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Buoyancy: Salt Water vs. Fresh Water February 26, 2009

Posted by Chris Sullivan in Training.
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Apparently this is a great source of confusion – or perhaps my previous writings about the subject have attracted many searches, and so biases my view of what divers are interested in. Anyway, I’m going to attempt to write something clear on the subject. Here are some of the searches on the topic that have found their way to my blog:

bouyancy of sea water
bouyancy salt
buoyancy salt water vs fresh scuba
buoyancy calculations salt water
buoyancy difference between salt water a
buoyancy fresh salt
buoyancy of salt & freshwater
buoyancy of sea water calculators
salt kg required for tank foot
salt water buoyancy calculation
salt water fresh water buoyancy
which is more buoyant fresh or salt water
why is bouancy more in salt water than fresh
why salt water is more buoyant than fresh
sea water density bar msw
why is salt water more buoyant
is salt or fresh water more buoyant
buoyancy salt water fresh water

Obviously people want to know. The answer is fundamentally simple – objects (including divers) are more buoyant in salt water than in fresh water because salt water is denser than fresh water. Why is salt water denser than fresh? Because salt is denser than water and so if you add it to water the resulting solution is denser than pure water. By how much? The salinity (saltiness) of the ocean varies, but the generally accepted average amount is 2.5%. So  salt water weighs 2.5% more than the same volume (a gallon or litre, for example) of fresh water.

Buoyancy is an upward force equal to the weight of water displaced by the object. A cubic foot of fresh water weighs 62.4 pounds, so an object with a volume of 1 cubic foot would experience 62.4 pounds of upward force due to buoyancy when immersed in fresh water. Gravity will exert an opposite (downward) force equal to the object’s weight, so if the object weighs less than 62.4 pounds it will float. If the object weighs more than 62.4 pounds it will sink. If the object weighs exactly 62.4 pounds it will be neutrally buoyant, and will stay where it is unless pushed by something (current, turbulence, a diver, etc.).

In salt water, that same 1 cubic foot will displace 64 pounds, because that’s what a cubic foot of sea water (which you recall is heavier by 2.5 percent) weighs. So there is 1.6 pounds more buoyancy in salt water than in fresh. That means if an object with a volume of 1 cubic foot weighs 63 pounds it will float in salt water and sink in fresh water. So objects in salt water are more buoyant than objects in fresh water because salt water is denser than fresh.

Note that it is incorrect to say that salt water is more buoyant than fresh water. Objects in salt water are more buoyant than objects in fresh water. The buoyant force is exerted on an object, not the water itself.

Hope that clears things up!

P.S. The density of salt is 2.16 grams per cubic centimetre vs. the maximum density of fresh water at 1 gram per cubic centimetre.  Sea water is about 3.5% salt by weight. A kilogram of sea water will have 35 grams of salt and 965 grams of fresh water (I’m ignoring the stuff that’s in sea water which isn’t water and salt). The volume of the water will be 965 cubic centimetres, while the volume of the salt will be 35 grams divided by the density of 2.16 which is 16.2 cc. The total volume of the components of this kilogram of sea water is 981.2 cc, versus a kilogram of fresh water at 1000 cc, so the combined density is almost 2% more with the salt than with fresh. This is a little less than the 2.5% real world difference – what’s going on?

My chemistry knowledge runs out at this point but my guess is that when salt dissolves in water, the salt molecules pack a little bit closer together with the water molecules than they do with each other, making the volume a bit smaller than the sum of the volumes of the salt and the water separately. If I find out the real answer I’ll post it here, unless someone beats me to it.

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Comments»

1. Alex - March 4, 2009

this is great!:P

2. Shawn - October 27, 2009

How much air (in cubic feet) is needed to lift 100 lbs under water. Ignoring the buoyancy of the weight material. How much does this differ in salt water?

deepstop - October 27, 2009

Hi Shawn,

To lift 100 lb you need to displace 100 lb of water, plus a little bit. For fresh water, which weighs 62.4 lb/cubic foot, you need 100 divided by 62.4 cubic feet of air, or just over 1.6 cubic feet. Remember though, that at depth, the volume of air from the tank that is required will increase with depth, so you need to multiply by the number of absolute atmospheres of pressure to get the volume used from the tank.

