp gave 400L/minute. (Multideco configuration max SAC rate cannot be set higher than 60L/minute)

None of the APEKS reached that flow. Do not why… Perhaps the venturi function (missing in the Golem Shrimp) is limiting the flow while redirecting the flow????

So I think I am convinced after this test. The GOLEM Shrimp has the OC capabilities I need which I already knew.
Last year I dived to 60m a couple of times using the BOV in OC while testing my homebuilt CO2 controller linearity.
The counterlung was filled with air all the dive.(0.04 - 0,28 kPa)



About gas switching moments:
Going down on a line to a wreck 60+, 15/52 from surface.
Going down slowly, perhaps a long shallow transport distance, switch to 15/52 at 20m before descending.
Always leaking proper amount of Dil from the BOV depending on descending speed.
Closing the ”leak” after target depth is reached and good contact with buddy is established.
(I have not used ADV for 1000+ hours since my breathing apparatus gets very stressed after sucking for dil all the way down - I’m not a big sucker...)


Going up I always switch to air at 40, make a slow loop content transition from 15/52 to 21/0 by a small BOV leak until reaching 20m.
IBCD problems are minimised should a BOV bailout be necessary.

Regards
Sven Becker
 
Closing the ”leak” after target depth is reached and good contact with buddy is established.
(I have not used ADV for 1000+ hours since my breathing apparatus gets very stressed after sucking for dil all the way down - I’m not a big sucker...)

Hi Sven,

I like the "leak" idea for the decent. Last time I was on my way down I was regularly reaching over to purge the ADV because the angle I was at made inhaling on the ADV difficult.

I've experienced the "leak" before by not fully opening the loop/closing the BOV switch and wondering where all the bubbles were coming from.

Interesting to see that it has a useful purpose too.
 
I really can't understand why you would need so many ways to get gas into the loop. JJ O2 MAV works pretty much as pefectly as you could ever want - put an inflator hose on your off board O2 and you're good to go. If you really want you could put inflators on deep gas also but in reality a quick bailout breath can be puffed into the loop and that will give you as much DIL as you'd ever need without all those connections.

For the BOV I understand why you might think you need to have a way to change gas particularly with very hypoxic DIL. But in reality you guys need to have done more OC deep trimix to understand how to dive those kinds of DIL - don't switch to BOV in shallow water (just like you can't switch to back gas in shallow water OC with hypoxic back gas). Its a trivial problem overcome by being aware in shallow water and not little DIL into loop above 6m.

Save yourself a bunch of money on swagelok etc and worrying about flow rates: just plumb your deepest gas into the BOV and have a way of adding O2 (like the inflator nipple on the JJ MAV). More complexity and options at stressful times increases the likelyhood of a dangerous choice or no choice (usually the worst option of all).

All that being said there is a certain geeky appeal to all those connectors and switchblocks - which I totally get (I fulfil that by going crazy with mixing equipment :) )



Bobjr, I will try and give you a lot of security reasons for my multiple gas connection points.

My greatest fear is entanglement caused by back mounted equipment.
I cannot even scratch my back between the shoulder blades so 10 years ago I begun tiding up my back profile.

Since Jan 2019 I dive a double canister, single CL home made rebreather (130 hours).
It is inspired by the KISS Sidewinder beauty and has a low back profile.
TecMe.de, Martin Tolksdorf has made most parts.
I due time I will present it on this forum.


For the last 10 years I carry ALL tanks side mounted with QC6 connectors.

I started with 1 QC6 for O2 and 1 QC6 for DIL.

DIL Manifold
Very soon I discovered the danger having that single point of failure
on the dil side when I got debris in the QC6 female and couldn’t connect the next gas.
The answer to that problem was the Omniswivel manifold with 2 parallell QC6 females.

The obvious security benefit is: Connect next DIL before removing the current DIL.

Usage either:
Connect 2 DIL tanks (with rather overlapping mixtures) as soon as possible. (I connect tank50/0 and 15/52 in the water to avoid bone fractures)
or:
(3 DIL tanks) Connect the 2 most suitable as soon as possible.
When switching to a third DIL tank,
disconnect the least suitable DIL tank
connect the bottom DIL.

At all times I am using the DIL MAV spraying 2 O2 sensors from 20 mm distance giving a quick verification of the dil status.
(cannot be done in most rebreather brands often having the DIL MAV on the inhale CL too far away from the sensors.)

During descent, all 5 O2 sensors constantly are testing the DIL choice.

Having such good knowledge of the current onboard DIL mix, the best security choice is to connect the BOV to the manifold.
In other words - the BOV is prepared for best bailout mix BEFORE you get stressed.

The BOV/needle valve is the heart in my RB diving.

O2 Manifold supporting the Chris Kennedy needle valve
1 QC6 male for O2 tank connection
1 QC6 female for DIL connection

Security:The O2 tank cannot be connected to the DIL manifold

Security and joy : The DIL tanks can be connected to the O2 manifold

Joy:
I often dive SCR none trimix dives with a 80cuf EAN50 replacing O2, and a 80cuf EAN21 as DIL

Security:
The possibility to dive SCR using 15/52, 21/0 50/0 is ”creating” an enormous gas depository!
(Help me to calculate how long it will last!)

The needle valve can deliver about 14L/minute max at surface.

