double tank equipment

[cool_hardware52]

Tobin, can you keep me honest on this? We don't need to check both posts for O2% after a fill if the isolator is open due to something like ideal gas laws. Is that right? Even if the tanks were filled separately with the isolator closed, if you open the isolator, in time the gas between both tanks will eventually, what's the word, mix?

As long as the Iso is open during filling you will get equal amounts of any of the fill gases into both cylinders, as long as you are adding only one gas at a time.

If you add the O2 via the right post and the He (or Air) via the left post the *initial* analysis may be a little weird, but a good blast on both posts will clear out the "last" gas from the fairly small internal passages of the valve and manifold.

I blend a lot of gas. I have a fairly unique fill station with banked He, and Air at ~6000 psi and O2 at ~3500. Introducing the "air top" at high speed, which requires a big pressure delta between the scuba cylinder and the Bank is enormously helpful in homogenizing high percentage He mixes.

OTOH if a compressor struggles to make 3500 psi and outputs 2 cfm when doing so you will see a change in analysis for hours after topping off Trimix tanks.

He is really interesting stuff.

Tobin

 

[PfcAJ]

The % of gases in each cylinder will effectively never balance. The "mixing front" is ~.070" diameter and the conduit is long. There was some real world testing done a few years ago and the percentages at each post essentially never changed, over many many days. Russell Edge did the tests IIRC.


Tobin

That sounds reasonable. I don't frequent places that close my isolator, thankfully.

 

[DaleC]

The situation was actually worse than that.

The SPG sits on the right post and therefore reads pressure from the right tank if the manifold is closed. The SPG was reading *full* at the beginning of the dive. So even if I trusted the previous fill which, as you say, cannot be trusted, there had to be gas introduced into the right tank as part of the fill process - the end result of which, I never analyzed. For all I know, they could have put O2 into the right tank which made the mix incredibly rich.

There is usually no need to check both posts for O2% and pressure if the manifold is open, especially if the tanks have been sitting for a while after the fill.

Help me out. SPG on right post? And I assume you are breathing off the primary on the right post? Either the SPG would move or you are breathing off the left or you meant the SPG is on the left. I suspect the latter.

View attachment 211077View attachment 211077

 

[Adobo]

Help me out. SPG on right post? And I assume you are breathing off the primary on the right post? Either the SPG would move or you are breathing off the left or you meant the SPG is on the left. I suspect the latter.

View attachment 211077View attachment 211077View attachment 211078

Sorry, SPG is on the left post as I wear the rig.

I have the same hose routing as what is on that pic except I have MK17 first stages.

I found an old pic:

 

[agilis]

Thanks guys. Somehow I missed this when it was covered in my open water class.

Keep in mind that the very limited mixing resulting from a long, small diameter conduit only applies to a static situation. Everything changes radically as the gas is drawn down when the unit is in actual use. Communicating tanks, unless they are isolated, will pressure equalize.

At least, that is what I would assume as a well trained OW certified diver with some understanding of diving physics, but with with no direct isolation manifold experience.

Despite my intention of leaving this discussion I thought this should be mentioned strictly as a diving safety issue.

 

[PfcAJ]

Pressure will equalize. Gases maybe not so much.

 

[agilis]

Pressure will equalize. Gases maybe not so much.

In order for pressures to equalize the gasses in each cylinder must mix, the degree of mixing increasing as the total amount of gas diminishes through use, especially assuming that there is a greater draw from the regulator being used by the diver, and also assuming that the amount of gas needed to equalize is flowing from the tank under the unused first stage to the other tank. The more you breathe the more mixing taking place.

This observation is based only on my simpleminded logic, after viewing a diagram of an isolation manifold and then applying the principles of basic compressed air physics as I imperfectly learned them during an extraordinarily lenghty OW certification course taken when Richard Nixon was president and I had not yet shaken the habit of measuring distance in kliks.

I understand that much has changed.

 

[cool_hardware52]

In order for pressures to equalize the gasses in each cylinder must mix, the degree of mixing increasing as the total amount of gas diminishes through use, especially assuming that there is a greater draw from the regulator being used by the diver, and also assuming that the amount of gas needed to equalize is flowing from the tank under the unused first stage to the other tank. The more you breathe the more mixing taking place.

You really need to stop. Try to get a grip on your compulsion to comment on things you have zero experience with, and apparently no ability to understand.

For gas to transfer between cylinders one tank *must* be at a higher pressure than the other. Gas will *always* flow from hi to flow pressure. The high pressure tank will lose pressure, and contents but no mixing takes place within the tank at an initially higher pressure. The lower pressure tank will increase in both pressure and contents, and some mixing will probably take place, but the degree of mixing can be largely a function of velocity. If the pressure differential is small, and the velocities low there may be very little mixing.

Once the tanks are at equal pressure, each will contribute equally to any gas delivered to a single regulator. This does nothing to further the mixing of the gases in each of the tanks. To claim otherwise suggests you either lack an understanding of physics, or still fail to grasp how a manifold functions. Can you please stop posting nonsense that only serves to confuse?

This observation is based only on my simpleminded logic, after viewing a diagram of an isolation manifold and then applying the principles of basic compressed air physics as I imperfectly learned them during an extraordinarily lenghty OW certification course taken when Richard Nixon was president and I had not yet shaken the habit of measuring distance in kliks.

