How to emulate 40ft (12 m) of water? I am part of an underwater robotics team, and our next year's challenge requires us to compete at a depth of 40ft. We don't have any pools nearby to test in, so I want to emulate this 40ft of pressure somehow. I found out that 40ft is about 17psi. 
Does anyone have any suggestions? 
My first thought would be a cylindrical tank with water, and put a weight on top of the water (how much weight, I'm not sure) to "compress" the water to 17psi. 
Ideas?
Edit: Thanks for the counter questions!


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*@steveverrill, The robot will be around 18x18x18"but even being able to test parts (maybe 4x6x6") would be great. 

*@CaptainCodeman, Traveling to a body of water may be the easiest, as you suggested. We are a college team, so traveling is a possibility. Though nearby options are limited (I will research our options though).

*@CuriousOne, To answer those regarding the design of the robot: we have found products to seal motors/other electronics with to take up the air space, with some success and will definitely be looking into it farther. As noted it wasn't asked, but IS a big issue we're also facing soon. 

*@steveverrill, thank you for the precautions on pressure safety. We will make sure to research and take precations.
 A: Fill the cavities of your vehicle with an environmentally friendly liquid like glycerin. This will take care of a possible water leak, at least to the extent necessary to survive for a short amount of time in a low pressure environment. Before you do that you have to ask the event organizer if that is an allowed design strategy (what "big oil" can do is not necessarily within your repertoire of engineering tricks). If you can't I would think of other ways to insulate all potentially sensitive electrical circuits from water, which shouldn't be a problem with exception of motors. For those you will have to find watertight housing solutions, albeit there are RC boat/submersible solutions out there which will do the trick for you without having to reinvent the wheel. In general I would not advise you to try any large scale pressurized tank solution, without professionally engineered hardware they are simply dangerous.
A: As several people have advocated using pressure vessels, I'll add my own take on this.
Pressure vessel safety
Compressed air in an uncertified vessel can be extremely dangerous. You need a pressure of about 1.2atm gauge to simulate 40ft of water. That's about 2.2atm absolute. I'm not going to perform an integration here, but air at these pressures is compressed to about half its original volume and contains about 120J of energy per litre, enough to accelerate a 100g projectile to 50m/s (180km/h,110mph.) Water on the other hand hardly compresses at all when pressurized to these pressures, and therefore hardly expands at all when it depressurizes, so the energy release in case of a rupture will be safe. 
It is normal practice before commissioning a pressure vessel to fill it completely with water, then pressurize it, in order to test its integrity (a "hydrotest"). On very rare occasions where this is absolutely impractical, a very thorough inspection of the welds (radiography, dye penetrant, ultrasound, magnetic permeability) and joints may be done before testing with air. Neverthless, professionals prefer to test vessels with water in order to avoid the risk of a pressure explosion. In my opinion, amateurs should take the same precautions. 
For anyone who thinks this is not a big issue, this is what happens when a small beer bottle explodes. https://www.youtube.com/watch?v=JqXh86ks_As
Note that engineers do sometimes get things wrong. Here's an example where it was impractical to perform a hydrotest. They thought they had a safety factor of 2.5 on pressure, but there was a weak point that they overlooked. https://en.wikipedia.org/wiki/BOAC_Flight_781 . Note that in the accident investigation they did perform hydrotests on an aircraft fuselage, until destruction. To perform the same tests with air would have been extremely dangerous. 

Pressure vessels
If your robot (or its components) are small enough, you can use a pressure cooker. It is quite likely that you can get 17-18psi from the mains water in your area. If not you can use a central heating pump to boost it. If you are sure the pressure cooker is rated for the required pressure you can ignore the next paragraph. 
Your pressure cooker likely has only one valve connection, which is not centred. This will make it difficult for you to expel the air once it is sealed. So immerse the pressure cooker in a bath, put the robot inside and put the lid on. This will ensure no trapped air. Pressurize with water from the mains or a pump. After use, do not use the pressure cooker again as a pressure cooker unless you are sure you have not exceeded its rated pressure.
If you manage to acquire a larger vessel, it's even more important to ensure all air is expelled. Try to arrange for the filling point to be at the top. If you have a hose of smaller diameter than the filling point, air can pass out around it until the vessel is full and water starts to pour out. Again mains water pressure or a small pump will be required.
The sidewalls (say 2mm thick) of an oil drum (say 400mm dia) would experience a stress of 17psi x (400mm/(2x2mm)) = 17000psi and should in theory hold (but I'm NOT saying they would be in code.) The top and bottom would however become severely domed. I would recommend something thicker, like Sch40 pipe, with some decent ends.
You can monitor your pressure with a tyre pressure gauge, or control it with a 40ft high overflow pipe (if your building is high enough this is easier because you can leave the flow of water on and not worry about exceeding pressure.)
EDIT
SkipBerne's suggestion of a fibreglass pool filter seems an excellent idea, much larger than a pressure cooker. http://www.pentairpool.com/pdfs/FNSPlusOM.pdf Seems this model will take 50psi. Note text in the manual "AIR ENTERING YOUR FILTER IS DANGEROUS" (capitalization theirs.) 

