H. C. McEvoy

New standards for devices used for the measurement of human body temperature

Significant changes in recording of human body temperature have been taking place worldwide in recent years. The clinical thermometer introduced in the mid-19th century by Wunderlich has been replaced by digital thermometers or radiometer devices for recording tympanic membrane temperature. More recently the use of infrared thermal imaging for fever screening has become more widespread following the SARS infection, and particularly during the pandemic H1N1 outbreak. Important new standards that have now reached international acceptance will affect clinical and fever screening applications. This paper draws attention to these new standard documents. They are designed to improve the standardization of both performance and practical use of these key techniques in clinical medicine, especially necessary in a pandemic influenza situation.

Thermodynamic temperature assignment to the point of inflection of the melting curve of high-temperature fixed points.

The thermodynamic temperature of the point of inflection of the melting transition of Re-C, Pt-C and Co-C eutectics has been determined to be 2747.84 ± 0.35 K, 2011.43 ± 0.18 K and 1597.39 ± 0.13 K, respectively, and the thermodynamic temperature of the freezing transition of Cu has been determined to be 1357.80 ± 0.08 K, where the ± symbol represents 95% coverage. These results are the best consensus estimates obtained from measurements made using various spectroradiometric primary thermometry techniques by nine different national metrology institutes. The good agreement between the institutes suggests that spectroradiometric thermometry techniques are sufficiently mature (at least in those institutes) to allow the direct realization of thermodynamic temperature above 1234 K (rather than the use of a temperature scale) and that metal-carbon eutectics can be used as high-temperature fixed points for thermodynamic temperature dissemination. The results directly support the developing mise en pratique for the definition of the kelvin to include direct measurement of thermodynamic temperature.

Traceability and calibration in temperature measurement: a clinical necessity

Patient temperature is a fundamental physiological measurement used primarily for observation and diagnosis, for example during surgery, intensive care, recuperation, or treatment. A variety of thermometers are used clinically and these can be separated into two categories, either contact (oral thermometers, rectal thermometers and temporal strips), or non-contact (ear thermometers, temporal thermometers and thermal imagers). To have the maximum confidence in the clinical performance of the temperature measurement instrument it is strongly desirable that the device be traceably calibrated to the International Temperature Scale of 1990 (ITS-90). Lack of traceable calibrations accredited to ISO17025 can lead to unreliability in temperature measurement and in some cases can have a deleterious effect on patient care. The National Physical Laboratory (NPL) maintains and disseminates the ITS-90 for contact and non-contact thermometry in the UK. The importance of accredited traceable calibrations and an outline of contact and non-contact thermometry standards are given here.

Comparison of the New NPL Primary Standard Ag Fixed‐Point Blackbody Source with the Primary Standard Fixed Point of PTB

Above the freezing point of silver (961.78 °C), the International Temperature Scale of 1990 is defined in terms of Planck’s radiation law. The scale is maintained and disseminated using a validated and linear pyrometer in conjunction with a blackbody reference source at either the Ag, Au (1064.18 °C) or Cu (1084.62 °C) freezing point. In order to realize the scale with the highest precision high quality, well‐characterised, reproducible fixed‐point blackbody sources are required. Such sources have been maintained at NPL for a number of years, but it was felt that improvements to the design would be beneficial. A new Ag point blackbody source has therefore been constructed. The new design will improve the quality and reproducibility of the melting and freezing plateaux and reduce errors due to the ‘out‐of‐focus’ size‐of‐source effect which is difficult to measure and to eliminate. Full details of the design of the new source, including results of the assessment of its performance, are described. Critical for the application of fixed‐point blackbodies as primary temperature standards is the precise knowledge of the emissivity of the cavity, which causes a correction to the melting and freezing temperature of the ingot. As blackbody emissivities are difficult to assess experimentally, two different numerical approaches developed at NPL and PTB are used to calculate the blackbody emissivity. In order to further validate the performance of the new Ag fixed‐point blackbody it has been compared with the Au primary fixed‐point blackbody of PTB. For the comparison the ratios of the spectral radiances of the fixed‐point blackbodies were measured at 650 nm and 950 nm using the PTB monochromator‐based spectral radiance calibration facility, and at 654 nm and 953 nm using the PTB interference filter‐based primary photoelectric pyrometer.

