Here's my theory:
As you decreased the distance between front and back, and the front
galss bowed out, the volume of the tank would increase and the water
depth would lower, decreasing the water pressure on the glass.

The relationship is between depth and pressure. If you built a tank that was a
mile long and a mile wide, but six inches deep, you would need glass thick
enough to support the water pressure associated with a six-inch depth.

One has to remember that, assuming the tank is level, there is equal pressure
on all the vertical structures containing the water, not just the front glass.
As the tank expands in length or width, the amount of
vertical structure to contain the water increases correspondingly, and
continues to support its share of the pressure.

Now, here's where the troubling part comes in. Lets say the
back of the tank, instead of being 72" away from the front is moved forward
until it is just 1/8 of an inch away from the front glass. Now there is less
than two gallons of water in the tank. I have trouble seeing the glass (3/4
in. thick) bowing from 2,500 lbs of "push" from less than two gallons of
water(16 lbs). I suppose we could even shrink the 1/8 in. to a few
thousandths and put a thimble of water in. Would there still be 2,500 lbs
of outward force from a gram or two of water? Would the heavy duty scale
across the room be forced all the way to the two thousand, five hundred
pound mark?

Dixon

Dixon,

Good question. Now, I have no physics background whatsoever, so don't
laugh if I'm way off...but regarding decreasing the depth (front to
back) and maintaining the same psi due to the same height...

Isn't the psi on the vertical surface walls based on overall volume of
the tank's contents? As in, it's calculated from a cubic measurement,
then translated to pressure on a square inch? For instance, the
pressure created by the approx. two gallons of water contained in the
very shallow 1/8th inch depth (front to back) would then
understandably be much less than the original 72"' measurement because
the volume is much less? It's not simply just the depth that makes the
difference, but the cubic volume rather, of which depth is only one
third of the equation.

Of course, the density of the contents, in this case water, makes a
big difference too - but we're dealing with the same contents across
the board here.

Anyway....I agree with the poster who said to check with the
experts...but that's my two (less than qualified) cents!

Capt. Rob Welling
Sarasota, FL

(Rob Welling) wrote in message . com...
Now, here's where the troubling part comes in. Lets say the
back of the tank, instead of being 72" away from the front is moved forward
until it is just 1/8 of an inch away from the front glass. Now there is less
than two gallons of water in the tank. I have trouble seeing the glass (3/4
in. thick) bowing from 2,500 lbs of "push" from less than two gallons of
water(16 lbs). I suppose we could even shrink the 1/8 in. to a few
thousandths and put a thimble of water in. Would there still be 2,500 lbs
of outward force from a gram or two of water? Would the heavy duty scale
across the room be forced all the way to the two thousand, five hundred
pound mark?

Dixon

Dixon,

Good question. Now, I have no physics background whatsoever, so don't
laugh if I'm way off...but regarding decreasing the depth (front to
back) and maintaining the same psi due to the same height...

Isn't the psi on the vertical surface walls based on overall volume of
the tank's contents? As in, it's calculated from a cubic measurement,
then translated to pressure on a square inch? For instance, the
pressure created by the approx. two gallons of water contained in the
very shallow 1/8th inch depth (front to back) would then
understandably be much less than the original 72"' measurement because
the volume is much less? It's not simply just the depth that makes the
difference, but the cubic volume rather, of which depth is only one
third of the equation.

Of course, the density of the contents, in this case water, makes a
big difference too - but we're dealing with the same contents across
the board here.

Anyway....I agree with the poster who said to check with the
experts...but that's my two (less than qualified) cents!

