Fluke 9010085 eng a w free download

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A New Method for the Calibration of the mV Ranges of an AC Measurement Standard Speaker/Author Neil Faulkner Fluke Corporation PO Box 9090, Everett, WA 98206 Phone: (425) 446-5538 FAX: (425) 446-5649 E-mail: [email protected] Abstract Presented are different methods for the calibration of an AC Measurement Standard from 20 mV to 2 mV, 10 Hz to 1 MHz. The two methods currently used for this calibration are the comparison to an AC/DC Transfer Standard and the use of the bootstrap method given in the manufacturer's service manual. A new method has been developed using the AC/DC Transfer Standard with a resistive voltage divider in a way that gives better Test Uncertainty Ratios (TURs) then the current methods. This paper describes these methods, sources of error and ways of reducing these errors. Also shown are test results comparing the methods [1]. 1. Introduction The Fluke 5790A AC Measurement Standard [2] is calibrated in the Fluke Labs by comparison to a Fluke 792A AC/DC Transfer Standard on all ranges from 1 kV down to the 70 mV range but is not used to verify the 20 mV range and below. The reason for this is low test uncertainty ratios (TURs) and noise. Instead the bootstrap method as given in the service manual is used. Tests run during the development of the 5790A showed this method to work with adequate TURs but some labs and accrediting bodies have been hesitant to accept this method without further verification. So a new method was developed which gives excellent results that can be used instead. This method uses a resistive voltage divider in conjunction with the 792A. This paper presents the challenges encountered in developing the divider method and how they were overcome. Once developed the divider method was used to evaluate the bootstrap method. This testing is still ongoing but the results so far indicate that the bootstrap method does work as originally intended The testing has also revealed ways of improving the method to get better results if so desired. Lastly test results with the different methods are presented. 2. Direct Measurements with 792A The 5790A is verified on the 70 mV range and above by connecting the input of a 792A to the input of the 5790A under test using a type N TEE and driving both with the output of a calibrator such as the 5700A. The calibrator supplies both the AC and DC Voltage needed for the test. The most important requirement for the 792A is that its uncertainties be low enough to give a good TUR at all the test points. At the time this product was developed the uncertainties for the 792A on its 20 mV range resulted in low TURs so it could not be used to do these ranges. The better uncertainties available t