Kenneth D. Hill

Estimating the influence of impurities on the freezing point of tin

The sum of individual estimates (SIE) and the overall maximum estimate (OME) are two methods recommended to estimate the influence of impurities on the temperatures of the liquid–solid phase transformations of high-purity substances. The methods are discussed starting with the basic crystallographic facts, and their application is demonstrated in detail by applying them to the freezing point of tin as a first example. The SIE method leads to a temperature correction with a corresponding uncertainty while the OME method yields only an uncertainty that is, perhaps not unexpectedly, larger than that of the SIE approach. The necessary sensitivity coefficients (derivatives of the liquidus lines) are tabulated, together with the equilibrium distribution coefficients. Other than the necessity of obtaining a complete elemental analysis of the fixed-point material using glow discharge mass spectrometry (or other suitable techniques), there remain no technical barriers to adopting the preferred SIE method. While the use of the method, and particularly the application of a temperature correction to account for the impurity influence, requires a paradigm shift within the thermometry community, improved interoperability and harmonization of approach are highly desirable goals. The SIE approach maximizes the application of scientific knowledge and represents the best chance of achieving these common goals.

The extension of the Practical Salinity Scale 1978 to low salinities

The Practical Salinity Scale (PSS) 1978 is defined only for salinities within the range 2-42. We have investigated the relationship between mass-determined salinity, electrical conductivity, and temperature for salinities between 0 and 2 with the aim of developing an extension to the Practical Salinity Scale 1978. The paper presents our data, on the basis of which the following correction is proposed to extend the validity of the equations defining the scale to the entire 0-42 range: S=\sum\min{i=0}\max{5} (a_{i}+b_{i}f(t))R_{t}^{i/2}-\frac{a_{0}}{1 + 1.5x + x^{2}}-\frac{b_{0}f(t)}{1+y^{1/2} + y + y^{3/2}} where f(t)=\frac{(t-15)}{1 + k(t-15) x=400R_{t} y=100R_{t} and the constant a_{i}, b_{i} , and k are defind by the Practical Salinity Scale 1978.

A gold/platinum thermocouple reference table

The thermoelectric voltage of the gold/platinum thermocouple has been measured over the range 0–962 °C by comparison with calibrated platinum resistance thermometers. From 0 to 500 °C, the measurements were carried out in stirred liquid baths; from 660 to 964 °C, they were carried out in a pressure‐controlled sodium‐filled heat pipe furnace that provided an isothermal intercomparison environment. Measurements were also made in the metal freezing points of indium, tin, zinc, antimony, and silver, at the melting point of gallium, and at the liquidus point of the silver‐copper eutectic. By fitting the measured thermoelectric voltages to a single eighth‐degree polynomial in temperature by the method of least squares, a reference function is obtained for the Au/Pt thermocouple that provides emf as a function of temperature (ITS‐90) to within ±10 mK from 0 to 962 °C. The Au/Pt thermocouple merits serious consideration for precise temperature measurements as its stability approaches that of the high‐temperature ...

Thermometry's dependence on chemical metrology: a needs-based assessment

The International Temperature Scale of 1990 assigns temperatures to the solid?liquid phase transitions (triple points, melting points, freezing points) of various substances. Since the ideally pure substances of the definition are unattainable in practice, the influence of impurities must be accounted for. Frequently, this is the dominant uncertainty component in realizing these most important reference temperatures. While a number of methods have been employed to estimate the uncertainty arising from the impurity effect, current thinking appears to have converged on methods that depend on chemical analyses of the materials employed. For the majority of cases, the demands of the thermometry community exceed the capabilities of routine analytical chemistry. It may, therefore, be necessary to consider a concerted action to rigorously characterize homogeneous batches of material using as many techniques as can reasonably be brought to bear on the problem in an effort to develop certified reference materials to serve both the thermometry and chemical metrology communities.