Capt. Rob Welling
Sarasota, FL

OK everyone, its basically a "statics" problem. Pressure is the same
at any given depth all over the tank regardless of its horizontal
extent. FOR SIMPLICITY and for over engineering, assume the total
outward force is given by pressure at the bottom X the area of the
front glass. With everything in balance, the vertical ends cannot
move and the middle is bowed out. The glass acts as a spring whose
restoring force wants to return to "straight" but it is stretched by
the force. The amount of stretch is given by the "spring constant" X
the horizontal extent of the glass. In other words, a longer span of
glass will stretch more (ie. bow out more). Theoretically, one could
measure the bowing of the glass and resolve the components of the
force at each infinitesimal point and solve for the "spring constant"
of the glass. Failure of materials occurs when they are stretched
beyond their "elastic limit". Unlike other materials that just
stretch more beyond this limit, glass fails catastrophically. Even a
scratch on the glass could result im local stresses beyond failure.
The bottom line is, ask your glass company what kind of load (in
pounds) a given span of your glass can support and make sure it is
less than the force we calculated above.
One analogy is to think of your glass as a very long stiff horizontal
spring supported at the ends. Put many, many, many little weights on
the spring at equal intervals all across it. The weight of the little
weights is the force due to a given pressure against a given area of
glass (spring) so greater depth would be like heavier weights. Due to
the weights, the spring stretches and sags (bows) and will sag more
for more pressure and more for a longer spring. I believe but am not
sure (have drowned too many brain cells with rum in this old punkin)
that the bowing is a catenary.

dixon wrote:
I wanted to build a 1000 plus gallon aquarium in the new house I was having
built. I wasn't sure how thick to make the front glass. The rest of the tank
would be fiberglass covered steel. The dimensions I arrived at were 8 ft.
across, 6 ft. front to back and 38 in. tall. This would be 1137 gallons. A
friend had a tank that was 38 in. tall but only 18 in. front to back. It was
also 8 ft. long. (284 gallons). The glass in his tank was 3/4 in thick. My
tank would be identical except the front to back dim. would be 72 in.
At first I was concerned that there would more pressure on the front glass.
After much research I became confident there would be no difference
regardless of front to back depth. For simplicity lets call depth the front
to back dim. and height the top to bottom dim. I found charts that told the
pressure at every height of water in inch increments. To figure the total
"push" on the front glass, you just need to find the pressure at the halfway
point(19 inches in this case) and multiply by the total sq. in. of the glass
(3,648). I believe
at 19" the pressure was about .686 or so psi. It figured out to be around
2,500 lbs. of force on the glass.

In fresh water, I guess because that seems a low number. Anyway the
force isn't "all" over the glass it's a water column 38" deep so along
the bottom of the front glass the pressure is what?? say 1.37 psi on
your figure & probably more like 1.55 psi on my figures.



It would be about 9,480 lbs of water.

The total weight of the water in the tank isn't relevant all the glass
sees is a PSI pressure starting at nothing at the surface, graduating to
say 1.55 psi along the bottom edge.

As
I
was filling the tank, a straight edge laid against the front glass showed
the glass bowing outward very noticeably even at 1/4 full.

How so??, did it start leaking?? what is the joint along the bottom
edge doing?? for the glass to "bow" it must be moving relative the the
tank bottom?? The bottom edge of the glass is about the only part that
sees full pressure & you should have that well tied to the bottom of the
tank (angle iron frame??? or similar) The top of the glass should have a
frame also but it's not as important as the bottom, because at the top
the glass sees very little psi of pressure.

I nervously
filled it full. Now it was very bowed even to the naked eye. The 2,500 lbs
was definitely there on the glass. Now, try to visualize the front glass as
a giant rectangle piston with 2,500 lbs of force on the inside. If you put a
4"x4" in the center of the glass that ran across the room to an opposite
wall and put a bathroom scale (quite a scale!) against the wall or better
yet, an "I" beam, the scale after filling, would read a total of 2500 lbs.
Or 25 men each pushing on the outside at 100 lbs would cancel the pressure
on the glass. Now, here's where the troubling part comes in. Lets say the
back of the tank, instead of being 72" away from the front is moved forward
until it is just 1/8 of an inch away from the front glass. Now there is less
than two gallons of water in the tank. I have trouble seeing the glass (3/4
in. thick) bowing from 2,500 lbs of "push" from less than two gallons of
water(16 lbs). I suppose we could even shrink the 1/8 in. to a few
thousandths and put a thimble of water in. Would there still be 2,500 lbs
of outward force from a gram or two of water? Would the heavy duty scale
across the room be forced all the way to the two thousand, five hundred
pound mark?

