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

[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........