[volt-nuts] Precision resistors

[Tony Holt]

On 11/08/2015 17:53, Richard Moore wrote:
> True of any resistor that you want to trust to better than 10ppm, including the Vishays.
Of course, but Dr Frank's experience with the oil filled Vishay foils 
has been good - from another eevblog post:

'Only the hermetically sealed, oil filled types (e.g. VHP202Z) give a 
big advantage. Their rate is typically 2ppm/6yrs., and therefore will 
add about 0.02ppm/yr only.
In picture 1 you’ll find long-term stability monitoring of 3 EA of my 5 
VHP202Z. After 2 years, they really remain within < 0.5ppm of their 
initial value, so that is obviously no fake advertisement.
(Remark: The measurement stability was improved also during that time.)'

That's pretty close to the SR104's typical drift of .2ppm/year, .5ppm 
max (1ppm first 2 years), but the TCR of his parts were much worse than 
the SR104's .1ppm typical ranging from .3ppm to 1ppm. He might well have 
got lucky but I read on a Chinese volt-nut type blog that Vishay 
originally specified < 2ppm/10 years but reduced it to 6 years - 
presumably due to complaints/experience. Personally I would be happy to 
trust that they would remain within 10ppm for many years but you would 
have to get them measured periodically to know for certain. Depending on 
how cheaply you could buy one, it might be cheaper to buy new ones for 2 
or 3 years rather than getting one calibrated and using them to 
determine the drift of the earlier parts. And you would have a 
collection of resistors to improve confidence in the secondary standard.

Does anyone know how much it would cost to get a 10k resistor measured 
to < 2ppm in the UK by a calibration company? Does anyone know how much 
it would cost to buy a 1% Vishay VH102Z/VHP101 or similar with a < 2ppm 
measurement?

Edwin Pettis quoted me $7.28 for one 10k resistor ($5.46 for 11 to 24) 
so they could be a viable way of getting accurately (approx 1ppm) 
measured resistors. The higher TCR, 3ppm/C would increase the 
uncertainties though as it would require them to be used within 1C or so 
of that when measured by Mr Pettis, but he can select for 1ppm TCR or 
less for an extra $2 which would easily be justified for this purpose. 
Obviously the uncertainties due to transport shocks/vibration (although 
Mr Pettis claims they are very rugged) and drift related to the time in 
transport would need to be considered.

>> On Aug 11, 2015, at 9:00 AM, volt-nuts-request at febo.com wrote:
>>
>> Edwin Pettis states his resistors drift is typically better than 2ppm in
>> the first year, so pretty good but you'd still need to have them
>> measured every few years. If you have to get them professionally
>> calibrated it may be cheaper to buy the Vishay parts. Edwin could
>> provide the measured values as could Vishay if you bought directly from
>> them.
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