Evaluation of guarded high-resistance Hamon transfer standards

An improved design for a guarded transfer standard in the resistance range 1 MΩ to 100 GΩ is described. Existing transfer standards and their limitations are reviewed along with a description of guard circuit theory. Measurements made to evaluate the effectiveness of guard networks are described and results are reported that verify the guard circuit behaviour. Guarded transfer standards have been tested with internal and external guard networks showing the benefits of guarding transfer standards for high-resistance measurements. Interchangeable guard networks are used in the improved transfer standards to ensure complete guarding during all phases of the measurement process thus reducing errors caused by leakages to ground. These improved transfer standards have been developed to support National Institute of Standards and Technology (NIST) calibration services over the range 10 MΩ to 1 TΩ and to support extension of the calibration service to 10 TΩ and 100 TΩ