Which is heavier? 1lb of lead or 1lb of feathers?

[HowardE]

More likely reasons for the difference would be trapped air in the BC

I agree with Walter. It's more likely that there was air in the BCD, and that when you hit 40 feet, the air in the BCD was mostly compressed, and made it easier to remain neutral. The same air (that probably didn't properly vent) at 14 feet would be more buoyant.

Another possibility is the compression of the wetsuit, and that at a shallower depth, the wetsuit is less compressed, and therefore more buoyant, but at depth, it's more compressed, and less buoyant, and easier to maintain neutral buoyancy, when the diver is underweighted.

[fire_diver]

specific gravity is completely seperate from weight the SG of lead will always be the same, regardless of the amount of lead.

[Charlie99]

Using the NIST they are the same.

But if you start using the avoirdupois scales, etc. then that's a different matter.

So, to which scale do you subscribe?

When measuring my immense pile of gold I always use the unit of troy pounds. Same for when I used to measure my "pounds sterling" of silver. So a pound of gold is about 373 grams, while a pound of feathers is about 454 grams.

On a more serious note, the commonly quoted price of an ounce of gold is indeed a troy ounce, not an avoirdupois ounce. Although the troy pound is smaller than the avoirdupois pound, since there are only 12 troy ounces per troy pound, the troy ounce (ozt) is slightly larger than the more common avoirdupois ounce (oz). 31.1034768 grams vs 28.349523125 g. You can easily check this by finding a source that quotes gold prices in both per ounce and per gram. You will find the ratio between the prices is 31.10x rather than 28.35.

I regret that the US Congress halted the metrification process back in the 1970s, leaving common 2 liter Coke bottle as one of the few relics of that program.

....... OK, now who is going to start the mass vs. weight thread hijack ...

[Hashime]

The SG of iron (depending one the composition and casting method) is about 7.2, compared to lead which has an SG of 11.3. That makes Lead about 40% denser. In terms of buoyancy, one Kg of lead (which produces appx. 9.8N of force down on the surface) only produces 9N of force down underwater. Iron on the other hand only produces about 8.43N down underwater.

Note: This is difficult for me to explain, I only know how to work in metric. The Imperial system uses the same term for both mass and weight (which are not the same) so don't complain.

Anyways, all converted, 1kg of lead (2.2 pounds) weighs 2.016 "pounds" (pounds-force really) compared to 1kg of iron, which weighs 1.882 pounds underwater.

So if you want a conversion factor (rough) multiply the amount of lead you use by 1.07 and you should weigh close to the same amount underwater.

Note: All calculations done using fresh water. (though since this is a ratio the factor will work the same for all fluids, It is actually more like 1.06 if I were to keep more decimal places, but unless you go custom the smallest increment you can really get is one pound, so it does not matter.)

If I did my math wrong it is because I should not be doing math at 00:40.

[Hashime]

My calculations come out the same as ss7's ---- the difference in negative buoyancy between 15 pounds of lead and 15 pounds of iron is less than 0.6 pounds.

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Calculations details for those that are curious.

specific gravity of lead is around 11.3, iron is 7.85.
Divide 15 pounds by 2.2 to convert it to 6.8kg.
6.8kg of lead will have a volume of 6.8/11.3 = 0.6 liters. The water displaced by that 0.6 liter volume will weigh just a smidgen over 0.6kg or 1.3 pounds.
The negative buoyancy from 15 pounds of lead is 15-1.3 = 13.7 pounds.

The same calculation with iron comes up with 1.9 pounds of buoyancy.

The the difference in negative buoyancy between 15 pounds of iron and 15 pounds of lead is 1.9-1.3 = 0.6 pounds.

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Of course, if one had been through the calculation before, then you would know that the negative buoyancy from something is simply = weight * (specific gravity - 1) / (specific gravity). Or to be more precise, replace 1 with the 1.03 for saltwater.

Damn, you did it first. Anyways, I had fun remembering my physics.

[knowone]

So this must be why ships, aren't made of lead or
feathers and kilograms apply to weight AND mass

[Walter]

kilograms apply to weight AND mass

The metric unit for weight is the rarely used newton.

[Leadking]

Reminds me of the woman at a DEMA show who picked up one of our weights and exclaimed that is was extremely heavy. She asked what they were made of. Lead I explained. She then asked "Why don't you make them out of aluminium, they would be so much lighter?"

[aquaregia]

The metric unit for weight is the rarely used newton.

Not to be confused with the nouveau metric newtonne.