For instance, with our 100lb lift, if we were starting from 102′, which is 4 ata (absolute atmospheres), we then need 4×1.6=6.4 cubic feet of air from the tank. As the object ascends, the air will expand and either spill from the lift bag or increase the lift (and therefore the rate of ascent) if the bag can hold the extra volume. But a 100lb lift bag will hold 1.6 cubic feet of air and work at any depth, the difference is how much air you consume from your tank to fill it.

In salt water, which weighs 64lb/cubic foot, the volume is slightly less at 100/64 or 1.56 cubic feet. If you wanted to take the buoyancy of the object you are lifting into account, you would just subtract the volume of the object from that volume. So if the object’s volume was .46 cubic feet, you would only need 1.1 cubic feet of air.
Chris

3. James Taylor - November 4, 2009

I have a camera witch is inside a glass container so it is submercable, am I to assume that the camera must wiegh at least 64lb to be nutrally bouyent in salt water?

deepstop - November 4, 2009

Only if it displaces a volume of 1 cubic foot or more. That would be one big camera!

4. Sanaa Drif - June 11, 2010

A small car ferry 6m wide, 12m long and 2m thick has a specific gravity of 0.80 how far, will it sink in fresh water then it is empty (unloaded)?
four cars, with a mass of 1500 kg each, are loaded on the ferry. how far will it sink in the water ??

Chris Sullivan - June 12, 2010

Sounds like a trick question! In fresh water the ferry displaces 6x12x2 cubic metres which is 144 metric tons (tonnes) of water. With a specific gravity of .8, the ferry weighs only 115.2 tonnes (80% of the displaced weight). Your 4 cars, weighing 6 tonnes, raises the total weight to 123.2, so it won’t sink (barring big waves and so on).

But that wasn’t the question. The question is how much further will it sink (or perhaps better put, how much lower will it lie in the water). Assuming the ferry is an oblong box, each tonne of additional weight will sink the ferry an additional 1/72 of a metre, because the ferry displaces 72 (6×12) tonnes for every metre it lies under the water. So 6 tonnes will sink it 1/12 of a metre which is 8 1/3 cm.

5. karena villegas - August 30, 2010

what would be the answer of 100 cubic ft. of water = _______pounds= _______kilo= _______ounces
100cubic ft. salt water = ________pounds= _____kilo
100ton of water=______pounds= ______kilo
100ton of salt water= ______pounds=_______kilo
1000ton fresh water = _______cubic ft.=______cubic meter
1000ton salt water=______cubic water=______cubic ft.=______cubic meter

Chris Sullivan - September 13, 2010

Hm, not sure why you ask, but…

100 cu ft of fresh water is 6,242.8 pounds and 2,831.68 kg
100 cu ft of sea water is 6,398.87 pounds and 2,902.48 kg
100 tons of water (or 100 tons of anything) is 200,000 pounds or 90,718.4 kg
100 tons of salt water is 200,000 pounds or 90,718.4 kg
100 tons of fresh water is 3,203.69 cubic feet or 90.7184 cubic metres
100 tons of salt water is 3,125.55 cubic feet or 88.5058 cubic metres

This is all very easy to calculate using calchemy.

Chris

6. haley - March 22, 2011

Why does salt water sink?

Chris Sullivan - March 23, 2011

The salt which is dissolved in the water makes it denser than fresh water. Anything denser than the liquid it is in will sink eventually (ignoring surface tension, which is why some insects don’t sink when they walk on water, even though they are more dense).

7. chi - April 4, 2011

i want to know why it is possible to have a fresh water and salt water interface and they don’t mix as in the salt water doesn’t dissolve in the fresh water?

Chris Sullivan - April 9, 2011

Take a look at this Wikipedia article on Haloclines. I think the answer you’re looking for is that there has to be some force that mixes the fresh and salty water together. If there is no mixing, then the different densities of the water will cause them to remain separate. In the article, it even suggests an experiment in which you can create a halocline in a drinking glass.

–Chris

8. Jane Chan - May 20, 2011

that sure saved my butt for the buoyancy project

Chris Sullivan - May 20, 2011

I’m glad I could help. What was the buoyancy project?

