Hogarthian rig balancing

[DevonDiver]

because you don't always have use of all of that gas. Regulators need a certain IP to function properly, typically at least 50psi over IP. So call it 200psi that isn't breathable, and if you are at depth, it is 200psi over ambient, so throw another 100psi in there depending on how deep you are going, but planning on the last 500psi being non-usable is not uncommon for that reason

I dive metric, which is probably why I haven't thought overly hard about this.

On a recreational dive, my minimum gas (reserve) is 50 bar. But I calculate my cylinder buoyancy based on 500psi.... that's only 35 bar.
So I'm actually calculating on being 15 bar below my minimum reserve.

As you say.... anything below that is debatable anyway... SPG readings, regulator IP differential...

Back to what @stuartv asked... let's not forget that balanced rig is (should be) implemented as part of a wider system. That system includes minimum gas calculations - Rock Bottom Gas Management. If that gas management is properly implemented, then running below minimum gas shouldn't be a major consideration in the balanced rig.

 

[PfcAJ]

I have never understood this. Why would you not weight yourself to allow the use of ALL your gas?

Imagine: You mess up (or your buddy messes up). As a result, you arrive at your safety stop with only 300psi left in your tank. Don't you want to be weighted so that you can hold your safety stop right down to the (almost-)dregs of your cylinder before you ascend? And to be able to do so without having to fight to hold the stop?

Admittedly, in this example, it only makes 1 # of difference (an empty AL80 being +4.4# buoyant), but still why would you not weight yourself based on the buoyancy of an empty cylinder versus a cylinder with 500 psi?

In a single tank 500psi of air is like 1lb. Eating a bean burrito before the dive will have a bigger impact.

Don't get wrapped up in being too precise with all this stuff.

 

[stuartv]

My question was specifically responding to an example of a recreational calculation. But, I've had the same question when weighting for a tech dive. I've had in-person conversations with a tech diver or two who also subscribed to the same philosophy - namely, "I'm not going to use that gas, so I don't need to weight for it."

I do my tech diving with back mount double 120s. Call that 18# of gas. Presuming that I could ditch any other cylinders, if they became light and were pulling me to the surface, I feel like I need to be weighted so as to be 18# negative at the start of the dive (not counting the buoyancy of any other cylinders). That way, if shtuff happens and I, for example, need to make an ascent where I hang at my last stop and literally decompress for as long as my remaining gas will allow, I am weighted sufficiently to do so. I.e. breathe my tanks down as far as they'll go.

Some tech divers that I've talked to would say that weighing to be 18# negative at the start is way too much and might start only 12# negative, because their dive plan is Rule of Thirds. Sure, it might mean that you are 6# or more heavy at the end (if you use 2/3 of your gas) if everything goes according to plan. But, shouldn't we plan for the worst case? It only takes 1 problem to cause you to blow your planned bottom time.

I mean, what good is it to even take the extra gas if you don't have enough weight to actually let you use it? (the same philosophy applies to a single AL80, in my mind - it's just that in that case, it's only 1#, so not critical either way)

 

[PfcAJ]

My question was specifically responding to an example of a recreational calculation. But, I've had the same question when weighting for a tech dive. I've had in-person conversations with a tech diver or two who also subscribed to the same philosophy - namely, "I'm not going to use that gas, so I don't need to weight for it."

I do my tech diving with back mount double 120s. Call that 18# of gas. Presuming that I could ditch any other cylinders, if they became light and were pulling me to the surface, I feel like I need to be weighted so as to be 18# negative at the start of the dive (not counting the buoyancy of any other cylinders). That way, if shtuff happens and I, for example, need to make an ascent where I hang at my last stop and literally decompress for as long as my remaining gas will allow, I am weighted sufficiently to do so. I.e. breathe my tanks down as far as they'll go.

