
06302020, 12:22 AM #121
For dummies
https://www.instructables.com/id/Sol...DambyWater/
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06302020, 12:27 AM #122

06302020, 12:30 AM #123
MKS
density in kilograms per meter cubed
acceleration in meters per second
depth in meters
pressure in Newtons
For dummies
https://www.instructables.com/id/Sol...DambyWater/
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06302020, 12:51 AM #124
It's possible that ol' Albert was smarter than he looked ....
The Rebel Physicist on the Hunt for a Better Story Than Quantum Mechanics

06302020, 01:29 AM #125
not quite correct on the units.
m/s^2 metres per second / per second accell
pressure has units of Pascals KPa is Kilo pascals (1000 x ) a conversion is 101.3 kpa is 14.7 psi or 1 bar. don't mix units must be SI units. so 1 Pa is a very small pressure.
Force has units of Newtons and a Newton is kg x 9.81 (x accel due to gravity)
as i said earlier its been a while since i looked at it so look it up in a good book is best especially if its critical to safety or life.


06302020, 01:45 AM #126
Originally Posted by JohnMartin
In that case, I guess you’d have to believe that a hydraulic cylinder develops more force the further out the piston is extended, and when it is retracted all the way there is no force at all. Intuition does not always serve us well.
How is that relevant here??
Well, if you believe that the pressure on a glass aquarium wall or on a dam face is somehow related to the amount of water pushing on it in the horizontal direction, then you would also have to believe that to be the case with the hydraulic cylinder as well. The more it extends the more fluid is behind it, so the force must increase as well. And, when the piston is fully retracted with very little fluid behind it, there would be very little force exerted. Does a bottle jack get easier to pump the more the ram is extended?

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06302020, 01:47 AM #127

06302020, 07:15 AM #128
If the thick glass bows as much as you say there could be another problem. Touching the outside with a diamond ring could cause it to break and make a huge mess. Best to cover the glass with one of those clear protective films used on cars. Expel is a good one, and not easy to scratch as it has some bit of self healing to it. If applied properly you will not know its there.

06302020, 07:41 AM #129


06302020, 08:23 AM #130

06302020, 09:03 AM #131
I'm still waiting for someone to expalin, in plain English, why the weight of the medium inside the tank does not matter.
Why does the tank have a ton of pressure with water (which is heavy at about 8lbs per gallon heavy) but hardly any with a liquid that weighs .005 lbs per gallon.

06302020, 09:14 AM #132
Weight doesn't matter, Density does.
Empty, the tank is filled with AIR a very "un dense" liquid. It pushes against the inside tank walls at 14.7 pounds per square inch . Just like the "fluid air" on the outside of the tank walls.
At suitable temperatures, you could fill the tank with liquid air. That would be "more dense" than gaseous air and would press against the glass with more pressure under the influence of GRAVITY.
Water is higher density yet, and will press against the tank glass harder yet. Also due to gravity attraction.
Take the tank and it's contents sufficiently distant from any single gravity source (like the earth) and all the pressure will disappear regardless of the density of the contents.
Like MAGIC
Gravity turns density into weight. Weight is a result, it is not a characteristic of any material. We just are accustomed to weight, because we seldom leave earths gravity with anything in our pockets. Check with the folks up in the international space station for comments..

06302020, 09:30 AM #133
Archimedes and Blaise Pascal would turn in their graves if they saw this thread.

06302020, 10:05 AM #134
Truer words never spoken.
I used to do a lot of scuba diving. Every 33 feet equals one atmosphere. So the pressure on the surface, 14.7 psi. At 33 feet, twice that. 66 feet, 3 atmospheres, etc.
If the pressure was many thousands of psi at say 100 feet deep, you would not be able to take a breath. Steel tanks are only pressurized to 2250 psi. The second stage regulator drops that down to equal the water pressure however deep you are, so your diaphragm can overcome the pressure.
80 ft^3 of air, pressurized at 3000 psi fits into a backpack size package will last about one hour at 60 feet with reserve. If the pressure was many thousands of psi, you would consume the entire tank of air in just a few breaths.

