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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!

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

  2. @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).

  3. @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.

  4. @steveverrill, thank you for the precautions on pressure safety. We will make sure to research and take precations.

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    $\begingroup$ How big is your robot? would it fit inside a domestic pressure cooker? otherwise you're going to have difficulty finding a vessel that can accomodate it and be sealed properly. As calculated by John Rennie, the forces on the lid are considerable. $\endgroup$
    – Level River St
    Commented Jul 1, 2015 at 10:43
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    $\begingroup$ Depending on your location, the cheapest option may be to travel to a river/lake/ocean. $\endgroup$
    – CaptainCodeman
    Commented Jul 1, 2015 at 12:55
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    $\begingroup$ I agree with @CaptainCodeman, I see a lake in your future. That's about 2.3 atms. On top of that, you have to be able to control and test the robot in a pressurized environment. Depending on the resources of your high school, that may be beyond what the school is willing to provide. $\endgroup$
    – jfa
    Commented Jul 1, 2015 at 13:02
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    $\begingroup$ You might want to look for a diving club near you. If there is one near you they will know where you can easily get to 40ft/13 meters of depth in reasonably clear water with an experienced diver near you. $\endgroup$
    – Thijser
    Commented Jul 1, 2015 at 15:40
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    $\begingroup$ This question seems to be about the application of scientific principles to solve a problem. Specifically, it asks for suggestions to use physics to find a way to create the same pressure as 40ft of water without having to use a 40ft deep tank of water. This is a textbook engineering question and I'm voting to close it as such $\endgroup$
    – Jim
    Commented Jul 2, 2015 at 15:08

9 Answers 9

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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.

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    $\begingroup$ +1 for the 40' hose for head. I was going to put that in a comment but I see you have done it. A "nice" method overlooked by many. $\endgroup$
    – Russell McMahon
    Commented Jul 1, 2015 at 14:31
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    $\begingroup$ By far the safest and easiest to implement solution. Any leaks or other issues that the OP will have with this method, he would have with any of the other methods as well. $\endgroup$
    – dotancohen
    Commented Jul 1, 2015 at 14:38
  • $\begingroup$ 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. $\endgroup$
    – SkipBerne
    Commented Jul 1, 2015 at 15:30
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    $\begingroup$ @JohnRennie You put water in your bicycle tyres? $\endgroup$
    – Bernhard
    Commented Jul 1, 2015 at 20:07
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    $\begingroup$ Wait so, if i had a 10m x 10m x 10m tank completly sealed, except a 10cm hole and attached to that hole is a 40ft hose going straight up. Now if i fill everything up with water to the top of the hose i would have the same pressure in the tank as if that hose and hole would be lets say 5m in diameter? $\endgroup$
    – Vajura
    Commented Jul 2, 2015 at 12:14
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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!

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    $\begingroup$ I guess I don't need to mention that working on pressure vessels requires special skills and shouldn't be done unless you know what you're doing. But I'll say it anyway. $\endgroup$
    – David Richerby
    Commented Jul 1, 2015 at 10:23
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    $\begingroup$ Air at 1.2 atmospheres =18 psi = 120kPa can store a considerable amount of energy, about 120J (equivalent of dropping a 12kg weight 1m) per litre of air (at a pressure of 12.2 tonnes per square metre!) You could have a significant pressure explosion. An air tank under these conditions in Europe would require certification if sold commercially. Don't use a compressor, use a water pump instead, and eliminate all air from the tank. Water doesn't compress so it doesn't store energy, and doesn't explode when released. Engineers prefer to test pressure vessels with water than air for safety reasons. $\endgroup$
    – Level River St
    Commented Jul 1, 2015 at 10:38
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    $\begingroup$ Hang on... if water doesn't compress, then what is "pressurized water"? o.O $\endgroup$
    – Lightness Races in Orbit
    Commented Jul 1, 2015 at 11:49
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    $\begingroup$ @LightnessRacesinOrbit pressurized water is water under pressure. But with the same density as water that is not under pressure. Unlike air, which when pressurized has the tendency to compress (it reduces its volume and increases its density, which causes it to store a lot of energy.) $\endgroup$
    – Level River St
    Commented Jul 1, 2015 at 11:52
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    $\begingroup$ @paul no he doesn't, he can use a sealed tank like John said, but he should use a water pump not an air compressor to pressurize it, and make sure he eliminates all air. That way he won't kill himself. My job involves testing pressure vessels, I know what I'm talking about. The feasibility of this approach depends on the size of the robot that he would want to put inside the vessel. Hence my comment on the question. Otherwise he might just as well find the nearest dam and dangle the robot off it. $\endgroup$
    – Level River St
    Commented Jul 1, 2015 at 12:05
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Find a rigid 40ft-long water hose, attach it to your tank vertically and fill it with water.

