19 Sep 2007 20:57:54
Sheldon
Interesting pressure question

Is the pressure on you the same, at any given depth, whether you dive in a
lake, a crater, a swimming pool or the ocean?

I'm thinking the larger the body of water the more water there is to exert
pressure on you.

Sheldon




19 Sep 2007 23:08:56
David In NH
Re: Interesting pressure question


"Sheldon" <sheldon@XXXXXXXXsopris.net > wrote in message
news:tI6dnYdm9ondfGzbnZ2dnUVZ_iydnZ2d@comcast.com...
> Is the pressure on you the same, at any given depth, whether you dive in a
> lake, a crater, a swimming pool or the ocean?
>
> I'm thinking the larger the body of water the more water there is to exert
> pressure on you.
>
> Sheldon
>
I'm gonna assume this isn't just a troll. Pressure on the body is not the
same at any given depth but doesn't depend on the location or quantity of
water. Rather it depends on the density of the water. The pressure on you
under x feet of ocean water will be higher than under x feet of lake or
swimming pool water because the density (weight) of salt water is higher
than that of fresh water.




20 Sep 2007 05:40:40
Conshelf
Re: Interesting pressure question

In rec.scuba, on Wed 19 Sep 2007 10:08:56p, "David In NH"
<dgintz@gmail.com > wrote:

> I'm gonna assume this isn't just a troll. Pressure on the body is not
> the same at any given depth but doesn't depend on the location or
> quantity of water. Rather it depends on the density of the water. The
> pressure on you under x feet of ocean water will be higher than under
> x feet of lake or swimming pool water because the density (weight) of
> salt water is higher than that of fresh water.

You also need to factor in altitude of the surface of the body of water and
barometric pressure. If you are wanting to be really accurate, you would
also factor in temperature so that you can take account for the thermal
expansion of water at the various temperatures. And while we're at it, we
might as well factor in the position of the moon and the sun. They're sure
to make the indicated depth change by at least a couple of nanometers,
right?

I believe that the fundamental law that deals with this is called "Pascal's
Law". It does take into account the acceleration due to gravity, so
technically, when the moon is on the same side of the planet where you are
taking measurements, the moon's gravity would slightly cancel out the
earth's gravity. How much it would affect it is left as an exercise for
the reader.


20 Sep 2007 16:17:33
dechucka
Re: Interesting pressure question


"Sheldon" <sheldon@XXXXXXXXsopris.net > wrote in message
news:tI6dnYdm9ondfGzbnZ2dnUVZ_iydnZ2d@comcast.com...
> Is the pressure on you the same, at any given depth, whether you dive in a
> lake, a crater, a swimming pool or the ocean?


basically yes if you are at the same altitude. Will slightly vary due to the
weight per M squared of the water + additives ( or maybe the mass, have to
think about it )


>
> I'm thinking the larger the body of water the more water there is to exert
> pressure on you.

it's the water ( + atmosphere ) above you not the water around you that
causes the pressure or we would all be crushed when we take a swim at the
local ocean beach.

didn't they explain this during your OW training?




20 Sep 2007 03:08:33
Douglas W. Popeye Frederick
Re: Interesting pressure question

"David In NH" <dgintz@gmail.com > wrote in message
news:HICdnUl1AL7NeWzbnZ2dnUVZ_iydnZ2d@comcast.com...
>
> "Sheldon" <sheldon@XXXXXXXXsopris.net> wrote in message
> news:tI6dnYdm9ondfGzbnZ2dnUVZ_iydnZ2d@comcast.com...
>> Is the pressure on you the same, at any given depth, whether you dive in
>> a lake, a crater, a swimming pool or the ocean?
>>
>> I'm thinking the larger the body of water the more water there is to
>> exert pressure on you.
>>
>> Sheldon
>>
> I'm gonna assume this isn't just a troll. Pressure on the body is not the
> same at any given depth but doesn't depend on the location or quantity of
> water. Rather it depends on the density of the water. The pressure on you
> under x feet of ocean water will be higher than under x feet of lake or
> swimming pool water because the density (weight) of salt water is higher
> than that of fresh water.

Altitude.



20 Sep 2007 11:01:30
-hh
Re: Interesting pressure question

"Sheldon" <shel...@XXXXXXXXsopris.net > wrote:
> Is the pressure on you the same, at any given depth, whether you dive in a
> lake, a crater, a swimming pool or the ocean?

No. Variations exist due to water density (think fresh/saltwater),
the topside air pressure (think altitude).

Obviously, some of these are more significant than others. For
example, the topside air pressure also includes variations in
barometric pressure (eg, weather) in addition to altitude.


> I'm thinking the larger the body of water the more water there is to exert
> pressure on you.

The size of the body of water does not have any direct bearing. It
might seem that way because the oceans are big, but they're also
always at lower altitudes than fresh water and they have higher salt
concentrations so are thus more dense.



-hh



20 Sep 2007 10:16:00
El Stroko Guapo
Re: Interesting pressure question

-hh wrote:
> "Sheldon" <shel...@XXXXXXXXsopris.net> wrote:
>
>>Is the pressure on you the same, at any given depth, whether you dive in a
>>lake, a crater, a swimming pool or the ocean?
>
>
> No. Variations exist due to water density (think fresh/saltwater),
> the topside air pressure (think altitude).
>
> Obviously, some of these are more significant than others. For
> example, the topside air pressure also includes variations in
> barometric pressure (eg, weather) in addition to altitude.
>
>
>
>>I'm thinking the larger the body of water the more water there is to exert
>>pressure on you.
>
>
> The size of the body of water does not have any direct bearing. It
> might seem that way because the oceans are big, but they're also
> always at lower altitudes than fresh water and they have higher salt
> concentrations so are thus more dense.
>
>
>
> -hh
>
>
And the practical application of that fact is that all non-pressurized
experimental diving tanks are silo shaped, i.e. the bulk of water is
above, not around, the diver.

esg



20 Sep 2007 10:47:07
Lee Bell
Re: Interesting pressure question

Sheldon wrote

> Is the pressure on you the same, at any given depth, whether you dive in a
> lake, a crater, a swimming pool or the ocean?
> I'm thinking the larger the body of water the more water there is to exert
> pressure on you.

