Summer STEM Challenge: Water Battles

Image credit: Neil Downie

As Ratty could have said to Mole in ‘The Wind in the Willows’, “There is nothing – absolutely nothing – half so much so worth doing as squirting ice-cold water on hot days.” You could buy a water pistol, but it’s more fun to make your own superior water weapon from a bike pump or use a drainpipe to make a giant Super Squirter.

STEM Challenge #52: Bike Pump Water Pistons, Super Squirters, and the Single Helix Pump

A mini bike pump can be good at squirting, but some work is needed. You can’t easily fill a bike pump with water. Bike pumps use an O-ring seal or conical seal which moves and lets air in when you pull back. So when you put the pump into the water and pull back to fill, it does... nothing. You can fix this by substituting the O-ring with a fatter one, or by putting hot-melt glue inside the large end of a conical seal, thus sealing the piston in both directions. Axial outlet pumps work as they are, but it’s better to saw off the end of right-angle-ended pumps.

What about a Super Squirter, again a cylinder and piston, but made from polypropylene 40/32mm plastic plumbing pipe? Wrap tape over an elastic band to make a homemade ‘O-ring’ for the seal on the 32mm piston, and glue a champagne cork in the tip. The nozzle can be a hole in a piece of plastic retained by half a 40mm compression pipe joiner.

Time for some target practice: use a piece of Chinese magic water paper – made for learning to draw Chinese writing with water. How far can you get your pumps to squirt? And can you get a better squirt by reducing – or increasing – the nozzle? (For small nozzle sizes, get a faster fill by adding a fill hole near the pump tip which you plug with your thumb when squirting.)

Now the theoretical jet speed from a nozzle, v = √(2 x pressure/density). So if you push hard and get 2bar pressure, you’ll get 20 ms-1. And 20ms-1 should give a range of v2/g, which is 40m.  But it won’t be as good as this, because nozzle efficiency is <100 per cent and there is air drag. In a vacuum, you get the longest range squirting up at 45° to horizontal. With air drag, the best angle is closer to horizontal. Look – or photograph – the jet and you’ll see the effect of drag: the water jet goes up at 30°-45°, but then comes down closer to vertical.

Here, Lottie Thomas and her dad Chris practice for a water battle:

Now even a Super Squirter only gives pulses of water. For a more continuous water jet, you can use the Single Helix Pump. This can be made from one of those hose reels that have an input pipe that is fixed while the hose reel rotates using a rotating joint. Take the hose off, and use a shorter (15m?) length of the hose. Wrap the reel centre with bubble wrap until there is just enough room for the shorter hosepipe to be plugged in and wound on. Tape the end of the hosepipe so it doesn’t come off. Now put a nozzle, maybe a biro end, on the end of what normally be the input hose, and half submerge the reel in a bath of water. Now turn the handle, and after dozen or two turns, water will come squirting out.

How does the Single Helix Pump generate pressure without pistons and cylinders and valves? The helix is like a line of connected U-tube manometers. Each turn of the helix is half-filled with water and when you turn it, more water added to the open end of the pipe forces the first ‘manometer’ up by say 10cm (10 millibar), and the same happens all down the line. At the end, the water is forced out under the pressure of all the ‘manometers’ added together. If there are 20 turns of hose, you get 20x 10 millibar, 0.2bar. If there were 40 turns of 70cm diameter, you could get 1bar (water main!) pressure.

If you liked this, you will find lots more fun science stuff in Neil Downie’s books, like ‘The Ultimate Book of Saturday Science’ from Princeton University, and for lots of other things (and a free copy of the ‘Exploding Disk Cannons’ book), visit www.saturdayscience.org. In line with this experiment, Neil’s current work includes developing a new ventilator system to support people with breathing difficulties – get more information on this great project here: Exovent.org

There is a back catalogue of STEM-related challenges from the past year to choose from if you are looking for more options. The IET also has a host of resources that adults can use to engage children with the world of STEM.

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