Water calorimeter design and correction factor analysis for RF medium power measurement

A water calorimeter is designed based on water for RF medium power measurement by National Institute of Metrology (NIM), based on water calorimetric method. Calorimeter system is designed with measurement capability of 1 W-100 W in power and 10 MHz-7.5 GHz in frequency. Since power range is as wide as 1 W-100 W, severe fluctuation occurs in thermopile output results, caused by temperature change in water bath, power sensor and laboratory ambient. Hence measurement is divided into two scenarios: 1 W -10 W and 10 W – 100 W, where heat exchange of water bath is turned off/on. Correction algorithm is schemed out using reference thermopile and PT100 platinum resistance thermometer, aiming to eliminate both fast and slow variation of calorimetric results. Corrected thermopile results show much more satisfying stability, with relative standard deviation decreased from 3% to 1% approximately. Measurement stability and repeatability are less than 0.6% and 0.5% in AC condition, in terms of relative standard deviation. Measurement in RF condition agrees with monitor RF power sensor well, with relative error less than 0.4%.