The pressure varies, but not as you suggest. Depth and density of anything
above you is all that matters.
If atmospheric pressure is higher or lower, pressure on you is equally
higher or lower.
If the density of the water is higher or lower, pressure on you is higher or
lower.

The density of salt water is higher than the density of fresh water,
therefore pressure is greater at the same depth. This could become a problem
for decompression calculations except for the fact that depth gauges and
computers are really reading pressure anyway. Since nitrogen transfer is a
function of pressure, everything works fine even though a fresh water depth
gauge will read deeper than you actually are when you're diving in salt
water. The reverse is true for a computer designed for fresh water. Some
computers allow you to indicate whether you are in fresh or salt water so
that they can at least partially adjust for the difference in density. It's
not precise since not all fresh or all salt water is the same density.

Lee




20 Sep 2007 10:49:02
Lee Bell
Re: Interesting pressure question

Conshelf wrote

> You also need to factor in altitude of the surface of the body of water
and
> barometric pressure.

Yep.

> If you are wanting to be really accurate, you would
> also factor in temperature so that you can take account for the thermal
> expansion of water at the various temperatures.

Thermal expansion of water? You planning on diving in boiling seas?

Lee




20 Sep 2007 15:09:39
Re: Interesting pressure question

On Sep 20, 10:49 am, "Lee Bell" <pleeb...@bellsouth.net > wrote:
> Conshelf wrote

> > If you are wanting to be really accurate, you would
> > also factor in temperature so that you can take account for the thermal
> > expansion of water at the various temperatures.
>
> Thermal expansion of water? You planning on diving in boiling seas?

You're lighting the tilt, Dad. The density of water does vary a bit
with the hot and cold, see http://www.simetric.co.uk/si_water.htm



20 Sep 2007 11:42:20
JOF
Re: Interesting pressure question

On Wed, 19 Sep 2007 20:57:54 -0600, "Sheldon"
<sheldon@XXXXXXXXsopris.net > wrote:

>Is the pressure on you the same, at any given depth, whether you dive in a
>lake, a crater, a swimming pool or the ocean?
>
>I'm thinking the larger the body of water the more water there is to exert
>pressure on you.

The height of the water column directly above you is the most
important factor.

And think about this - what is your depth when you are diving under
20' waves?

JF



20 Sep 2007 11:44:57
El Stroko Guapo
Re: Interesting pressure question

kookie.kookson@gmail.com wrote:

> On Sep 20, 10:49 am, "Lee Bell" <pleeb...@bellsouth.net> wrote:
>
>>Conshelf wrote
>
>
>>>If you are wanting to be really accurate, you would
>>>also factor in temperature so that you can take account for the thermal
>>>expansion of water at the various temperatures.
>>
>>Thermal expansion of water? You planning on diving in boiling seas?
>
>
> You're lighting the tilt, Dad. The density of water does vary a bit
> with the hot and cold, see http://www.simetric.co.uk/si_water.htm
>
>
Even more so with salinity, which varies from area to area and from day
to day.

esg



20 Sep 2007 18:17:19
Conshelf
Re: Interesting pressure question

In rec.scuba, on Thu 20 Sep 2007 09:49:02a, "Lee Bell"
<pleebell@bellsouth.net > wrote:

> Thermal expansion of water? You planning on diving in boiling seas?

Admittedly, it would be a very minor difference, but water does vary in
density even over normal temperature ranges. Fresh water has maximum
density at 4C. Abover or below that point, it expands. Technically, there
would be a difference, but I suspect that our depth gauges are not accurate
enough to see the difference.

http://www.simetric.co.uk/si_water.htm

The record low barometric pressure (during a hurricane, of course) is 25.69
inches of mercury. Standard sea level atmospheric pressure is 29.92. If
you were diving at that time, it would result in a change of actual depth
to pressure depth of approximately 4.5 ft. I suspect that you would have a
few other things to be worried about diving in that level of a hurricane.

As you pointed out earlier though, we are diving to a pressure depth which
may or may not be equivalent to a true physical depth. Since nitrogen
absorption is related to pressure depth, not true physical depth, this
works fairly transparently for us. To show an extreme of this, let's
assume that it was possible to dive on the moon. If you were physically
120 ft below the surface of the water, your earth calculated pressure gauge
would read around 20 ft instead since the gravity of the moon is 1/6th of
earth.


20 Sep 2007 16:18:21
Chris Guynn
Re: Interesting pressure question


"JOF" <jofrancis@gmail.com > wrote in message
news:sm45f3l0pgogrfvm13pmvjhuqvoin3n810@4ax.com...
> On Wed, 19 Sep 2007 20:57:54 -0600, "Sheldon"
> <sheldon@XXXXXXXXsopris.net> wrote:
>
> >Is the pressure on you the same, at any given depth, whether you dive in
a
> >lake, a crater, a swimming pool or the ocean?
> >
> >I'm thinking the larger the body of water the more water there is to
exert
> >pressure on you.
>
> The height of the water column directly above you is the most
> important factor.
>
> And think about this - what is your depth when you are diving under
> 20' waves?
>
> JF

How much does the pressure change when you're diving under 20' waves?

At first, you'd think that as the crest of the wave passed you'd be subject
to 2/3 of an extra ATM over when the trough was over you, but on closer
examination I'm not entirely sure that's true. I can remember sitting
through a class and discussing wave action (as it pertains to water) and
that there are some unusual propoerties involved (particularly involving the
movement of the water itself), but that's about the extent of what I
remember. Is some of the pressure reduced because of the motion of the
energy through the water? I really don't have an answer for that.




20 Sep 2007 10:59:01
Scott
Re: Interesting pressure question


"El Stroko Guapo" <omgray@earthlink.net > wrote in message
news:13f55d4t3o048bd@corp.supernews.com...
> kookie.kookson@gmail.com wrote:
>
> > On Sep 20, 10:49 am, "Lee Bell" <pleeb...@bellsouth.net> wrote:
> >
> >>Conshelf wrote
> >
> >
> >>>If you are wanting to be really accurate, you would
> >>>also factor in temperature so that you can take account for the thermal
> >>>expansion of water at the various temperatures.
> >>
> >>Thermal expansion of water? You planning on diving in boiling seas?
> >
> >
> > You're lighting the tilt, Dad. The density of water does vary a bit
> > with the hot and cold, see http://www.simetric.co.uk/si_water.htm
> >
> >
> Even more so with salinity, which varies from area to area and from day
> to day.