Again the total volume of water in the tank is not relevant, what does
matter is that the glass be strong enough to resist the pressure at the
deepest point, if it's OK there then it's OK at any lesser depth. At
6ft X 3ft+ it's a big sheet of heavy glass.

Most boat windows are not 6ft long in one go & if built to survey
standards are surprisingly thick, the forward facing windows in my boat
are 3/8+" thick & nothing like 6 ft in any one direction.

I'm sure you've noticed most "in survey" paying passenger carrying open
water vessels don't have large areas of unsupported glass, even
laminated etc?? There are specific rules which govern max window size vs
material, thickness & support in the USL code & it seems smaller windows
are a safer, lighter (no pun intended) & probably a cheaper choice than
trying to persist with huge picture windows that pleasure craft can get
away with.

I think 3/4" glass is just not thick enough unsupported over that
length (again I'm still wondering how it bows away from the bottom of
the tank??) & probably thicker wouldn't help much (as glass gets thicker
it doesn't get stronger in proportion, it's still the outer skin that
carries the load & it fails in tension; also it starts to have problems
supporting it's own weight), so maybe consider some better framing??

Again the pressure is mostly along the lower edge.

K

Dixon

"K. Smith" wrote in message
...
dixon wrote:
I wanted to build a 1000 plus gallon aquarium in the new house I was
having
built. I wasn't sure how thick to make the front glass. The rest of the
tank
would be fiberglass covered steel. The dimensions I arrived at were 8
ft.
across, 6 ft. front to back and 38 in. tall. This would be 1137 gallons.
A
friend had a tank that was 38 in. tall but only 18 in. front to back. It
was
also 8 ft. long. (284 gallons). The glass in his tank was 3/4 in thick.
My
tank would be identical except the front to back dim. would be 72 in.
At first I was concerned that there would more pressure on the front
glass.
After much research I became confident there would be no difference
regardless of front to back depth. For simplicity lets call depth the
front
to back dim. and height the top to bottom dim. I found charts that told
the
pressure at every height of water in inch increments. To figure the
total
"push" on the front glass, you just need to find the pressure at the
halfway
point(19 inches in this case) and multiply by the total sq. in. of the
glass
(3,648). I believe
at 19" the pressure was about .686 or so psi. It figured out to be
around
2,500 lbs. of force on the glass.

In fresh water, I guess because that seems a low number. Anyway the
force isn't "all" over the glass it's a water column 38" deep so along
the bottom of the front glass the pressure is what?? say 1.37 psi on
your figure & probably more like 1.55 psi on my figures.

The info I am using for fresh water says; 2'=.87 psi / 3'=1.30 psi / 4'=1.73
psi. It seems linear. I don't have salt figures, but I wouldn't think there
would the that much diff.
An interesting note, after the tank was full I added the commercial salt mix
and it mixed with the water but didn't noticeably raise the level. I guess
this explains why salt water is heavier. I believe it took 3 or 4 hundred
pounds of salt.

For you folks that take salt water for granted, it cost me almost $500.00 to
turn 1140 gallons of fresh into salt. So much for living in Michigan!



It would be about 9,480 lbs of water.

The total weight of the water in the tank isn't relevant all the glass
sees is a PSI pressure starting at nothing at the surface, graduating to
say 1.55 psi along the bottom edge.

As
I
was filling the tank, a straight edge laid against the front glass
showed
the glass bowing outward very noticeably even at 1/4 full.

How so??, did it start leaking?? what is the joint along the bottom
edge doing?? for the glass to "bow" it must be moving relative the the
tank bottom?? The bottom edge of the glass is about the only part that
sees full pressure & you should have that well tied to the bottom of the
tank (angle iron frame??? or similar) The top of the glass should have a
frame also but it's not as important as the bottom, because at the top
the glass sees very little psi of pressure.

