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White D.R.,New Zealand Institute for Industrial Research and Development
Metrologia | Year: 2012

This paper discusses the effects of non-linearity, some of the mechanisms responsible for non-linearity, and methods for measuring non-linearity in Johnson noise thermometry. Mechanisms considered include quantum tunnelling, bipolar junction transistor and junction field-effect transistor amplifiers, feedback, clipping, output-stage crossover, quantization and dither. It is found that even- and odd-order effects behave differently in correlator-based noise thermometers, with the dominant even-order effects contributing as intermodulation products whereas the dominant odd-order contributions are third-order and at the same frequencies as the parent signals. Possible test methods include the use of discrete tones, changes in spectral shape, and direct measurement using reference noise powers. For correlators operated at constant noise power, direct measurement of non-linearity using reference noise powers enables corrections to be made with negligible additional uncertainty and measurement time. © 2012 BIPM & IOP Publishing Ltd. Source

White D.R.,New Zealand Institute for Industrial Research and Development
International Journal of Thermophysics | Year: 2015

It is well known that a single negative-temperature-coefficient thermistor can be linearized over a narrow temperature range by connecting a single resistor in parallel with the thermistor. With the linearizing resistor properly chosen for the operating temperature, the residual errors are proportional to the cube of the temperature range and have a peak value of about 0.2∘C for a 30∘C range. A greater range of temperatures can be covered or greater linearity be achieved by cascading thermistor–resistor combinations. This paper investigates the limits of the linearity performance of such networks by using interpolation to model their behavior. A simple formula is derived for estimating the residual non-linearity as a function of the number of thermistors, the temperature range covered by the network, and the constant characterizing the exponential temperature dependence of the thermistors. Numerical simulations are used to demonstrate the validity of the formula. Guidelines are also given for circuit topologies for realizing the networks, for optimizing the design of the networks, and for calculating the sensitivities to relative errors in the component values. © 2015, Springer Science+Business Media New York. Source

Saunders P.,New Zealand Institute for Industrial Research and Development
International Journal of Thermophysics | Year: 2014

A detailed analysis of the double-wavelength radiation thermometry technique for determining the thermodynamic temperature is presented. This technique provides an alternative method to absolute filter radiometry without the requirement of traceability to the watt. The analysis derives an algebraic expression for the uncertainties in the temperatures measured with the double-wavelength technique, which shows that the optimum strategy is to employ one narrowband and one broadband spectral responsivity, and that the center wavelengths do not need to be widely separated. With current best estimates for signal and spectral responsivity measurements, it is shown that the double-wavelength method can achieve total uncertainties only about four times larger than the current best absolute radiometric methods. Improvements in the signal measurement in the future could possibly reduce the total uncertainty to a level comparable to absolute radiometry. © 2014 Springer Science+Business Media New York. Source

White D.R.,New Zealand Institute for Industrial Research and Development | Fischer J.,Physikalisch - Technische Bundesanstalt
Metrologia | Year: 2015

The 26th General Conference on Weights and Measures (CGPM) will redefine the Kelvin by fixing the value of the Boltzmann constant in 2018 if the plans of the International Committee on Weights and Measures (CIPM) are fulfilled. This will replace the current definition of the Kelvin based on the triple point of water (TPW), which has been in use since 1954. The change in definition is not expected to make an immediate or significant difference to measurement practice, except perhaps at very low and very high temperatures. This change will enable much greater improvements in practical thermometry in the long-term. Source

Saunders P.,New Zealand Institute for Industrial Research and Development
AIP Conference Proceedings | Year: 2013

The majority of general-purpose low-temperature handheld radiation thermometers are severely affected by the size-of-source effect (SSE). Calibration of these instruments is pointless unless the SSE is accounted for in the calibration process. Traditional SSE measurement techniques, however, are costly and time consuming, and because the instruments are direct-reading in temperature, traditional SSE results are not easily interpretable, particularly by the general user. This paper describes a simplified method for measuring the SSE, suitable for second-tier calibration laboratories and requiring no additional equipment, and proposes a means of reporting SSE results on a calibration certificate that should be easily understood by the non-specialist user. © 2013 AIP Publishing LLC. Source

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