Meanwhile, back at the lab: Measured buoyancy varies from one cylinder to another

[2airishuman]

I've never been able to get the math to work out exactly on weighting -- actually dialing in the best amount of lead on a dive was leaving me a few pounds away from what it should be, in some cases. I would add up the experimentally determined buoyancy of the wetsuit I'm using, the published empty buoyancy of the cylinder, add an adjustment for salt vs. fresh, add the effects of any stage or doubles configuration, and so on. I feel the top of my wing at the end of each dive to see how much air it has, and there were variations I couldn't explain.

I have a large number of cylinders, because I shore dive and in some cases am over a hundred miles from the nearest compressor, and because I share my cylinders with my kids who dive.

I decided to measure the buoyancy of my cylinders to eliminate that as a variable. Besides, I have a few older cylinders where published data is unavailable.

In the lab (also known as the "garage"), I set up a kitchen scale accurate to the nearest 1/4 ounce on a high shelf, and rigged a line down from it. Below I set a large, clean trashcan filled with tap water. I measured the pressure and temperature in each cylinder so I could accurately determine the mass of the gas in the cylinder, then put it in the water bath, suspended from the line.

Results were repeatable, and I believe I was accurate to within under an ounce. I believe I now have a very accurate assessment of the buoyancy of each of my two twinsets (LP72 and HP100), and I've confirmed the expected results in most other areas.

The outlier is a pair of Worthington X7-120s that were originally part of a twinset. Serial numbers are close. Production month and year the same. Valves are mirror images of each other. But the buoyancy values adjusted to empty by subtracting the mass of the air fill, differ by 1.6 pounds. That seems like a lot, and it's enough to make me wonder if I need to start checking the serial number when I determine the ideal amount of lead.

Is this common? How much variation should there be?

 

[Kharon]

Factors that change buoyancy during a dive that are not included in your experiment: Amount of air trapped under your wetsuit, your density (varies during the day), the temperature of the water (it's density varies with temperature), probably other factors as well. I don't even expect two "identicle" dives with the exact same kit on the same day to have the same results.

Factors within your experiment: the actual weight of the completely empty tank (including valve) needs to be included, calculations adjusted to empty not as accurate as using completely empty tanks, unless the tank was completely submerged measurements are suspect.

Just my 2 cents.

 

[The Chairman]

I've never been able to get the math to work out exactly on weighting

I've never understood the need for some to "get the math to work out exactly". There are too many variables, even from dive to dive, for this to be an exact calculation. I can deal with anything from about 20 pounds overweight (Yeah, boo hiss, and that's still a conservative number) to about 4 or five pounds light. Being too light, even by a pound, consumes a lot more gas at depth trying to compensate for it. Consequently, if I'm diving at a certain amount of weight with unfamiliar tanks, I'll play with my weights from dive to dive. I'll lighten up by two or four pounds each dive until I'm a tad too light on my safety stop. Then I'll add back what ever I just took out. If you're diving with a BC, then this is as close as you need to get so stop stressing. Diving is about seeing the pretty little fishes and not a competition to see who has the least amount of weight, the smallest BC or the lowest air consumption.

 

[Storker]

How much variation should there be?

You're giving a variation of 1.6 lbs. In proper units, that's less than 1 kg. The smallest lead weight I've seen is 1 kg.

Whatchagonnado if you're weighted to the nearest gram (ounce or grain for those of you who insist on using obsolete units) and suddenly has a little more intestinal gas one day? Or is slightly on the cold side and would like a little more loft in your undersuit?

Diving is about seeing the pretty little fishes cool stuff and having fun and not a competition to see who has the least amount of weight, the smallest BC or the lowest air consumption.

FTFY. Seconded, BTW.

 

[tursiops]

Are you sure there is no water in the heavier HP100?

 

[2airishuman]

Thanks for all the replies.

Factors that change buoyancy during a dive that are not included in your experiment: Amount of air trapped under your wetsuit, your density (varies during the day), the temperature of the water (it's density varies with temperature), probably other factors as well. I don't even expect two "identicle" dives with the exact same kit on the same day to have the same results.

Correct, and that's by design.

Factors within your experiment: the actual weight of the completely empty tank (including valve) needs to be included.

I did measure this but it has nothing to do with buoyancy, since I am measuring buoyancy directly.

calculations adjusted to empty not as accurate as using completely empty tanks

True enough although I doubt if I'm off by more than a few ounces.

unless the tank was completely submerged measurements are suspect.

