Equation for water pressure at various depths?

domlee

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Can anyone help?

I am making an underwater housing for a camera and am trying to calculate the pressure of water at various depths - say 15 metres and 30 metres etc.

I know there is a simple equation to calculate this but cannot remember/find a reference to it. Can anyone help?
 

Evadne

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If you want to be deadly accurate then you use something called the Carter Tables (used to be the Matthews Tables before David Carter revised them).
A good rule of thumb though is 1 metre = 1 decibar.
 

AliM

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At a depth of h (in metres for the present) the pressure is
9810 h + 101325 Pascals (or Newtons per sq. m)
The 101325 is about 1 atmosphere which adds onto the weight of the water above you (which is where the first term comes from). If you are making a box, then inside will be at atmospheric pressure, so you are just worried about the difference in pressure between the inside and outside (so lose the second term, and say P~10000 h).

If you want it in psi, 1 psi = 6895 Pa

Well, you did ask!
 

Talbot

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[ QUOTE ]
Your worst pressure is in the first 10 meters because the pressure doubles !

[/ QUOTE ]

I am sure you meant the worst pressure change! The worst actual pressure is the deepest you go.
 

pete

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Hi
Atmospheric presure is roughly 1 bar at sea level +
at 10mtr + 1bar
at 20mtr+2bar
at 30mtr+3bar
This does vary with fresh or sea water and weather conditions but is near enough for what you are doing but you must build in some safety margin.
Watches stating water resistant to 100mtrs are not generally good enough for scuba divers who go nowhere near that deep.something to do with the static pressure in the manufacturers test and dynamic pressure when in use.(most divers watches are pressure proof to 200 mtr)
Pete
 

allanc

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Atmospheric pressure, at surface, is held to be 14.7 pounds per square inch and is known as 1 Bar, or 1 atmosphere.
At 10m of water depth, the pressure on a submerged object is 2 Bar ie twice 14.7 psi. Pressure increases by 1 Bar/1 atmosphere every 10m of depth.
At 100 ft, (33m) the pressure is therefore 4.3 times 14.7psi. The problem with homemade housings is not the acrylic case, it's the seals and holes for the control rods. That's why they are expensive. It's generally not worth making your own and risking flooding your camera kit.
Buy Diver magazine. This month's has plenty of used housings for sale.
Hope this helps.
 

bbg

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I agree with the other posters - every 10 m of depth adds one bar (atmosphere) of pressure. I suspect this is actually somewhat academic unless you are actually designing for a specific pressure, as opposed to building the case as best you can and then testing to see if it works at various depths.

I also agree with allanc that the savings you might achieve in making your own will be eaten up very quickly with the cost of new equipment if it fails.
 

domlee

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I knew I'd get the answer here......!

FYI constructing the box is just part of an experimental programme I'm designing as an alternative education project during the simmer break. The idea is to construct and test the box and pressure test on a bench thus introducing kids to applied math. The idea then is to place inside the box an old Epsom PC800 camera set to time-lapse mode and leave it on the sea/lough bed for 24 hours - similar in idea to a lobster pot - and bait the area, thus the kids will also have to learn to navigate to and from the site. Time-lapse will enable a flash photo to be taken automatically at regular intervals. Images will then be processed via Photoshop and a report compiled using MS Word & Powerpoint (the IT bits).

All this is done on a budget of approximately zero £ but I think this beats the bells out of the ECDL in terms of capturing interest and creating motivation in kids that, shall we say, wouldn't normally give a second thought to an educational environment.

Thanks again to all for the info, just what I wanted.

Regards..
 

Heckler

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Hurrah for this man, exactly right, worked in the oil industry and the only measurements i ever used was that one for fresh water and pi r squared h to work out the contents of pipes etc
stu
 

AliM

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Well, if it's an education project, you'd better get the kids to look up the theory (very simple) and do the sum themselves. Although, obviously, you'll have had to get it right in advance.

It sounds great fun - I hope it inspires a kid or two!
 

wooslehunter

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Great idea. Would be nice to see some of the pics. Your question on pressure is already answered: 1 bar (ish) per 10m depth - easy. Here are some more thoughts.

As a diver, Im used to handling things under water & they are guarenteed to not do what you expect. It would be a good idea to mount the camara on a large perforated plate. Add something to attract the fish & weigh down with lots of weight, preferably lead. You have to make sure that the whole affair lands up the right way on the bottom and stays where it's put. I assume you intend to have a surface bouy to mark the position & retrieve the camera. Bear in mind that this will exert quite a force if there's any wind or waves. You have to be aware of what the camera will weigh under water to make sure it's heavy enough. I once attended a commercial photo shoot where the set builders made a phoney coral reef out of foam & weighed everything down with bricks - they weigh hardly anything under water! The whole lot took 4 guys to lift a floated wonderfully. To sink it would have had to weigh over 3 tonnes - I guessed.

So there's something else for your students to work out: the difference between weight and mass. If an object weighs 1kg in air but occupies 0.5l volume, it will actually weigh 0.5kg under water because it displaces 0.5l or 0.5kg of water. Hence if a 1kg object ocupies a volume of 1l, it's actually neutrally bouyant in water. Therefore use something like lead for weighing down in water rather than bricks. My underwater camera plus flash is quite a lump on land but really light under water.

Also try to get really clear water or don't go down too deep. The visibility is likely to be only a few meters & everything will be green in natural light. There's something else for the guys. Put some painted coloured strips in the picture. The red ones will start to go black as the depth increases. That's because the water filters out the red light. There's likely to be some algy in the water too & that's what makes it green rather than the blue you would expect from the clearest sea water.

Good luck with a great idea.

Dave
 

domlee

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Thanks for the advice - some great info to work on particularly the perforated plate and the red trace..

The original idea was to anchor a line with a buoy at the other then slide the weighted camera box down the line to a predetermined stop on the anchor line (say 0.5 / 1m from the bottom). The whole lot can then be lifted as one when the time has expired. The logistics of getting the camera at the correct depth/angle will add extra interest to the project. Another aspect would be to lower the camera to a depth above the bed and trawl the boat slowly as the camera automativally takes images.
 

Evadne

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You can find some video and time lapse pictures from deep water on our scientists' web site here which might be of some interest. For info on the time lapse piccies look under "instruments" and "bathysnap" on the site.
 

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