[time-nuts] Capacitive temperature sensing

David t_list_1_only at braw.co.uk
Sat Aug 23 10:21:26 UTC 2008


Bruce,

A totally trivial correction for you :
"the work of RV Jones at the university of Glasgow in the 50's, 60's and
70's. "

Jones was at Aberdeen university post war. A curious character, his
recollections of his wartime research are fascinating and worth reading but
I fear are best read with care alongside some other histories to maintain
balance. 

Regards
David

> -----Original Message-----
 
> Hej Magnus
> 
> A capacitive sensing AC bridge can be very sensitive, one only has to 
> look at the work of RV Jones at the university of Glasgow in 
> the 50's, 
> 60's and 70's. He and his collaborators used capacitive sensors to 
> detect (among other things) the extrusion of a micrometer shaft as it 
> was clamped to realise just how sensitive it can be.
> They found it possible to detect length changes of less than 
> 1E-11m with 
> a suitably designed sensor.
> 
> Even more sensitive capacitance bridge displacement sensors 
> have since 
> been constructed. Off course the critical bridge components 
> have to be 
> maintained at a reasonably constant temperature. Since the 
> most critical 
> component the balancing capacitor in the other bridge arm can be very 
> small it can easily be located in the same controlled temperature 
> environment as the sensor itself. One of the major contributors to 
> instability will be creep in the glass capillary and bulb dimensions. 
> Even with a relatively crude guard ringed coaxial sensor electrode 
> surrounding a capillary tube is capable of submicron 
> sensitivity without 
> undue effort.
> 
> If one uses a mercury in glass thermometer with say 0.1C 
> resolution with 
> the 0.1C graduation say 0.5mm apart then 1micron change in mercury 
> column length is equivalent to a temperature change of 200uK. The 
> performance will be determined largely by mechanical 
> instabilities not 
> the bridge sensitivity.
> 
> Bruce






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