[volt-nuts] Thermistor value for 732A Reference 10V Standard

[Tom Leedy]

Rob Klein <rob.klein at ...> writes:

> 
> Hi Tom,
> 
> Definitely not good! The thermistor is supposed to be an NTC, so seeing 
> the value
> rise from cold means there's something very wrong indeed.
> 
> If you want to replace it, this might be a good alterative:
> http://www.digikey.nl/product-detail/en/192-103LET-A01/480-3158-ND/1886044
> 
> Kind regards,
> Rob Klein.
> 
> Op 29-6-2012 1:04, Leedyt at ... schreef:
> >
> > Hi!
> >   
> > Can anyone give me a rough value of the thermistor resistance that is
> > brought out to the front panel of a Fluke 732A DC Reference Standard (lower
> > left-hand side)?  The thermistor is for the user to monitor the temperature  
of
> > the oven that contains the voltage reference.  I see a resistance that
> > starts out at 6600 Ohms (unit was nearly cold) and climbs and levels out at
> > about 8400 Ohms after a day, or so.  Is this reasonable?  The  thermistor is
> > given in the parts list as RT2, a Fenwal JA41J1 (no longer made)  and on the
> > schematic as a 10K @ 25C unit.  So this doesn't make  sense.
> >   
> > Does anyone have any experience with this?  Or is just simpler to  replace
> > the thermistor with a 100 Ohm RTD and be done with it?  Or would  this drive
> > the calibration labs nuts?
> >   
> > Any references or advice would be appreciated.
> >   
> > Tom Leedy
> > Clarksburg, MD
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> 


Many thanks to all for the advice.  I suspect that somebody "replaced" the 
original thermistor with another unit and didn't know what they were doing.  
Since the unit seems to put out 10 Vdc (+/- less than a ppm or so) at ambient 
temperature, I suspect that the internal oven is operating correctly.  But I 
might investigate further by raising and lowering its ambient a few degrees and 
checking to see that temperature of the oven remains constant.  

Has anyone actually measured the internal temperature of the oven (in degrees 
C)?  Since the operating temperature of the 732A is spec'd at 40 C, I suspect 
that it is about 45-50 C, or so.  And I also suspect that temperature stability 
of the oven is more important than absolute accuracy.

I'll let you know what I find.  Thanks to all again -- I didn't expect the 
large outpouring of knowledge!

Regards -- Tom