National Physical Laboratory

Johnson noise thermometry

NPL's Temperature Group is involved in a joint project with Metrosol Limited to develop a practical Johnson noise thermometer. These activities have now resulted in a working prototype which can measure temperature with an uncertainty of the order of +/- 1 °C in just a few seconds.

Johnson noise thermometer

The impact of mechanical and chemical changes in conventional sensors such as thermocouples and resistance thermometers, which causes calibration drift, can be avoided by instead using temperature sensors based on fundamental thermometry. A prime example of this is Johnson noise thermometry, which is based on measurement of the fluctuations in the voltage of a resistor arising from thermal motion of charge carriers – i.e. the 'Johnson noise'. A Johnson noise thermometer never needs calibration and is insensitive to the condition of the sensor material. It is therefore ideally suited to long-term temperature measurements in harsh environments, such as nuclear reactor coolant circuits, in-pile measurements, nuclear waste management and storage, and severe accident monitoring.

There have been a number of previous attempts to develop a Johnson noise thermometer for the nuclear industry, but none have reached commercial exploitation because of technical problems in practical implementation. The main challenge is to extract the tiny Johnson noise signal from ambient electrical noise influences, both from the internal amplification electronics, and from external electrical noise sources. Recent advances in electronics technology and digital signal processing techniques have opened up new possibilities for developing a viable, practical Johnson noise thermometer, and this is the subject of the current developments.

See this recent poster for more details Adobe Acrobat PDF file

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For further information, please contact Jonathan Pearce

Last Updated: 22 Dec 2016
Created: 18 May 2016


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