The tanks were, in fact, completely submerged.

I've never understood the need for some to "get the math to work out exactly". There are too many variables, even from dive to dive, for this to be an exact calculation. I can deal with anything from about 20 pounds overweight (Yeah, boo hiss, and that's still a conservative number) to about 4 or five pounds light. Being too light, even by a pound, consumes a lot more gas at depth trying to compensate for it. Consequently, if I'm diving at a certain amount of weight with unfamiliar tanks, I'll play with my weights from dive to dive. I'll lighten up by two or four pounds each dive until I'm a tad too light on my safety stop.

I think we agree that there is a point beyond which it doesn't matter.

I was experiencing differences of as much as 4 pounds between what I expected and what I actually experienced in the water. That's more than I would like. I think, based on the information I collected about actual cylinder buoyancy, I'll be much closer. I'll probably end up choosing a median value for the X7-120s and call it good enough.

You're giving a variation of 1.6 lbs. In proper units, that's less than 1 kg. The smallest lead weight I've seen is 1 kg.

Whatchagonnado if you're weighted to the nearest gram (ounce or grain for those of you who insist on using obsolete units) and suddenly has a little more intestinal gas one day? Or is slightly on the cold side and would like a little more loft in your undersuit?


FTFY. Seconded, BTW.

I would like to enter the water reliably weighted about a pound heavier than I need to be.

Are you sure there is no water in the heavier HP100?

You mean the X7-120? I am quite sure it has only air.

 

[Luis H]

I actually measure the displacement of my cylinders in a very similar way. Below is some of the data I have taken on a few of my steel 72 and a couple of HP80s. The salt water (SW) displacement is calculated and so is the weight of the air, but the air is based on the actual tank volume (not published data).

You may notice that not only the displacement varies, but also the actual internal volume. I measured the internal tank volume by accurately measuring the water capacity during the hydro test process.

Most people don’t understand what an engineer may do for fun.
But, one of my favorite things to do is observing the pretty little fishes and spending time studying their behavior. That is why it is so nice having my bubbles behind me. I can get very close without scaring them.

Here is having some face time with Nemo.

This summer I am planning on taking a lot more data on some other different cylinders that I own.

I dive too many different configurations (different dry suits, different wet suits, different cylinders in singles, doubles, etc) that it is just easier to keep track of the gear. I am going to also be experimenting with a new configuration this summer.

 

[Storker]

I would like to enter the water reliably weighted about a pound heavier than I need to be.

Just diving a site at the coast instead of in the fjord close to home would affect my buoyancy by about half a kg (about one pound) due to the difference in salinity. I don't see any point in tuning my weighting that closely. Besides, since weight=warmth when you dive dry, I'm usually a couple of kilos "overweighted" during the winter. Another reason for not tuning my weighting to within less than a kg.

 

[NAM001]

2airis....

if you need to know that bad do this. empty a bottle to about 100 psi attach a 10 # weight to it hang it from a scale with it submerged in a pool. if it weighs 7# then it is 3# light when empty without the 10# weight. why calculate when you dont have to. You can use a postal scale and weigh the valve if you need to. I too have found many flaws in the tables compared to actual weight and buoyancy's. When you are done mark your tank with the mt buoyancy of the tank, or 500 psi buoyancy if you want. I prefer MT. Then you know the info for your tank with your valve. I use H valves on my tanks. NOw a word of caution in your calculations. remember your wet suit looses lift as time goes on, so you cant use the lift value of your WS forever as a constant.

 

[northernone]

I actually measure the displacement of my cylinders in a very similar way. Below is some of the data I have taken on a few of my steel 72 and a couple of HP80s. The salt water (SW) displacement is calculated and so is the weight of the air, but the air is based on the actual tank volume (not published data).

You may notice that not only the displacement varies, but also the actual internal volume. I measured the internal tank volume by accurately measuring the water capacity during the hydro test process.


View attachment 404655


Most people don’t understand what an engineer may do for fun.
But, one of my favorite things to do is observing the pretty little fishes and spending time studying their behavior. That is why it is so nice having my bubbles behind me. I can get very close without scaring them.

Here is having some face time with Nemo.

View attachment 404656



This summer I am planning on taking a lot more data on some other different cylinders that I own.

I dive too many different configurations (different dry suits, different wet suits, different cylinders in singles, doubles, etc) that it is just easier to keep track of the gear. I am going to also be experimenting with a new configuration this summer.

Thank you for sharing your findings. Would it be ok if I download your results for future reference.