Alert Diver Misstatement

[donacheson]

In the Nov/Dec 2003 issue of Alert Diver on page 32 the following statement appears: "A diver displaces an amount of water equal to his combined body and equipment masss." Ignoring the mispelling of the word "mass" and the failure to distinguish between mass and weight, the statement is WRONG. It's true ONLY for a diver or any other object which is floating.

If it were always true, nothing would ever sink since, as the two sentence which follow it, correctly state, "The weight of this displaced water acts as an upward force. The weight of the diver and his equipment is the downward force." and the upward force of buoyancy would always exactly balance the downward force of gravity.

[JRScuba]

What I think that statement says is true, the amount of water a diver displaces is equivalent to him and his equipment when in the water. It does not says the diver is floating. It you want to know the mass of an object, place it in water and the water it displaces is the size of the mass. Since object can have irregular shapes and their mass cannot be calculated, the above process is used. So, the article in DAN is correct. QED

[pipedope]

A floating diver displaces the exact amount of water as his (her) mass, diver and equipment.

A submerged diver displaces the exact same volume of water as his volume.

The relationship between mass and volume determines bouyancy condition. Technically the bouyancy is the upward force of the water. As divers we use the term to mean the condition of the diver to float or sink.

[Groundhog246]

the statement is WRONG. It's true ONLY for a diver or any other object which is floating. .

I agree the original statement is not quite correct, but I think you're interpretation is completely wrong.

In the context of the original statement, the definition of mass is: The quantity of matter which a body contains, irrespective of its bulk or volume.

Note: Mass and weight are often used, in a general way, as
interchangeable terms, since the weight of a body is
proportional to its mass (under the same or equal
gravitative forces), and the mass is usually
ascertained from the weight. Yet the two ideas, mass
and weight, are quite distinct. Mass is the quantity of
matter in a body; weight is the comparative force with
which it tends towards the center of the earth. A mass
of wood and a mass of lead are assumed to be equal
when they show an equal weight by balancing each other
in the scales.

The confusion arises from using the same unit for different measures (mass & weight). One Kilogram of wood will float upon 1KG of water. 1 KG of lead will sink in 1 KG of water. Yet all three have the same mass, the missing ingredient is VOLUME.
If the diver and equipment displaces a VOLUME of water of equal mass, then you have neutral buoyancy, which is what we all spend a lot off time and effort achieving.
Knowing a bit about the print industry, what you read may not be what the original author wrote.

[Charlie99]

It you want to know the mass of an object, place it in water and the water it displaces is the size of the mass. Since object can have irregular shapes and their mass cannot be calculated, the above process is used. So, the article in DAN is correct. QED

Eureka! You have just repealed Archimedes law of buoyancy.

100kg of gold and 100kg of copper will NOT displace the same amount of water.

A fully submerged object displaces water equal to it's volume, not its mass. OTOH, a floating object will displace water equal to its mass.

[Rick Murchison]

Donacheson is correct, Alert Diver is wrong.
Let me try to make this simple.
For simplicity's sake, let's assume that one cubic foot of water weighs 64 pounds.
If I have an object that weighs 50 pounds and occupies one cubic foot of volume and throw it into the water, it will not sink. When it has displaced 50 pounds of water (50/64 cubic feet) it will be buoyed up by the water it has displaced and float.
If I increase the weight of the object to more than 64 pounds - let's say 100 pounds, it will still only be able to displace a maximum of one cubic foot of water - because that's how big it is - but that won't be enough water to buoy it up because that cubic foot of water still only weighs 64 pounds. So the object will sink - and if you weighed it under water it would weigh 36 pounds (100 pounds of "mass" buoyed up by 64 pounds of water leaves 36 pounds of weight).
What Alert Diver meant to say is that an object submerged in water displaces an amount of water equal to its volume. If that volume of water weighs less than the object, the object is negatively buoyant and will sink. If that volume of water weighs more than the object, then the object will be positively buoyant and will rise until enough of it is above the water that the weight of the water displaced equals the weight of the object. If the object displaces exactly its own weight then it is neutrally buoyant and will neither sink nor float, but remain submerged and stationary unless disturbed.
Rick

[Charlie99]

DAN definitely needs some better editors. The author's other statements show that he does understand Archimedes law. I also suspect that the author knows that many tanks (mostly steel) are negative buoyant when empty, even though the article says

Tanks are a little more complicated. Place a full tank in the water and it will sink, but an empty tank will float

The chart with data on 65 and 80 cu ft. aluminum tanks is a bit weird too. It has 65 cu ft of air weighing 5.9 pounds, while 80 cu ft is 6 pounds. It also lists size and buoyancy info for the short, fat 8"dia x 23" tank rather than the more common 7.25" x 26" AL80.


picky, picky, picky..... but that's what they should have editors doing before it is printed.

Charlie

[kingprawn]

Let’s see:
An object displaces a volume of water. How can this volume be equal to a mass.

The mass of the water the diver displaces must be equal to the mass of the diver and his equipment.

Personally, I wrote them a comment about the article concerning the diver that got bent. While they went to great lengths to describe how a 5 minute Neuro exam would have helped him, they mentioned nothing about why a diver was at 120ft without a backup air supply. The cause of his accident was not equipment failure, but bad dive practices.

[queentrigger]

Let’s see:
An object displaces a volume of water. How can this volume be equal to a mass.

The mass of the water the diver displaces must be equal to the mass of the diver and his equipment.

Personally, I wrote them a comment about the article concerning the diver that got bent. While they went to great lengths to describe how a 5 minute Neuro exam would have helped him, they mentioned nothing about why a diver was at 120ft without a backup air supply. The cause of his accident was not equipment failure, but bad dive practices.

1. Your physics is failed. A submerged diver (except while escending) will have equal or negative bouyancy. If bouyancy is neutral, the mass of water displaced will be equal to mass of diver. DENSITY of diver will be same as water. But if diver is negatively bouyany, eg. when going down or finning away above the bottom, his density is greater than water. The sea/ lake can only displace the same volume of water as diver. If it displaces the same mass as a negatively bouyant diver (as you claim), there will be more water by volume than diver. Where does this excess water go to?

2. From what depth is it good diving practice to have a backup air supply? Why?

[Rick Murchison]

1. Your physics is failed. A submerged diver (except while escending) will have equal or negative bouyancy. If bouyancy is neutral, the mass of water displaced will be equal to mass of diver. DENSITY of diver will be same as water. But if diver is negatively bouyany, eg. when going down or finning away above the bottom, his density is greater than water. The sea/ lake can only displace the same volume of water as diver. If it displaces the same mass as a negatively bouyant diver (as you claim), there will be more water by volume than diver. Where does this excess water go to?

2. From what depth is it good diving practice to have a backup air supply? Why?

Great Zot!
And now we bring up density! I suppose you could say that the overall density of the diver and all his gear (as a unit) has changed, but genreally we use the term "density" when referring to a specific homogenous substance, like air or water or lead, or in some cases to a single object, as in "body density." We could possibly more accurately use "specific gravity of the diver/gear package" too.... but I'd just as soon not.
2. My personal criteria for a backup air supply is when the question "If I have a total sudden loss of breathing gas, can I make a safe ascent to the surface from where I am right now?" has a "no" answer. For me that's about 100' or any overhead environment. As with most things there is a grey area in which absolute redundancy isn't required - a buddy that's in sight and reach qualifies (for me) down to about 130' or where the "overhead" is a nice big clear cavern.
Rick