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Metrology Meeting in Stuttgart/Germany

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I had time to run one more test to continue the 3458A guard switch saga, which was to check if the same problem also occurs if I use a different voltage reference than the Fluke 7001. So now I used the same 3458A #1 DMM and cable, but I used a Fluke 732A running off mains power (it does not have batteries yet) instead of the Fluke 7001 running of battery I used for the previous experiments. I tried both with connecting the guard lead to the F732A guard terminal, which is isolated from the 10 V HI/LO terminals. Unsurprisingly the guard switch did not make a difference there, and the difference are way down in the noise (0.05 ppm). Note that the vertical scale spans 0.2 ppm here:

Then I connected the guard to the same terminal as the low input lead to the DMM (so 10 V LO when measuring positive polarity and 10 V HI when measuring negative polarity). Here I observed a very similar effect to what I saw on the F7001, -6 to -8 ppm for this meter. So clearly the effect isn't just that the setting upsets the F7001 output stage:

Error bars again indicate standard deviation, n >= 20.


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