The NASA Metrology and Calibration program’s Programmable Josephson Voltage Standard (PJVS) recently was routed to three other government organizations as part of an Inter-Laboratory Comparison (ILC). The PJVS is the most accurate Direct Current (DC) voltage calibration standard available. The comparison validated that NASA’s PJVS and the Josephson voltage standards at those locations are working properly and providing accurate measurements.
“The ILC helps us validate that we are achieving accurate measurements and using the standard correctly, which helps us turn out better products for the customers,” said Kenny Mathews, Metrology and Calibration program manager.
The ILC benefits NASA by ensuring measurement quality by providing a means to compare similar calibration capabilities at different organizations, resulting in an increased confidence in the accuracy of Direct Current (DC) voltage measurements performed at NASA’s calibration labs and operations floors.
NASA has one PJVS that periodically is routed to six NASA centers to calibrate their DC voltage reference standards. The PJVS ensures the absolute accuracy of these reference standards, which in turn are used to calibrate high-end Measuring and Test Equipment (MTE), such as digital voltmeters, data acquisition systems, linear voltage transducers, power supplies and load cells. NASA’s programs and projects then use this MTE to measure flight hardware, ground systems, research projects and safety operations.
“Once you get to the level of the PJVS, there is nothing more accurate, so national laboratories like the National Institute of Standards and Technology [NIST], inter-compare these systems to establish equivalence of the systems,” explained Jim Wachter, metrology engineer with APT Research. “These top-tier measurement systems are compared in a lateral sense, as opposed to the hierarchical comparisons associated with measurement traceability.”
During the ILC, NASA’s PJVS traveled to the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland; the U.S. Air Force Primary Standards Laboratory (PSL) in Heath, Ohio; and the U.S. Navy PSL in San Diego, California. These organizations use non-portable, conventional Josephson systems, which use different technology but have accuracies equivalent to the PJVS.
“In addition to the confidence the ILC provided NASA for its calibration program, it was also a great opportunity to collaborate with other government organizations within the metrology and calibration community,” said Mathews.
The results from the ILC demonstrate that the NIST, NASA, Air Force and Navy systems are in close agreement and are all producing valid reference voltages. The ILC also gave the organizations an opportunity to assess the measurement uncertainty of the systems — important because no measurement is perfect.
NASA likely will look to repeat the ILC process in three to five years, unless needed sooner, to ensure the PJVS continues to work properly.
“If we are suspicious for any reason that something isn’t working as it should, we’ll coordinate a direct comparison sooner — we wouldn’t wait,” said Wachter.
In 2010, the NASA Metrology and Calibration program office purchased the PJVS from NIST. Josephson voltage standards, like the PJVS, are intrinsic standards based on the Josephson Effect and produce reference voltages through a superconducting microchip array. The operating principles of the PJVS give it several advantages over conventional Josephson systems, including improved stability. NASA is the only organization that regularly ships this type of system between locations.
“Our PJVS is the only traveling one in the United States,” said Mathews.