Automatic Self-Certification of a Computer-Controlled Calibration System

The theory, techniques, and apparatus that are used to perform an automatic self-certification of a computer-controlled calibration system are described. This process determines the linearity and bias deviations of the instrumentation of the system. These deviations are stored in the digital computer for subsequent use as corrections to the nominal instrument values when the system is applied to external devices. Dc measurements and stimuli, the uncertainties of which are of the order of 0.001 percent, have been achieved using this technique, exceeding the specifications normally associated with the uncorrected instrumentation. A similar reduction in uncertainty results from the self-certification process in ac voltage and resistance measurements. Using a 1-volt dc source, 1000-ohm resistor, and ac-dc thermal transfer as references, the scales of measure are automatically reestablished over a wide dynamic range in less than five minutes. Laboratory applications of this computer-controlled system have been made to classical metrology problems that include measurements of standard resistors, saturated standard cells, and resistance thermometers. Semi-automatic calibrations of test and measuring instruments have been performed to demonstrate potential productivity and accuracy.