How do you figure dispalcment (H20)

[Mr. Bubble]

I am studying my DM manual, and it goes into great detail about physics and bouyancy, weights, lift...etc....but they always seem to "give" you the displacement figure e.g." the motor displaced 3.2cfsw. The diver displaces 4.5 cfsw.

Is there a way to figure displacement of an object precisley without dropping it into a giant water tank with measurments on the side? Can you size up a student with a measuring tape, or any object that you want to lift just by measuring it first?

 

[rakkis]

Not really. You can roughly approximate with people, but the only exact way to get that value is to measure displaced volume directly.

Sometimes its hard to do the same with objects... they got nooks and crannies for air to escape and water to get in, so an object may displace much less water than outwards appearance may indicate.

 

[miketsp]

I am studying my DM manual, and it goes into great detail about physics and bouyancy, weights, lift...etc....but they always seem to "give" you the displacement figure e.g." the motor displaced 3.2cfsw. The diver displaces 4.5 cfsw.

Is there a way to figure displacement of an object precisley without dropping it into a giant water tank with measurments on the side? Can you size up a student with a measuring tape, or any object that you want to lift just by measuring it first?

Only if you can parse the object into a number of regular geometric volumes, estimate each one and then sum them.

 

[Mr. Bubble]

So, is there a professional manual that gives this info to someone who mat wish to be precise? Like a seamans manual for salvage or something? Maybe an instructors manual with hight and weight averages = x ? Not that I really need it now, but if I'm going this route, (you know me) I gotta have it right....or at least know where I can find out.

Thanks

 

[The Kraken]

Well, if you can just do some rough measurments to get your basic volumes and do a total, that will get you in the vicinity of your required lift. Ignore the nooks and crannies because if you do that, your lift bag will have more than enough capacity to lift your object.

Knowing the approximate weight of the object is essential, however.

Keep in mind that in the DM manual, you're concentrating much more deeply into these figures than you would in a real life situation.

You'll have 25# lift bags, 50# lift bags, 100# lift bags and so on.

One doesn't have to determine that the object to be lifted requires 41.75# of lift and then run out and find a 41.75# lift bag.

the K

 

[Mr. Bubble]

Good point. Just use lift bags til it floats! I guess I wanted the precision more so for helping student later on, getting their weights right. I have read a few threads on this BB that kinda scare me about teaching, being over weighted, or underweighted and rocketing to the top. Also, for me, I would really like to get my trim down to perfection. I've been having a hell of a time getting my trim right using these possitively bouyant fins. I dive like I'm standing on my head! (LOL) ...not really.....but close....especially in shallower depths. When I was DM assist on the rescue class, they rescued me and I kept my assent feet first! LOL They finally got me straightend out. I don't think that I am too heavy, its just those fins are really bouyant. I have yet to determin my displacement. But I don't think it matters too much because I can't change my tec rig around too much. Maybe change the BP if it weighs too much...but I am okay with the front half, now just to fix the other half.

Thanks..............................

 

[miketsp]

Good point. Just use lift bags til it floats! I guess I wanted the precision more so for helping student later on, getting their weights right. I have read a few threads on this BB that kinda scare me about teaching, being over weighted, or underweighted and rocketing to the top. Also, for me, I would really like to get my trim down to perfection. I've been having a hell of a time getting my trim right using these possitively bouyant fins. I dive like I'm standing on my head! (LOL) ...not really.....but close....especially in shallower depths. When I was DM assist on the rescue class, they rescued me and I kept my assent feet first! LOL They finally got me straightend out. I don't think that I am too heavy, its just those fins are really bouyant. I have yet to determin my displacement. But I don't think it matters too much because I can't change my tec rig around too much. Maybe change the BP if it weighs too much...but I am okay with the front half, now just to fix the other half.

Thanks..............................

Don't forget one thing. Most DMs and instructors that I know always carry extra weight anyway when diving with/ leading groups as they often need to give a couple of pounds to someone that's underweighted. As you gain experience, diving overweighted becomes less of a problem; the extra air in your BC makes buoyancy adjustments more sensitive but you will take this in your stride.

 

[Medic0506]

What you are trying to do is figure volume. One way of doing this is determining the mass of an object in two mediums (air and water). You need to figure out density, however. Density = mass / volume, d = m / v. Because you are not the same density throughout your body your density is not a constant number and changes from person to person. To figure out density weigh yourself in water (maybe a pool) and on land (no bc, tanks, or weights, just you). Then take a small sample of the water you were weighed in and figure out the density of it (d = m/v), remembering that mass is in grams. You take your mass on land and divide by your mass in the water and that will equal your density divided by your density minus the density of water. Ma / Mw = Dy / (Dy - Dw) Where Ma is mass of the air, Mw is the mass of the water, Dy is the density of you, and Dw is the density of water. Using algebra you can manipulate the equation in order to solve it. That is the really long way of esitmating your density.

...or you could look up the average density of a human which is around 1 g/cm^3 (around the density of water). D = m/v or V = m / d. Once you have your volume which will be in cubic centimeters, convert to cubic feet and your good (multiply by .033).

 

[Charlie99]

Determining volume is a multistep process.

First fill up a bathtub.
Then go find some old geezer named Archimedes.
Have him jump into the bathtub, making it overflow.
After he shouts "Eureka" and runs off naked, grab him and throw him back in the tub.

You should now have a bathtub of water filled right to the brim.

Throw the object to be measured into the tub, and catch any water that spills out.

If the object sinks, the displaced water gives you the volume.

If the object floats, then you need to force it underwater to get water equivalent to its total volume to spill out of the tub.

===========

Or for nice rectangular objects, you just measure the LxWxH and multiply.

===========

In real life, lifting something typically requires a lot more lift than calculated, since you have to break the suction of the mud.

 

[Mr. Bubble]

What you are trying to do is figure volume. One way of doing this is determining the mass of an object in two mediums (air and water). You need to figure out density, however. Density = mass / volume, d = m / v. Because you are not the same density throughout your body your density is not a constant number and changes from person to person. To figure out density weigh yourself in water (maybe a pool) and on land (no bc, tanks, or weights, just you). Then take a small sample of the water you were weighed in and figure out the density of it (d = m/v), remembering that mass is in grams. You take your mass on land and divide by your mass in the water and that will equal your density divided by your density minus the density of water. Ma / Mw = Dy / (Dy - Dw) Where Ma is mass of the air, Mw is the mass of the water, Dy is the density of you, and Dw is the density of water. Using algebra you can manipulate the equation in order to solve it. That is the really long way of esitmating your density.

...or you could look up the average density of a human which is around 1 g/cm^3 (around the density of water). D = m/v or V = m / d. Once you have your volume which will be in cubic centimeters, convert to cubic feet and your good (multiply by .033).

Sorry I asked

I think I prefer the ..."bag it til it floats" method. I will defenitly make note of it, and even try it out...one rainy night.

Thanks for the detailed reply