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    $\begingroup$ How would this help? $\endgroup$
    – Kyle Kanos
    Commented Jul 1, 2015 at 13:02
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    $\begingroup$ This creates 40ft of water pressure. $\endgroup$
    – B.Pascal
    Commented Jul 1, 2015 at 13:20
  • $\begingroup$ I like this one. It is a simpler way to pressurize a tank than using a pump. No need for the hose to be rigid. If it is flexible, you can control the pressure by raising and lowering the free end. $\endgroup$
    – mmesser314
    Commented Jul 1, 2015 at 13:22
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    $\begingroup$ This is a very good suggestion. I like that your user name matches the answer... $\endgroup$
    – Floris
    Commented Jul 1, 2015 at 14:01
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    $\begingroup$ By far the best answer. $\endgroup$
    – user121330
    Commented Jul 1, 2015 at 14:43
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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.

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    $\begingroup$ I don't see how this answers the question. You seem to be advising on how to build a robot that can operate at a depth of 40ft; the question is asking about how to test such a robot. $\endgroup$
    – David Richerby
    Commented Jul 1, 2015 at 10:12
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    $\begingroup$ @DavidRicherby: If they follow the instructions then they don't need to test. Having said that, I wrote pretty much what JohnRennie wrote in a comment which then got erased by DavidZ with the comment that this should have been an answer. To be honest with you, kids have no business in working with pressurized vessels. That's business for adults. Running a robot competition in four feet of water is just as good as in 40. $\endgroup$
    – CuriousOne
    Commented Jul 1, 2015 at 16:00
  • $\begingroup$ Who said anything about kids? $\endgroup$
    – David Richerby
    Commented Jul 1, 2015 at 16:13
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    $\begingroup$ @DavidRicherby: The question. It's a kid's question about pressure. Engineers know how to deal with this and so to properly prepared students. $\endgroup$
    – CuriousOne
    Commented Jul 1, 2015 at 16:19
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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.

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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.

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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.

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  • $\begingroup$ Excellent suggestion. pentairpool.com/pdfs/FNSPlusOM.pdf Seems this model will take 50psi. Note text "AIR ENTERING YOUR FILTER IS DANGEROUS" (capitalization theirs) which confirms what I've been saying. $\endgroup$
    – Level River St
    Commented Jul 1, 2015 at 22:01
  • $\begingroup$ The reason for that is simply most pumps will not be able to re-prime if there is too much air and will eat up the seals due to the loss of water for lubrication/cooling. $\endgroup$
    – SkipBerne
    Commented Jul 2, 2015 at 15:15
  • $\begingroup$ also now is the best time to find one that was replaced and would be headed for the trash. go to a pool supply store and ask $\endgroup$
    – SkipBerne
    Commented Jul 2, 2015 at 15:17
  • $\begingroup$ I quote again "AIR ENTERING YOUR FILTER IS DANGEROUS. 1. Air entering your filter can cause the lid to blow off." $\endgroup$
    – Level River St
    Commented Jul 2, 2015 at 18:01
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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.

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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.

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  • $\begingroup$ You would want to fill with water nearly full. To raise the pressure to 1.2 atm, you would need to add 1.2 times more air than already is in the tank. It would take most compressors a long time to fill half the tank. Or fill completely and go the garden hose route. $\endgroup$
    – mmesser314
    Commented Jul 1, 2015 at 13:31
  • $\begingroup$ If it is a very small robot, a cylinder of compressed gas could be repurposed. $\endgroup$
    – mmesser314
    Commented Jul 1, 2015 at 13:35
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    $\begingroup$ I'll have to say this again, as I did on John Rennie's answer: the propane tank, regardless of design & prior use, should be 100% FULL OF WATER. This doesn't affect what the OP wants to do at all. Leaving 2 cubic metres of compressed air adds up to around 240kJ of energy. Probably nothing will go wrong, but if it did, bits of metal will be flying about like bullets. Any Vessel containing compressed GAS must be tested and inspected by a professional. In most cases that means testing with 100% water first, though with Thorough inspection (radiography etc) it may be possible without. $\endgroup$
    – Level River St
    Commented Jul 1, 2015 at 13:50
  • $\begingroup$ You want to take a rejected tank and pressurize it?!? That's... not very safe. $\endgroup$
    – David Richerby
    Commented Jul 1, 2015 at 16:26
  • $\begingroup$ @DavidRicherby take a tank designed for 200psi (plus safety factor -> rupture is 1000psi) and put 1atm/ 18psi in it. Unless it has massive amounts of rust it won't be a problem. You need a better sense of perspective. $\endgroup$
    – paul
    Commented Jul 1, 2015 at 23:49

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