Intercomparison of blackbodies for calibration of infra-red ear thermometers (IRETs)

Project Description

The intercomparison will be conducted to determine the level of agreement between the blackbody of the National Physical Laboratory (NPL), United Kingdom and the blackbody with different cavities of the Laboratory of Metrology and Quality (LMK), Slovenia. Three cavity shapes are suggested in different standards (CEN, ASTM, JIS) as suitable for calibration of infrared ear thermometers (IRETs), while one cavity shape was proposed by the LMK. The agreement between blackbody cavities will be determined with the help of platinum resistance thermometers (PRTs) and two reference IRETs. Measurements will be performed with two IRETs, at the NPL in one cavity and at the LMK in four different cavities. The comparison is initiated to solve the problem of assuring proper traceability for IRETs.

Final Report 2005-03-30

The intercomparison was performed in two stages. The first stage was performed at the NPL, where two reference IRETs (NPL and MIRS/FE-LMK) were measured against the NPL reference blackbody. The calibration set-up at the NPL consisted of a commercial stirred water bath to which the blackbody cavity of EN shape was mounted vertically. The copper cavity was painted with a high-emissivity black paint (Nextel). The calculated isothermal emissivity of the cavity was 0,9995. Isothermal emissivity was larger than the emissivity, where temperature gradients along the blackbody cavity were taken into account. Temperature homogeneity of the bath at 42 °C was 0,002 °C and the stability within ± 0,02°C. Temperature of the bath was measured with the reference PRT with the uncertainty of 0,01°C. The PRT was connected to the a.c. resistance bridge with the relative uncertainty of better than 0,01. The average of ten readings of an IRET in the calibration mode (instrumental emissivity was presumed to be set to 1; from manufacturer specs or the display it was not obvious) and with the enhanced resolution of 0,01°C was directly compared to the average of ten readings of the reference PRT. Measurements were performed twice (on 19th and 20th January 2004) in the temperature range from 35,5°C to 41,5°C in steps of 1°C. Differences between the radiation temperature (T_rad) of an IRET and the true temperature of the blackbody (T_prt) are presented in Figure 1. Agreement between measurements in two successive days was better with the NPL’s IRET than with the MIRS/FE-LMK’s IRET.

The second stage was performed at the MIRS/FE-LMK. The same reference IRETs were measured against four cavity shapes (EN, ASTM, JIS, LMK) of the LMK blackbody facility. A special stirred water bath was designed, in which four different shapes (EN, ASTM, JIS, LMK) of blackbody cavities were mounted horizontally. The cavity wall material was identical for all cavities that was, copper coated with three layers of a high-emissivity black paint (Pyromark 800). The emissivity of each cavity was modelled based on its configuration, measured emissivity of the copper disc with a FTIR spectrometer, corresponding to the spectral range from 8 µm to 16 µm and at the temperature 50°C, and temperature conditions in the bath.
The effective emissivity was calculated by the programme, which was used in the TRIRAT project. In Table 1 the results of the temperature stability and homogeneity, and of the calculated directional emissivity for all cavities under investigation are presented. It was taken into account that the opening angle of an IRET is 7 degrees, and the front of it was placed at the aperture of a blackbody cavity. It was assumed that in the worst case the field of view was twice as large, therefore the directional emissivity was calculated at the angle of 14 degrees. The emissivity value was stated as the worst case, taking into account the temperature gradients. The uncertainty of emissivity was considered as the rectangular distribution, where the emissivity of an isothermal cavity, temperature gradients at high temperatures (42°C) and temperature gradients at low temperature (35°C) were taken into account. The emissivity value for the EN shape of cavity was not calculated because it was not symmetrical around the horizontal axis. The value of its emissivity and the associated uncertainty was estimated as the value between the values of ASTM or JIS shape (higher emissivity values) and the value of LMK shape (lower emissivity value). The average of ten readings of an IRET was directly compared to the average of ten readings of corresponding two reference contact thermometers. Measurements were performed on 17th and 18th February 2004 in the temperature range from 35,5°C to 41,5°C in steps of 1°C. Differences between the radiation temperature (Trad) of an IRET and the true temperature of the blackbody (T_prt) are presented in Figure 2. Agreement between measurements against different shapes of blackbody cavities was similar with the NPL thermometer and the LMK thermometer. Especially in good agreement were measurements of both thermometers against EN, ASTM and JIS shape of the blackbody cavity (within 0,05 °C at individual temperature). Readings of both IRETs against the LMK shape of cavity differed up to 0,16 °C from readings against other cavities, which was likely caused by lower emissivity of the cavity. The difference of 0,15°C we would have got in calculation by using the Planck law, if the emissivity value was 0,975, with the effective wavelength of 11 µm at 37°C. We could assume that the emissivity of LMK cavity was lower than the calculated value due to the reflected radiation from not perfectly round surface of the sphere. The program for emissivity calculation was designed for calculation of emissivity of cavities with a conical bottom, therefore calculation of cavities with round bottom was questionable.

