The DC voltage calibration room is by far the largest of the labs I visited. Several benches, each with several calibration setups, populate the room. Sims uses those setups mostly to calibrate Zener voltage references, but she also calibrates voltage standard cells. NIST requires that standard voltage cells remain in Gaithersburg for about six weeks. Standard cells, because of their chemical properties, need the first four weeks to settle.
Measurements take about two weeks.
Thermal Voltage and Current Figure 5. Joe Kinnard calibrates AC-DC thermal converters. My next stop took me to the AC-DC thermal voltage and current lab. Labs then use the DMMs to calibrate multifunction calibrators that industry calibration labs use to calibrate test equipment. Lipe and Kinnard calibrate about 25 to 30 thermal converter sets each year.