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Session Topic: 7B, Resistance Session

   

Paper Title: An Automatic Calibration System for 10 MW to 1 TW DC Standard Resistors

   

Date & Time: WEDNESDAY, August 7, 2002

 

10:45am - 12:15pm - PARALLEL SESSIONS - SESSION 7

   

Speaker: D.W.K. Lee Standards & Calibration Laboratory, Hong Kong

Email: wklee@itc.gov.hk

   

An automatic system is implemented based on a modified Wheatstone bridge. With this system, measurement uncertainty, uc , of less than 10 ppm can be achieved for calibrating 10 MO to 1 GO resistors. uc for resistance calibration above 1 GO become larger, mainly due to the large temperature coefficients of the resistors. The system comprises a control software written in Visual Basic and a modified Wheatstone bridge, whose two arms are programmable dc calibrators supplying test voltages at selectable magnitudes (1 V to 1000 V) and ratios (1:1 to 1: 1000) to the unknown and reference resistors that form the remaining two bridge arms. To minimise the leakage effect, ‘3-terminal’ configuration is used for resistance connection. A programmable electrometer, Keithley 6517, in current mode with a resolution of 0.1 pA is used to measure the current imbalance (i.e. the difference in currents that flow through the unknown and standard resistors). Parameter setting, instrument control, error handling and real-time monitoring of outcomes are performed via a user-friendly graphical user interface.

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