BCD and tanks

[tbone1004]

that doesn't make it ideal for cold water diving because it is still positively buoyant. vs 6lbs or more negative where you get to distribute the weight significantly better

 

[diversteve]

Sure about that? FTR I don't know either but:

I believe it's a back inflate, so a BP&Wing doesn't have any real benefit over it. I'm not a big fan of the vest style, but at least it has neutral buoyancy padding.

 

[The Chairman]

That's what they call it. It's supposed to be neutral.

 

[REVAN]

The HP 120 steel tanks are a nice option. 120 cubic feet when you can get the high pressure fill, and if all you can get is a standard pressure fill it is still a 103 cubic foot tank at 3000 psi. 7.25 inch diameter means less drag in the water than the LP tank at 8 inches and you won't have to adjust your straps if you rent an aluminum 80 because they are the same diameter.

 

[tbone1004]

the drag is insignificant for 7.25 vs 8 inch tanks for reference. We don't move fast enough through the water for that to be of concern, especially compared to the BC's that most people use acting like sea anchors. Having a dangling SPG or octopus will cause more drag than going from 7.25 to 8 inch tanks. There is the mass penalty of going to 8" and that comes at energy required to accelerate and decelerate, and that is noticeable.

 

[cool_hardware52]

^^Just click up a gear.

Tobin

 

[REVAN]

the drag is insignificant for 7.25 vs 8 inch tanks for reference. We don't move fast enough through the water for that to be of concern, especially compared to the BC's that most people use acting like sea anchors. Having a dangling SPG or octopus will cause more drag than going from 7.25 to 8 inch tanks. There is the mass penalty of going to 8" and that comes at energy required to accelerate and decelerate, and that is noticeable.

Poppycock!

The difference in drag for a 7.25 vs. 8 inch diameter tank is a 22% increase in the tank's intrinsic drag. That's before you take into account the increased interference drag between the larger tank and the diver. The severity of that depends on many factors.

 

[tbone1004]

ok, so assuming 22% increase is valid, I'm sure you have done the math, however what is that compared to most divers less than perfect trim, and the drag of the bouncy compensator?
Sure a 22% increase is significant, but if the tank is only causing say 10% of the overall drag, and say drag is 100 units, drag increases from 100 to 103, that is pretty insignificant in the grand scheme of things.
You can't just say the tank has a 22% increase and therefor you can notice it without qualifying what percentage of the total drag the tank is causing, which for an elongated cylinder, compared to a diver, I'm willing to be is pretty insignificant in the grand scheme of things

 

[Colliam7]

Poppycock!

The difference in drag for a 7.25 vs. 8 inch diameter tank is a 22% increase in the tank's intrinsic drag. That's before you take into account the increased interference drag between the larger tank and the diver. The severity of that depends on many factors.

Can we presume that your '22%' value is a reflection of the change in the area of a circle, from a 7.25" diameter (area=41.3 square inches), to an 8" diameter (area=50.2 square inches)? The actual value of the change (increase) is 21.758%, but 22% is acceptable as a reasonable round-off. And, that really sounds impressive. But, I think that tbone's point was that the increase in cylinder size (and therefore, the cylinder area exposed during horizontal movement - which is the relevant factor in the drag equation) is not the defining factor in total drag associated with a diver moving underwater. In fact, it is relatively trivial by comparison with other factors.

The difference in drag is instead a reflection of the change in the total vertical plane area of the diver and cylinder. (In the drag equation, the area is a direct multiplier.) And, the vertical plane area of the cylinder is quite small in comparison to that of the diver, so even though the change in cylinder diameter contributes to what sounds like an impressive change - 22% - the overall change is minor. Take a diver who presents an elliptical vertical plane area (which most of us do) as s/he moves horizontally through the water. For the sake of discussion in the context of the drag equatiuon, the diver's measured semiminor axis (e.g. front of the abdomen, to the back) is measured to be 22 inches, while the semimajor axis (shoulder tip to shoulder tip) is measured to be 14 inches. The calculated elliptical area is 241.9 square inches. So the total area of diver and 7.25" cylinder would be 282.3 square inches). If we put an 8" diameter cylinder on that same diver, the total vertical plane area would increase to 291.2 square inches. And, THAT percentage change (increase) is (only) 3.2%.

Therefore, tbone's point is well taken. While the increase in cylinder diameter does add an element of drag, the total change in drag associated with a diver moving from a 7.25" diameter cylinder to an 8" diameter cylinder, is relatively minor.

That doesn't even account for additional drag associated with the fact that the diver is not a perfect, symmetrical shape - ellipse, or circle - and the diver's actual vertical plane area presented in the water (with various 'danglies', etc.) is probably bigger than the ellipse calculation, above, would suggest.

In fairness, we don't know the coefficiant of drag for a diver. That is something that needs to be determined experimentally. So actual drag is an unknown. But, the relative change associated with using an 8" cylinder compared to a 7.25" cylinder is relatively trivial in the big picture.

 

[REVAN]

ok, so assuming 22% increase is valid, I'm sure you have done the math, however what is that compared to most divers less than perfect trim, and the drag of the bouncy compensator?
Sure a 22% increase is significant, but if the tank is only causing say 10% of the overall drag, and say drag is 100 units, drag increases from 100 to 103, that is pretty insignificant in the grand scheme of things.
You can't just say the tank has a 22% increase and therefor you can notice it without qualifying what percentage of the total drag the tank is causing, which for an elongated cylinder, compared to a diver, I'm willing to be is pretty insignificant in the grand scheme of things

The tank is usually a lot more than 10% of the divers total drag. The actual percentage will depend on many factors relating to other gear choices and how it is configured, but the average recreational scuba diver kit will approximately double the drag of the diver's body. Of that doubling, the tank is probably contributing about half. The cleaner the kit, the larger impact the choice of tank will have on the outcome. My kit is very clean, so the tank contribution is much more than half of the kits drag contribution (probably more like 80%).

So... if you are a wanker that pays no attention to hydrodynamics and also pays no attention to results, you are probably correct on your assessment that you will not notice the difference. But, that is because you are not paying attention, not because there is no difference.

If you dive a clean rig, you will be able to notice the drag increase of the larger tank. It is not just that the tank has 22% more frontal area, the interference zone between the diver's body and the tank is also larger and that creates more incremental drag.

Back to the OP's topic though, the HP 120 has a lot of advantages. Truth is a HP 100 is probably better for most situations, but if you want the larger volume and especially if you still want more air than typical even when HP fills are not available, the 120 is a nice option. Plus, it's the same diameter as a standard tank, so no more drag than usual and you don't need to adjust tank straps to switch back and forth to a rented 80. I do a lot of diving in Mexico and they don't do HP fills there. My 120s make nice 103s in Mexico. The tank is almost the same size as an 80 and, personally, I like my 120s.