Bingo.




20 Sep 2007 18:17:15
-hh
Re: Interesting pressure question

Conshelf <Consh...@127.0.0.1 > wrote:
> "Lee Bell" <pleeb...@bellsouth.net> wrote:
> > Thermal expansion of water? You planning on diving in boiling seas?
>
> Admittedly, it would be a very minor difference, but water does vary in
> density even over normal temperature ranges.

Agreed. And there's also minor variations (as already pointed out) in
local gravity, as well as variations from that variation caused by
relative position of the moon and sun (as is illustrated by day-to-day
differences in tide heights). Fortunately, most of these are 2nd and
3rd order variables that we can pragmatically ignore.


> The record low barometric pressure (during a hurricane, of course)
> [versus] Standard sea level atmospheric pressure...would result
> in a change of actual depth to pressure depth of approximately 4.5 ft.

Nice analysis.


> To show an extreme of this, let's assume that it was possible
> to dive on the moon. If you were physically 120 ft below the
> surface of the water, your earth calculated pressure gauge would
> read around 20 ft instead since the gravity of the moon is 1/6th of
> earth.

Since we're having the fun of being pendantic, this isn't quite
right.

The gage wouldn't read 20ft because we don't have a 15psia atmosphere
on the moon like we have on Earth.

The Earth gage is calibrated with the assumption of the approximately
15 psia atmosphere present being equal to 0ft depth. Thus, 15psia =
0fsw, 30psia = 33fsw, 45psia = 66fsw, etc.

As such (and using the round-off of 33ft = 1 ATM), the depth gage
that's reporting 120fsw is doing so based on a measured absolute
pressure of:

15+ (120/33*15) = 69.5 psia.

On the moon, the atmosphere is zero psia (ie, none), and assuming
1/6th the gravity, then each 6*33fsw = 198fsw of water = 15psi, so a
depth of 120fsw on the moon would be calculated as:

0 + (120/198*15) = 9 psia.

Since 9psia is less than 15psia (Earth Sea Level), this would be
interpreted by said Earth gage as less than a zero depth (in water
units, approximately negative 13fsw). If it was a fancy dive
computer, it might translate this pressure into a perceived altitude,
which would be equivalent to being at roughly 13,000ft elevation on
Earth...can you say "Dive Lake Titicaca"? :-)


-hh



20 Sep 2007 14:16:51
El Stroko Guapo
Re: Interesting pressure question

Chris Guynn wrote:
> "JOF" <jofrancis@gmail.com> wrote in message
> news:sm45f3l0pgogrfvm13pmvjhuqvoin3n810@4ax.com...
>
>>On Wed, 19 Sep 2007 20:57:54 -0600, "Sheldon"
>><sheldon@XXXXXXXXsopris.net> wrote:
>>
>>
>>>Is the pressure on you the same, at any given depth, whether you dive in
>
> a
>
>>>lake, a crater, a swimming pool or the ocean?
>>>
>>>I'm thinking the larger the body of water the more water there is to
>
> exert
>
>>>pressure on you.
>>
>>The height of the water column directly above you is the most
>>important factor.
>>
>>And think about this - what is your depth when you are diving under
>>20' waves?
>>
>>JF
>
>
> How much does the pressure change when you're diving under 20' waves?
>
> At first, you'd think that as the crest of the wave passed you'd be subject
> to 2/3 of an extra ATM over when the trough was over you, but on closer
> examination I'm not entirely sure that's true. I can remember sitting
> through a class and discussing wave action (as it pertains to water) and
> that there are some unusual propoerties involved (particularly involving the
> movement of the water itself), but that's about the extent of what I
> remember. Is some of the pressure reduced because of the motion of the
> energy through the water? I really don't have an answer for that.
>
>
>
If you maintain the same position relative to the bottom, the pressure
does indeed increase from trough to crest. If you ride with the waves,
there's little or no pressure difference.

That's why doing deco on an anchor or tied off line in high seas will
beat hell out of yer ears, but hanging from a float or lift bag won't.

esg



21 Sep 2007 00:04:44
Conshelf
Re: Interesting pressure question

In rec.scuba, on Thu 20 Sep 2007 01:17:15p, -hh
<recscuba_google@huntzinger.com > wrote:

> Agreed. And there's also minor variations (as already pointed out) in
> local gravity, as well as variations from that variation caused by
> relative position of the moon and sun (as is illustrated by day-to-day
> differences in tide heights).

That would be why Pascal's Law just mentions the gravitational constant.
It's going to vary dependent upon where on the planet you are, where the
sun and moon are, and possibly tons of other things that might actually
make the difference of plus or minus a nanometer. :)

> Nice analysis.

Rough estimate actually. I just used a ratio of the normal 29.92
measurement for barometric pressure and factored in the number of ft per
ATM of water. I figure someone somewhere managed to create some long ass
equation for it that is more accurate and takes into account moles and
other such things. For 1 or 2 digits of accuracy, it's probably a good
ballpark type estimate.

> Since we're having the fun of being pendantic, this isn't quite
> right.
>
> The gage wouldn't read 20ft because we don't have a 15psia atmosphere
> on the moon like we have on Earth.

Correct, I was intentionally assuming that the water on the moon was in
some sort of compartment that had 1 ATM of pressure above it. If the water
was to be exposed to the moon's 'atmosphere', the water would probably boil
off in the near perfect vacuum (1 earth-ATM = approximately 1E-12 lunar-
ATM). If it was not open to the lunar 'atmosphere', then we would have to
factor in the vapor pressure of water and that is going to depend upon the
temperaure. Since even if we're on the moon, we want to be warm water
divers, let's use 77F as a good water temperaure. As such, if I'm reading
my tables right, then that would mean that the vapor pressure is 23.76 in-
Hg and would have to adjust the depth appropriately.

Yeah, we're being pedantic.


20 Sep 2007 18:06:08
David In NH
Re: Interesting pressure question

True that you'd need to factor all those in to make an absolute measurement.
However, the difference between salt and fresh water is significant. All the
other things mentioned are minimal.