I nervously
filled it full. Now it was very bowed even to the naked eye. The 2,500
lbs
was definitely there on the glass. Now, try to visualize the front glass
as
a giant rectangle piston with 2,500 lbs of force on the inside. If you
put a
4"x4" in the center of the glass that ran across the room to an opposite
wall and put a bathroom scale (quite a scale!) against the wall or
better
yet, an "I" beam, the scale after filling, would read a total of 2500
lbs.
Or 25 men each pushing on the outside at 100 lbs would cancel the
pressure
on the glass. Now, here's where the troubling part comes in. Lets say
the
back of the tank, instead of being 72" away from the front is moved
forward
until it is just 1/8 of an inch away from the front glass. Now there is
less
than two gallons of water in the tank. I have trouble seeing the glass
(3/4
in. thick) bowing from 2,500 lbs of "push" from less than two gallons of
water(16 lbs). I suppose we could even shrink the 1/8 in. to a few
thousandths and put a thimble of water in. Would there still be 2,500
lbs
of outward force from a gram or two of water? Would the heavy duty scale
across the room be forced all the way to the two thousand, five hundred
pound mark?

Again the total volume of water in the tank is not relevant, what does
matter is that the glass be strong enough to resist the pressure at the
deepest point, if it's OK there then it's OK at any lesser depth. At
6ft X 3ft+ it's a big sheet of heavy glass.

Most boat windows are not 6ft long in one go & if built to survey
standards are surprisingly thick, the forward facing windows in my boat
are 3/8+" thick & nothing like 6 ft in any one direction.

I'm sure you've noticed most "in survey" paying passenger carrying open
water vessels don't have large areas of unsupported glass, even
laminated etc?? There are specific rules which govern max window size vs
material, thickness & support in the USL code & it seems smaller windows
are a safer, lighter (no pun intended) & probably a cheaper choice than
trying to persist with huge picture windows that pleasure craft can get
away with.

I think 3/4" glass is just not thick enough unsupported over that
length (again I'm still wondering how it bows away from the bottom of
the tank??) & probably thicker wouldn't help much (as glass gets thicker
it doesn't get stronger in proportion, it's still the outer skin that
carries the load & it fails in tension; also it starts to have problems
supporting it's own weight), so maybe consider some better framing??

Again the pressure is mostly along the lower edge.


The bowing is in the center of the glass, the frame is to rigid to flex.
Dixon

Judging from all the various viewpoints about how water can affect an
aquarium glass, it's a good thing we all aren't hired to build a submarine
or an airplane. This problem is old science.

"K. Smith" wrote in message
...
dixon wrote:
I wanted to build a 1000 plus gallon aquarium in the new house I was
having
built. I wasn't sure how thick to make the front glass. The rest of the
tank
would be fiberglass covered steel. The dimensions I arrived at were 8
ft.
across, 6 ft. front to back and 38 in. tall. This would be 1137 gallons.
A
friend had a tank that was 38 in. tall but only 18 in. front to back. It
was
also 8 ft. long. (284 gallons). The glass in his tank was 3/4 in thick.
My
tank would be identical except the front to back dim. would be 72 in.
At first I was concerned that there would more pressure on the front
glass.
After much research I became confident there would be no difference
regardless of front to back depth. For simplicity lets call depth the
front
to back dim. and height the top to bottom dim. I found charts that told
the
pressure at every height of water in inch increments. To figure the
total
"push" on the front glass, you just need to find the pressure at the
halfway
point(19 inches in this case) and multiply by the total sq. in. of the
glass
(3,648). I believe
at 19" the pressure was about .686 or so psi. It figured out to be
around
2,500 lbs. of force on the glass.

In fresh water, I guess because that seems a low number. Anyway the
force isn't "all" over the glass it's a water column 38" deep so along
the bottom of the front glass the pressure is what?? say 1.37 psi on
your figure & probably more like 1.55 psi on my figures.



It would be about 9,480 lbs of water.

The total weight of the water in the tank isn't relevant all the glass
sees is a PSI pressure starting at nothing at the surface, graduating to
say 1.55 psi along the bottom edge.

As
I
was filling the tank, a straight edge laid against the front glass
showed
the glass bowing outward very noticeably even at 1/4 full.

How so??, did it start leaking?? what is the joint along the bottom
edge doing?? for the glass to "bow" it must be moving relative the the
tank bottom?? The bottom edge of the glass is about the only part that
sees full pressure & you should have that well tied to the bottom of the
tank (angle iron frame??? or similar) The top of the glass should have a
frame also but it's not as important as the bottom, because at the top
the glass sees very little psi of pressure.

