Dual-mode auto-calibrating resistance thermometer: A novel approach with Johnson noise thermometry.

A dual-mode auto-calibrating resistance thermometer (DART) is presented. The novel DART concept combines in one instrument the fast and accurate resistance thermometry with the primary method of Johnson noise thermometry. Unlike previous approaches, the new thermometer measures the spectral density of the thermal noise in the sensing resistor directly in a sequential measurement procedure without using correlation techniques. A sophisticated data analysis corrects the thermometer output for both the parasitic effects of the sensor wiring and the amplifier current noise. The instrument features a highly linear low-noise DC coupled amplifier with negative feedback as well as an accurate voltage reference and reference resistor to improve the gain stability over time and ambient temperature. Therefore, the system needs only infrequent calibrations with electrical quantum standards and can be operated over long intervals and a wide temperature range without recalibration. A first prototype is designed for the industrially relevant temperature range of the IEC 60751 (-200 °C to +850 °C); a later extension of the measurement range is being considered. A proof-of-principle measurement with a calibrated Pt100 sensor at room temperature yielded an uncertainty of about 100 µK/K. The final device is expected to reach uncertainties of below 10 µK/K, suitable for accurate measurements of the difference between thermodynamic temperatures and temperatures traceable to the International Temperature Scale of 1990 (ITS-90).