[time-nuts] Restoring GR 1120-AB Frequency Standard
Max Robinson
max at maxsmusicplace.com
Thu Aug 21 13:37:05 EDT 2008
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: max at maxsmusicplace.com
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----- Original Message -----
From: "Magnus Danielson" <magnus at rubidium.dyndns.org>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
Sent: Thursday, August 21, 2008 12:00 PM
Subject: Re: [time-nuts] Restoring GR 1120-AB Frequency Standard
> Mike Monett wrote:
>> wa3frp at aol.com 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
>
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