The Non-Uniqueness of the ITS-90: 13.8033 K to 273.16 K

Estimates of the non‐uniqueness of the ITS‐90 are reported based on comparisons of capsule‐style standard platinum resistance thermometers at more than eighty temperatures between 13.8033 K and 273.16 K. Using the measurements reported here, as well as those of Ward and Compton and Head, we conclude that the non‐uniqueness takes on its largest value (± 0.4 mK) in the range from 83.8058 K to 234.3156 K. This result may have been anticipated due to the fact that the argon and mercury fixed points are separated in temperature by more than 150 K, with no intermediate calibration points. Discussions concerning the details of the temperature scale that will eventually replace the ITS‐90 must consider this fact if the non‐uniqueness is to be minimized. At the present time, however, there are no candidate fixed points within this temperature range that are realizable to the required level of accuracy for inclusion into a revised International Temperature Scale.

Performance Assessment of Resistance Ratio Bridges used for the Calibration of SPRTs

Automatic balancing dc and ac resistance ratio bridges are the primary measurement devices used by National Measurement Institutes for the calibration of standard platinum resistance thermometers (SPRTs) on the International Temperature Scale of 1990 (ITS‐90). Performance assessment of these resistance ratio bridges is critical to the determination of both the uncertainties of the measurements and for identifying if a resistance bridge is exhibiting non‐compliant behavior. NIST and NRC investigated the performance of 18 resistance ratio bridges consisting of 14 ASL F18s, 2 ASL F900s, and 2 MI 6010Bs. The assessment techniques included the use of an AEONZ resistance bridge calibrator, an ASL ratio test unit, and complements checks. Additionally, the possible effects from using different ac frequencies were investigated. This paper presents the methods of assessment employed, indicates the performance results obtained for the 18 resistance ratio bridges, describes the determination of measurement uncertaint...

Inconsistency in the ITS-90 and the triple point of mercury

The triple point of mercury appears to be assigned an incorrect temperature on the ITS-90. This conclusion is based on fixed-point correlations and the disagreement of the a-coefficients for the deviation functions associated with the calibration of platinum resistance thermometers above and below 273,16 K. The disagreement of the a-coefficients is particularly important as it implies a slope discontinuity in the ITS-90 at 273,16 K. The proposed remedy involves modification of the ITS-90 platinum resistance thermometer low-temperature reference function. With the revised reference function, it is also necessary to lower the argon triple point temperature by 0,7 mK to 83,8051 K to preserve acceptable sub-range consistency. The mercury point retains its ITS-90 assigned value of 234,3156 K. With this modification, the a-coefficient obtained from any sub-range calibration above 273,16 K may be used in the low-temperature deviation functions in place of a measurement at the mercury triple point.

Observations on the velocity scaling of thermistor dynamic response functions

The dynamic response of thermistor temperature sensors is known to be sensitive to the relative velocity between the sensor and the surrounding fluid on both theoretical and experimental grounds. The majority of the experimental investigations derive empirical velocity scaling relationships which have little basis in theory. We have determined that the dynamic response of a particular fast response thermistor scales with the square root of velocity, in accordance with the predictions of the boundary layer theory, over a speed range of 0.5–2 m/s with water as the working fluid.

The NRC Blackbody-Based Radiation Thermometer Calibration Facility

The utilization of radiation thermometers by industry is increasing at a significant rate. At the same time, the need to establish the traceability of the resulting measurements to national standards has never been more important. NRC has responded to this demand by establishing a calibration facility comprising both variable‐temperature and fixed‐point blackbody cavities spanning the range from −50 °C to 2500 °C.

The dynamic response of the two-electrode conductivity cell

The dynamic response of the two-electrode conductivity cell in the dual-needle configuration of M.C. Gregg et al. (1981) was investigated both theoretically and experimentally. A theory is presented which answers fundamental questions regarding the effective cell volume and the mathematical form of the transfer function. These characteristics are shown to be functionally dependent on the spacing between the wires. Experimental determinations of cell response were performed with a thermal plume as the stimulus rather than with the step-response-in-salinity method of Gregg et al. The data are shown to confirm the theoretical form of the transfer function. >