[time-nuts] Improving the stability of crystal oscillators

John Miles jmiles at pop.net
Sun Oct 14 21:15:10 UTC 2007


> The required depth depends on the soil diffusivity and the temperature
> stability required.
> It is instructive to install thermometers at depth intervals of a foot
> or so and record the temperature fluctuations experienced by each
> thermometer.
> This was first done around 1860 by Forbes.
> I repeated the experiment in 1966.

There was an interesting bit in the last Agilent Measurement Journal about a
product that uses an ordinary communications-grade fiber as a thermometer.
>From what I remember, they send a laser pulse down the fiber, then look at
the backscatter, correlating time-of-flight with the Raman-scattering lines
(Stokes and anti-Stokes).  One of those spectral lines is
temperature-dependent while the other isn't, so by recording the separation,
they end up with is a graph of temperature versus distance along the fiber,
gathering up to a few kilometers' worth of data with what looked like
sub-meter resolution.

No doubt this effect is old hat to physicists on the list, but I'd never
heard how it worked before.  So if you buried a fiber like this, you'd
presumably get a great picture of what happens with temperature at various
depths.  Plotting the temperature-versus-distance on a waterfall display
gives a nice diurnal picture.  The article used it to study water
temperature along the course of a stream, but you could think of plenty of
other uses for 2D remote temperature sensing.

-- john, KE5FX





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