water clock

How do digital water clocks work?

Recently I started seeing Groupon offers and Amazon ads for “digital water clocks” that “run entirely off of the ions in the water.”

Being skeptical, I did a little research. I found some pictures of the insides of these devices. Turns out, the answer should’ve been obvious. I assumed that the clock would have an internal battery that was shorted into the circuit via water and that the entire apparatus was a scam. Instead, I discovered that it actually works somewhat according to how the manufacturers claim. These clocks actually do make use of the ionic impurities in water to provide power in a way. However, it is incorrect to say that they’re powered by water. Instead, they use something called “galvanic corrosion.”

Galvanic cell
Used under Creative Commons Attribution-Share Alike 3.0 Unported license. Credit: Ohiostandard

If you look at the above picture, you’ll see that there’s two metal plates separated by a divider. That looks pretty similar to the chart on the left. What happens is that two electrodes are submerged in an electrolytic solution. In this case, tap water is used since it contains ionic impurities which allow for electrical conduction. These electrodes are composed of dissimilar metals. I don’t know what each particular alarm clock contains but we can pretend that it’s Zinc and Copper so that it matches  the chart. Unfortunately, we’re just using impure water so our galvanic cell performance is going to be drastically reduced. I guess packing in a bottle of sulfuric acid detracts from the “green” image. Anyway, these metals possess different levels of electronegativity. That’s a fancy word for saying that one metal really wants more electrons than the other. As a result, electrons will flow from one electrode to the other then back through a porous membrane. In doing so, electrons flow through the clock at a high enough volume to allow a simple IC chip to display to a 7 segment display. This gradually depletes one metal while gradually adding to the mass of the other metal. As a trivia point, galvanic corrosion is a major problem in many industries which often requires the use of “sacrificial anodes.” These are hunks of metal with a high susceptibility to galvanic corrosion. Instead of a pipe or wall corroding, these sacrificial anodes corrode. That brings me to my next point.

Over time, the electrodes would need replacing which is probably not possible without destroying the clock. Oddly enough, using one of these clocks will cause the metals to leech out into the water that you must periodically pour down the sink. Zinc isn’t especially bad for you in small quantities but copper is toxic to both plants and animals. Free copper ions can also combine with other things you might flush down the sink to make some much nastier compounds (like copper (II) chloride from the chlorination of free copper ions by free chlorine ions in an aqueous solution). Regardless, the EPA sets a limit of 1.3mg of copper per liter of water for drinking water so try to avoid it. Ultimately, you’re paying a chunk of money for a clock with a built in battery where you change the electrolyte out by pouring it down the drain. It’s an interesting novelty but not what I would call “green” and definitely not “free energy.”

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