"Conshelf" <Conshelf@127.0.0.1 > wrote in message
news:Xns99B0E6A56A400conshelf@194.177.96.26...
> In rec.scuba, on Wed 19 Sep 2007 10:08:56p, "David In NH"
> <dgintz@gmail.com> wrote:
>
>> I'm gonna assume this isn't just a troll. Pressure on the body is not
>> the same at any given depth but doesn't depend on the location or
>> quantity of water. Rather it depends on the density of the water. The
>> pressure on you under x feet of ocean water will be higher than under
>> x feet of lake or swimming pool water because the density (weight) of
>> salt water is higher than that of fresh water.
>
> You also need to factor in altitude of the surface of the body of water
> and
> barometric pressure. If you are wanting to be really accurate, you would
> also factor in temperature so that you can take account for the thermal
> expansion of water at the various temperatures. And while we're at it, we
> might as well factor in the position of the moon and the sun. They're
> sure
> to make the indicated depth change by at least a couple of nanometers,
> right?
>
> I believe that the fundamental law that deals with this is called
> "Pascal's
> Law". It does take into account the acceleration due to gravity, so
> technically, when the moon is on the same side of the planet where you are
> taking measurements, the moon's gravity would slightly cancel out the
> earth's gravity. How much it would affect it is left as an exercise for
> the reader.




21 Sep 2007 00:19:42
Conshelf
Re: Interesting pressure question

In rec.scuba, on Thu 20 Sep 2007 05:06:08p, "David In NH"
<dgintz@gmail.com > wrote:

> True that you'd need to factor all those in to make an absolute
> measurement. However, the difference between salt and fresh water is
> significant. All the other things mentioned are minimal.

We're attempting to reach a Zen level of pedantry. I think we're nearly
there.


21 Sep 2007 00:18:39
Matthias Voss
Re: Interesting pressure question

Sheldon wrote:

> Is the pressure on you the same, at any given depth, whether you dive in a
> lake, a crater, a swimming pool or the ocean?
>
> I'm thinking the larger the body of water the more water there is to exert
> pressure on you.
>
> Sheldon

Sometimes the pressure is so great that it would squeeze out
the brains of dive instructors for having given certs for no
obvious reason.

Matthias


--
VK exquis. mod. Gesch.haus in ruh. Wohnl.3 WE, ehem. Praxis,
ca. 350/1000 qm WF/Grdst. 19km nördl. Braunschweig , hochw.
einger; Nah VW, Hafen; frei, 0160-4433698 o. 0911-7809081.
Gut für Arzt/RA/Ing.büro, oder ruhiges Wohnen, Bilder vorh.



21 Sep 2007 00:22:47
Matthias Voss
Re: Interesting pressure question

-hh wrote:

> "Sheldon" <shel...@XXXXXXXXsopris.net> wrote:
>
>>Is the pressure on you the same, at any given depth, whether you dive in a
>>lake, a crater, a swimming pool or the ocean?
>
>
> No. Variations exist due to water density (think fresh/saltwater),
> the topside air pressure (think altitude).
>
> Obviously, some of these are more significant than others. For
> example, the topside air pressure also includes variations in
> barometric pressure (eg, weather) in addition to altitude.
>
>
>
>>I'm thinking the larger the body of water the more water there is to exert
>>pressure on you.
>
>
> The size of the body of water does not have any direct bearing.

Yes it does. And season. Proportional to the size of a given
water body is the number of marinas and moorings for
ships. The more ships are launched into the water, the
higher the level, thus the pressure.

Matthias

--
VK exquis. mod. Gesch.haus in ruh. Wohnl.3 WE, ehem. Praxis,
ca. 350/1000 qm WF/Grdst. 19km nördl. Braunschweig , hochw.
einger; Nah VW, Hafen; frei, 0160-4433698 o. 0911-7809081.
Gut für Arzt/RA/Ing.büro, oder ruhiges Wohnen, Bilder vorh.



20 Sep 2007 18:31:45
David In NH
Re: Interesting pressure question


"Matthias Voss" <spammat.voss@gmx.de > wrote in message
news:fcuru8$fcl$00$4@news.t-online.com...

> Yes it does. And season. Proportional to the size of a given water body is
> the number of marinas and moorings for ships. The more ships are launched
> into the water, the higher the level, thus the pressure.
>
> Matthias
>

That clarifies things now. As long as we're talking about boats and marinas,
can anyone tell me who lines up all the boats so that they all point the
same way?




20 Sep 2007 20:34:05
JOF
Re: Interesting pressure question

On Fri, 21 Sep 2007 00:22:47 +0200, Matthias Voss
<spammat.voss@gmx.de > wrote:

>-hh wrote:
>
>> "Sheldon" <shel...@XXXXXXXXsopris.net> wrote:
>>
>>>Is the pressure on you the same, at any given depth, whether you dive in a
>>>lake, a crater, a swimming pool or the ocean?
>>
>>
>> No. Variations exist due to water density (think fresh/saltwater),
>> the topside air pressure (think altitude).
>>
>> Obviously, some of these are more significant than others. For
>> example, the topside air pressure also includes variations in
>> barometric pressure (eg, weather) in addition to altitude.
>>
>>
>>
>>>I'm thinking the larger the body of water the more water there is to exert
>>>pressure on you.
>>
>>
>> The size of the body of water does not have any direct bearing.
>
>Yes it does. And season. Proportional to the size of a given
> water body is the number of marinas and moorings for
>ships. The more ships are launched into the water, the
>higher the level, thus the pressure.

And heaven help you if one of those boats drives over your particular
water column, eh?

JF



20 Sep 2007 21:33:51
Douglas W. Popeye Frederick
Re: Interesting pressure question

"David In NH" <dgintz@gmail.com > wrote in message
news:Ccmdnf0yFeBNaW_bnZ2dnUVZ_sGvnZ2d@comcast.com...
>
> "Matthias Voss" <spammat.voss@gmx.de> wrote in message
> news:fcuru8$fcl$00$4@news.t-online.com...
>
>> Yes it does. And season. Proportional to the size of a given water body
>> is the number of marinas and moorings for ships. The more ships are
>> launched into the water, the higher the level, thus the pressure.
>>
>> Matthias
>>
>
> That clarifies things now. As long as we're talking about boats and
> marinas, can anyone tell me who lines up all the boats so that they all
> point the same way?


