Original Date: 01/23/1995
Revision Date: 01/18/2007
Information : Sandia Volt Map Program
Sandia has used a Volt Map program for more than 20 years to certify Contractor Standards Lab cell banks. Specially selected transport standards are used for the maps.
The Sandia program was originally designed around the NIST program where a parent lab cell bank is shipped to the customer. Several years ago it became apparent that transporting the cell banks containing hazardous materials such as mercury, mercurous sulfate, and cadmium sulfate would become more difficult. Sandia investigated the possibility of using the 1.018 volt taps of Zener solid state voltage standards as the transport standards for the Volt Map. About the same time, Sandia implemented the Josephson Array Voltage Standard (Jo Volt) into their operation which made it simpler to accurately characterize the Zener standards that were being considered for transportation standards. The new Zener transportation plan was reviewed with John Fluke Inc. because of the company's extensive experience conducting a Volt Map program for Zeners at the 10V level. Based on the company's experience with shipping Federal Express - Overnight, Sandia chose Federal Express to eliminate one of the variables in the transportation process.
Sandia evaluated several different brands of Zeners. The Zeners underwent tests for short and long term stability, noise on the output terminals (which can make it difficult to calibrate with the Jo Volt), their sensitivity to changes in environments (i.e., temperature, humidity, and also pressure, since Sandia is at 5400 feet altitude), and how they could be shipped. The Zeners with the greatest potential were checked at Sandia using the Jo Volt standard, then shipped to NIST in Gaithersburg, MD, where they were checked with the Jo Volt standard and then returned to Sandia. Once again, they were checked and the process closed. The typical shift between Sandia and NIST was less than 0.2 ppm.
The initial evaluations consisted of measuring the 1.018 volt taps of three 732A Zeners, once each on five consecutive days with the Jo Volt for opening runs. If the results were within the expected system uncertainties, each Zener was packed with its external battery pack for shipment. Each Zener and battery pack weighed approximately 100 pounds. The external battery packs were necessary to supplement the internal batteries to allow the Zeners to stay powered for at least 48 hours. This was done to avoid a potential loss of power which had an unpredictable effect on the Zeners output voltage. Low thermal switches were also shipped to the customer for interconnecting the Zeners to their cell bank. To simplify the transportation of standards, a change was made to Fluke 732B Zeners. The Fluke 732B Zeners weigh 12 pounds, ship as well as the 732A, and their internal batteries alone are sufficient to power the Zener for more than 48 hours.
The customer runs a 3 by X (where "X" is the number of cells in the customer bank) test pattern every day for at least five days on each of the banks to be certified. The data is called into Sandia daily. Sandia checks each day's results using the opening run data. When five satisfactory runs have been completed on five separate days, the Zeners are returned to Sandia for five days of closing runs.
The average of the opening and closing runs data on the Sandia Zeners is used to help calculate the customer cell values used to certify the banks. The uncertainty assigned to the customer cells is an uncertainty for an interval, typically one year, that includes an allowance for the drift of the cell bank, from the last certification, and for the uncertainty of the transfer, which is determined from the differences in the opening and closing runs.
Sandia also calibrates customer Zeners that are sent in under battery power, typically the Zeners 1.018 and 10V taps voltage. Customer Zeners are calibrated using a Zener calibration system which was built for Sandia by NIST. The Sandia Zener calibration system is identical to the NIST Zener calibration system. The as-measured uncertainty for these Zeners is increased for drift over the assigned calibration interval and for the effect of transport upon the Zeners. Because customer Zener transport characteristics are never well known (i.e., temperature, humidity, and pressure effects), the increase in uncertainty for transport can be substantial. The smallest uncertainty assigned to a customer Zener given a one year certification interval is three ppm. Since Sandia is already shipping well-characterized transfer Zeners to their customers, they also calibrate the 10V as well as the 1.018V taps on their Zeners so the customers can use them to calibrate their Zeners and cell banks. The customer Zener uncertainties will be significantly reduced because the customers' Zeners never leave the facility. Sandia expects to offer this new service in 1996.
Sandia is developing a portable, user friendly Josephson Volt calibration system for Sandia and NASA. They are being assisted in this effort by NIST, Boulder, the developer of the 1V and 10V Josephson array chips. The Jo Volt standard is an intrinsic standard that converts frequency to voltage. The customer need only supply a 10 MHz frequency source that is known to an accuracy of 1 ppb, and a Dewar of liquid helium to have a standard that would be accurate to better than 0.1 ppm over a voltage range of 0.5V to 11V. This will improve the as-measured uncertainty by a least a factor of ten. Sandia will offer these services to their customers in three to five years.
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