Electronics > Metrology

Vishay VHP101 out of tolerance? (or HP 3458A)

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today my VHP101 10k 0.001% resistors arrived (2 pieces). I tried to measure the resistors with a HP 3458A and the result was 0.0018% (0.0019%) below 10k. The reading was very stable. The 1-year specification for the 3458A is 10.5ppm for 10k. I had a look into the calibration sheet from last year (10 montah ago) and the meter had 7ppm difference to the calibrator. The calibration uncertainty was 0.12R (12ppm) (Isn't it to bad for a 3458A calibration?).

I'm very suprised with this result.

Whats going wrong here? Is the resistor out of spec or the meter? Or is this the sum of tolerances? In this case both have to be in the same direction with maximum tolerance.

3458A settings: 4W, 100NPLC, Offset Compensation ON, the meter was running till days, the fan is clean and I performed a AUTOCAL before the measurement

Thanks in advance

I will try another 3458A in the next days. If the resistors is out of tolerance I would ask Vishay whats going on.

Dr. Frank:
In 2010, I also ordered several VHP202Z, 9k9998, 0.001%, complete with a measurement protocol.
First problem was a stable environment, a stable 4W assembly, and reliable measurement of temperature.
You can't trust Vishay for their claimed low T.C. (VHP101 maybe better, anyhow)

Without that, you can already have several ppm deviation by temperature.

Later, when I compared my finished 10k standard against 'quarks' one, it turned out, that all resistors were 10ppm lower than measured by Vishay, so that even the +/-10ppm spec was not met.

Second problem, Vishay might have cycled the resistors to +125°C to check the butterfly T.C.
This caused about +5ppm hysteresis to both resistors, which I sent in for re-check.

Do you have a measurement protocol from Vishay?

For the 3458A, Keysight offers several, very different level of calibration.

I have lurked into several calibration certificates, and one of them also contained a detailed Test Report.
The standards used were really bad, and they could really not definitely test to the original 1 yr. limits..

So your test report should tell, which references they used, and maybe they also gave you the as found values/deviation, so you might find out, in which direction your instrument deviates.
(This is normally not given, very strange). 
Also, take into account any temperature deviation from cal temperature.. might be as high as 1..2ppm/K!

What is the vintage of your 3458a?
If it's before about 1995, you might have the old, less stable 40k reference inside. After 1995, it should also be a VHP101, so T.C. and ageing is much lower than specification.
If you dare to open your unit and have a peek on this resistor, you might tell for sure.

In the end, I agree to DiligentMinds, that everything is inside spec, especially as you did not tell anything about the direction of your 3458As deviations.

In the end, a practical and straight forward proposal, to decide what's going on:

I could measure your resistor under controlled temperature conditions, for a 2nd opinion.
I estimate my own reference resistors to have about 1ppm uncertainty.

Unfortunately, I have no protocol from Vishay.

I think even with the old resistor the 3458A should meet the specs? I'll try to find out how old the units are (both HP brand).

I don't trust my 3458A, I use it mainly for linearity, repeatability and low noise.
I don't trust Vishay neither, so I buy 0.1% only for their hermetic foils when ever possible.


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