Gordon Edwards

Waveguide effects on quasispherical microwave cavity resonators

The perturbing effect of a waveguide on the boundary of a quasispherical cavity resonator is investigated both theoretically and experimentally. Expressions for the frequency perturbation to the triply degenerate TM1mn and TE1mn modes are derived using cavity perturbation theory. The fields in and around the waveguide are calculated in the static limit using finite-element software. Experiments performed using quasispherical and cylindrical cavity resonators confirm the accuracy and generality of the approach. The impact of this study on attempts to re-determine the Boltzmann constant (kB) by an acoustic resonance technique is briefly considered.

Measurement of the Thermal Diffusivity of Solids with an Improved Accuracy

A photothermal radiometry technique is being developed at the NPL with the goal of improving the accuracy of thermal diffusivity measurements. The principle is to perform a laser-induced thermal experiment while simultaneously making accurate measurements of the experimental boundary conditions. A numerical three-dimensional heat diffusion model based on thermal transfer functions has been developed to account for the measured boundary conditions. The thermal diffusivity is determined from the experimental data by a nonlinear, least-squares fit to the model. Experiments carried out on pure metals at 900 K demonstrate good agreement between the theoretical predictions and experimental data, and uncertainties of about 1.5% for the thermal diffusivities of platinum, titanium, and germanium were obtained.

The electromagnetic fields of a triaxial ellipsoid calculated by modal superposition

The electromagnetic fields and eigen-frequencies of triaxially ellipsoidal resonators have been calculated. The method used is modal superposition, based on the decomposition of the fields inside the resonator into a series expansion, with unknown coefficients, of the eigen-states of a spherical resonator. The coefficients and eigen-frequencies have then been calculated by minimizing the rms electric field at the resonator surface using software designed within MatLab®. Accuracies of parts in 1010 have been obtained for the eigen-frequencies, and the dependence of the calculations on the ellipsoidal eccentricities and the resonator modes has been analysed. The results are compared with those of Mehl (2009 Metrologia 46 554–9), who used a second-order approximation. The method of modal superposition has the potential for solving many similar problems in resonators of arbitrary shapes.

New temperature references and sensors for the next generation of nuclear power plants

In preparation for the new challenges posed by the higher temperature environments which are likely to be encountered in the next generation of nuclear power plants, to maintain the safety and to ensure the long-term reliability of such plants, it is crucial that new temperature sensors and methods for in-situ measurement are investigated and developed. This is the general objective of the first workpackage of the joint research project, ENG08 MetroFission, funded in the framework of the European metrology research program. This paper will review the results obtained in developing and testing new temperature sensors and references during the course of the project. The possible continuation of these activities in the future is discussed.

Development of the laser absorption radiation thermometry technique to measure thermal diffusivity in addition to temperature

A comparative technique based on photothermal radiometry has been developed to measure thermal diffusivity of semi-infinite targets with arbitrary geometry. The technique exploits the principle that the frequency response of the temperature modulation induced by a periodic modulated heating source (in this case a laser spot) scales with thermal diffusivity. To demonstrate this technique, a photothermal radiometer has been developed, which detects modulated thermal radiance at a wavelength of 2 μm due to a small temperature modulation induced on the target surface by a modulated erbium fiber laser of power 1 W. Two frequency responses were measured for platinum and oxidized Inconel 600 targets (the frequency response is a scan of the amplitude of the modulated thermal radiance over laser modulation frequency). Scaling the two responses with respect to frequency gives a ratio of thermal diffusivities Dplatinum/DInconel of 4.45(33) which compares with a literature value of 4.46(50). The aim is to combine thi...

Practical acoustic thermometry with twin-tube and single-tube sensors

Accurate measurement of high temperatures in a nuclear environment presents unique challenges. All secondary techniques inevitably drift because the thermometric materials in thermocouples and resistance sensors are sensitive not just to temperature, but also their own chemical and physical composition. The solution is to use primary methods that rely on fundamental links between measurable physical properties and temperature. In the nuclear field the best known technique is the measurement of Johnson Noise in a resistor (See Paper 80 at this conference). In this paper we describe the measurement of temperature in terms of the speed of sound in a gas confined in a tube - an acoustic waveguide. Acoustic thermometry is the most accurate technique of primary thermometry ever devised with the best uncertainty of measurement below 0.001 °C. In contrast, the acoustic technique described in this work has a much larger uncertainty, approximately 1 °C. But the cost and ease of use are improved by several orders of magnitude, making implementation eminently practical. We first describe the basic construction and method of operation of thermometers using twin-tubes and single tubes. We then present results using a twin-tube design showing long-term stability (i.e. no detectable drift) at 700 °C over periods of several weeks. We then outline how the technique may be developed for different nuclear applications.

Practical acoustic thermometry with acoustic waveguides

Acoustic thermometry is the most accurate technique of primary thermometry ever devised. However, resonator based techniques are not practical and are rarely used in practice. Previously we have demonstrated the basic functionality of an acoustic thermometer operating at up to 1000 °C based on the time-of-flight measurements of short pulses in an acoustic waveguide. Here we report progress in assessing the practical and theoretical performance limitations of such a device. In this paper we report improvements in timing techniques which significantly improve the signal-to-noise ratio, a feature likely to be important in many industrial settings. We discuss the inference of the free field speed of sound from timing measurements and report results from a simple twin tube thermometer operating up to 500 °C. Finally we consider the shortcomings of current designs and prospects for future improvements including frequency domain analysis of the system.

Twin-tube practical acoustic thermometry: theory and measurements up to 1000 °C

We present details of a Practical Acoustic Thermometer (PAT), in which temperature is inferred from measurements of the speed of sound along acoustic waveguides. We describe both the theory of operation, and measurements on three devices at temperatures up to 1000 °C. Because the relationship between the speed of sound in a simple gas and absolute temperature is well understood, the mean temperature along a tube may be estimated from measurements of the frequency-dependent propagation constant. A PAT device made from two tubes of different lengths allows the temperature measurement region to be localised, creating an instrument functionally similar to conventional contact thermometers. Three twin-tube PAT devices were constructed and tested. PAT-A, made of silica, served to validate the technique with differences between the acoustic thermometer and a reference thermocouple of less than 2 °C at temperatures in the range from 100 °C to 1000 °C. PAT-B and PAT-C were made of Inconel-600, potentially more suitable for use in harsh environments. The Inconel devices deviated from expected behaviour in a reproducible manner, which after calibration allowed measurements with errors of less than ±1 °C in the range to 700 °C. No drift was observed up to 700 °C. The drift observed during prolonged exposure to higher temperatures is described and its likely causes discussed. In the longer term, similar technology may provide a means for the measurement of temperature in harsh environments such as those found in the nuclear industry.