Stability of some DC reference standards

Some 15 years ago, after 80 years of utilization of the Weston cell as volt age standard, commercial designs of sources with Zener diodes, the so-calle d "dc reference standards" appeared on the market. They have much more favo rable properties compared to Weston Standard Cells. The stability of one co mmercial de reference standard Fluke 732A at the 10 V output was analyzed i n relation to the calibration data obtained in the Fluke, NIST, and PTB lab oratories during a period of seven years. Two approaches to accessible data for regression line calculation were compared. In the first approach all t he data were taken into consideration, whereas in the second approach only the mean values of particular calibration expressed for the mean date were taken into account and were calculated with weights, Both approaches show a very good agreement. The voltage of the analyzed source at the 10 V output changes for about 0.1 x 10(-6)/year, which is several times less than the value given in the manufacturer's specifications.