9. Thomas Houston - August 4, 2011

Im currently taking a scuba class, and on a recent test I got an answer wrong that I disagreed with; the instructor was able to tell me it was wrong, but didn’t really address my reason for disagreement… the question was:
“If an object is neutrally buoyant in salt water, it would mostly _______ in fresh water.”
a. Float (positively buoyant)
b. Sink (negatively buoyant)
c. Neither float nor sink (neutrally buoyant)
d. The question lacks enough information

-I chose d. based on the fact that the question didn’t address what the ‘object’ was… is it not at all possible for an object to be both neutrally buoyant in salt water AND neutrally buoyant in fresh water? Aren’t there several different levels of neutral buoyancy? Does the weight of the object in relation to the difference in weight/density of salt water not play a part?

Thanks
Thomas Houston

Chris Sullivan - August 8, 2011

Hi Thomas,

The correct answer is (b) sink. Buoyancy is equal to the weight of the water displaced by the object, so as the volume is the same in both cases, but that volume of fresh water weighs less than the same volume of salt water, the buoyancy is less and the formerly neutrally buoyant object will now sink. The object is question has exactly the same density as salt water, which is why it’s neutrally buoyant. This explains why we need less weight when diving in fresh water.

Chris

P.S. A physicist might argue the point. Some “objects” may have special properties, like individual atoms for instance, or stuff that dissolves.

10. David - November 12, 2011

Can you get my sister to go in the water? She’s afraid that she’ll drown.

Chris Sullivan - November 13, 2011

Maybe. The owner of my local dive shop is really good at getting people to overcome their fears. I’ve been successful as well. Usually it’s a matter of being reassuring and patient, and starting off with little things in the shallow end of the pool.

11. Abraham Patrick - February 18, 2012

Will an object completely immersed sink
a) more in water
b) more in salt water
c) the same

Chris Sullivan - March 6, 2012

There’s a very similar question on the PADI open water exam. Goes something like “A neutrally buoyant object in salt water will ____ when in Fresh Water”
(a) sink (b) float (c) stay the same (d) impossible to tell.

Because salt water is denser, it provides more buoyancy for an object of a given volume. So in fresh water there will be less buoyancy, so the upward force is less. So the object will sink in fresh water. We know this intuitively I think because it is easier for a swimmer to float in the sea than in a fresh water lake, on in the Dead Sea, which has more salinity than regular sea water, people float really well, almost like they’re on a air mattress.

Sorry it took so long to respond. Had some issues to deal with.

Chris

12. Oe Li - March 26, 2012

Why does a marble still sinks when put in salt water but not float?

Chris Sullivan - April 15, 2012

The marble won’t sink as fast as it will in fresh water, but marble (or glass, which “marbles” are made from) is more dense than salt water (a marble is heavier than its weight in water, salt or fresh).

The difference between salt and fresh water densities is only about 3%.

Hope that helps…

13. Gala Holliday - June 10, 2012

Hi Chris,
I am wondering about a boat lift on fresh water. I am buying a boat that weighs 4300 dry weight, has a 90 gallon fuel tank, which full is basically another 720 lbs., and say 500 pounds more for other objects. Total together 5,520. How big of a boat lift would we need to lift the boat? Do we need to take into consideration the weight of the tank? I’m confused. We looked at a tank that was 9X6X30″- would this work and how big (lbs) is this. – THanks – Gala

Chris Sullivan - June 12, 2012

I don’t know a lot about boat lifts, but if you’re lifting it out of the water, it would at least have to be capable of lifting the full weight, as once it’s out of the water there’s no buoyancy available. I doubt the weight of the tank would be significant, and your calculation for the weight of the fuel is on the high side because you used the specific gravity of water (i.e. 1.0) instead of the specific gravity of gasoline which is .739 (unless it’s diesel, which is still <1 but higher than gasoline and covers a fairly wide range).

So the fuel, if gasoline, would actually weigh only about 532 lbs. The tank size of 9'x6'x2.5' is 270 cubic feet, and there's 0.133680556 US gallons in a cubic foot, so your proposed fuel tank would hold just over 2,000 gallons. For 90 gallons you need 12 cubic feet, say 2'x2'x3'.

Just for fun, let's think about the weight of the tank. Assume it is steel, which weighs .2836 lbs per cubic inch. The 2x2x3 tank from the previous paragraph has a surface area of 17 square feet, which is 2448 square inches. If the thickness of the walls of the tank is say 1/8th of an inch (that would be thicker than I'd expect), then the tank would contain 2448/8 or 306 cubic inches, so would weigh 306 x .2836 or just under 87 pounds. So you'd have about 620 lbs of tank and gasoline, all told.

