The "random noise" uncertainty that the 3456A's null function can remove, can be under 1 count even on the lowest range which is 0.1uv The "So What" is that anyone can measure resolution and differences AND accuracies between two similar voltages to 0.1 uv if you know what you're doing. As far as the stability and transfer accuracy of my 3456A, In a constant environment, it will hold day to day within a couple tenths of a uv at zero on the 0.1 volt range and a couple of PPM at full scale on the 10 volt range after taking several days to fully stabilize. This has Nothing to do with absolute accuracy or any spec, 24 hrs, 90 days or any other. It is just what this meter does for repeatability. It takes some skill to be able to use this stability and repeatable to compare the differences between two similar voltages that are close together, but certainly NO trouble doing it to under 0.1PPM when both are near 10 volts As far as "accuracy" being what volts nuts are interested in, I must disagree somewhat for the extreme volt nut. Stability and repeatable is what is important when testing and choosing voltage references. Accuracy can be Calibrated in against a known voltage reference standard, BUT ONLY if the stability is there first. And yes I can say with certainly that I can measure voltage differences and changes to 0.1uv at 10 volts, see attached graph. The ultimate limit is the noise and stability of the voltage reference, Not the resolution of the null meter. ws ********************* Charles P. Steinmetz charles_steinmetz at lavabit.com >As far as zeroing the meter with its leads shorted, using "Math >Null" (aka Tare) is the only way to zero the 3456A's small offset >that is left over after "auto zero". >When using the Null command with the 3456A's slow update rate, high >resolution, and filter off, you can measure voltages down to 0.1 uv >if you're real careful. My point was that doing this most likely does nothing to reduce systematic errors -- rather, it just adjusts readings by the particular random noise sample it takes when you activate the Null function. Certainly, you can READ a 3456A to 0.1 uV on its lowest range, but so what? There is a big difference between resolution and accuracy. You have to be very careful that those glowing red digits don't lull you into thinking you have measured something with an accuracy of 0.1 uV just because the last digit reads tenths of a uV. In fact, the ACCURACY (uncertainty) of the reading is much, much poorer than 0.1 uV -- in this case, well over +/- 2 uV. If you look at the 3456A specs, you will see that the best specified accuracy on the lowest range is +/- 22 ppm +/- 24 counts -- and that's only for 24 hours. (At 90 days it is +/- 34 ppm +/- 24 counts.) On the lowest range, 24 counts is 2.4 uV. So even without any scale errors, you can only expect useful accuracy of +/- 2.4 uV. The scale error can add another 2 or 3 uV to this, depending on the voltage being measured. So, you can read a 3456A down to 0.1 uV, but you can't say that you MEASURED a voltage down to 0.1 uV with it. We need to be very careful not to get sucked into the resolution trap. It is accuracy that volt nuts are interested in. Best regards, Charles