Other options
You can simulate a pressure differential of 1 atm (14.7psi) by fitting a valve to your robot and drawing the air out of it. That's nearly 17psi. Given the possibility that the vacuum pump will have to handle water if something goes wrong, I would not recommend using your physics lab's best vacuum pump. Go to the chemistry lab and get a cheap eductor type vacuum pump. These use a jet of water and the venturi effect to produce a vacuum for removal of air from dessicators. If you dye the water surrounding your robot, you will be able to detect any breach by seeing the colour in the hose to the vacuum pump.
You may find finally that the cheapest option is to find a reasonably deep lake (possibly a reservoir with a dam, they tend to be deep) and try it out for real there. They may not like you hanging your robot off a rope at the dam, but many reservoirs are used for boating so you could hire a boat and do it in the middle somewhere. Obviously don't offend the relevant authorities. Failure to recover your robot, besides being a setback for you, would be unauthorised waste disposal. Be careful with toxic substances such as batteries. 
A: I think John Rennie's is a bit misleading. You don't actually need 12 tons of weight. You can get the same 18 psi from a 40 feet garden hose hung vertically. Attach it to the lid of a pressure cooker, and you'll have a "pool" that's 40 feet deep. The pool doesn't need to have the same diameter throughout.
The two challenges are (1) attaching the garden hose such that it does not leak - may require drilling a hole in the lid and fitting a proper connector and (2) scaling this up. You can't just replace the pressure cooker by an oil drum, it would burst. You can dig in the oil drum to support it sideways, but you'd still need to weight down the top. That gets us back to those 12 tons.
A: It's easy to work out what weight you need, because it's the weight of 40 feet of water. The pressure at a depth of 40 feet is simply due to the weight of the 40 feet of water above.
Let's work in SI units, so 40 feet is 12.2 metres. Suppose the top of your tank has an area of one square metre, then the amount of water above it would be 12.2 cubic metres and this weighs 12.2 tonnes. That's a lot of weight!
The pressure is just 12.2 tonnes per square metre, which is about 1.2 atmospheres or about 18 psi. I'd be inclined to use a sealed tank and use a compressor to pressurise the headspace.
A footnote:
Several comments have warned about the dangers of pressurising a tank to 18 psi. I know nothing about working with pressurised equpment, never having had to do it, so you should not take my answer as licence to start pressurising old oil drums or whatever stuff you have lying around. Proceed with extreme caution and don't sue me if you blow yourself up!
A: Find a rigid 40ft-long water hose, attach it to your tank vertically and fill it with water.
A: Fit a valve onto the body of your submarine, and use a vacuum pump to pump out all the air.  1 atmosphere is equivalent to 10.3 meters (33.8ft) of water.  Then you only need 6-7 feet of water to reach the pressure difference you want to test.
A: A 2 piece fiberglass pool filter is plenty large and can withstand the pressure. A stainless steel belt goes around the sections after you put the RUT in there. Add a hose and use a bicycle pump if you cant achieve 40 foot hose head. It is only 18 psi. 
A: I was going to suggest building a centrifuge. I am guessing you don't need to attach cables to your robot. And yes, a centrifuge can be dangerous. 
But Paul has less complicated solutions.
A: How much space do you need, and what shape does it need to be in?
If it's a stationary bot, call an excavating company and ask them for a quote on drilling a 24 inch hole 40 feet deep. They will bring a humongous machine out to your place, punch a rather large hole in the ground, and be gone before lunchtime. Visit a home center for some 6mil plastic rolls, tape up a 40 foot long cylinder and slide it into the hole. Fill with water, lower robot and camera.
Need salt water? 50kg of road salt from the same home center will be close enough.
How much will this cost? More than a little and less than a lot.
If you need to drive R2D2 around down there you will need to go the pressurized route. Call a propane company and get a reject tank they used for commercial buildings and filling stations. The one I'm thinking of is a horizontal cylinder a meter in diameter and 5m long, you can drive that far across the bottom. Propane tanks work at far higher pressures than you need, so unless there's massive damage you'll be fine. Insert bot, goPro and lights, fill halfway with water, bolt inspection hatch closed, add air to 18psi. A mechanic's grade air compressor can do this easily. Don't overfill with water or you will have to increase the pressure to balance the reduced area.
Another way to get the pressure up is to drop in some dry ice or an open flask of liquid N2. As the gas boils off the pressure goes up. Open the relief valve at the right value. Probably not cheaper (assuming you have to pay for it), but does not require power. 