The current?

Was that a trick question?



20 Sep 2007 23:03:54
Lee Bell
Re: Interesting pressure question

Douglas W. "Popeye" Frederick wrote

>> That clarifies things now. As long as we're talking about boats and
>> marinas, can anyone tell me who lines up all the boats so that they all
>> point the same way?

> The current?
> Was that a trick question?

The current and/or the wind . . . or a second anchor, or a dock, or . . .

Lee




21 Sep 2007 08:41:30
JOF
Re: Interesting pressure question

On Thu, 20 Sep 2007 23:03:54 -0400, "Lee Bell"
<pleebell@bellsouth.net > wrote:

>Douglas W. "Popeye" Frederick wrote
>
> >> That clarifies things now. As long as we're talking about boats and
>>> marinas, can anyone tell me who lines up all the boats so that they all
>>> point the same way?
>
>> The current?
>> Was that a trick question?
>
>The current and/or the wind . . . or a second anchor, or a dock, or . . .

The drivers?

JF



21 Sep 2007 13:38:40
Chris Guynn
Re: Interesting pressure question


"Conshelf" <Conshelf@127.0.0.1 > wrote in message
news:Xns99B1B03A9123Econshelf@194.177.96.26...
> In rec.scuba, on Thu 20 Sep 2007 05:06:08p, "David In NH"
> <dgintz@gmail.com> wrote:
>
> > True that you'd need to factor all those in to make an absolute
> > measurement. However, the difference between salt and fresh water is
> > significant. All the other things mentioned are minimal.
>
> We're attempting to reach a Zen level of pedantry. I think we're nearly
> there.

If you want a Zen level, follow Depak Chopra's teaching.

You are as deep as you really want to be deep down inside.




21 Sep 2007 10:13:28
Scott
Re: Interesting pressure question


"Chris Guynn" <chris.guynn@gamil.com > wrote in message
news:APPIi.771$6p6.34@newssvr25.news.prodigy.net...
>
> "Conshelf" <Conshelf@127.0.0.1> wrote in message
> news:Xns99B1B03A9123Econshelf@194.177.96.26...
> > In rec.scuba, on Thu 20 Sep 2007 05:06:08p, "David In NH"
> > <dgintz@gmail.com> wrote:
> >
> > > True that you'd need to factor all those in to make an absolute
> > > measurement. However, the difference between salt and fresh water is
> > > significant. All the other things mentioned are minimal.
> >
> > We're attempting to reach a Zen level of pedantry. I think we're nearly
> > there.
>
> If you want a Zen level, follow Depak Chopra's teaching.
>
> You are as deep as you really want to be deep down inside.

*****




21 Sep 2007 17:30:59
David In NH
Re: Interesting pressure question


"Lee Bell" <pleebell@bellsouth.net > wrote in message
news:kwGIi.49875$wN3.43004@bignews2.bellsouth.net...
> Douglas W. "Popeye" Frederick wrote
>
> >> That clarifies things now. As long as we're talking about boats and
>>> marinas, can anyone tell me who lines up all the boats so that they all
>>> point the same way?
>
>> The current?
>> Was that a trick question?
>
> The current and/or the wind . . . or a second anchor, or a dock, or . . .
>
> Lee
>

Just wondering what kind of answers I'd get from this group. I'm a little
disappointed that nobody had a really good one. Oh well, enough on this
topic. On to some more real stuff????




22 Sep 2007 00:17:28
Matthias Voss
Re: Interesting pressure question

David In NH wrote:

> "Lee Bell" <pleebell@bellsouth.net> wrote in message
> news:kwGIi.49875$wN3.43004@bignews2.bellsouth.net...
>
>>Douglas W. "Popeye" Frederick wrote
>>
>>
>>>>That clarifies things now. As long as we're talking about boats and
>>>>marinas, can anyone tell me who lines up all the boats so that they all
>>>>point the same way?
>>
>>> The current?
>>> Was that a trick question?
>>
>>The current and/or the wind . . . or a second anchor, or a dock, or . . .
>>
>>Lee
>>
>
>
> Just wondering what kind of answers I'd get from this group. I'm a little
> disappointed that nobody had a really good one. Oh well, enough on this
> topic. On to some more real stuff????
>

Current..Wind... gross forces... It is much more subtle.
A moonshadow... Remembrances of St. Elmo's fire at the mast
top. Ships. Boats. Let them keep their secrets.

Matthias



--
VK exquis. mod. Gesch.haus in ruh. Wohnl.3 WE, ehem. Praxis,
ca. 350/1000 qm WF/Grdst. 19km nördl. Braunschweig , hochw.
einger; Nah VW, Hafen; frei, 0160-4433698 o. 0911-7809081.
Gut für Arzt/RA/Ing.büro, oder ruhiges Wohnen, Bilder vorh.



20 Sep 2007 07:47:15
Dennis (Icarus)
Re: Interesting pressure question

"Sheldon" <sheldon@XXXXXXXXsopris.net > wrote in message
news:tI6dnYdm9ondfGzbnZ2dnUVZ_iydnZ2d@comcast.com...
> Is the pressure on you the same, at any given depth, whether you dive in a
> lake, a crater, a swimming pool or the ocean?

As others have said, it depends on the conents of the water above you.
You'd have the altitude of the water (weight of the atmosphere) and contents
(salt water is more dense than fresh)

>
> I'm thinking the larger the body of water the more water there is to exert
> pressure on you.

While I'm pretty sure that all bodies that contain mass have a gravitational
field, the field you create is miniscule compared to that of the earth.
The vast majority of the water pressure in a body of water here is directed
towards the center of the earth, not towards you.
:-)
You only have to worry about what's above you.

Dennis




21 Sep 2007 23:45:03
Douglas W. Popeye Frederick
Re: Interesting pressure question

"David In NH" <dgintz@gmail.com > wrote in message
news:gbidnYt9rOCOpWnbnZ2dnUVZ_tKinZ2d@comcast.com...
>
> "Lee Bell" <pleebell@bellsouth.net> wrote in message
> news:kwGIi.49875$wN3.43004@bignews2.bellsouth.net...
>> Douglas W. "Popeye" Frederick wrote
>>
>> >> That clarifies things now. As long as we're talking about boats and
>>>> marinas, can anyone tell me who lines up all the boats so that they all
>>>> point the same way?
>>
>>> The current?
>>> Was that a trick question?
>>
>> The current and/or the wind . . . or a second anchor, or a dock, or . . .
>>
>> Lee
>>
>
> Just wondering what kind of answers I'd get from this group. I'm a little
> disappointed that nobody had a really good one. Oh well, enough on this
> topic. On to some more real stuff????

