How hard is WOB on rebreathers?

dave t

whats the volume of your loop in total?
whats the volume of each counterlung?

the inspo scrubber is about 2:1 ratio (hope thats correct Phi) but what more important is dwell time. too short and you loose dwell time, too long and WOB is too high An example is the inspo scrubber against the evo, inspo is 2.45kg of sofnolime, evo is 2kgs. Inspo scrubber duration is 3hrs (under test conditions) evo is only two hrs both scrubbers are a the same diameter, evo one is a little shorter

HTH
Dave

marekm

The counterlungs are two 2L Dromedary bags. (Will change them to 2x 4L)
Total loop volume is 6.5L.
My max lung volume is 3.5L.

At the moment comfort of breathing thanks to a low WOB is my top priority. Scrubber inefficiency leading to shorter run times is not that big a concern.

What good is a long scrubber duration if your you can't breathe that long without getting tired

dave t

ok I think the main reason your WOB is high is the loop is too small, I would try 2 x 4l lungs and see how you get on if that still no good make the scrubber bigger, I dont mean just the sorb pack but the total scrubber volume.
I know you will be thinking well my lung volume is only 3.5l and my loop is almost double that so whats the problem!
the problem is its not as simple as that!!!

best

Dave

Bosh

Have a look at these links.

Loading Trimix Divers

Go to the AARG section and find the construction tips.

www ppo2.com

Look at reference material and under files you will find some articles from the US.Navy

Have fun reading

Best regards

Bo

Nemo1957

The loop (seen here http://home.iprimus.com.au/marekm/rb/100_0548.jpg) has a minimum of 25mm internal diameter in all its parts (with 38mm for hoses). The DSV check valves are very soft so it has a minimal WOB. I've tried various size mouth pieces and they have had no effect on the WOB. Basically, when exhaling, the exhale CL fills up, and then the WOB becomes difficult. On the inhale, the inhale CL empites, and then again the high WOB kicks in. So unless I've missed something, all this seems to point to the scrubber being the problem.


Based om this discription your CL's are too small. The inhale is bottoming out before you get a lung full and the exhale is topping out before you fully exhale. Also from the photo your loop looks very long.

Nemo

AD_ward9

I agree, CLs appear to be too small (should be 2x 4.5l ideally), and there is another restriction. Look carefully at the mushroom valve cross section. Try to achieve a minimum cross section anywhere of equivalent to a 32mm pipe. 38mm hoses are more than what you need. Loop length is excessive, but this is unlikely to affect WOB much: up to 40 times the diameter of hose, the flow drops only 10% compared to a simple hole of that diameter.

Alex

marekm

Thanks for the tips guys. I'll try test out the larger counter lungs and make bigger checkvalves as soon as I have time.

In the mean time, I have some questions about hose lenghts. The hoses that go from the T-pieces to the DSV are 50cm (20") long. How short long should they be?

jradomski

dave I just measured the Inspo cartridge and its 5.7 inches (internal- 5.90 external) diameter, length 7.0 inches (bottom of cart to lip inside) Now take off the thickness of the spider, and 4mm extra so that the spider is not at the exact edge, thats about .35 inches..

so the area of the sorb would be 5.7 by 6.65 approximately 1:1.17 (diameter to length)

__________________
Joe Radomski
CCR Trimix Instructor Trainer
ANDI Instructor Trainer Director #10

All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.

dave t

Joe, agreed but 2:1 is a good start when you dont have any real means to test the scrubber, WOB may be a bit higher but dwell time is a bit longer and the scrubber "should" take a bit more abuse!

Dave

sorry better clarify that, what I meant to the origional poster was 2:1 as a maximum is a good starting point (his was 3:1) the inspo scrubber was 2:1 origionally but with proper test equipment AP ended up with an ideal size for the inspo loop. that is as you measured

AD_ward9

As short as possible.

One reason for long hoses on some units is that EN14143:2003 requires hoses not to stretch more than 30% with a 1kg load. This means for 150mm head movement, hoses have to be long. I hope this is fixed in EN14143:200X but for now, that is what it is. Only the Draeger Dolphin meets this spec but other companies try.

The better solution is very flexible hoses then have the hose go onto the CL, by having the CL as close to the mouth as possible, subject only to visibility issues.

Alex