06302020, 10:06 AM #135
When I was putting this tank together I did research on glass, not as basic as metals. It seems every piece of glass is it's own entity. A flawless piece would be exceptionally strong but a piece with a sharp 'V' scratch however small would lead the a stress riser that would weaken it considerably. The figure for the strength spread in glass from strongest to weakest due to imperfections "could" be from 100% to 10%, or so I vaguely remember reading. When my tank was filled I remember tiptoeing around the house whispering as I talked. I was thinking the tank could be at 99.99% of it's breaking strength. If someone slammed a door I would flip out. Now it has lived through terrible thunder storms and all sorts of threats. It would be nice to know, in percentage, how close it is to letting go. My living room is 14 feet by 28 feet and if the tank let go there would be 4" of water to mop(?) up. I must be confident now as I have slept like a baby on the couch beneath it several times.

06302020, 10:24 AM #136
This discussion reminds me of one here a few years back when someone mentioned that Young's Modulus for steel doesn't (on any practical level) change with heat treatment.

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06302020, 10:39 AM #137
Jancolic, The basis of the question here, if I can remember it through the noise level, is that a small hydrostatic head may only have a small pressure but acting on a lot of square inches can generate a lot of force. The actual question is whether it will still generate the same head when the column becomes very narrow. The answer is that it will until the column becomes so narrow that other forces dominate. These forces are always there but for most spacings they are so small that they can be ignored. There is no magic dimension where there is a sudden change.
Bill

06302020, 11:35 AM #138
So, first of all, calm down and get less pissed off: your equation is correct. That said, this is a fluid STATICS equation. Nothing moving. Fluid dynamics would involve moving fluid.
Units is a stumbling block for many. I'll give you a simple factor below, but r * g * d has units
mass/volume * length/(time^2) * length
When you collect terms you get mass * length/(time^2) * length/volume, or mass * length/(time^2) /(length^2), or
mass * length/(time^2) /area
the mass*length/(time^2) is force, so this checks out: Pressure = force/area.
If P = r * g * d, then using metric (meters, seconds, grams) we have
P = g/(m^3) * m/(s^2) * m, or
P = g *m /(s^2) /(m^2)
The g * m/(s^2) unit is called a gram force unit and is analogous to the kg force unit. But both of these are nonstandards. 1 kg force = 9.80 N. One g force = 9.80 mN.
If you want simple, in psi, P = 0.036 * d, where d is inches assuming water at 4° C (1.000 g/cm^2).
in Bar, you get P = 0.098 * d, where d is in meters, again assuming that water is at 4° C
If you want to work out the equations yourself, I suggest writing out the equations with the units but with variables and value, like this:
P [ g *m /(s^2) /(m^2) ] = r [g/m^3] * 9.80 m/(s^2) * d [m]
Then remember that x/y is 1 if x = y and y is not zero. Why is this important? Because it allows you to calculate conversion factors. Suppose you want to get pressure in the same units but using a depth in inches. A conversion factor from d in meters, to d'' in inches. We know that 100cm = 1m. So we can multiply any factor in the equation by 1m/100cm. And by 2.54 cm/inch. So
P [ g *m /(s^2) /(m^2) ] = r [g/m^3] * 9.80 m/(s^2) * d'' [inches] * [m/100cm] * [2.54 cm/inch]
P [ g *m /(s^2) /(m^2) ] = r [g/m^3] * 9.80 m/(s^2) * d'' (2.54/100) [m]
The point is that d'' in inches * 2.54 /100 gives you d in meters.
Clear as mud?

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06302020, 11:37 AM #139
The force can never exceed the weight of the water, and it's just the weight divided by the square inches or whatever unit you want to use.
A column of water one square inch and 33 feet high weighs about 13.7 lbs. Pressure at the base is 13.7 psi.
If you spread that same volume of water in a column 1 square foot in cross section, you get 13.7/144 or 1.3 psi.
OP's tank has a base cross section of 48 ft^2 and holds 9445 lbs of water. That's 196lbs/ft^2 or 1.36 psi.
The pressure on the sides is equally divided on all sides. So the average pressure is .68 psi on the sides and the total force is about 8680 lbs. (average force .68 psi times total side area of 12,768 in^2). This is lower than the 9445 lbs of water, so we're okay.
For our 1" thick aquarium, the weight of the water is only 131 lbs. you get the same 1.36 psi at the base. The total side area is 7372 in^2, so multiply the .68 psi times 7372 in^2 (the total side area) and you get 5012 lbs as the total force.
This is a nonsense number, you cannot generate 5000 lbs of force with 131 lbs. of water.

06302020, 11:48 AM #140
Jancollc, you seem usually pretty able &smart in technical matters so take a deep breath and think this over once more.
People on this thread are mixing up weight, pressure and force in amazing amounts. 😬

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