Sorry.

Next time you ask a question, I'll drop some 4way Windowpane, and -then-
I'll answer.

Did you have a better response in mind?



21 Sep 2007 22:33:17
Sheldon
Re: Interesting pressure question


"dechucka" <dechucka@vomithotmail.com > wrote in message
news:46f21054$0$28170$5a62ac22@per-qv1-newsreader-01.iinet.net.au...
>
> "Sheldon" <sheldon@XXXXXXXXsopris.net> wrote in message
> news:tI6dnYdm9ondfGzbnZ2dnUVZ_iydnZ2d@comcast.com...
>> Is the pressure on you the same, at any given depth, whether you dive in
>> a lake, a crater, a swimming pool or the ocean?
>
>
> basically yes if you are at the same altitude. Will slightly vary due to
> the weight per M squared of the water + additives ( or maybe the mass,
> have to think about it )
>
>
>>
>> I'm thinking the larger the body of water the more water there is to
>> exert pressure on you.
>
> it's the water ( + atmosphere ) above you not the water around you that
> causes the pressure or we would all be crushed when we take a swim at the
> local ocean beach.
>
> didn't they explain this during your OW training?

Thanks for being the first person to give a simple answer to a simple
question. They obviously covered the differences between salt water and
fresh water, and covered high altitude diving. I just never though to ask a
question regarding the actual size of the body of water (I think about stuff
too much). But you make a good point. I don't "feel" much difference
between being in the ocean or being in a pool.

Thanks.

Sheldon
>
>




21 Sep 2007 22:35:49
Sheldon
Re: Interesting pressure question


"JOF" <jofrancis@gmail.com > wrote in message
news:sm45f3l0pgogrfvm13pmvjhuqvoin3n810@4ax.com...
> On Wed, 19 Sep 2007 20:57:54 -0600, "Sheldon"
> <sheldon@XXXXXXXXsopris.net> wrote:
>
>>Is the pressure on you the same, at any given depth, whether you dive in a
>>lake, a crater, a swimming pool or the ocean?
>>
>>I'm thinking the larger the body of water the more water there is to exert
>>pressure on you.
>
> The height of the water column directly above you is the most
> important factor.
>
> And think about this - what is your depth when you are diving under
> 20' waves?
>
> JF

That is an interesting thought. :-)
>




22 Sep 2007 09:13:15
Conshelf
Re: Interesting pressure question

In rec.scuba, on Fri 21 Sep 2007 11:33:17p, "Sheldon"
<sheldon@XXXXXXXXsopris.net > wrote:

> Thanks for being the first person to give a simple answer to a simple
> question. They obviously covered the differences between salt water
> and fresh water, and covered high altitude diving. I just never
> though to ask a question regarding the actual size of the body of
> water (I think about stuff too much).

Perhaps they made the mistake of thinking that any high school science
course worth a damn would have taught you about Pascal's Law. But since
they've dumbed down the courses so much these days, what with letting young
(i.e. pre-high-school) kids get certifications they can't really assume
that the kids have taken a high school level science course.

> But you make a good point. I don't "feel" much difference between
> being in the ocean or being in a pool.

That's not a good measure since you don't feel much different at 10 ft vs
150 ft. Well, other than possibly having a bit more of a nitrogen buzz
starting to develop at that point. At least you don't feel different from
the actual pressure. Assuming you know how to equalize that is. Oh, wait
a minute. You had trouble equalizing also, didn't you? Oh well... Never
mind.


22 Sep 2007 09:35:19
Sheldon
Re: Interesting pressure question


"Conshelf" <Conshelf@127.0.0.1 > wrote in message
news:Xns99B3168BF3EC5conshelf@194.177.96.26...
> In rec.scuba, on Fri 21 Sep 2007 11:33:17p, "Sheldon"
> <sheldon@XXXXXXXXsopris.net> wrote:
>
>> Thanks for being the first person to give a simple answer to a simple
>> question. They obviously covered the differences between salt water
>> and fresh water, and covered high altitude diving. I just never
>> though to ask a question regarding the actual size of the body of
>> water (I think about stuff too much).
>
> Perhaps they made the mistake of thinking that any high school science
> course worth a damn would have taught you about Pascal's Law. But since
> they've dumbed down the courses so much these days, what with letting
> young
> (i.e. pre-high-school) kids get certifications they can't really assume
> that the kids have taken a high school level science course.
>
>> But you make a good point. I don't "feel" much difference between
>> being in the ocean or being in a pool.
>
> That's not a good measure since you don't feel much different at 10 ft vs
> 150 ft. Well, other than possibly having a bit more of a nitrogen buzz
> starting to develop at that point. At least you don't feel different from
> the actual pressure. Assuming you know how to equalize that is. Oh, wait
> a minute. You had trouble equalizing also, didn't you? Oh well... Never
> mind.

Actually, my OW instructor helped me with that. Had no problems during
certification or the next time I took a dive in the pool. I've actually had
three instructors -- remember I started with a NAUI course. Each instructor
was different and taught me different things. Still, no way a novice can
learn "everything" in that short a period of time.

I will say the NAUI course was longer and more involved -- missed the last
day because he wouldn't let me wear contacts and I couldn't get my hands on
a prescription mask overnight, but the instructor said I was doing fine,
considering I was pretty much blind. The NAUI instructor got way more into
the dangers of diving (can't believe I kept going after the first chalk
talk) and also went a bit overboard on calculating multiple dives and high
altitude diving. He doesn't believe in computers.




22 Sep 2007 14:49:05
Lee Bell
Re: Interesting pressure question

Sheldon wrote

>> And think about this - what is your depth when you are diving under
>> 20' waves?

> That is an interesting thought. :-)

It's a meaningless question from a diving perspective. Depth is not the
critical factor. Pressure is. Remember, what your depth gauge shows is
nothing more than a conversion of pressure to a number based on standard
density and atmospheric pressure.