Some tech divers that I've talked to would say that weighing to be 18# negative at the start is way too much and might start only 12# negative, because their dive plan is Rule of Thirds. Sure, it might mean that you are 6# or more heavy at the end (if you use 2/3 of your gas) if everything goes according to plan. But, shouldn't we plan for the worst case? It only takes 1 problem to cause you to blow your planned bottom time.

I mean, what good is it to even take the extra gas if you don't have enough weight to actually let you use it? (the same philosophy applies to a single AL80, in my mind - it's just that in that case, it's only 1#, so not critical either way)

Your thinking is spot on. Being too light is bad news bears on a cave or technical dive. Basing your weight on not using that last third is asking for trouble.

But for sorting out your weighting, 500psi is close enough, especially for single tank stuff.

 

[RainPilot]

because you don't always have use of all of that gas. Regulators need a certain IP to function properly, typically at least 50psi over IP. So call it 200psi that isn't breathable, and if you are at depth, it is 200psi over ambient, so throw another 100psi in there depending on how deep you are going, but planning on the last 500psi being non-usable is not uncommon for that reason

A first stage will limit tank pressure down to IP until the tank pressure is equal to IP. After that the IP will be equal to the tank pressure. You will be able to breathe the tank down to ambient pressure.

 

[tbone1004]

A first stage will limit tank pressure down to IP until the tank pressure is equal to IP. After that the IP will be equal to the tank pressure. You will be able to breathe the tank down to ambient pressure.

you ever tried breathing the second stage when it gets down to that point? There is a reason that IP is x-pressure OVER ambient. If you choose to believe that you can breathe the tank down to ambient pressure, take the cap off of your first stage and breathe through the second stage. Let me know how many breaths you get through, you won't get very far

 

[RainPilot]

you ever tried breathing the second stage when it gets down to that point? There is a reason that IP is x-pressure OVER ambient. If you choose to believe that you can breathe the tank down to ambient pressure, take the cap off of your first stage and breathe through the second stage. Let me know how many breaths you get through, you won't get very far

If you are at 5m on a safety stop, the ambient will be 1.5 bar or around 20 psi. even an additional 10 psi will be breathable so at 5m you could easily get the tank down to 30 psi if not a bit less. Next time you have a visual due, pop a reg set on and breathe the tank down to the dregs on the surface. You will be able to remove the first stage while the tank valve is on and you MIGHT hear a tiny bit of gas escape but unlikely. The SPG will be zero long before then, they arent super accurate that low.

 

[PfcAJ]

If you are at 5m on a safety stop, the ambient will be 1.5 bar or around 20 psi. even an additional 10 psi will be breathable so at 5m you could easily get the tank down to 30 psi if not a bit less. Next time you have a visual due, pop a reg set on and breathe the tank down to the dregs on the surface. You will be able to remove the first stage while the tank valve is on and you MIGHT hear a tiny bit of gas escape but unlikely. The SPG will be zero long before then, they arent super accurate that low.

Confirmed. Have sucked a few tanks totally empty before.

 

[DevonDiver]

If you are at 5m on a safety stop, the ambient will be 1.5 bar or around 20 psi. even an additional 10 psi will be breathable so at 5m you could easily get the tank down to 30 psi if not a bit less. Next time you have a visual due, pop a reg set on and breathe the tank down to the dregs on the surface. You will be able to remove the first stage while the tank valve is on and you MIGHT hear a tiny bit of gas escape but unlikely. The SPG will be zero long before then, they arent super accurate that low.

Confirmed also.

With my SP MK25s in the shallows, I can 'feel the pull' for a good few minutes before I reach the point where I actually can't drag a small breath out of the tank.

 

[stuartv]

It goes without saying to most folks, nevertheless (for posterity).... I think we all agree that this talk of breathing a tank down to 30psi (or pick your really-low-practically-empty number) is based on the presumption that the 1st and 2nd stage regs are both balanced regs. I think, with an unbalanced piston 1st stage (for example), you would not get very close to the 30psi (or whatever) like you could with a balanced reg.