Electronics > Metrology
resistance standard - which Vishay resistor series to use ?
rob77:
Hi Guys,
i decided to build myself a resistance standard box a very simple one - just a single 10K resistor in a hermetically sealed box with binding posts. it's supposed to be used as a reference for a hobbyist.
the main question is which resisor series to use from Vishay ? one would say Z series with 0.005% accuracy but there are S series and VAR series which are half the price of the Z series but still has excellent 0.01% accuracy.
tempco is the same 0.05 ppm/C for Z and VAR series and a bit higher of 2ppm/C for S. the load life stability of all 3 series is the same 0.005% / 2000hrs.
so considering the price, accuracy and tempco one would choose the VAR series, but that's an "audio resistor" as per datasheet. but is the VAR series suitable for a resistance standard box (hobby grade) ? as per datasheet the VAR series is a "naked" Z foil resistor , but if i put it into a hermetically sealed box, it shouldn't be a problem.
any suggestion or idea is very welcome from you Guys.
Rob.
TiN:
I don't think that you will be able to find hermetic connectors for your hermetic box (they exist, but cost is outside of hobbyist realm), so I'd go for already hermetic VPG VHP202Z or VHP203Z resistor. They cost 20-30$/pc but since you just need one, should be ok. If you have access to, or know a friend with precision DMM, you could take your resistor box and get measurement of it's value to use as reference. Then you can just have 0.1% tolerance without paying extra. ;)
rob77:
thanks for your reply, i don't have anyone with a precision DMM, that's why aiming for the higher accuracy vishay resistors.
the price difference is kind of significant between the mentioned Z series and VAR series... Z series 10K is 37eur + VAT = 44.4Eur for a single resistor while VAR series 10K resistor is 18Eur + VAT = 21.6Eur (prices @ farnell).
regarding the hermetical seal, that won't be a big deal. there will be no pressure difference between inside and outside of the box (apart from standard atmospheric pressure deviations) and it won't be exposed to big temperature differences, therefore sealing ordinary binding posts is pretty easy (or i can even use a box inside a box to improve the seal but i think that won't be necessary).
Dr. Frank:
--- Quote from: rob77 on August 12, 2016, 10:16:04 am ---Hi Guys,
i decided to build myself a resistance standard box a very simple one - just a single 10K resistor in a hermetically sealed box with binding posts. it's supposed to be used as a reference for a hobbyist.
the main question is which resisor series to use from Vishay ? one would say Z series with 0.005% accuracy but there are S series and VAR series which are half the price of the Z series but still has excellent 0.01% accuracy.
tempco is the same 0.05 ppm/C for Z and VAR series and a bit higher of 2ppm/C for S. the load life stability of all 3 series is the same 0.005% / 2000hrs.
so considering the price, accuracy and tempco one would choose the VAR series, but that's an "audio resistor" as per datasheet. but is the VAR series suitable for a resistance standard box (hobby grade) ? as per datasheet the VAR series is a "naked" Z foil resistor , but if i put it into a hermetically sealed box, it shouldn't be a problem.
any suggestion or idea is very welcome from you Guys.
Rob.
--- End quote ---
Hello Rob,
buy VHP202Z, or better VHP101, which has a better guaranteed (!) T.C.
Please, forget about this 0.05ppm/K fairy tale, that is typical data, or wishful thinking only.
No, that's Vishay advertising only, but not real.
(We had a long discussion about that in a long, long precision resistor thread)
VHP101 has a guaranteed max. T.C. of <10ppm over 15..45°C, so essentially < 0.3ppm/K
ALL other similar types have a max. spread of their TCs of 2ppm/K.. I once bought 5 EA VHP202Z, in reality I measured between 0.3 to 1.0ppm/K.
Buy an oil filled type (VHP...) for best long term stability of < 1ppm/year.
Anyhow, these resistors are extremely stable and precise, beyond of amateur use, so they deserve a good mounting.
Buy a small Hammond alu box, and 4 Pomona 3770 jacks, to connect in 4W configuration.
Do not cut the legs of the resistors, and avoid any heating of the resistor element.
Then you might want to add a precision thermistor or PT100, to determine the resistors temperature. Craft an alu block as heat reservoir and coupling between resistor and thermometer.
That way, you can also measure its R(T) curve, for precision resistor calculation. This also let you determine its real T.C.
I recommend also to let some volt-nuts or cal lab determine the absolute value, at a defined temperature, and then you may buy a cheaper 0.1% type.
Please search for the precision resistor thread, there you will find a lot of additional information.
Frank
rob77:
thank you very much for the reply.
but it yields one question.. where to buy those ? as per Vishay datasheet the VHP100 10k 0.01% part number is Y007810K0000T9L - when i search it on google it throws only digikey at me and they have 500pieces minimal order quantity and 0 on stock - and it's the same for VHP101 part number Y407810K0000T9L and the very same for VHP202Z part number Y607110K0000T9L.
where can one buy those in single piece quantities ? i mean from distributors, not individuals on e-bay.
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