Table 1: Temperature stability, temperature homogeneity, and effective emissivity of ASTM, EN, JIS and LMK cavity

Figure 3 shows the comparison results of both infrared ear thermometers against the EN shape of blackbody cavity measured at NPL and MIRS/FE-LMK. Measurements at different dates show also the possible drift of IRETs, which were hand-carried during the transport. The knowledge about the drift of an IRET is very important to determine its appropriate recalibration interval.

Based on results of comparison measurements performed at the NPL and at the MIRS/FE-LMK we can conclude that calibration of IRETs against the blackbody cavities proposed in the relevant standards (ASMT standards 1998, Japan Measuring Instruments Federation 2002, CEN 2003) enables agreement of calibration results in order of 0,05°C up to 0,2°C (Figure 3), depending on the quality of performance of an IRET. Such a large difference results mainly due to instable reading of IRETs although the temperature stability of the blackbody is better than 0,01°C. Both reference IRETs were chosen among several other types of IRETs as far the most reliable. It is important to emphasise that readings of IRETs were taken in a calibration mode, where their instrumental emissivity is presumably set to 1,00. Also their resolution was enhanced from 0,1°C to 0,01°C. Access to the calibration mode and the enhanced resolution are typically not available to users of IRETs, therefore they are not able to calibrate them properly. The majority of IRETs don’t even have the possibility of setting the instrumental emissivity or enhanced resolution. During the comparison measurements some other unfavourable effects of IRETs were detected, i.e. short-term drift due to temperature influence, influence of different lens caps, which in practice are changed for each measurement of a new patient. During the comparison each IRET had the dedicated lens cap. These effects will be investigated thoroughly also with other IRETs and presented elsewhere.

The blackbody cavities, suggested in the EN, ASTM and the JIS standard, are suitable for calibration of IRETs, providing that manufacturing of blackbodies follows certain important steps. These would be the use of material with a good thermal conductivity, the use of a high-emissivity paint for painting the cavity in such a way that the surface of a cavity is completely covered with the paint. Finally, also the choice of an appropriate bath, to which the cavity is mounted, is important. All these steps were taken before the blackbody cavities were made and prepared for comparison measurements. And yet we were faced with the temperature differences of up to 0,2°C between the radiance temperature of an IRET and the true temperature of the same blackbody (EN-shape, Figure 3), using probably the best available IRET. Similar differences were observed between the radiance temperature of an IRET measured against the LMK shape of cavity and the radiance temperature of the same IRET measured against other shapes of cavity, because the LMK shape of cavity has slightly lower emissivity than other shapes. If we want to achieve the total uncertainty of 0,2°C in calibration of an IRET, we must have a blackbody with the uncertainty well below 0,1°C.

Results of intercomparison were presented at 9th Temperature Symposium TEMPMEKO in Cavtat, Croatia, in June 2004 and published in journal Physiological Measurement of the Institute of Physics, Bristol, UK (see attached paper)

• Figure 1: Comparison results at the National Physical Laboratory, Teddington, UK
• Figure 2: Comparison results athe the Laboratory of Metrology and Quality, Ljubljana, Slovenia
• Figure 3: Comparison results at the NPL, Teddington, UK and at the LMK, Ljubljana, Slovenia