"about" 10M or 33ft??

[blackwater]

O.K, silly question, but it bothers me none the less,

If I'm playing with various decompression models, I come to the point where I need to convert depth to pressure in ATA, or Distance of seawater. Every reference I find say's that the conversion is "About" 10 meters or 33ft.
Where can I find a more accurate answer?
How does this relate to Bar's (or Pascals). most references equate 1bar to 1 ata, but I found one that gave two close but different mmhg values.
how much does salinaty changes affect this in normal Recreational ocean water?

I'm away from my books right now, but would like to know..

Thanks
GT

 

[The Iceni]

Confusing, isn't it Blackwater.

I normally assume a metre is 39 inches

So 10 meters = 10x39/12 feet = 32 feet 6 inches

1 atmosphere at mean sea level = 1013 millibar (1.013 bar)*

Most of us assume the pressure in 10 metres of sea water is 1 bar but of course this varies with salinity so it is much less in fresh water and greater in the dead sea. For the purposes of decompression it is, of course, the actual pressure that counts, not the absolute depth.

If your depth guage reads 30 metres in fresh water you are actually deeper than this but does it matter? Your depth guage is a manometer telling you ambient pressure is 4 bar.

However take a look at

http://www.ex.ac.uk/cimt/dictunit/dictunit.htm

All will be revealed and you will find accurate conversion factors for almost all the common units of measurement.

We are pretty dumb here so in UK diving we use metres for depth, bar for cylinder pressure and litres for volume. This is simply because 10 metres (of sea water!!!) = 1 bar and a cylinder fill to 200 bar contains about 200 times the cylinder capacity. If this is 7 litres you have 1400 litres.

If your SAC is 14 l/min this is enough for 100 minutes on the surface, 50 minutes at 10 metres, 33 mins at 20 metres and so on.

You can use the following links to calculate the true density of sea water if you wish. Happy hunting!

http://visumod.freeshell.org/thermo/density.html

http://geosun1.sjsu.edu/~dreed/105/exped7/8.html

Hope this helps.


*Amendment corrected to 4 sig figures!

 

[Walter]

The way Paul started out, I thought he was going to clarify the concept.

10 m does not equal 33 ft

10 msw does equal 33 fsw

Why the difference?

m (meters) and ft (feet) are linear measures

msw (meters of sea water) and fsw (feet of sea water) are measurements of pressure.

Pressure changes with density of water, that density varies from place to place and in the same place over time. Linear distance from the surface (depth) is not important, pressure is.

By defination 10 msw = 33 fsw.

HTH.

 

[The Iceni]

You are - of course - quite right, Walter.

Not being a physicist I forgot to point out, as you did, that metres and feet are units of length, while metres of sea water and feet of sea water are units of pressure but are used only in diving circles. For instance Frank Tapson does not include feet of sea water as a unit of pressure in his tables although he does include inches of fresh water.

I am sure blackwater will have great fun working out his actual depth during his indicated 3 metre stop in fresh water at mean sea level, where the absolute pressure is 1.3 bar. ( I presume depth gauges are calibrated to a mean surface pressure of 1 bar.)

Like most of us, blackwater is playing with deco tables and alogorithms. Of real interest is the changes to decompression profiles when diving in a freshwater lake at altitude where the surface pressure is far less than one bar.

If this surface pressure is 0.8 bar, the absolute pressure at an indicated 3 metre stop is still 1.3 bar. Therefore the pressure due to the water column is 1.3 - 0.8 = 0.5 bar.

How deep is that in fresh water?

201 inches or 5.11 metres. Not 3 metres!

Now, I just wonder what changes to deco might that require?

 

[Rick Murchison]

Measure with a micrometer.
Mark it with chalk.
Cut it with an ax.
That's essentially what we try to do with decompression and decompression models. The variance in individuals and in one individual depending on diving conditions - workload, barometric pressure (what's one actual atmosphere right now, on this dive), temperature, what one had for breakfast, wave height & period, salinity, etc etc etc - plus the mere size of the average human body (what's your real depth? Measured where? In your flattest posture you're still a foot or so thick, even if you're skinny) makes any decompression model a gross approximation of what's happening deep within our tissues at best.
Bottom line... Using 10M or 33FSW as "one atmosphere" for dives in the sea is just fine, and there's nothing to be gained by shaving that measurement any closer. On any given day either figure is liable to be closer to reality than any "more accurate value".
Rick

 

[Dr Deco]

Dear blackwater :

The use of 33 feet for one atmosphere is fine.

A quote from my decompression physiology class, “Because dive tables are expressed in precise numbers for time and pressure, they give the impression of a precision in physiology that does not actually exist.”

Dr Deco :doctor:

 

[Uncle Pug]

Rick Murchison once bubbled...
Measure with a micrometer.
Mark it with chalk.
Cut it with an ax.

I like that!!! (did you just make that up Rick?)

That and Dr. Deco's post sum it up folks.

 

[Rick Murchison]

Re: "measure with a micrometer etc"

Uncle Pug once bubbled...
I like that!!! (did you just make that up Rick?)

Can't claim credit for that one.. I think its origins are probably in the Army weapons engineers; we used it in the Navy - I think I heard it first from some engineering students at Auburn in the 60's and got the impression it was already quite old.
Rick

 

[rmediver2002]

Sounds like a demolitions calculation to me...

Sappers!!

Jeff Lane

 

[pescador775]

Actually, pressure does increase linearly at diving depths. Unfortunately, this doesn't tell us much that is useful unless units are defined. For example, those who say that 33 feet of salt water equals one atmosphere are very close to exact and certainly good enough for divers' concerns. However, on the face of it saying that 10m is equal to one bar is equal to one atm is open to examination. After all it sounds like the same thing. It is not. One atmosphere of pressure is equal to 14.7 psi. One bar of pressure is equal to 14.5 psi or about 99% of one atm. Multiply this by 33 feet and the answer is 32.5 which is correct for one bar. So, one concludes that 10m is equivalent to one bar but only roughly equivalent to one atmosphere. Still good enough though.