When I was at the university I saw and worked on a thermostatic switch that used a standard mercury thermometer. The little thing that clipped on to the thermometer was the capacitor in an oscillator. The detector for opening and closing the relay was a high Q tuned circuit. I saw them regularly because I had to keep teaching lab instructors how to tune them up before each use. In a controlled environment it might be possible to optimize such a circuit for stable operation and tight control. My intuition tells me that 1 degree is about as fine a control as you could get unless you had a very narrow range thermometer with a small capillary. You might be able to convert the existing thermostat to capacitive sensing. That is apparently what Magnus is suggesting.
Regards. Max. K 4 O D S. Email: [EMAIL PROTECTED] Transistor site http://www.funwithtransistors.net Vacuum tube site: http://www.funwithtubes.net Music site: http://www.maxsmusicplace.com To subscribe to the fun with tubes group send an email to, [EMAIL PROTECTED] ----- Original Message ----- From: "Magnus Danielson" <[EMAIL PROTECTED]> To: "Discussion of precise time and frequency measurement" <time-nuts@febo.com> Sent: Thursday, August 21, 2008 12:00 PM Subject: Re: [time-nuts] Restoring GR 1120-AB Frequency Standard > Mike Monett wrote: >> [EMAIL PROTECTED] wrote: >> >> >Phil, >> >> > I didn't believe that the thermoswitch was the problem, at first, >> > chiefly because of the simplicity of operation. Eventually, after >> > checking wiring, a carbon resistor that is in series with the >> > thermoswitch, and components around the inner oven control >> > circuitry, I removed the thermoswitch to the bench. >> >> > After hooking up to a ohmmeter and using a 60 watt light bulb as >> > the heat source, I found that I could duplicate the a pulsating >> > open / close as before. I first focused on the bulb leads and >> > eventually completely removed the old leads and rebuilt each one >> > and did all new soldering under magnification. The problem remains >> > the same. >> >> > I'm ready to move on at this point noting that this component >> > failure has me stumped and that the fault is most likely internal >> > to the thermoswitch (as strange as this seems). Years ago, when I >> > first saw how internal temperature worked using the mercury >> > thermometer switch, I remarked that it was one component that >> > would never fail. HA! That statement came back to haunt me. >> >> >Best, >> >> >Russ >> >> I have been following this thread with some interest, as I expect to >> have similar equipment in the future. What is amazing is how you >> discovered the problem! >> >> Like you, I would not have believed a mercury switch could fail. But >> a quick search showed the contact can oxidize, and gave several >> patents aimed at solving the problem: >> >> 1. Reduction of oxides in a fluid-based switch - US Patent 7071432, >> 07/04/2006 >> >> Often, oxides may form within the switch and inhibit proper >> functioning of the switch. For example, the oxides may increase or >> decrease the surface tension of the liquid metal, which may increase >> or decrease the energy required for the switch to change state. >> >> Oxides can lead to poor switch performance, and even switch failure, >> because they lessen or prevent a switching fluid from wetting >> surfaces it is supposed to wet. > > Hmm... but capacitive sensing should still work well. > > Cheer, > Magnus > > _______________________________________________ > 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.