Sounds like a nice boat. I hope you go diving on it.

Chris

14. dee - June 20, 2012

please sir i got a problem of a floating ship floats on surface of sea water of density 1030 transfers to river of density 1000 the immersed volume changes by 3 metres cube g equal 10 find immersed volume in sea , buoyant force in river and the weight of the ship plz help

Chris Sullivan - June 21, 2012

Hello Dee. I think I understand the problem.

  • The sea water has a specific gravity of 1.03, and the river, being fresh water has a specific gravity of 1.
  • The ship has an unknown weight
  • The volume under the water line increases by 3 cubic metres when the ship moves from the sea to the river.
  • The value of g is rounded to 10, although I can’t see the relevance of including this in the problem.
  • This is very much like what happens to a diver moving from salt to fresh water, the diver will tend to sink more.

    Let’s say the ship’s weight in tonnes (thousands of kg) is x. The ship therefore displaces x/1.03 cubic metres of sea water, and x cubic metres of fresh water. We know that the displacement in fresh water is 3 cubic metres more than in salt water, so xx/1.03 = 3. Therefore 1.03x-x = 3; .03x = 3, x=3/.03; x=100 tonnes (100,000 kg).

    I hope this helps
    Chris

    15. aadesh - November 7, 2012

    can you explain floatation in sea water

    Chris Sullivan - November 9, 2012

    Sure. Things that float take up space. Take an iceberg for example. The iceberg takes up space so it has a certain volume. It also has weight. If the weight of iceberg is less than the weight of the same volume of water, it floats. We know that icebergs float, so we know that the same volume of water weighs more than the iceberg.

    You may have heard that most of an iceberg is under water. This is where we get the saying “it’s only the tip of the iceberg” meaning there’s much more of the iceberg you can’t see than you can (unless, of course, you are diving near the iceberg, which can be very dangerous).

    The part of the iceberg that is under water takes up space. If that space consisted of water, it would weigh exactly the same as the entire iceberg. That’s why we say the iceberg displaces its weight in water. Not every object displaces its weight in water. A lead weight, for instance, displaces much less than its weight in water. That’s why if you release a dive weight in water it drop to the bottom (quickly enough that you don’t want to be under it, so be careful if you ever need to drop your weight belt).

    In summary the reason the iceberg (or anything else) floats, is that it is less dense than water, so it can displace more than its own weight in water. The extra volume, therefore, sits on top of the water instead of in it.

    16. Meli - December 10, 2012

    How does salt effect water?

    Chris Sullivan - December 11, 2012

    When it comes to buoyancy, the addition of salt increases the density of water, making objects in it more buoyant. It also makes water more electrically conductive.

    17. Tim Hanks - August 30, 2013

    Hey Chris,
    I have a question about weighting in Scuba. Reading through your posts, sea water imparts 2.5% more bouyancy on objects (divers) than in fresh water. So if you’ve determined the correct amount of weight while in a fresh water pool you would need 2.5% more weight for sea water. If you needed 10 lbs of weight, would you need 10.25 lbs for sea water, or do you have to add 2.5% of weight to the entire weight of the diver? i.e. if the diver weights 250 lbs with all his or her gear and weights, then would you need 6.25lbs of extra weight in sea water? If the answer is the latter (which I believe is correct), then it seems like dive shops with pools should have scales to make this precise conversion between their pools and the ocean…

    Chris Sullivan - September 6, 2013

    You need to add 2.5 percent of the total weight. So if you’re 200lbs with all your gear on with your fresh water setup, then you need to add about 5lbs because that’s how much extra water you’ll be displacing (or 250lb total with 6.25 in your example)

    Precision is not all that important. If you know what you weigh, add about 50lbs to that and go from there. A few ounces here and there isn’t going to make a huge difference. Remember that your lungs can vary your buoyancy by 5-10 lbs.

    Thanks for the question,
    Chris

    18. Dhyey - September 30, 2013

    i have a question sir did buoyancy affect water animals ?if yes or no?how

    Chris Sullivan - October 5, 2013

    Buoyancy affects anything in the water, including water animals and even water itself. The principle of buoyancy applies to anything that takes up space in water. The object, like a water animal, displaces some weight of water, so there is an equivalent upward force.


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