Of course the sorb is finished long before the gas but breakthrough will be prolonged depending on the ”SCR gas overflow”

14L/minute should be enough to dilute the CO2 to an acceptable level?????
(MultiDeco Deco gas consumption = 20L/min)

I measure the CO2 between my 2 canisters so perhaps I will do a test with the primary canister empty and the spare canister fresh ...




Attending my first IANTD course many years ago I was told it is OK to change your configuration if you have good reasons to do it.

I have many good reasons…..

Regards
Sven Becker

PS
As you can see, I also use both belt and braces securing the 15kg weight belt!
My rebreather is nearly neutral in salt water so I have almost all weights on the belt.

This enables an entanglement security excerise I occasionally perform.
I park my rebreather on the bottom.
Hook up all tanks on the weight belt D-rings.
Swim a short loop away from my rebreather.
Swim back and restore the configuration.

DS
Manifolds.jpg
 
Bobjr, I will try and give you a lot of security reasons for my multiple gas connection points.

My greatest fear is entanglement caused by back mounted equipment.
I cannot even scratch my back between the shoulder blades so 10 years ago I begun tiding up my back profile.

Since Jan 2019 I dive a double canister, single CL home made rebreather (130 hours).
It is inspired by the KISS Sidewinder beauty and has a low back profile.
TecMe.de, Martin Tolksdorf has made most parts.
I due time I will present it on this forum.


For the last 10 years I carry ALL tanks side mounted with QC6 connectors.

I started with 1 QC6 for O2 and 1 QC6 for DIL.

DIL Manifold
Very soon I discovered the danger having that single point of failure
on the dil side when I got debris in the QC6 female and couldn’t connect the next gas.
The answer to that problem was the Omniswivel manifold with 2 parallell QC6 females.

The obvious security benefit is: Connect next DIL before removing the current DIL.

Usage either:
Connect 2 DIL tanks (with rather overlapping mixtures) as soon as possible. (I connect tank50/0 and 15/52 in the water to avoid bone fractures)
or:
(3 DIL tanks) Connect the 2 most suitable as soon as possible.
When switching to a third DIL tank,
disconnect the least suitable DIL tank
connect the bottom DIL.

At all times I am using the DIL MAV spraying 2 O2 sensors from 20 mm distance giving a quick verification of the dil status.
(cannot be done in most rebreather brands often having the DIL MAV on the inhale CL too far away from the sensors.)

During descent, all 5 O2 sensors constantly are testing the DIL choice.

Having such good knowledge of the current onboard DIL mix, the best security choice is to connect the BOV to the manifold.
In other words - the BOV is prepared for best bailout mix BEFORE you get stressed.

The BOV/needle valve is the heart in my RB diving.

O2 Manifold supporting the Chris Kennedy needle valve
1 QC6 male for O2 tank connection
1 QC6 female for DIL connection

Security:The O2 tank cannot be connected to the DIL manifold

Security and joy : The DIL tanks can be connected to the O2 manifold

Joy:
I often dive SCR none trimix dives with a 80cuf EAN50 replacing O2, and a 80cuf EAN21 as DIL

Security:
The possibility to dive SCR using 15/52, 21/0 50/0 is ”creating” an enormous gas depository!
(Help me to calculate how long it will last!)

The needle valve can deliver about 14L/minute max at surface.

Of course the sorb is finished long before the gas but breakthrough will be prolonged depending on the ”SCR gas overflow”

14L/minute should be enough to dilute the CO2 to an acceptable level?????
(MultiDeco Deco gas consumption = 20L/min)

I measure the CO2 between my 2 canisters so perhaps I will do a test with the primary canister empty and the spare canister fresh ...




Attending my first IANTD course many years ago I was told it is OK to change your configuration if you have good reasons to do it.

I have many good reasons…..

Regards
Sven Becker

PS
As you can see, I also use both belt and braces securing the 15kg weight belt!
My rebreather is nearly neutral in salt water so I have almost all weights on the belt.

This enables an entanglement security excerise I occasionally perform.
I park my rebreather on the bottom.
Hook up all tanks on the weight belt D-rings.
Swim a short loop away from my rebreather.
Swim back and restore the configuration.

DS
View attachment 10394


Your risk analysis is completely different to mine. I am not worried even slightly about entanglement (and I have done a lot of diving in areas prone to lost /discarded fishing equipment) - I like my rig to be as simple as possible, at stressful times i want easy decisions not complexity. Each to their own ;)
 
Your risk analysis is completely different to mine. I am not worried even slightly about entanglement (and I have done a lot of diving in areas prone to lost /discarded fishing equipment) - I like my rig to be as simple as possible, at stressful times i want easy decisions not complexity. Each to their own ;)

Bobjr
My risk analysis was conciderably changed when a line formed a perfect Clove hitch around a tank valve on my back at 55m many years ago...
Regards
Sven Becker
 
Your risk analysis is completely different to mine. I am not worried even slightly about entanglement (and I have done a lot of diving in areas prone to lost /discarded fishing equipment) - I like my rig to be as simple as possible, at stressful times i want easy decisions not complexity. Each to their own ;)
+1
 
Sven, would he interested in seeing more details or your configuration and parts.

DeepUnderground,
With pleasure I will add a thread " My OAK rebreather" soon. In January i bought my first UW camera and have collected rather much material ( and have spent days trying to understand Filmora, Filmmaker, Quick and other tools)

I miss the glory days of CCRexplores and similar forums.
It would be nice if we can participate in bringing this forum to a more active status.

Regards
Sven Becker
 
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