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If you generate hydrogen with electrolysis, methane from compost, or just need to compress regular air so you can use a divers bell this system will be suitable as both a storage system and a compression system.  When the zombies attack it may not be as simple as hooking up a compressor and filling a cylinder.

This project will utilize some simple physics which will enable you to  store a gas and compress it so you can use it on demand.  It may require some machining skills, unless you find a suitable storage tank that has a drain on it that you can make an air tight seal with.

This is ideal  for flammable gases since it will keep them away from oxygen.  Hydrogen, methane, propane, and many other flammable gases are only flammable when combined with oxygen.  Since the air is about 1/5 oxygen this poses a problem.  This system will separate the gas you want from the oxygen, although it can also be used to store oxygen for your post-zombie medical clinic allowing the hospital to administer pressurized oxygen to people in cardiac or respiratory distress.

gas storage tank

In the simplest terms available to me at this moment, you just need two containers, one slightly smaller than the other and hoses.  The larger container (red) only needs to be water tight.  It can be natural or artificial.  A swimming pool or a bottle.  The larger this tank, the more water it can hold and the more pressure you can create.

The smaller tank (green) should be close to the size of the larger tank.  This will help prevent it from tipping over when it is filled with gas.  As the gas bubbles in, the smaller tank will rise.  As the gas is allowed to escape the smaller tank will sink.  This lets you measure how much gas you have left with a simple glance.  When you first place the smaller tank in, the outlet valve should be open so that water can fill as much of the tank as possible.  If you need oxygen separation you should fill this with gas and then vent it in a controlled manner so that only your flammable gas is present.  It would be safer to have the smaller tank submerged before you start filling it so that there is no chance of oxygen mixing with your flammable gas.

Gas is generated and pumped into the “in” hose (black).  Since gases are lighter than water, they will bubble to the surface and be trapped in the smaller tank.  When you open the “out” hose the weight of the smaller tank will force the gas out.  The more the smaller tank weighs the more pressure can be contained or generated.  There are some limits to this pressure, namely the gas pressure is fighting against the water pressure. 

Nature is lazy.  It will always take the path of least resistance.  If compressing the gas requires more energy than pushing the water back, the water will move and the gas will escape between the two tanks.  You can visually tell if this is occuring by looking for bubbles.  If the gas is flammable you will want to discontinue generation of the gas.

If you only want to compress regular air, you can open the “out” valve, raise the smaller tank using a rope and pulley, lever, or just brute force, then close the “out” valve and let the smaller tank sink to its natural buoyancy.  If you need to ensure that the output gas is dry, you may want to place some steel wool or something else that is not reactive with the gas in sufficient density to let the gas through but trap the larger particles, which will usually be water.  This is optional but sometimes required.

If you need additional pressure, you can apply weight to the smaller can.  This can be bags of rocks, dead zombies, or survivors.  Gravity treats them all the same.  Likewise if you need less pressure all you have to do is remove some weight on the smaller tank.  If you have no weights on it, you will have to rig a rope and pulley, lever system or something else to raise it.

This system can also work as a pump, although this will be really inefficient and is only mentioned more of a science project type experiment.  The smaller tank is large you may expend a lot of energy to raise the tank which might be extremely wasteful.  If you have a one way valve on the input only allowing air in, and a one way valve on the output only allowing air out, you only have to raise and lower the smaller tank.

Home Experiment

I made a system following this model with a clear plastic jug (red in the image), a soda can (green) with a hole drilled in the bottom and a pipe fitting attached into that hole (black “out”), and a drinking straw to work as the in hose (black “in”).  By opening the pipe fitting inverting the can so the pipe  fitting is on top and dropping the can into the water filled clear plastic jug I had a system ready to be filled.  I then blew on the straw with one end inside the can.  This caused the soda can to float.  When I released the pipe fitting valve on the can the gas rushed out and the can sank.  Closing that valve stopped the gas flow and the can remained where it was.  This is a really simple model that can usually be made out of items you already have in your home so that you can get a good understanding of how this works.

One thing to pay attention  to is how much water goes up into the straw.  If you do not have pressure on the straw, water will naturally back up through the straw.  Normally the pressures will not be great enough to force the water above the water line in the larger tank, so if you make friends with gravity you can have him keep the water where it belongs.  If you quickly try to force the smaller tank down, you can force some water out the straw.

One Way Valves

There are a couple of ways to make a one way valve.  One method would be to make a cage at the end of the hose you want the valve on and placing a ball so that it can move freely in that cage in a vertical alignment, gravity will pull the ball down if the pressure is less in the hose than it is outside that end of the hose.  As the pressure increases inside the hose it will push the ball out of the way (against gravity) and gas can escape.  Once the pressure in the hose is less than the force of gravity the ball will fall back down and form a seal.  Remember gravity is the force, the heavier the ball the more the pressure that is required to push open this valve.  Using a spring over the ball can increase the pressure that holds the ball in place, help it to form a better seal.  You can adjust the pressure that is required by slightly compressing the spring on the side not touching the ball.  There are many factory made one way valves that work in exactly this fashion.

Another way to make a one way valve is similar to how a bicycle pump works.  You have a rubber flap, perhaps from the mud flaps on the truck outside, now unmanned since the zombies got to the driver.  Connect them on one side so they are hinged.  If the gas pressure on one side is greater than the  other side it will push against the rubber flap.  If the flap is hinged, and not pressing against a solid object it will move enough to allow the gas to flow through.  When the pressure changes the flap will be pushed into a closed position.  Most of the cylinder shaped bicycle pumps work exactly this way, although they tend to use softer rubber so that they form a good seal.