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

[trailblazer1229]

Ok, It may seem like a strange question, and I risk sounding like an idiot. But am I really this stupid???

My wife and I conducted a boat dive in salt water a couple of weeks ago. Buoyancy checks revealed her to need 12lbs of lead and I needed 15lbs. We had a great dive to about 40 feet and we were beautifully neutrally bouyant. We used the lead and weight belts from a LDS sponsoring the dive.

The next day we conducted a shore dive to about 14 feet. We had the exact same equipment as the day before. Some suits, ballast of gear, tanks, air, etc. The difference was that instead of using the LDS lead and weight belts, we used our pocket weight belts with weights I pulled from my weighted pull-up vest. They are a pound each and I think they are made out of iron with a glossy black paint. When I weighed them with my luggage scale, they were just over 15 lbs and 12lbs.

On the shore dive, my wife could not descend as she was too buoyant. She tried everything and the only way should could submerge is if I pulled her down with me and then she slowly floated back up.

So the only variables changing were the makeup of the weights and the column of water we were swimming in since it was a shore dive.

My question is: Does the composition or make up of the weight material (such as lead compared to iron) make a difference underwater if they weighted the same on the surface?

Or, is the underlying factor the column of saltwater (depth) under us?

Or, could it be she used an anchor line to lower her to a point on the boat dive where her wetsuit compressed enough?

What weighs more underwater? A pound of surface lead or a pound of surface feathers (or iron)?

[dkktsunami]

No scientific background here and I doubt that this would make enough difference ............. but the volume of iron necessary to match the same weight of lead would be more thereby displacing more water and creating a greater buoyant force. Am just guessing though. Someone with real knowledge will come along soon.

[gcbryan]

You are both new divers and that is why your wife had problems on that particular day. You didn't have problems and you were using different weights as well...correct?

[trailblazer1229]

Correct. We were both using new weights. I initially had some trouble getting down but worked it out.

Interestingly enough after doing some more internet searching I found this interesting piece of information.
http://freymanart.com/scale.htm

Turns, out it is a reverse experiment of the question.

[DennisW]

If you weigh purchased weights, you will find that they are not the weight they claim to be. A 3 lb weight will be around 2 1/5 lbs. This differs widely, so you can't just take a percentage off the weights you get from the LDS. So, if you used weights from the LDS and had 14 lbs (stated), I would bet they were actually about 2 to 3 lbs less than stated total unless the LDS makes their own weights. If you used weights that are actually weighed, then they are the weight you saw on the scale assuming the scale is Ok. Does this make any sense?

So about your problem, the weights you used in the second dive could be as much as 2 to 3 lbs different than the ones on the first dive. 2 to 3 lbs can make you have problems getting down.

So a pound of feathers weighs the same as a pound of lead, unless someone stamped 1lb on the lead and they lied.

[fire_diver]

Yes, there is difference, but I wouldnt think it would be enough of a difference to be noticable.

[fire_diver]

So a pound of feathers weighs the same as a pound of lead, unless someone stamped 1lb on the lead and they lied.

All bets are off when you change the medium you weigh them in. Don't forget the bouyancy factor invloved. One pound of lead and one pound of plastic would have VASTLY different "weight" underwater.

[ss7]

Lead and iron will give you different results. Dkktsunami was on target about the buoyancy effects. Lead will help your decent more than iron as its about 40% more dense.

If you had 15lbs of ice as weight, you would float like a champ, eh?

[Walter]

While there is a slight difference between buoyany of iron and lead, it's not enough for it to be a factor in your weighting. More likely reasons for the difference would be trapped air in the BC or the diver kicking up.

[Hatul]

I've never heard of iron diving weights. Wouldn't they rust with the slightest leak of the coating?

[spectrum]

Commercial dive weight like the ubiquitous Sea Pearls are as close spot on as you would hope for. I have measured a sampling on a precision scale and they are darned close. Those with vinyl coatings weigh more due to the coatings but that material is nearly neutral in buoyancy and cancels it's self out of the equation.

