User avatar
DaveMann
Lung Diver
Posts: 61
Joined: Mon Aug 15, 2016 11:12 am
First Name: Dave
Location: Fort Myers, Fla., USA

Calculating the Buoyancy of Individual Objects

Sun Sep 04, 2016 11:21 am

When discussing buoyancy the first thing that comes to my mind is the tank. Charts abound for the buoyancy of various tanks, but what about the other stuff?

If one wants to be as precise as is possible with regard to buoyancy shouldn't the positive or negative buoyancy of every object worn or borne by the diver be calculated? To that end I ask this question:

What is a practical method for determining the buoyancy, positive or negative, of any object in pounds and ounces?

Dave

User avatar
Herman
VDH Moderator
Posts: 1176
Joined: Thu Oct 25, 2007 1:45 pm
Location: Raleigh NC

Re: Calculating the Buoyancy of Individual Objects

Sun Sep 04, 2016 3:01 pm

Calculation is not really practical, it can be done but not worth the effort IMO. The easiest and most precise method is to get a set of fishing scales and some string. Suspend the item in question on the string, submerge the item and weight it.....assuming it sinks, this will give you an accurate buoyancy. For floaty objects, putting them in a net bag and weighting it down with weights is the simplest way or you can put a pulley on a big weight, submerge it and pull the item under with the string through the pulley and measure the force on the string with the scales.
Herman

User avatar
DaveMann
Lung Diver
Posts: 61
Joined: Mon Aug 15, 2016 11:12 am
First Name: Dave
Location: Fort Myers, Fla., USA

Re: Calculating the Buoyancy of Individual Objects

Sun Sep 04, 2016 6:59 pm

Ya know, Hermann, that's pretty much what I thought before I posted this, and then I followed up that thought with - nah, that'd never work. Too easy. Sometimes the simplest way is the best way. Thanks for your reply.


Dave

User avatar
luis
VDH Moderator
Posts: 1536
Joined: Thu Jun 30, 2005 9:28 pm
First Name: Luis
Location: Maine

Re: Calculating the Buoyancy of Individual Objects

Sun Sep 04, 2016 7:22 pm

What Herman described is what I have done with some of my gear and all my tanks. I have a deep water container and a digital fish scale.

For buoyant items, I just attach a weight to it and measure the combined in water weight. Then I measure the weight by itself in the water and take the difference.

I dive with so many different configurations (wet suits and dry suits) and different tanks that I keep a spread sheet with this data.
Luis

Buceador con escafandra autónoma clásica.

User avatar
DaveMann
Lung Diver
Posts: 61
Joined: Mon Aug 15, 2016 11:12 am
First Name: Dave
Location: Fort Myers, Fla., USA

Re: Calculating the Buoyancy of Individual Objects

Sun Sep 04, 2016 8:40 pm

Excellent method for weighing floaty things. Thanks, Luis.


Dave

User avatar
Herman
VDH Moderator
Posts: 1176
Joined: Thu Oct 25, 2007 1:45 pm
Location: Raleigh NC

Re: Calculating the Buoyancy of Individual Objects

Mon Sep 05, 2016 7:21 am

One thing to keep in mind for things like wetsuits, you must get any trapped air out of them, otherwise the additional air will add quite a bit to their apparent buoyancy.
Never thought about overweighting a positive item and measuring the differential before but it makes perfect sense . Also, don't forget that tanks need to be checked full and empty to get their buoyancy swing
Herman

User avatar
luis
VDH Moderator
Posts: 1536
Joined: Thu Jun 30, 2005 9:28 pm
First Name: Luis
Location: Maine

Re: Calculating the Buoyancy of Individual Objects

Mon Sep 05, 2016 6:57 pm

I just measure the tank pressure before I weight the tank in water.
Knowing the tank pressure it is easy to calculate the amount of air in it (and the weight of the air at that pressure). I have precisely measure the actual volume of most of my tanks, but if I don’t have exact volume I just used the published data.

For a steel 72, I use 71 cu ft at 2475 psi, which is close to the average that I have measured. That means that you get 0.0287 cu ft per psi (or 2.87 cu ft/ 100 psi).

The weight of air is about 0.075 pounds per cu ft.

This works to about 0.215 pounds per 100 psi.

The difference between a completely empty steel 72 (71 cu ft) and a full one is about 5.3 pounds.

The weight change (from full to empty) of any cylinder is just related to the air volume in it. It has nothing to do with the type or the material of the cylinder.
Luis

Buceador con escafandra autónoma clásica.

User avatar
8dust
Master Diver
Posts: 547
Joined: Wed Aug 05, 2009 11:39 am
Location: Nashville's North Shore

Re: Calculating the Buoyancy of Individual Objects

Wed Sep 07, 2016 7:58 pm

+1 on the fish scale, although mine is a cheap spring-type. Advantage to it is I can take it under water for floaty things. Just have to dry it off good.
Freddo
NAVED member #201

User avatar
Bronze06
Master Diver
Posts: 617
Joined: Tue Jul 01, 2014 5:33 am
First Name: Russell
Location: Tabuk, Saudi Arabia

Re: Calculating the Buoyancy of Individual Objects

Wed Sep 14, 2016 11:25 am

AHHH, but then we get into the weighty nuances of how floaty things like wet suits change displacement at various depth, a possible example ; 16lbs of weight for a 1/4 inch full suit at 50 feet, but 18lbs. at 30. Heh, heh, heh............ :twisted:( I betcha Luis has all his measurements for any depth or configuration.) :D
"Where'd ya get that ol' thang, don't cha' know them thare things ill kill ya!"

Live From the Red Sea,

Russ

User avatar
Herman
VDH Moderator
Posts: 1176
Joined: Thu Oct 25, 2007 1:45 pm
Location: Raleigh NC

Re: Calculating the Buoyancy of Individual Objects

Wed Sep 14, 2016 1:26 pm

In a spreadsheet! :)
Herman

Return to “New Vintage Equipment Diver's Forum”

Who is online

Users browsing this forum: No registered users and 1 guest