Evaluation of guarded high-resistance Hamon transfer standards

An improved design for a guarded transfer standard in the resistance range 1 M Omega to 100 G Omega is described. Existing transfer standards and thei r limitations are reviewed along Kith a description of guard circuit theory . Measurements made to evaluate the effectiveness of guard networks are des cribed and results are reported that verify the guard circuit behavior, Gua rded transfer standards have been tested Kith internal and external guard n etworks showing the benefits of guarding transfer standards for high-resist ance measurements. Interchangeable guard networks are used in the improved transfer standards to ensure complete guarding during all phases of the mea surement process thus reducing errors caused by leakages to ground. These i mproved transfer standards ha re been developed to support National Institu te of Standards and Technology (NIST) calibration services over the range 1 0 M Omega to 1 T Omega and to support extension of the calibration service to 10 TR and 100 To.