This is a theory that I have been thinking about for some time. A possible physical process for crystal ageing is the desorption of water from surfaces in the oscillator circuit. Most solid surfaces, particularly those with oxygen in the molecular structure, have at least a monolayer of water over the surface. In high vacuum laboratory work you bake glassware at 300*C for 4 hours in a hard vacuum to remove this water to prevent future degassing.
When you have an oven with air interchange to the ambient, the humidity in the oven becomes very low. For example air at 20*C and 65% RH drops to 3.2% RH if heated to 80*C. (the vapour pressure of water is still 1520 Pascals) Relative humidity is a good predictor of the amount of moisture absorbed and adsorbed by a solid. The heating of the oven reduces the relative humidity, so much water would leave a surface on warm-up but the last of the water is tightly bound and to reach a thermodynamic equilibrium under these conditions is very slow. If the air at room temperature had the humidity reduced to 32.5%, then the RH in the oven would reduce to 1.6%, so the oscillator would still respond to the change in humidity. The main evidence to support this theory is that it is a plausible physical process with slow enough time constants and known hysteresis that could explain the days it takes for my HP 10811A to return to the control voltage it formerly required to be on frequency after it has been allowed to cool down for 2 days. The best experiment I can think of to prove this is to run the oscillator in a paper bag until it is stable, then trickle a flow of dry nitrogen into the bag for a day or two and watch for oscillator drift as the humidity in the oven drops to extremely low values. It is a pity that I do not have bottled gas on tap any more. cheers, Neville Michie On 16/02/2008, at 7:24 AM, Rick Karlquist wrote: > For ovenized crystals, any oven temperature change will > cause aging to temporarily increase (and may change the > direction as well). Restabilization may take an hour > to a day, depending on how much the temperature was changed. > We haven't noticed that powering up the oscillator or not > makes a big difference. (This refers to leaving the oven > on, and turning off the oscillator circuitry. This experiment > This is a theory that I have been thinking about for some time.is > easy to do on a 10811 since the supplies are separate.) > Does that answer your question? > > Rick Karlquist N6RK > > > [EMAIL PROTECTED] wrote: > >> Hi all, >> the aging rate of crystals is known to be not constant. >> Is there any known external cause affecting the rate, or it is only a >> matter of casuality? >> Thanks, >> Antonio I8IOV >> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> >> > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/ > time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
