RCABs air volume

[daveee]

Could someone please tell me ;
1) What is the ratio of WTC volume to bladder volume when inflated?

2) Would I get more flotation from a RCABS-R than a RCABS everything else being equal?

I am presently getting about 11 cubic inches flotation from a ~ 38 cubic inch WTC with an RCABS system and it isn't nearly enough. I think I need to at least double it.
Using a "Micro Air Pump" AP-3PO1

Thanks!
Dave

[raalst]

about Q1,

as far as

about Q1,

as far as I understand, it depends on your pump.
the pump is rated for some pressure difference.

you start out with equal pressure and the known
volume of the WTC. due to the pump the pressure in
the wtc decreases as the bladder expands. the pressure in the bladder is always the water pressure
at that depth.

the pressure in the WTC drops 11/38th of a bar.
the pump needs to overcome this plus the water pressure. A stronger pump might be able to suck
more air out of the WTC. All this about RCABS.

and this is where they say : "the rest is left as an exercise for the reader".

[crazy ivan]

daveee,

Down here in Subron

daveee,

Down here in Subron 8 country, we have done some testing with these pumps. They are much more efficient at creating pressure than pulling a vacuum. When pumping air from an open source into a closed container, they will develop between 11 and 15 psi. However, when drawing air from a sealed container to an open outlet, they will produce a pressure differential of only 5 psi at best.

When I plug your numbers into the Ideal Gas Law calculations, it shows that you are only developing around a 3.3psi differential. Not too surprising, as the AP-3PO1 is not designed to be a vacuum pump and it likely becomes quickly starved for air molecules to pump as the vacuum develops in your WTC. You say you need to double the amount of air pumped to the bladder to be around 22 cu.in. Then with your present configuration you would need to double the size of your WTC. Any equipment inside will serve to reduce the WTC volume somewhat as well. At 38 cu.in. your WTC would be something like 3 in. by 5.5 in. or something like that. I think most regular RCABS systems use a much larger WTC so they can provide plenty of air without pulling too much of a vacuum.

In a Reverse RCABS setup, that same 22 cu.in of air would be contained in a pressure vessel of 29.4 cu.in at 11 psi. That is volume used up in the wet area of the hull in addition to the WTC's 38 cu.in. unless you can fit the pressure vessel inside the WTC.

[daveee]

Crazy Ivan
I like your idea,

Crazy Ivan
I like your idea, Thanks.
I am re-designing for a RCABS-R, only using the original WTC as the air reservoir. Attached is a picture of a modification I made to the pump that now allows me to isolate the pump from the WTC and hook up a suction line to the bladder. The WTC is about 49 cu-in but I estimated it at 38 ci-in with the stuff inside. The 1/16th brass fitting will be replaced with a 1/8th hose barb.
Do you see any reason this woulden't work?
I have ordered two separate bladders so that I can put one on each side of the WTC and there fore have little or no inflation underneath the WTC, and the top edge will now be at the water line. I had trouble with the single bag inflating on one side first causing the sub to come up crooked.
This is great fun solving problems.
Dave

[Guest]

RCABS air volume

Davee,
Another simple way to solve the problem of the single bladder not inflating evenly, is to cover the bladder with a curved plastic section cut from a 1 or 2 liter soft drink bottle, and held down around the WTC with 3 rubber bands. That not only allows adjustment for even inflation, but also helps insure complete deflation for diving. I think this wqs explained in the original RCABS article in the Mar. 2004 SCR, and in subsequent posts. Hope this helps you and others.

[crazy ivan]

Daveee,

Using the numbers you give

Daveee,

Using the numbers you give and staying with the 22cu.in. bladder for the moment, my calculations show that when the bladder has emptied, the WTC would be at around 8.5 psi using the 38 cu.in figure, and 6.5 psi for 49 cu.in.
This is a workable pressure, though not optimal. If the total bladder volume is actually more than that, then the psi will be higher.

I calculate pressure this way: with the bladder inflated, the total volume of air in the system is initially that of the bladder plus the volume of the WTC, and the initial pressure is 0 psi (gauge). With the bladder empty, the total air volume is just that of the WTC. I then go to this website:

http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/idegasc.html#c1

and plug in those values using the alternate values section at the page bottom, then hit the "Final Pressure" button to get that value.

Just a warning about using the WTC to store pressure]external[/i] pressure. You can always isolate that section from the rest of the WTC I suppose.

BTW, nice work on adding the pump inlet fitting. What is that black end cap made from?

I agree, this is a lot of fun!

[daveee]

Thanks guys
Crazy Ivan
If I change

Thanks guys
Crazy Ivan
If I change the lip seals to o-rings, wouldn't the seals hold either way? In the hydraulics world o-rings easily hold hundreds of pounds. What about turning the lip seals around? At least it will be easy to see where the leaks are.

Art, thanks for the advise, a great idea I never would have thought of.

I don't know what type of plastic it is, it was just laying around and is easy to machine. I just siliconed it on as it is always negative pressure. (I think)
Dave

[crazy ivan]

If you turn the cup

If you turn the cup seals around then they would leak water in when you dive. The O-rings will work bi-directionally. They just have a little more friction with your pushrods when they slide. I use O-ring seals myself, though my outputs are rotary, not linear.

[bigdave]

Once you see an end

Once you see an end cap straining to break free from the cylinder with 11 PSI behind it you may want to go with a pressure vessil. It did not give me a warm and fuzzy feeling! BD.

[daveee]

Big Dave
I just looked over

Big Dave
I just looked over my notes and see the WTC is 59 Cu-In, not 49. I suppose that will help keep the pressure down.
I don't see any way to fit a pressure chamber anywhere without starting all over again, which would result in a Divorce. (Can't afford)
I'll strap it good and give it a try. I'll measure the pressure and let everyone know how it turns out.

[bigdave]

I wish you much success

I wish you much success in your quest Dave.

[daveee]

My first test yesterday worked

My first test yesterday worked great.
The air volume almost doubled and now the sub nearly achieves the correct water line. I expect to receive the new bladders soon to correct the list problem.
The pressure inside the WTC can't be too high because the 1" diameter access cover on top doesn't even push out, merely held in place by the o-ring friction. (It will get a safety of some sort.)
I held the two WTC sections and end caps together simply by using aircraft .025" SS safety wire, in a loop lengthwise around the tube, held in place by tiny holes drilled into the edges of the end caps. It is neat, simple and fast.
The 1/8" push rod seals are now o-rings and they all hold perfectly.
That's all for now.
Dave