Home / local pour weights on the other hand are all over the board.

If you used shop weights that were non commercial they may have been above spec. This your true need would be understated.

Also if your weights from home were of a different material you would be giving up some ballast capacity. the true effect of the weights you wear is the weight of the weights minus that of the water it displaces.

The depth below you is of no consequence.

Using the rope to get down is a huge risk. Some BC's are very buoyant but get much closer to neutral once saturated. In this case pulling down would not be the end of the world but it's no way to dive. The big risk is that she is what she is and after pulling down she will only get more buoyant as she breathes the cylinder down. Eventually she will be helplessly ascending past a safety stop or into boat traffic etc.

A diver is much better off being 4 pounds overweight than 2 under. Until the configuration is certain I tend to add a little more. Just remember to dial it in subsequently by doing end of dive weight checks.

Weight is only part of the answer the difference in density relative to the water your in tells the rest of the story.

Pete

[trailblazer1229]

Hatul,
Yes, they would, and they did a bit. I was improvising as they were not supplying weights on the shore dive and I didn't feel like paying $30 to rent them or over $100 to buy them. I had some weights from a weight vest that fit into the belt pockets and figured I'd give them a try. Worked fine for me, not so much for my wife.

[trailblazer1229]

Thanks for the replies, starting to make sense as a combination of a bunch of things.
Interesting physics question though. Makes sense when you talk about 5 pounds of ice in water, but then again the specific gravity of water in its solid form is less than 1, which means it floats. The specific gravity of Iron and Lead are pretty close to one another at 7 and 11, so they both sink pretty nicely.

Didn't even think the weights from the LDS were not true to their stamped weight I just assumed. I knew the iron I used was at 15 because of the scale. That is interesting too. Next time I guess we will do a buoyancy check with the iron as well. Diving physics never ceases to amaze.

[spectrum]

Thanks for the replies, starting to make sense as a combination of a bunch of things.
Interesting physics question though. Makes sense when you talk about 5 pounds of ice in water, but then again the specific gravity of water in its solid form is less than 1, which means it floats. The specific gravity of Iron and Lead are pretty close to one another at 7 and 11, so they both sink pretty nicely.


7/11 is significant when your dialed in with only 15 or less.

-water it's 6/10 if my fuzzy physics is right for 15 pounds it is comparable to only 9 pounds of lead .

[ss7]

7/11 is significant when your dialed in with only 15 or less.

-water it's 6/10 if my fuzzy physics is right for 15 pounds it is comparable to only 9 pounds of lead .

I calculated the difference to be only 0.58 lbs.. which is something, but a bit different than 6lbs.

Buoyancy lift of 15lbs of lead and iron would be 1.31 lbs and 1.89 lbs, respectively.

[annieols]

a little off topic but the comment about ice cubes as weight makes me think of a silly DM story... Working on a boat in a shore diving area, all divers were told to do a shore dive and get weighting down first because there would be no extra weights on board.. Of course, we did keep about 4- 2lb weights just in case. A diver forgot his belt and needed 14 lbs, we gave him the 8 lbs, and i gave him the 2 lbs spare i carried. The DM then had a brilliant moment when he told the group that he would fill his sandwich ziplocks with water and that should bring up the last 2 lbs... unbelievable part is he couldnt figure out why we all laughed so hard!!

[Charlie99]

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.

==================================
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.

--------

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.

[Charlie99]

The DM then had a brilliant moment when he told the group that he would fill his sandwich ziplocks with water and that should bring up the last 2 lbs... unbelievable part is he couldnt figure out why we all laughed so hard!!It was not a DM, but an instructor that after she saw me dump a lot of water out of my BCD after a dive commented that I must have been very negatively buoyant with all that water in my BC.

Being a whiz at physics is definitely not a requirement to becoming a DM or an instructor.

