[time-nuts] Laser Temperature System
WB6BNQ
wb6bnq at cox.net
Fri Aug 22 19:32:06 UTC 2008
WAS Re: [time-nuts] Restoring GR 1120-AB Frequency Standard
Mike,
I fail to see how you would apply the laser interferometer system to
the measurement of temperature. At least as a viable means of
temperature control to any reasonable degree.
At the Navy's Primary Standards Lab, here in San Diego, there is a very
complete laser interferometer measurement system. It is housed in its
own room that is "completely" controlled for environmental conditions,
i.e., temperature, humidity, air filtration, etc. The actual laser
platform is a huge, highly machined micro-flat granite table of
considerable weight mounted to a fine grain concrete pillar that goes
down to the bedrock, some 30 feet below, and is completely separate
from the rest of the building's foundation.
When the operator enters this laser room, it takes a little over 30
minutes for his body heat to be stabilized into the matrix of the rooms
environment before any real measurements can be made. Even though this
granite table is, like in the neighborhood of, around 2000 lb., laying
a standard U.S. Navy glass coffee cup (empty) on the very, very corner
of the granite will deform the light beam quite visually.
My reason for the above description is to point out the scale of the
installation, the sensitivities and the problematic issues one would
have in trying to control temperature sans everything else changing. I
suppose it can be reduced in size, but then doing so also reduces the
sensitivity of the system. Did I mention that the granite table is 3
feet square ? That allows for folded beams to increase the length for
the needed sensitivity.
Now equate all of the above to a small oven in an instrument. Not
likely possible.
Bill....WB6BNQ
Mike Monett wrote:
Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:
> Max
> A capacitance bridge using a transformer would be a lot
more
> stable than merely using the capacitance to vary the frequency
of
> an LC oscillator.
> The advantage of a capacitive sensing technique over a light
beam
> is that it has much simpler and potentially more stable
mechanical
> system than when a the mercury colum interrupts a light beam.
> Unless of course on has the Mercury column move a moire grating
or
> similar setup such as making the top of the mercury column
a
> reflector in an interferometer system.
> Bruce
Very clever! According to this web page, interferometry could give
a
resolution of 0.15 nm, or close to the radius of a silver atom:
[1]http://www.aerotech.com/products/engref/intexe.html
This video shows the basic principle in case you want to make
your
own:
[2]http://www.metacafe.com/watch/1381543/laser_interferometer_homema
de_for_20/
You can see from the movement of the fringes how
sensitive
interferometry can be. This should give unprecedented
temperature
control, so maybe someone has already done it. A google search
gives
lots of hits, but it is difficult to distinguish between
controlling
the termperature of something, and the need for accurate control
of
temperature to get stable fringes!
Great idea!
Regards,
Mike Monett
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References
1. http://www.aerotech.com/products/engref/intexe.html
2. http://www.metacafe.com/watch/1381543/laser_interferometer_homemade_for_20/
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