Hogarthian rig balancing

[DevonDiver]

I can swim my kit up pretty easily, I just don't know how heavy it actually is at depth. But that's using a single AL80 with 6lbs of lead in a 3/2 fullsuit. But if I switched to a 5mm in winter, or an HP steel tank, obviously that will change. I was wondering if there way a good way to figure it all out before going to depth and then finding out that no, it's not balanced. I would rather not ever be down there with an unbalanced rig.

1. The weight of your cylinder (full/half/low/empty) is available online. i.e. Scuba Cylinder Specifications | Metric & Imperial

2. Put your wetsuit in a mesh bag, add/remove weight to make it sink. This calculates surface buoyancy.

3. Apply Boyle's Law ( The Physics of Diving - Scuba Gas Laws ) to determine loss of that buoyancy as you descend. Obviously, wetsuits don't compress in a perfectly linear manner, as Boyle's Law suggests. Because a wetsuit isn't a perfectly flexible container for the air trapped within it.... but this is as close as you're likely to approximate in advance of a dive. Wetsuit buoyancy properties vary depending on thickness, size and even the brand of wetsuit (quality of neoprene used)

4. The most negative you'll be is the weight of the gas you carry + the loss of wetsuit buoyancy at depth.

5. That assumes that your core weighting (neutral with minimum gas, empty BCD on last stop) is correct.

 

[seeker242]

1. The weight of your cylinder (full/half/low/empty) is available online. i.e. Scuba Cylinder Specifications | Metric & Imperial

2. Put your wetsuit in a mesh bag, add/remove weight to make it sink. This calculates surface buoyancy.

3. Apply Boyle's Law ( The Physics of Diving - Scuba Gas Laws ) to determine loss of that buoyancy as you descend. Obviously, wetsuits don't compress in a perfectly linear manner, as Boyle's Law suggests. Because a wetsuit isn't a perfectly flexible container for the air trapped within it.... but this is as close as you're likely to approximate in advance of a dive. Wetsuit buoyancy properties vary depending on thickness, size and even the brand of wetsuit (quality of neoprene used)

4. The most negative you'll be is the weight of the gas you carry + the loss of wetsuit buoyancy at depth.

5. That assumes that your core weighting (neutral with minimum gas, empty BCD on last stop) is correct.

Thanks. Although, what I'm still not sure about is the maximum value for #4. But I guess the only way to figure that out is just to dive with more weight and see what happens when swimming up.

 

[PfcAJ]

The dir recommendation is against steel *doubles* with a wetsuit.

You can easily have a balanced rig with a steel single.

 

[DevonDiver]

Thanks. Although, what I'm still not sure about is the maximum value for #4

Here's an example:

AL80 (full gas) = 0.64kg / 1.4lbs negative *
AL80 (500 psi) = 2.00kg / 3.4lbs positive *
5mm Wetsuit buoyancy at surface = 12lbs positive **
5mm Wetsuit buoyancy at 4 atm = 3lbs positive **

* As per Luxfer specifications
** You need to measure the buoyancy of your wetsuit (in mesh bag, and add weights reqd to sink it)

Obviously, you should aim to end the dive neutral on your last stop (empty BCD). So you must add weight to account for your exposure protection and cylinder buoyancy characteristics. On your last stop, your wetsuit will return to full buoyancy, but your cylinder will be at 500psi (in this example, you'd be 12lbs + 3.4lbs positive - that's 15.4lbs of lead needed).

So... what's the most negative buoyancy you'd need to contend with?

You have 15.4lbs of lead (negative)
You have 1.4lbs of gas (negative)
You have 3.00lbs of wetsuit buoyancy (positive)

Thus, on descending to 4 atm (30m/100ft) with a still-full cylinder, you are 13.8lbs negative (16.8lbs - 3lbs)

But I guess the only way to figure that out is just to dive with more weight and see what happens when swimming up.

That's how you'd test your math.

