In my previous post I mention than I would like to buy an 8½ digits metrology grade DMM. The options were HP/Agilent/Keysight 3458a, Fluke 8508A, Datron 1281/1271 and Advantest 6581. Even as second hand, they cost several thousand USD. Finally I bought Advantest R6581T which is part of Advantest 2000 ATE system. It is not metrology grade DMM, but in the sub 2K USD range this is the only 8½ digits alternative.
I received the R6581T DMM today and fortunately I got yesterday also my first calibration protocol for my LTZ1000 based voltage reference from the Bulgarian Metrology Institute (it is in Bulgarian Language, but in the table is shown the value and the expanded uncertainty : 2.2 ppm). The DMM was turn on several hours ago and for now shows less than 1ppm difference from the calibrated LTZ1000 Voltage reference.
I will need time to make DMM performance tests like Linearity/TCR/Drift/Noise/Input resistance and also DIY GPIB hardware/software to get result in automatic way. For now I only made noise test and it shows around 0.1 uV RMS and 0.5uV Vp-p on 10V range with AZero On/100PLC. I read the eevblog.com forum thread about the TCR/Linearity problems about this model and I hope that performance is not so bad.
I checked that the R6581T has also DCV ratio function and RTD temperature measurement which is good feature to have.
I'm almost ready with my 10K Reference Resistor and together with the 10V Voltage reference I can use them for R6581T calibration. This will save me a lot of money for calibration services.