If you want the answer to the question, when you visit that warm Caribbean
dive location, hand on the anchor or other line at a fixed depth and watch
what your depth gauge says about the pressure as the depth varies due to
passing waves. I think you may be surprised.

Lee




22 Sep 2007 17:58:09
JOF
Re: Interesting pressure question

On Sat, 22 Sep 2007 14:49:05 -0400, "Lee Bell"
<pleebell@bellsouth.net > wrote:

>Sheldon wrote
>
>>> And think about this - what is your depth when you are diving under
>>> 20' waves?
>
>> That is an interesting thought. :-)
>
>It's a meaningless question from a diving perspective. Depth is not the
>critical factor. Pressure is. Remember, what your depth gauge shows is
>nothing more than a conversion of pressure to a number based on standard
>density and atmospheric pressure.
>
>If you want the answer to the question, when you visit that warm Caribbean
>dive location, hand on the anchor or other line at a fixed depth and watch
>what your depth gauge says about the pressure as the depth varies due to
>passing waves. I think you may be surprised.

Perhaps meaningless in terms of the immediately observed effect on yer
physiology but it was only posed as a thought provoker, to help get a
perspective on what happens as yer body rides up and down in the water
column. I suspect that if a member of the metoo gallery had made the
suggestion, it would have been deemed worthy of note. Don't hesitate
to tell me if I'm wrong. 8)

JF



22 Sep 2007 17:29:06
Sheldon
Re: Interesting pressure question

>>If you want the answer to the question, when you visit that warm Caribbean
>>dive location, hand on the anchor or other line at a fixed depth and watch
>>what your depth gauge says about the pressure as the depth varies due to
>>passing waves. I think you may be surprised.
>
> Perhaps meaningless in terms of the immediately observed effect on yer
> physiology but it was only posed as a thought provoker, to help get a
> perspective on what happens as yer body rides up and down in the water
> column. I suspect that if a member of the metoo gallery had made the
> suggestion, it would have been deemed worthy of note. Don't hesitate
> to tell me if I'm wrong. 8)
>
> JF
>
I heard today that some divers were down when that tsunami hit and it didn't
effect them at all, except they couldn't understand why their depth gauges
went from 50 ft to 150 ft in a few seconds. They had quite a shock when
they came up. That might make for an interesting decompression problem.

Sheldon




22 Sep 2007 17:02:04
Scott
Re: Interesting pressure question


"Sheldon" <sheldon@REMOVEsopris.net > wrote in message
news:QfmdnfOOhM_VNWjbnZ2dnUVZ_s2tnZ2d@comcast.com...
> >>If you want the answer to the question, when you visit that warm
Caribbean
> >>dive location, hand on the anchor or other line at a fixed depth and
watch
> >>what your depth gauge says about the pressure as the depth varies due to
> >>passing waves. I think you may be surprised.
> >
> > Perhaps meaningless in terms of the immediately observed effect on yer
> > physiology but it was only posed as a thought provoker, to help get a
> > perspective on what happens as yer body rides up and down in the water
> > column. I suspect that if a member of the metoo gallery had made the
> > suggestion, it would have been deemed worthy of note. Don't hesitate
> > to tell me if I'm wrong. 8)
> >
> > JF
> >
> I heard today that some divers were down when that tsunami hit and it
didn't
> effect them at all, except they couldn't understand why their depth gauges
> went from 50 ft to 150 ft in a few seconds. They had quite a shock when
> they came up. That might make for an interesting decompression problem.

Got a link for that?




22 Sep 2007 20:07:47
nitespark
Re: Interesting pressure question

Sheldon wrote:
>>>If you want the answer to the question, when you visit that warm Caribbean
>>>dive location, hand on the anchor or other line at a fixed depth and watch
>>>what your depth gauge says about the pressure as the depth varies due to
>>>passing waves. I think you may be surprised.
>>
>>Perhaps meaningless in terms of the immediately observed effect on yer
>>physiology but it was only posed as a thought provoker, to help get a
>>perspective on what happens as yer body rides up and down in the water
>>column. I suspect that if a member of the metoo gallery had made the
>>suggestion, it would have been deemed worthy of note. Don't hesitate
>>to tell me if I'm wrong. 8)
>>
>>JF
>>
>
> I heard today that some divers were down when that tsunami hit and it didn't
> effect them at all, except they couldn't understand why their depth gauges
> went from 50 ft to 150 ft in a few seconds. They had quite a shock when
> they came up. That might make for an interesting decompression problem.
>

One of our rec.scuba regulars was on a dive boat when the tsunami hit.
As I recall he said it was barely detectable on his boat. Where did the
100 ft difference come from?


22 Sep 2007 20:42:52
Dan Bracuk
Re: Interesting pressure question

"Sheldon" <sheldon@XXXXXXXXsopris.net > pounded away at his keyboard
resulting in:

: I don't "feel" much difference
:between being in the ocean or being in a pool.

You won't. In fact, I don't even feel the difference between 10 ft
and 100 ft.

Dan Bracuk
Never use a big word when a diminutive one will do.

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22 Sep 2007 20:45:36
Dan Bracuk
Re: Interesting pressure question

"Sheldon" <sheldon@XXXXXXXXsopris.net > pounded away at his keyboard
resulting in:
:
:I will say the NAUI course was longer and more involved -- missed the last
:day because he wouldn't let me wear contacts and I couldn't get my hands on
:a prescription mask overnight, but the instructor said I was doing fine,
:considering I was pretty much blind. The NAUI instructor got way more into
:the dangers of diving (can't believe I kept going after the first chalk
:talk) and also went a bit overboard on calculating multiple dives and high
:altitude diving. He doesn't believe in computers.

Calculating multiple dives with dive tables is a basic skill. Hi
altitude diving was relevent for your course because you live at
altitude and since it's conceivable that you will dive locally, it's
important information.

Dan Bracuk
Never use a big word when a diminutive one will do.

----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
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22 Sep 2007 21:09:36
Dan Bracuk
Re: Interesting pressure question

"Sheldon" <sheldon@REMOVEsopris.net > pounded away at his keyboard
resulting in:
:I heard today that some divers were down when that tsunami hit and it didn't
:effect them at all, except they couldn't understand why their depth gauges
:went from 50 ft to 150 ft in a few seconds. They had quite a shock when
:they came up. That might make for an interesting decompression problem.