Dissemination of thermodynamic temperature above the freezing point of silver

The mise-en-pratique for the definition of the kelvin at high temperatures will formally allow dissemination of thermodynamic temperature either directly or mediated through high-temperature fixed points (HTFPs). In this paper, these two distinct dissemination methods are evaluated, namely source-based and detector-based. This was achieved by performing two distinct dissemination trials: one based on HTFPs, the other based on absolutely calibrated radiation thermometers or filter radiometers. These trials involved six national metrology institutes in Europe in the frame of the European Metrology Research Programme joint project ‘Implementing the new kelvin’ (InK). The results have shown that both dissemination routes are possible, with similar standard uncertainties of 1–2 K, over the range 1273–2773 K, showing that, depending on the facilities available in the laboratory, it will soon be possible to disseminate thermodynamic temperatures above 1273 K to users by either of the two methods with uncertainties comparable to the current temperature scale.

Radiation temperature scales between the indium and silver points realized at IMGC, NPL and UME using a fixed-point calibration technique

Radiation temperature scales were realized with a precision infrared thermometer at IMGC, NPL and UME using fixed-point blackbodies at the freezing points of indium, tin, zinc, aluminium and silver. From the fixed-point calibration in each laboratory, continuous scales were obtained with a four-coefficient interpolation equation. The differences between the three scales were well within 0.1°C at all temperatures between the indium and silver points. Calibrations of the same thermometer performed at NPL and UME using a more traditional calibration scheme based on the comparison with contact thermometers showed differences of up to more than 0.4°C. This suggested that the fixed-point scheme, that has been used for the first time in an international comparison, should be more reliable for accurate calibrations of precision infrared thermometers.

Use of an InGaAs Radiation Thermometer To Verify the Accuracy of the NPL Blackbody Reference Sources from 156 °C to 600 °C

A transfer standard radiation thermometer based upon an InGaAs detector was developed at the NPL in 2001. The purpose of this instrument was to provide a compact high‐resolution device, which could be used to maintain and disseminate a radiance temperature scale between 156 °C and 962 °C. The thermometer requires calibration at the ITS‐90 fixed points. For this purpose fixed‐point blackbody sources have been designed and constructed using high‐purity metals comprising In, Sn, Zn, Al and Ag. Using the results of the calibration, an interpolating equation based upon the Wien function is calculated which relates the thermometer output to the radiance temperature of the source. This paper describes the development of the InGaAs thermometer with results which illustrate its stability between successive calibrations. The InGaAs thermometer was used to verify the calibration of the NPL variable‐temperature water and caesium heat‐pipe blackbody sources, whose temperatures are normally derived from ITS‐90 calibrat...

New blackbody standard for the evaluation and calibration of tympanic ear thermometers at the NPL, United Kingdom

The use of infrared tympanic thermometers for monitoring patient health is widespread. However, studies into the performance of these thermometers have questioned their accuracy and repeatability. To give users confidence in these devices, and to provide credibility in the measurements, it is necessary for them to be tested using an accredited, standard blackbody source, with a calibration traceable to the International Temperature Scale of 1990 (ITS-90). To address this need the National Physical Laboratory (NPL), UK, has recently set up a primary ear thermometer calibration (PET-C) source for the evaluation and calibration of tympanic (ear) thermometers over the range from 15 °C to 45 °C. The overall uncertainty of the PET-C source is estimated to be ± 0.04 °C at k = 2. The PET-C source meets the requirements of the European Standard EN 12470-5: 2003 Clinical thermometers. It consists of a high emissivity blackbody cavity immersed in a bath of stirred liquid. The temperature of the blackbody is determined using an ITS-90 calibrated platinum resistance thermometer inserted close to the rear of the cavity. The temperature stability and uniformity of the PET-C source was evaluated and its performance validated. This paper provides a description of the PET-C along with the results of the validation measurements. To further confirm the performance of the PET-C source it was compared to the standard ear thermometer calibration sources of the National Metrology Institute of Japan (NMIJ), Japan and the Physikalisch-Technische Bundesanstalt (PTB), Germany. The results of this comparison will also be briefly discussed. The PET-C source extends the capability for testing ear thermometers offered by the NPL body temperature fixed-point source, described previously. An update on the progress with the commercialisation of the fixed-point source will be given.

Design and Construction of a New Primary Standard Pyrometer at NPL

For many years the temperature scale at NPL has been realized and maintained using the NPL primary pyrometer. This instrument has reached the end of its useful life, and a replacement instrument has been built and is currently being validated. This new instrument will give reduced uncertainties in disseminating the temperature scale. The new pyrometer uses lens optics, designed to be achromatic at the two operating wavelengths of 650 nm and 900 nm. The wavelengths are defined by interference filters. The target size of 0.75 mm has been chosen to allow for calibration of tungsten ribbon lamps. The filters, stops, detector and amplifier are all temperature stabilised. The following tests have been carried out: linearity of the detector and amplifier, the temperature coefficient of the instrument, size‐of‐source effect measurement, determination of the effective wavelength of each interference filter and the short‐term stability. This paper gives a detailed description of the instrument design and the result...