However, a balanced rig is more than just 'being able to swim up the rig'... it's about fine tuning your overall configuration to achieve an end result. You should be able to hold stops, not just swim up.

Part #1 Getting up to the stop. What can you comfortably swim up against? For what distance?
Part #2 Holding a stop. What depths are your stop/s? What is your wetsuit buoyancy at that depth? How does that effect your overall positive/negative balance?

Assuming a shallow stop (3-6m).. your wetuit is only marginally compressed... so add back 9lbs of buoyancy since you left the bottom.

You have 15.4lbs of lead (negative)
You have 1.4lbs of gas (negative)
You have 12.00lbs of wetsuit buoyancy (positive)

Thus, on ascending to 1.5 atm (5m/18ft) with a still-full cylinder, you are just 4.8lbs negative (16.8lbs - 12lbs)

So... to hold that stop (without ditching ballast, or using redundant lift) you only have the buoyancy control permitted by your lungs. Breath deeply and you can add around 2-3 kgs (3-7lbs) of buoyancy. You should be able to hold that stop comfortably.

Anything beyond that lung volume compensation requires ditching ballast. Not all the ballast... just enough to get yourself neutral on the last stop. This allows you to identify how much ditchable ballast you will need. The remainder can be permanently fixed into your rig.

 

[stuartv]

AL80 (500 psi) = 2.00kg / 3.4lbs positive *

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?

 

[DevonDiver]

You could, I guess.

As you said... it's 1lb... very easily compensated with breathing. An experienced diver wouldn't even notice they were doing that.
We're talking about single-tank recreational divers on a voluntary safety stop.

That's the thing about balancing a rig... it's like double-entry book keeping. What you add for the end, you have to carry over when you balance the rig at the start. So that's another 1lb, or whatever, that needs to be resolved for your worst case BCD failure scenario.

Which is the bigger problem?
Being 1lb more overweight at the start if your BCD fails?
Or 1lb underweight at the end, if you're low on gas?

Two different contingencies, with two contrary demands. I'd guess being low on air was the more common.

Start factoring for higher capacity cylinders, then you've got to think it through more cautiously.
And you've got to test that it's workable.

If we're talking about technical divers, then (at 500psi) they're weighting based on consumption well above what they've planned to surface with anyway (Rule of 3rds gas). They've also got different gasses..multiple cylinders which is the empty one? Or do we weight for all the tanks, every gas carried being empty?

There's lots of parameters to play with, including that you may be able to dispose of deco tanks to reduce (if empty) or increase (if full) buoyancy. On sidemount, you can do that with your primaries also.

 

[Lorenzoid]

I'll venture a guess: If some emergency, er, I mean unforeseen event leaves you with less than 500 psi in your tank, then the assumption is you skip the safety stop and get to the surface.

 

[seeker242]

The dir recommendation is against steel *doubles* with a wetsuit.

You can easily have a balanced rig with a steel single.

That's what I was thinking too. But Kathy Dicker kinda frowned when I mentioned wanting a steel tank, lol.

That's how you'd test your math.

Thanks. Haven't really needed to do any math yet. I just been using the minimum amount of weight needed to hold a stop and it just happens to be easy to swim up. But I've been playing with that wing lift calculator excel spreadsheet, which I think I got off your website, and it seems to be doing the same math?

 

[tbone1004]

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?

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

 

[DevonDiver]

I've been playing with that wing lift calculator excel spreadsheet,...it seems to be doing the same math?

It does the same math as the balanced rig is a contingency for BCD failure/buoyancy loss... and the wing lift calculator tells you the minimum lift you need in a BCD.

- The first stage of configuring a balanced rig is about getting your weighting spot-on.

- The second stage is about understanding how much lift you lose when your BCD won't hold gas anymore.

- Subsequent stages are about how you minimize that figure to ensure you can (a) ascend from depth and (b) hold necessary stops.

- The final stage is about physically confirming your calculations.