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Sink rate in freshwater versus seawater


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#1 pointy

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Posted 26 January 2013 - 06:57 AM

I attached floats to my camera which allows it to sink at a rate of about 1 meter/minute in freshwater. Is there any way to predict if it will float in seawater on the basis of its freshwater sink rate? Is there a freshwater sink rate that would predict neutral buoyancy in seawater? As a landlocked float carver this would be good to know.

John McCracken

#2 Glasseye Snapper

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Posted 26 January 2013 - 08:35 AM

Hi John,

Sink rate depends on both negative buoyancy and friction based on rig size and shape. I can't see how you can extrapolate its buoyancy in sea water from that. What you need is the specific density for you rig, which is its weight divided by its volume, and make it close to that of sea water (1.020 to 1.029 kg·liter). You can use Archimedes' trick to dunk your rig in a tub and see how much the water level rises (or in a full-to-the-rim tub and measure and weigh how much water overflowed the tub when putting the rig in). If you have kids it could be a fun exercise to do together.

Bart

Edited by Glasseye Snapper, 26 January 2013 - 08:36 AM.

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#3 pointy

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Posted 26 January 2013 - 10:16 AM

Hi John,

Sink rate depends on both negative buoyancy and friction based on rig size and shape. I can't see how you can extrapolate its buoyancy in sea water from that. What you need is the specific density for you rig, which is its weight divided by its volume, and make it close to that of sea water (1.020 to 1.029 kg·liter). You can use Archimedes' trick to dunk your rig in a tub and see how much the water level rises (or in a full-to-the-rim tub and measure and weigh how much water overflowed the tub when putting the rig in). If you have kids it could be a fun exercise to do together.

Bart

Hi John,

Sink rate depends on both negative buoyancy and friction based on rig size and shape. I can't see how you can extrapolate its buoyancy in sea water from that. What you need is the specific density for you rig, which is its weight divided by its volume, and make it close to that of sea water (1.020 to 1.029 kg·liter). You can use Archimedes' trick to dunk your rig in a tub and see how much the water level rises (or in a full-to-the-rim tub and measure and weigh how much water overflowed the tub when putting the rig in). If you have kids it could be a fun exercise to do together.

Bart


Hi Bart,

I agree that water resistance would affect the rate of sinking, but don't you think resistance would be a relatively minor factor when the rate of descent is very slow? When I released the camera in the pool, it initially took a few seconds for me to determine whether it was going up or down, and then it drifted down a foot in about 20 seconds.

Of course, my present rig may be a floater in seawater. If the floats need to be pared down to achieve the target of seawater neutral buoyancy, then maybe I should expect the camera to sink much quicker in the pool. So you might be right that friction will confound my effort to adjust buoyancy based on the camera's sink rate.

John

#4 diver dave1

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Posted 26 January 2013 - 03:04 PM

What you need is the weight in fresh water. A cheap fish scale can be used, which is what I did. Connect it to your rig and submerge the rig in the pool or tub. Then you can get the actual weight in fresh water and predict the weight in salt water. You get it to the buoyancy you want in the pool or tub and it will likely be close enough to satisfy you in salt water... unless you are diving the dead sea.:-)

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#5 pointy

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Posted 26 January 2013 - 04:01 PM

What you need is the weight in fresh water. A cheap fish scale can be used, which is what I did. Connect it to your rig and submerge the rig in the pool or tub. Then you can get the actual weight in fresh water and predict the weight in salt water. You get it to the buoyancy you want in the pool or tub and it will likely be close enough to satisfy you in salt water... unless you are diving the dead sea.:-)


I'm OK with the camera's very slight negative buoyancy in freshwater. If that's unlikely to change much in the sea, then I guess I can call the job done. Thanks for the info.

John

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#6 Glasseye Snapper

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Posted 28 January 2013 - 04:13 PM

I certainly think you can consider the job done. My intuition is that you will find your rig to be slightly positive in the sea, but within such a small margin that it doesn't really matter. My own rig is substantially negative and I've never felt the need to make it more neutral so you will be way ahead of the game.

Bart
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#7 pointy

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Posted 29 January 2013 - 01:35 PM

I certainly think you can consider the job done. My intuition is that you will find your rig to be slightly positive in the sea, but within such a small margin that it doesn't really matter. My own rig is substantially negative and I've never felt the need to make it more neutral so you will be way ahead of the game.

Bart


Actually, I'm a bit embarrassed by my original post. Sink rates - Jeez, what a red herring! Once I got the flotation right in fresh water, all I had to do was eyeball the volume of water they would displace (about 1.5 litres) in order to estimate the amount of extra lift they would provide when immersed in the sea (1.5 x 1.025 which is about 40 grams - not that much). Archimedes was the right man for this job.


John