Displacement of Scooters at Depth - Spun off from the A&I Discussion about Nothernone

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IOW, pretty much negligible for all practical purposes. At least within scuba.

Agreed. I tried to be more precise to avoid confusion and debate based on converting rounded data from other sources.

I sincerely believe that the US would be a Metric country if the Metric system were developed 10-20 years earlier, or our revolution were later. There was a strong "decimal" movement in the years after the revolution. We were the first to use decimal money (1 dollar = 100 cents) and there was an attempt to use the inch as the base unit and make all others a multiple of 10 of the inch.

On the other hand, timing of the French revolution was perfect; along with the absence of a dominant standard in Europe. Metric easily spread because continental inter-state (country) commerce was virtually unregulated or standardized -- sort of a vacuum. The US was able to stick with the Imperial system because we were relatively geographically isolated, trade with the UK remained dominant, and the system was standardized throughout the nation. I consider the US a victim of momentum, which could easily have swung in the other direction in the late 1700s.
 
Fully agree except I believe water temp for density purpose is 4 degrees because at that temp density is maximum (the polar molecules pack more tightly) and also because at 0 degrees you would have ice :) and not water!

It depends on the industry. I understood that the Metric standard is (or was?) based on STP (Standard Temperature and Pressure), which is 0° C, or 32° F. That is the transitional temperate between liquid and solid states of freshwater. Seawater' transitional temperature averages -2.22° C or 28° F.

The standard method for calibrating thermometers and thermistors is to immerse the sensor into a dense icy freshwater slush at sea level for the freezing end of the scale and in boiling freshwater for the boiling (steam transition) end of the scale -- 100° C, or 212° F.

Of course the difference is well within tolerance for recreational or even commercial saturation diving purposes. I do find the discussion interesting and am thankful for any corrections when I am wrong. First and foremost, I am here to learn.
 
I seem to fail explaining the buoyancy concept to some of you. Let me try another way in terms of something that is more general in nature, i.e., density.

Do you all agree that if an object is neutrally buoyant in the water, that means, its bulk density is the same as that of water = 1.0 g/ml or 1.0 kg/l or 62.4 lb/cft? Let's use Sw = 1 for water and the object bulk density = Sb. If Sb < 1, it floats, i.e., it has positive buoyancy. If Sb > 1, it sinks, i.e., it has negative buoyancy. How much negative buoyancy it will be, depends on the difference between its density and the water density multiply by the bulk volume of the object.

Negative buoyancy = submerged weight = (Sb - Sw) x Vb = (Sb -1) x Vb
Where Sb = density of the object in kg/l & Vb = Bulk volume of the object in kg.

Here is where the confusion starts, that is, it is not easy to estimate the bulk density when you have an object such as DPV, with internals composing of battery, electronic, air space, etc., with each of the components has different density. If we know each component weight and volume, we can estimate Sb by:

Sb = (W1 + W2 + W3 + ....+ Wn) x Vb
Where W1 is the weight of component 1 in kg,..., subscript n = component n

Air density is so much smaller than the rest of the component and we can practically ignore, unless we are dealing with compressed air as in the SCUBA tank.

Let's now do a simple example of the dreaded flooded AL80. If its empty weight is say 14.2 kg as @Storker estimated in post #28, page 3
W1 = 14.2 kg

We ignore the weight of atmospheric air in the tank.
W2 = 0 kg

Vb is about 16.3 liters as @Storker estimated previously.

Therefore, empty AL80 bulk density becomes
Sb = (14.2 + 0) / 16.3 = 0.87 < 1 the AL80 will float if the nozzle is capped.

Now let's see what happen if it is completely flooded. If the cavity is 11 liters, then
W2 = 11 kg.

Therefore, the flooded AL80 density becomes
Sb = (14.2 + 11) / 16.3 = 1.55

Negative buoyancy = Submerged weight = (1.55 - 1) x 16.3 = 8.9 kg

That is not equaled to the weight of the water inside the AL80, which is 11 kg.

In conclusion
Don't use the weight of water occupying the original cavity of an object (scuba tank, DPV, etc.) for estimating the negative buoyancy.

When the DPV imploded, Vb changes, i.e., it gets smaller, but Sb also changes, i.e, it gets bigger, accordingly. Whether it is flooded or imploded or exploded, it doesn't matter as now the Vb is the volume of the solid remains once the air is out of the cavity.

For imploded, exploded or flooded AL80, Sb = density of aluminum = 2.7 kg/l and Vb = 5.26 liters, estimated by @Stroker previously.

Negative buoyancy = Submerged weight = (2.7 - 1) x 5.26 = 8.9 kg.
 
Negative buoyancy = Submerged weight = (1.55 - 1) x 16.3 = 8.9 kg

That is not equaled to the weight of the water inside the AL80, which is 11 kg.

In conclusion
Don't use the weight of water occupying the original cavity of an object (scuba tank, DPV, etc.) for estimating the negative buoyancy.

Definitions:
1. A downward force is negative
2. An upward force is positive.
3. The empty tank weighs 14.2 kilograms.
4. The tank volume (including the inside volume) is 16.3 liters.
5. Buoyant force on this tank is 16.3 kilograms.
6. The inside volume of the tank is 11 liters.
7. 11 liters of water weighs 11 kilograms.
8. Buoyancy equals the sum of the various force components. [i.e weight of tank (negative), weight of water in tank (negative), and buoyant force (positive)]

Therefore:
Buoyancy = buoyant force (+16.3 kg) + tank weight (-14.2 kg) + 11 liters of water weight (-11 kg)
Buoyancy = 16.3 kg - 14.2 kg - 11kg
Buoyancy = -8.9 kg

The change in buoyancy is exactly equal to the weight of the water inside the AL80, which is 11 kg.