I nervously
filled it full. Now it was very bowed even to the naked eye. The 2,500
lbs
was definitely there on the glass. Now, try to visualize the front glass
as
a giant rectangle piston with 2,500 lbs of force on the inside. If you
put a
4"x4" in the center of the glass that ran across the room to an opposite
wall and put a bathroom scale (quite a scale!) against the wall or
better
yet, an "I" beam, the scale after filling, would read a total of 2500
lbs.
Or 25 men each pushing on the outside at 100 lbs would cancel the
pressure
on the glass. Now, here's where the troubling part comes in. Lets say
the
back of the tank, instead of being 72" away from the front is moved
forward
until it is just 1/8 of an inch away from the front glass. Now there is
less
than two gallons of water in the tank. I have trouble seeing the glass
(3/4
in. thick) bowing from 2,500 lbs of "push" from less than two gallons of
water(16 lbs). I suppose we could even shrink the 1/8 in. to a few
thousandths and put a thimble of water in. Would there still be 2,500
lbs
of outward force from a gram or two of water? Would the heavy duty scale
across the room be forced all the way to the two thousand, five hundred
pound mark?

Again the total volume of water in the tank is not relevant, what does
matter is that the glass be strong enough to resist the pressure at the
deepest point, if it's OK there then it's OK at any lesser depth. At
6ft X 3ft+ it's a big sheet of heavy glass.

Most boat windows are not 6ft long in one go & if built to survey
standards are surprisingly thick, the forward facing windows in my boat
are 3/8+" thick & nothing like 6 ft in any one direction.

I'm sure you've noticed most "in survey" paying passenger carrying open
water vessels don't have large areas of unsupported glass, even
laminated etc?? There are specific rules which govern max window size vs
material, thickness & support in the USL code & it seems smaller windows
are a safer, lighter (no pun intended) & probably a cheaper choice than
trying to persist with huge picture windows that pleasure craft can get
away with.

I think 3/4" glass is just not thick enough unsupported over that
length (again I'm still wondering how it bows away from the bottom of
the tank??) & probably thicker wouldn't help much (as glass gets thicker
it doesn't get stronger in proportion, it's still the outer skin that
carries the load & it fails in tension; also it starts to have problems
supporting it's own weight), so maybe consider some better framing??

Again the pressure is mostly along the lower edge.

K

Dixon




Actually Cruise ships have large, very thick windows. Boarding a cruise
ship in Valpariso, Chile one time and they were replacing one of the side
windows that was about 6' long and was broken during a storm off the
Shetlands. The window was at least 1.5" thick. The 2 bridge windows were
not replaced at that time. I guess lack of suitable replacements. And the
glass of the aquarium will always keep bending more, until it breaks.
Glass is actually a liquid. Very high viscosity, but non the less a liquid.
Windows in 1400's buildings in Europe are actually thicker at the bottom
from the flow.

"Calif Bill" wrote in message link.net...
"K. Smith" wrote in message
...
dixon wrote:
I wanted to build a 1000 plus gallon aquarium in the new house I was
having
built. I wasn't sure how thick to make the front glass. The rest of the
tank
would be fiberglass covered steel. The dimensions I arrived at were 8
ft.
across, 6 ft. front to back and 38 in. tall. This would be 1137 gallons.
A
friend had a tank that was 38 in. tall but only 18 in. front to back. It
was
also 8 ft. long. (284 gallons). The glass in his tank was 3/4 in thick.
My
tank would be identical except the front to back dim. would be 72 in.
At first I was concerned that there would more pressure on the front
glass.
After much research I became confident there would be no difference
regardless of front to back depth. For simplicity lets call depth the
front
to back dim. and height the top to bottom dim. I found charts that told
the
pressure at every height of water in inch increments. To figure the
total
"push" on the front glass, you just need to find the pressure at the
halfway
point(19 inches in this case) and multiply by the total sq. in. of the
glass
(3,648). I believe
at 19" the pressure was about .686 or so psi. It figured out to be
around
2,500 lbs. of force on the glass.

In fresh water, I guess because that seems a low number. Anyway the
force isn't "all" over the glass it's a water column 38" deep so along
the bottom of the front glass the pressure is what?? say 1.37 psi on
your figure & probably more like 1.55 psi on my figures.



