[time-nuts] Thunderbolt stability and ambient temperature

Robert Atkinson robert8rpi at yahoo.co.uk
Wed Jun 10 14:52:22 EDT 2009


Interestingly, the packing instructions for the  Solartron 7081 81/2 digit voltmeter shows two "thermal inertia bottles" close to the instument. These provide a thermal mass that works with the insulation to reduce the rate of temperature change at the unit. This will also filter short duration transients. So a cardboard box with a couple of bottles of water in it would make a good enclosure. Or just leave the beer in the cooler :-)

Robert G8RPI.
--- On Tue, 9/6/09, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:

> From: Poul-Henning Kamp <phk at phk.freebsd.dk>
> Subject: Re: [time-nuts] Thunderbolt stability and ambient temperature
> To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
> Date: Tuesday, 9 June, 2009, 9:03 PM
> In message <BLU125-W9E23FDF3E1618C93E603ECE440 at phx.gbl>,
> Mark Sims writes:
> 
> >I think that using a well insulated box [...]
> 
> The point for timenuts is not just the thermal resistance,
> but more
> importantly the thermal *impedance*: you want to low-pass
> filter
> the thermal changes so that they all happen in the area
> where the
> PLL can cope with them.
> 
> Thermal resistance is about insulation, thermal impedance
> is
> about (thermal) mass.
> 
> So you significant mass and volume (like a fridge) not
> light and
> small (like a cardboard box).
> 
> When metrology people really want to keep things at the
> same
> temperature, they mount them in oil-baths (for good thermal
> contact)
> in the middle of a block of aluminium, typically 2'x2'x1'
> (for
> thermal impedance) which is again insulated with 1"
> styrofoam, all
> of this mounted in a plywoodbox, set on rubber-wheels to
> get it off
> the floor (for thermal resistance).
> 
> If they are really into this, they cover the plywood with
> high-quality
> (noble-) metal foil, to maximize reflectivity and minimize
> emissivity,
> so that the black-body radiation from devices and humans in
> the lab
> does not affect the temperature interface as much.
> 
> Then they leave it alone for "some weeks" in their
> temperature
> controlled lab so the temperature can stabilize.
> 
> At this point they may start to wonder how they can verify
> the pt100
> temperature sensor they put in the middle of it all
> actually works
> when the temperature never changes...
> 
> Poul-Henning
> 
> -- 
> Poul-Henning Kamp       | UNIX
> since Zilog Zeus 3.20
> phk at FreeBSD.ORG 
>        | TCP/IP since RFC 956
> FreeBSD committer       | BSD
> since 4.3-tahoe    
> Never attribute to malice what can adequately be explained
> by incompetence.
> 
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