In conclusion
Don't use the weight of water occupying the original cavity of an object (scuba tank, DPV, etc.) for estimating the negative buoyancy (unless the original scuba tank, DPV, etc. are neutrally buoyant to begin with).

Edited to correct the tank weight.
 
Definitions:
1. A downward force is negative
2. An upward force is positive.
3. The empty tank weighs 14.2 kilograms.
4. The tank volume (including the inside volume) is 16.3 liters.
5. Buoyant force on this tank is 16.3 kilograms.
6. The inside volume of the tank is 11 liters.
7. 11 liters of water weighs 11 kilograms.
8. Buoyancy equals the sum of the various force components. [i.e weight of tank (negative), weight of water in tank (negative), and buoyant force (positive)]

Therefore:
Buoyancy = buoyant force (+16.3 kg) + tank weight (-16.3 kg) + 11 liters of water weight (-11 kg)
Buoyancy = 16.3 kg - 16.3 kg - 11kg
Buoyancy = -11 kg

That is equaled to the weight of the water inside the AL80, which is 11 kg.

In conclusion
Don't use the weight of water occupying the original cavity of an object (scuba tank, DPV, etc.) for estimating the negative buoyancy (unless the original scuba tank, DPV, etc. are neutrally buoyant to begin with).

Nope. The tank weight is 14.2 kg, not 16.3 kg. :)

Buoyancy = 16.3 - 14.2 - 11.0 = - 8.9 kg

Review @Storker post # 28 in page 3 of this thread for more detail.
 
I see now another confusion. The buoyancy of flooded AL80 (- 8.9 kg) is close to the weight of its cavity when it is filled with water (- 11 kg), people just erroneously assume that the weight of the flooded object in the water equals to the weight water in that flooded cavity, when the unflooded object is neutrally buoyant in the water.
 
@Charles2 here is the @Storker post if you don't feel like going back & look for it :)

Let's run the numbers and look at an Al80.

As best as I can find out, the mass of an Al80 is 31.3 lbs, or 14.2 kg. Let's disregard the mass of the valve, because it doesn't matter for the net result here. The internal volume of an Al80 is about 11L, and the total volume is internal volume plus the volume of the aluminium. Now, Al has a density of 2.7 kg/L, so the volume of the material is 5.26L. Total volume of an intact Al80 is thus 11+5.26=16.26L, and the net buoyancy is 16.26-14.2=2.06kp. It's some 2kp or some 4lbf positive, minus the weight of the tank valve. And of course neglecting the mass of the air, which is only 0.0135kg, so it's quite ok to neglect that.

First, I'll flood the tank. Let's disregard that the density of sea water is slightly higher than that of freshwater, because once again it doesn't matter for the calculations. Now its total mass is 14.2+11=25.2kg, and its net buoyancy is 16.26-25.2=-8.94kp.

Next, I'll thoroughly implode the tank. I'm so good at that that what I have now is a solid Al ingot with a mass of 14.2kg. The volume of that ingot is 5.26L. And its net buoyancy is 5.26-14.2=-8.94kp.

So no matter if I flood the tank - or scooter - or if I implode it completely, its (negative) buoyancy is exactly the same. As not only I said, physics.
 
It depends on the industry. I understood that the Metric standard is (or was?) based on STP (Standard Temperature and Pressure), which is 0° C, or 32° F. That is the transitional temperate between liquid and solid states of freshwater. Seawater' transitional temperature averages -2.22° C or 28° F.

The standard method for calibrating thermometers and thermistors is to immerse the sensor into a dense icy freshwater slush at sea level for the freezing end of the scale and in boiling freshwater for the boiling (steam transition) end of the scale -- 100° C, or 212° F.

Of course the difference is well within tolerance for recreational or even commercial saturation diving purposes. I do find the discussion interesting and am thankful for any corrections when I am wrong. First and foremost, I am here to learn.

Hello Akimbo,
from the link you posted
“The International Standard Metric Conditions for natural gas and similar fluids are 288.15 K (15.00 °C; 59.00 °F) and 101.325 kPa.”

From my knowledge standard atmosphere (I am a flying pro) is 15 degrees.
Water density happens to be 1 at 4 degree celsius or 277.15K ... because of instrinsec characteristics of water.
Mass, Weight, Density or Specific Gravity of Water at Various Temperatures
As you say absolutely irrelevant for diving ... but nice to know :)

Density of water (and the temperature at which is measured) is unrelated to the thermometric scale definition which is the freezing point (0 degrees) and the boiling point (100 degrees) of distilled water.

E19ED881-6F38-4A1E-8FA8-21B1CC17BD96.jpeg
 
This pains me to see this - it's simple physics.

Scooter weighs 20kgs on land at sea level
Intact scooter under seawater displaces approx 20 liters of seawater - it's neutrally buoyant.

Flooded, imploded or damaged scooter under seawater displaces 6 liters of seawater - what's the scooters relative weight under seawater now? 20kgs (original weight) minus 6kgs (6 liters of displaced seawater) equals 14kgs.

The above calculation doesn't take into account any material used in making the scooter that has a specific gravity less than one (actually less than 1.02) - that material floats anyways, so it'd effect actual weight under water and make it lighter - heavy batteries have no bearing on the weight under water.

Weights are purely made up and don't relate to any scooter - the one I picked up the other day was heavy......

Chuck, if your numbers are correct, the weight underwater should be 6 kg.

Buoyancy = buoyancy force - weight on dry land - weight of water in flooded cavity
Buoyancy = (20 L x 1 kg/L) - 20 kg - (6 L x 1 kg/L) = 20 - 20 - 6 = - 6 kg.

Weight underwater = - Bouyancy = 6 kg, not 14 kg.

How do you come up with 6 liter cavity?
 
https://www.shearwater.com/products/peregrine/

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