It would be about 9,480 lbs of water.

The total weight of the water in the tank isn't relevant all the glass
sees is a PSI pressure starting at nothing at the surface, graduating to
say 1.55 psi along the bottom edge.

As
I
was filling the tank, a straight edge laid against the front glass
showed
the glass bowing outward very noticeably even at 1/4 full.

How so??, did it start leaking?? what is the joint along the bottom
edge doing?? for the glass to "bow" it must be moving relative the the
tank bottom?? The bottom edge of the glass is about the only part that
sees full pressure & you should have that well tied to the bottom of the
tank (angle iron frame??? or similar) The top of the glass should have a
frame also but it's not as important as the bottom, because at the top
the glass sees very little psi of pressure.

I nervously
filled it full. Now it was very bowed even to the naked eye. The 2,500
lbs
was definitely there on the glass. Now, try to visualize the front glass
as
a giant rectangle piston with 2,500 lbs of force on the inside. If you
put a
4"x4" in the center of the glass that ran across the room to an opposite
wall and put a bathroom scale (quite a scale!) against the wall or
better
yet, an "I" beam, the scale after filling, would read a total of 2500
lbs.
Or 25 men each pushing on the outside at 100 lbs would cancel the
pressure
on the glass. Now, here's where the troubling part comes in. Lets say
the
back of the tank, instead of being 72" away from the front is moved
forward
until it is just 1/8 of an inch away from the front glass. Now there is
less
than two gallons of water in the tank. I have trouble seeing the glass
(3/4
in. thick) bowing from 2,500 lbs of "push" from less than two gallons of
water(16 lbs). I suppose we could even shrink the 1/8 in. to a few
thousandths and put a thimble of water in. Would there still be 2,500
lbs
of outward force from a gram or two of water? Would the heavy duty scale
across the room be forced all the way to the two thousand, five hundred
pound mark?

Again the total volume of water in the tank is not relevant, what does
matter is that the glass be strong enough to resist the pressure at the
deepest point, if it's OK there then it's OK at any lesser depth. At
6ft X 3ft+ it's a big sheet of heavy glass.

Most boat windows are not 6ft long in one go & if built to survey
standards are surprisingly thick, the forward facing windows in my boat
are 3/8+" thick & nothing like 6 ft in any one direction.

I'm sure you've noticed most "in survey" paying passenger carrying open
water vessels don't have large areas of unsupported glass, even
laminated etc?? There are specific rules which govern max window size vs
material, thickness & support in the USL code & it seems smaller windows
are a safer, lighter (no pun intended) & probably a cheaper choice than
trying to persist with huge picture windows that pleasure craft can get
away with.

I think 3/4" glass is just not thick enough unsupported over that
length (again I'm still wondering how it bows away from the bottom of
the tank??) & probably thicker wouldn't help much (as glass gets thicker
it doesn't get stronger in proportion, it's still the outer skin that
carries the load & it fails in tension; also it starts to have problems
supporting it's own weight), so maybe consider some better framing??

Again the pressure is mostly along the lower edge.

K

Dixon




Actually Cruise ships have large, very thick windows. Boarding a cruise
ship in Valpariso, Chile one time and they were replacing one of the side
windows that was about 6' long and was broken during a storm off the
Shetlands. The window was at least 1.5" thick. The 2 bridge windows were
not replaced at that time. I guess lack of suitable replacements. And the
glass of the aquarium will always keep bending more, until it breaks.
Glass is actually a liquid. Very high viscosity, but non the less a liquid.
Windows in 1400's buildings in Europe are actually thicker at the bottom
from the flow.


First, pressure in fresh water is .43 psi/ft. This tells you why
water towers are so high to get the 43 psi we have in our homes.
Second, ship portholes in the past were fairly thick, in fact, at one
time they were the choice material for amateur telescope makers for
primary mirrors.
NEXT, lets dispell this idea of glass being liquid, it is not. Glass
is amorphous but does not flow at all at room temp. Old windows
really are not thicker at the bottom due to flow. Much old glass has
thickness variations due to primitive manufacturing processes.