Si[hon clocks experiment by Neil Downie

Summer STEM Challenge: Digital water clocks

Image credit: Neil Downie

In this week’s experiment, Neil Downie talks us through how to make a digital water clock using a siphon as the oscillator. A ‘Symphony of Siphons’ if you will!

The clock was invented in the Stone Age when a troglodyte stuck a stick into the ground outside the cave and noticed the shadow. But when the sun doesn’t shine… you need a water clock. So, water clocks were the next invention in timepieces. In ancient Greece, water clocks called ‘clepsydra’ were used, famously to stop lawyers from arguing for too long.

Water clocks and sundials are analogue clocks. But 700 years ago, the human race got around to digital clocks, clocks based around an oscillator, a pendulum, whose oscillations were counted up on gearwheels, and hands pointing at a circle of numbers. Curiously, these used the 5000BCE sexagesimal system with 60 seconds per minute, 60 minutes per hour, that we still use.

But you don’t have to use pendulums – or quartz crystal oscillators. You can make a digital water clock using a siphon as the oscillator, and further siphons for the counter: a veritable Symphony of Siphons!

Equipment needed for siphon clocks experiment

Image credit: Neil Downie

A siphon is an upside down ‘U’-tube sitting with one leg in a tank of water, and one (longer) leg outside. It will sit in a tank doing nothing until the level in the tank goes up to the top of the siphon. Then, after a bit of dribbling, Whoooosh! Out goes the water in a rush until it gets to the level of the leg inside the tank, when, with a bit of gurgling, it stops.

Siphons are used in toilets of course. The excellent – leak-free – siphon design of Mr Crapper (his real name!) has been used since early Victorian times. The handle or pull-chain pushing water into the top of the siphon ‘U’ tube operates the toilet siphon. Now, if you supply water continuously to a tank with a siphon, the water level will go up slowly and then down quickly, repeating automatically. The siphon oscillator is the first part of the digital siphon clock.

Bottle being filled up with water as part of siphon clock experiment

Image credit: Neil Downie

But the magic of siphons doesn’t stop there. You can also make siphons to count the flushes from an oscillating siphon. A larger siphon receiving the water from the oscillator, will go up in level in steps, and then flush down fast ready to count again. And this counter siphon can be followed by another larger siphon to receive the flush from that counter siphon, and this counter siphon can be followed… and so on.

You’ll need a big vessel like a 30-litre plasterer’s bucket, two or smaller ones, like a 5-litre plastic container and soda bottles, 15mm copper tube, and a garden hose with water supply. And you’ll need a gate valve for finely adjusting the water flow.

Bucket for siphon clocks experiment

Image credit: Neil Downie

The timer siphon and the counter siphons are based on upside-down ‘U’-tubes in different-sized vessels. Setting the flow rate for the timer siphon is key: not too low, not too high. It must be lower than the siphon outflow. And it must be high enough that a siphoning action happens; at low incoming flow rates, the water may simply dribble out, never emptying the vessel. For a smaller oscillator siphon, a small tube, 5mm or so in bore can be used, but for faster flows, as seen in the larger siphon counting part of the Siphon Symphony, you will need a larger tube, e.g. 15mm.

Some practical tips: you can join a garden hose to a 15mm copper pipe by softening the hose end in boiling water. A U-tube can be made in copper using elbows, or by bending the tube with a bending spring. Alternatively, buy 15mm steel tubing, pre-bent into a ‘U’ shape for use as a wall hook. Finally, the ’U’-tube can be sealed into a soft plastic vessel by pushing it into an undersized hole.

Once you have the components assembled, it’s time to test and adjust. With the hose on, let the water out in a steady dribble and check that the oscillator vessel is filling and flushing. Now measure the oscillator cycle time and adjust it.

After some earlier versions, we built the Symphony of Siphons using a heptimal counting system for time. Instead of the powers-of-60 sexagesimal time system, this uses powers of 7. For example, with 4 on the big tank and 2 on the medium one, that’s 42 heptimal, which is 4x7+2 = 30 decimal. The system counts up to 77 heptimal flushes, or 56 in decimal.

For a more accurate water clock, you can make a ‘constant head of water’ vessel or use a water pressure regulator on the hose. You can make the indication of time clearer using a float. The simplest is a float and rod with an arrow pointing to a linear scale above the vessel, or maybe a clock dial operated by string and a float.

Can you make a Symphony of Siphons which could tell the time pretty much correctly? An oscillator siphon that runs from a minute to several minutes is easily possible. With a 4.2 (decimal) minute oscillation time, you only need to have 3 7-flush siphon counters to count up to a day, which is 777 heptimal. But you’ll have to get used to the heptominute and the heptohour (29.4 decimal minutes). To do Sumerian use a 5-minute oscillator with two 12-count siphonic counters for minutes and hours. With an oscillator siphon of 100ml, however, you would still need a big final counter: 72 litres!

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 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 at

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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