[time-nuts] Temp/Humidity control systems?

[Dr. David Kirkby (Kirkby Microwave Ltd)]

On 31 October 2016 at 10:37, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:

> --------
> In message <CANX10hCrGmtUuO+uX=vTuRHtiZTpzHK8Vz0kybFiq1fQ-wr6rg at mail.
> gmail.com>, "Dr. David K
> irkby (Kirkby Microwave Ltd)" writes:
>
> >> > [...] so it might be a relatively cheap way to measure humidity.
> >>
> >> 80m wave-guide is neither cheap, nor in most circumstances, practical
> :-)
> >
> >But you don't need 80 m of waveguide. 100 mm or so would be sufficient.
>
> Then it would require a quite high frequency...
>

If you try to do an attenuation measurement in the normal operating
wavelength of the guide, then I agree you would need a huge length unless
at very high frequency (100 GHz+), where water absorption becomes high  But
that's not the approach I would take.

I would look at trying to exploit the variation of cutoff frequency of
waveguide with humidity. WR90, which has an internal width of 0.900"
(that's where the 90 comes from),  has a cutoff frequency around 6.5 GHz. I
would expect that to be pretty dependent on the permittivity in the guide.
6.5 GHz is not a ridiculously high frequency to generate, and WR90 is not
unmanageably large. Making the guide bigger would reduce the frequency you
need to generate.

. .