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Comment by deanmen | Hacker News Reader

There is an actual atomic clock watch on the market that is a little bit smaller:

https://www.hodinkee.com/articles/introducing-the-bathys-ces...

These days it's not as complicated. Microchip has a line of "chip scale" atomic clock modules you could use to make one. Not sure what the pricing is.

https://www.microsemi.com/product-directory/clocks-frequency...

Only $5,100.20 from Mouser

https://www.mouser.com/ProductDetail/Microchip-Technology/09...

Is that the same one as this is using?

https://www.hodinkee.com/articles/richard-hoptroffs-atomic-w...

Indeed it is!

I wonder if it's safe to wear a radioactive source on the wrist like that. Or is it not strong enough to matter?

Cs 133 isn't radioactive. Tritium is, but at least until recently, it was still commonly used in watches for illumination. Low-level beta source, expensive but not especially hazardous.

Cesium atomic clocks are not radioactive, nor are they based on radioactive decay. The isotope used for cesium beam atomic clocks is natural and stable Cs-133. The clock is based on an ultra-precise energy transition near 9.192 GHz.

You may be thinking of the nasty radioactive isotope Cs-137, which is unnatural and often a byproduct of nuclear tests or accidents.

It's not unlike Carbon or Potassium; Carbon 12 is the safe stuff, the rarer Carbon 14 is radioactive. Potassium 39 is the safe one and rare Potassium 40 is radioactive. Which is why bananas are slightly radioactive:

https://en.wikipedia.org/wiki/Banana_equivalent_dose

You can make a cesium clock radioactive by placing a banana on top of it. See page 1, 36, 37 of:

http://leapsecond.com/ptti2020/2020-PTTI-tvb-Atomic-Timekeep...

What is the banana equivalent dose of working 8 hours a day in an old office building with granite stone floors, and granite wall paneling? I've read that all granite is slightly radioactive.

Roughly 0.18 to 3 mSv per annum [0]. (EU regulations limit it to 3 mSv/a maximum [0]).

One banana is 0.1 µSv [1].

The granite is therefore 0.18 mSv / 0.1 µSv = 1800 bananas.

This is compared to 4 mSv normal yearly background dose [1]. The granite increases background radiation by 0.18 / 4 * 100 = 4.5%. The yearly dose from natural potassium in the body is 0.39 mSv, so I wouldn't worry about the extra 0.18 mSv if I were you.

You're at much greater risk if you live in a house which used reinforced concrete with steel from the Ciudad Juárez accident - after that accident, 109 houses were demolished. [2] [3]

[0] https://www.nature.com/articles/jes200944

[1] https://xkcd.com/radiation/

[2] https://en.wikipedia.org/wiki/List_of_civilian_radiation_acc...

[3] https://news.ycombinator.com/item?id=24891947

> which is unnatural

There's nothing unnatural about Cs-137. Fission occurs naturally in U-235.

It draws quite a lot of power. From what I remember, the lipo battery required for 16hrs w/ no display would be the same size as the already chunky module itself, requiring wristband integration. Better to have a quartz oven and fire up the CSAC only once per hour as a reference, still yielding one factor of 10 in precision.

All in all it's better suited for a nice pocket watch with a 10MHz out connector. Any takers at 15k$+?

Hah. Admittedly I didn't look up the power consumption. I came across these modules a few years ago randomly and seeing this article reminded me of them.

I wasn't seriously suggesting it's a practical answer by any means =)

Note that bathyshawaii.com is likely no longer owned by the maker, and it most certainly is NSFW (oh, keep scrolling).

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