[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.


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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