Poul-Henning Kamp wrote: > In message <[email protected]>, 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
The thermal time constant (not the thermal impedance per se) is what matters when one is trying to reduce the effective amplitude of temperature fluctuations due to air conditioner cycling. Adding mass increases the thermal capacitance adding insulation increases the thermal resistance. It is possible to construct an enclosure with a long thermal time constant together with relatively low thermal resistance so that the temperature of a GPSDO or similar device within the enclosure only increases by a relatively small amount. Multiple alternating layers of thermal conductor and thermal insulator reduce thermal gradients as well as temperature fluctuations. Having an outer conductive layer reduces the temperature gradients over the insulator surface. Readily available inexpensive aluminium foil is a cheaper alternative to expensive noble metal foils. Silica aerogel is one of the most effective insulators. Balsa wood has been used as the insulator in portable temperature controlled ensclosures for standard cells. Bruce _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