[Bubbletrubble]

While there is a slight difference between buoyany of iron and lead, it's not enough for it to be a factor in your weighting. More likely reasons for the difference would be trapped air in the BC or the diver kicking up.
Walter pointed out that her BCD might have had a trapped bubble or that she was kicking upwards. It's also possible that the OP's wife had some trapped air in her wetsuit. I suppose holding a larger than normal breath (shallow breathing?) during the weight check...or any combination of the above could be responsible for the weighting discrepancy.

When it comes to novice divers experiencing issues like this, you have to address the simplest explanations first. I'd investigate the presence of trapped bubbles (inside the wetsuit or BCD) during the weight check first. At the same time, I'd recommend that she cross her legs/feet to avoid unintentionally kicking upward during the weight check.

[diver 85]

OP, now you know why 'chickens' make poor divers........

[AbyssalPlains]

The answer lies in the equipment configuration that you used for your 14-foot shore dive. You said you were using "the same suits, ballast of gear, tanks, air, etc. The difference was that instead of using the LDS lead and weight belts, we used our pocket weight belts with weights I pulled from my weighted pull-up vest. They are a pound each and I think they are made out of iron with a glossy black paint. When I weighed them with my luggage scale, they were just over 15 lbs and 12lbs."

What you should have used is a snorkel.

:D

[DevonDiver]

On the shore dive, my wife could not descend as she was too buoyant. She tried everything and the only way should could submerge is if I pulled her down with me and then she slowly floated back up.

From my experience as an instructor, the most common problem that novice divers have with descending is incorrect technique, not insufficient weighting. This could explain the discrepency in this instance.

It could be that the BCD was not fully deflated. Not raising the LPI properly, can leave some air remaining.

It could be that your wife did not fully exhale to begin her descent. This is common when there is some apprehension before the dive.

It could be that your wife was instinctively finning or sculling as she tried to descend. Even a small amount of fin movement can provide substantial upwards thrust - which can be mistaken for excess buoyancy.

Lastly, of course, it could be a difference in the stated weights. Some weights are not accurately cast..leading to differences. Some divers mistake lbs for kgs and vice versa.

[The Kraken]

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

1 pound, as set forth by the NIST, is an expession of weight which must comply with a national standard, therefore, regardless of the molecular make up of the material, the weights must be the same . . . the 1 pound standars as set forth by the NIST.

1 Lb. = 1 Lb.

the K

[fire_diver]

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

1 pound, as set forth by the NIST, is an expession of weight which must comply with a national standard, therefore, regardless of the molecular make up of the material, the weights must be the same . . . the 1 pound standars as set forth by the NIST.

1 Lb. = 1 Lb.

the K

1 Lb in air is NOT = 1 Lb in water.

[mrmagoo560]

All bets are off when you change the medium you weigh them in. Don't forget the bouyancy factor invloved. One pound of lead and one pound of plastic would have VASTLY different "weight" underwater.

That is true palstic would weigh less underwater,but lead and iron should in theory weigh the same underwater.

[Charlie99]

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

1 pound, as set forth by the NIST, is an expession of weight which must comply with a national standard, therefore, regardless of the molecular make up of the material, the weights must be the same . . . the 1 pound standars as set forth by the NIST.

1 Lb. = 1 Lb.

the K
So which is heavier, a pound of feathers or a pound of gold?

(yes, it IS a trick question).

[Charlie99]

.......,but lead and iron should in theory weigh the same underwater.Mr Magoo, you should put on your glasses and read the post by ss7, above.

[The Kraken]

So which is heavier, a pound of feathers or a pound of gold?

(yes, it IS a trick question).

Using the NIST they are the same.

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

Perhaps I should have been more clear in my statement that one must presume that you are referencing the U.S. Customary unit (which is the Avoirdupous scale) of the pound wich is equivalent to 16 ounces, 7.0000 grains or 453.59237 grams.

So, to which scale do you subscribe?

the K

[The Kraken]

Weight is a factor of the pull of gravity.

Take it up with Einstein.

the K

[The Kraken]

Do not misconstrue weight with specific gravity.

the K

[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. :D 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.

==================================
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.

--------

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.