Development of a Four-Channel Johnson Noise Thermometry System

Long integration periods are necessary to reach low uncertainty when measuring temperature through Johnson noise thermometry (JNT). The main goal of the National Institute of Standards and Technology JNT program is to achieve a combined relative uncertainty of 6 × 10^-6 in the measurement of the water triple point temperature, which, in turn, allows determination of Boltzmann´s constant at the same uncertainty. To this end, a four-channel JNT system, which will reduce the measurement period twofold, is being developed with new components, including a switchboard, analog-to-digital converters (ADCs), and a programmable recharging power supply system. A significant source of systematic error that was discovered while implementing the new ADCs is described. New measurements are presented using a doubled ADC sampling rate, which show the potential for higher measurement bandwidth.