And they were at 150 ft for less than a minute. It's hardly a big
deal.

Dan Bracuk
Never use a big word when a diminutive one will do.

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23 Sep 2007 18:07:46
Sheldon
Re: Interesting pressure question


"nitespark" <nitespark@cox.net > wrote in message
news:o7iJi.12648$OD5.10187@newsfe22.lga...
> Sheldon wrote:
>>>>If you want the answer to the question, when you visit that warm
>>>>Caribbean
>>>>dive location, hand on the anchor or other line at a fixed depth and
>>>>watch
>>>>what your depth gauge says about the pressure as the depth varies due to
>>>>passing waves. I think you may be surprised.
>>>
>>>Perhaps meaningless in terms of the immediately observed effect on yer
>>>physiology but it was only posed as a thought provoker, to help get a
>>>perspective on what happens as yer body rides up and down in the water
>>>column. I suspect that if a member of the metoo gallery had made the
>>>suggestion, it would have been deemed worthy of note. Don't hesitate
>>>to tell me if I'm wrong. 8)
>>>
>>>JF
>>>
>>
>> I heard today that some divers were down when that tsunami hit and it
>> didn't effect them at all, except they couldn't understand why their
>> depth gauges went from 50 ft to 150 ft in a few seconds. They had quite
>> a shock when they came up. That might make for an interesting
>> decompression problem.
>>
>
> One of our rec.scuba regulars was on a dive boat when the tsunami hit. As
> I recall he said it was barely detectable on his boat. Where did the 100
> ft difference come from?

Not sure. It's a number that just stuck in my head. I heard the story from
another diver. Might be an urban legend.




24 Sep 2007 04:38:50
Douglas W. Popeye Frederick
Re: Interesting pressure question

"JOF" <jofrancis@gmail.com > wrote in message
news:ue3bf3dgjt6nen64f3sv1nachl5h3lkkip@4ax.com...

> Perhaps meaningless in terms of the immediately observed effect on yer
> physiology but it was only posed as a thought provoker, to help get a
> perspective on what happens as yer body rides up and down in the water
> column. I suspect that if a member of the metoo gallery had made the
> suggestion, it would have been deemed worthy of note.

Wow.

My work here is done.

What were you saying last week about being thin-skinned?

> Don't hesitate to tell me if I'm wrong. 8)

Who does?

It's the prevalent recreation here.

>
> JF
>




23 Sep 2007 22:27:38
Dennis (Icarus)
Re: Interesting pressure question


"Sheldon" <sheldon@XXXXXXXXsopris.net > wrote in message
news:fK-dnX4p3Z5On2rbnZ2dnUVZ_v7inZ2d@comcast.com...
>
> "nitespark" <nitespark@cox.net> wrote in message
> news:o7iJi.12648$OD5.10187@newsfe22.lga...
> > Sheldon wrote:
> >>>>If you want the answer to the question, when you visit that warm
> >>>>Caribbean
> >>>>dive location, hand on the anchor or other line at a fixed depth and
> >>>>watch
> >>>>what your depth gauge says about the pressure as the depth varies due
to
> >>>>passing waves. I think you may be surprised.
> >>>
> >>>Perhaps meaningless in terms of the immediately observed effect on yer
> >>>physiology but it was only posed as a thought provoker, to help get a
> >>>perspective on what happens as yer body rides up and down in the water
> >>>column. I suspect that if a member of the metoo gallery had made the
> >>>suggestion, it would have been deemed worthy of note. Don't hesitate
> >>>to tell me if I'm wrong. 8)
> >>>
> >>>JF
> >>>
> >>
> >> I heard today that some divers were down when that tsunami hit and it
> >> didn't effect them at all, except they couldn't understand why their
> >> depth gauges went from 50 ft to 150 ft in a few seconds. They had
quite
> >> a shock when they came up. That might make for an interesting
> >> decompression problem.
> >>
> >
> > One of our rec.scuba regulars was on a dive boat when the tsunami hit.
As
> > I recall he said it was barely detectable on his boat. Where did the
100
> > ft difference come from?
>
> Not sure. It's a number that just stuck in my head. I heard the story
from
> another diver. Might be an urban legend.
>
>

http://observe.arc.nasa.gov/nasa/education/teach_guide/tsunami.html
http://news.nationalgeographic.com/news/2004/12/1228_041228_tsunami.html
http://www.nws.noaa.gov/om/brochures/tsunami3.htm

So, based on the above, if they were relatively close to shore when the wave
came in, it could happen. Could be they were outside the range of any
current effects?
On the open ocean it might be only 12 inches high, but when they get close
to shore they can get to 100 ft or more.

Dennis




24 Sep 2007 08:38:50
JOF
Re: Interesting pressure question

On Mon, 24 Sep 2007 04:38:50 -0400, "Douglas W. "Popeye" Frederick"
<Popeye@finalprotectivefire.com > wrote:
>
> Wow.
>
> My work here is done.

That'll be the day.

JF



02 Oct 2007 16:15:03
Dillon Pyron
Re: Interesting pressure question

Thus spake "David In NH" <dgintz@gmail.com > :

>
>"Sheldon" <sheldon@XXXXXXXXsopris.net> wrote in message
>news:tI6dnYdm9ondfGzbnZ2dnUVZ_iydnZ2d@comcast.com...
>> Is the pressure on you the same, at any given depth, whether you dive in a
>> lake, a crater, a swimming pool or the ocean?
>>
>> I'm thinking the larger the body of water the more water there is to exert
>> pressure on you.
>>
>> Sheldon
>>
>I'm gonna assume this isn't just a troll. Pressure on the body is not the
>same at any given depth but doesn't depend on the location or quantity of
>water. Rather it depends on the density of the water. The pressure on you
>under x feet of ocean water will be higher than under x feet of lake or
>swimming pool water because the density (weight) of salt water is higher
>than that of fresh water.
>

Given that our "depth" measuring devices are actually pressure
measuring devices, the answer is that, if your gauge says 60 feet,
then the pressure will be exactly the same, regardless of where you
are diving.

If you're hanging off a line, then things change.
--
dillon

Elvis is still dead