Developments towards an industrial Johnson noise thermometer

Abstract

In this paper we present new developments in our practical, driftfree Johnson noise thermometer as it moves from a proofofprinciple prototype towards a practical implementation to become a viable primary thermometer for industrial applications. We will discuss bandwidth optimisation to obtain the lowest possible uncertainty of temperature measurements. The concept of weighting the crosscorrelation frequency bins to improve the uncertainty will be introduced and it will be shown that this can decrease the measurement uncertainty in our system by 28%. Weighting fundamentally changes the optimum bandwidth calculation such that information from higher parts of the measured spectrum can always contribute to a reduction in total measurement noise. We will present the design approach used to provide extreme immunity to electromagnetic interference, a problem that has severely degraded previous efforts. We describe the results of tests by a thirdparty test laboratory demonstrating immunity up to field strengths of 10 Vm −1 , as required in standards for equipment operating in heavy industrial environments. We have also looked at interference at frequencies that could directly interfere with our thermometer that are not covered in the standards. The latest test results will be presented showing a fivefold improvement in accuracy on an extrapolation to absolute zero compared with the original proofofprinciple prototype with similar measurement parameters. Developments in techniques for providing traceability of the thermometer's calibration to electrical rather than temperature national standards will be presented.

Emerging technologies in the field of thermometry

“…One of the aforementioned techniques, the superposition thermometer, has been commercially developed by Metrosol 5 in collaboration with the National Physical Laboratory (NPL), UK (see figure 6) [56][57][58]. It uses pseudo random current injection for the calibration signal, which can be separated from Johnson noise in the frequency domain.…”

Towards Realising the Redefined Kelvin

“…Such an approach will be needed to make autonomous production truly a reality. Such approaches could be through practical Johnson Noise Thermometry (where practical demonstrator devices already exist) [69,70] and in the longer-term small-scale Doppler Broadening Thermometry [71,72] as well as other photonic based approaches [73].…”

Towards Digitalization of Temperature Measurements

“…To turn one of these into a practical, commercially available reality, NPL has been collaborating with Metrosol Limited to develop a practical Johnson noise thermometer [8][9][10]. The Johnson noise voltage is related to temperature by Nyquist's relation <VT 2 > = 4kTRf, where <VT 2 > is the mean squared Johnson noise voltage, k is the Boltzmann constant, R is the sensor resistance and f is the frequency bandwidth which is a function of the sensing electronics and cables.…”