I understand that much has changed.

The laws of physics haven't changed.



Tobin

 

[agilis]

You really need to stop. Try to get a grip on your compulsion to comment on things you have zero experience with, and apparently no ability to understand.

For gas to transfer between cylinders one tank *must* be at a higher pressure than the other. Gas will *always* flow from hi to flow pressure. The high pressure tank will lose pressure, and contents but no mixing takes place within the tank at an initially higher pressure. The lower pressure tank will increase in both pressure and contents, and some mixing will probably take place, but the degree of mixing can be largely a function of velocity. If the pressure differential is small, and the velocities low there may be very little mixing.

Once the tanks are at equal pressure, each will contribute equally to any gas delivered to a single regulator. This does nothing to further the mixing of the gases in each of the tanks. To claim otherwise suggests you either lack an understanding of physics, or still fail to grasp how a manifold functions. Can you please stop posting nonsense that only serves to confuse?




The laws of physics haven't changed.



Tobin

The mixing must take place, either in the tank with a decreasing pressure as it's breathed from, or in in the manifold being breathed from, or, most likely, in both, with the phenomenon occurring most markedly as the overall system volume decreases.

When two tanks are freely connected it is not possible to draw from only one.

If they are freely connected each cylinder will have the identical pressure. When breathing, you are either drawing mostly from one tank, lowering its pressure causing the other tank to contribute what's required to equalize, or drawing more of less equally from both, with the mix taking place in the manifold. The only way this would not happen is if the isolating valve were closed. The mix taking place may be slight, at least initially, but it must take place. The laws of physics have not changed. When you remove more air from one of two openly connected paired tanks, you lower its pressure, causing gas from the other tank to effect equalization on however small a scale. If most of the mix takes place in the manifold the safety issue is, if anything, amplified.

There is nothing wrong with my ability to understand. It is churlish of you to make that kind of remark, especially when any calm consideration would compel you to admit that I am right.

I don't expect that to happen anytime soon.

 

[cool_hardware52]

In order for pressures to equalize the gasses in each cylinder must mix, the degree of mixing increasing as the total amount of gas diminishes through use, especially assuming that there is a greater draw from the regulator being used by the diver, and also assuming that the amount of gas needed to equalize is flowing from the tank under the unused first stage to the other tank. The more you breathe the more mixing taking place.

Wrong again. When you open the Iso valve gas will move from the tank at higher pressure *to* the tank at lower pressure, there will be *no* mixing in the tank that was at a higher pressure. If no new gas enters this tank *what* does it mix with?

When two tanks are freely connected it is not possible to draw from only one.

True, I've never said anything to the contrary, but you seem to think it happens with every breath.

If they are freely connected each cylinder will have the identical pressure.

True again, yet you contradict yourself by claiming that one is "drawing mostly from one tank" Can't keep your nonsense straight?

When breathing, you are either drawing mostly from one tank,

Complete and utter nonsense. By what mechanism does this magical gas flow "mostly from one tank"? It's PRESSURIZED GAS. Physics demands the pressure to be equal *everywhere* inside both tanks and the manifold. The internal passage ways of the manifold are easily sufficient to maintain this equal pressure at any consumption rate a diver would impose. It would require a *tiny* restriction, like the hole drilled in the reg end of a HP hose to produce the phenomena you claim. This just isn't true. *Anybody* who has ever assembled a manifold knows this. You know, actually had their hands on one. Not quite the same as walking disinterestedly past a set of dubs at the LDS.


I've designed, built and operated fill stations, and I have designed and manufactured special purpose manifolds for rebreather. Manifolds based on the exact type we are now discussing. You are quite simply completely wrong. Both the designs I produced involved extending the length of the center section of the manifold, one using HP stainless tubing. I researched the cross sectional area need to avoid any measurable pressure differential across the manifold. I don't have my notes with me, but it's quite small, on the order of .035" IIRC. The internals of a typical manifold offer about 5 times that cross section.

What actually happens when a diver cracks a reg is gas is drawn from the manifold and the pressure in the manifold drops slightly. Both tanks, due to the drop in the manifold pressure supply gas to the manifold. Apparently you believe that when a diver cracks a reg gas flows "mostly" from one tank, and not from both due to to some as yet unspecified magical force, then it flows from the "other" tank, through the manifold, PAST the post orifice and into the first tank, mixes and then makes a U turn and flows back to the orifice. Remarkable, just remarkable. Magical thinking on display.

The mix taking place may be slight, at least initially, but it must take place. The laws of physics have not changed. When you remove more air from one of two openly connected paired tanks, you lower its pressure, causing gas from the other tank to effect equalization on however small a scale. If most of the mix takes place in the manifold the safety issue is, if anything, amplified.

More nonsense, you can't preferentially remove gas from one tank, the flow rates and internal cross section of the manifold won't allow it. Yet another illustration of your profound ignorance of the internals of a manifold.

There is nothing wrong with my ability to understand. It is churlish of you to make that kind of remark, especially when any calm consideration would compel you to admit that I am right.

I don't expect that to happen anytime soon.

Only a fool or a troll would repeatedly post on topics they know nothing about.

Tobin