Evaluation of measurement uncertainty in testing laboratories

Transcription

1 Elektrotehniški vestnik 68(5): , 2001 Electrotechnical Review, Ljubljana, Slovenija Evaluation of measurement uncertainty in testing laboratories Gaber Begeš, Janko Drnovšek, Igor Pušnik, Jovan Bojkovski University of Ljubljana, Faculty of Electrical Engineering, Laboratory of Metrology and Quality Tržaška 25, 1000 Ljubljana, Slovenia Abstract. The objective of the paper is to elaborate new elements related to metrological analyses in the field of testing, such as measurement uncertainty and traceability. Until now the international standard ISO did not explicitly required uncertainty specifications in the area of testing in the same extent as the newly implemented standard ISO/IEC 17025, [1]. Therefore several additional steps should be taken in specifying the measurement and testing results especially those concerning performance of testing and other conformity assessment activities. The following contribution is focused on uncertainty analyses of a test procedure regarding electrical safety of household appliances developed according to the European standard EN The example serves as a useful case study, as well as a very illustrative example, highlighting many dilemmas. This particular case is relatively straightforwardly to evaluated due to the relative ease of traceability of its electrical and thermal quantities. Key words: measurement uncertainty, testing, ISO/IEC Ovrednotenje merilne negotovosti v preskusnih laboratorijih Povzetek. Cilj članka je predstaviti nove elemente metrološke analize na področju preskušanja, kot sta merilna negotovost in sledljivost. Mednarodni standard ISO do sedaj ni izrecno vseboval zahteve po specificiranju negotovosti na področju preskušanja v takem obsegu kot novi standard ISO/IEC 17025, [1]. Zato je treba dodatno ovrednotiti merilne in preskusne rezultate s poudarkom na aktivnostih pri ugotavljanju skladnosti s specifikacijami. Prispevek članka je osredotočen na analizo negotovosti pri postopkih za preskus varnosti gospodinjskih aparatov v skladu z evropskim standardom EN Izbran je primer, ki je ilustrativen in na katerem se da prikazati različna problematika. Konkreten primer se relativno hitro ovrednoti zaradi dokaj preproste sledljivosti električnih in termičnih veličin. Ključne besede: merilna negotovost, preskušanje, ISO/IEC Introduction The objective of the paper is to elaborate new elements related to metrological parameters in testing laboratories. The fact is that there is a lack of equivalence among national testing and measurement capabilities and therefore very often absence of mutual acceptance of calibration certificates and test reports. As a result, there are existing and potential new barriers to international trade and free movement of goods, due to required additional or repeated testing procedures. Received 11 March 2001 Accepted 4 September 2001 From the technical point of view, major requirements for mutual acceptance of test results are related to traceability and estimation of uncertainty of all measurements related to test reports and certificates. The new international standard ISO/IEC directly resolves several dilemmas. It explicitly requires that testing laboratories shall apply procedures for estimating uncertainty of measurements. Therefore, it is of utmost importance to define essential requirements of a test method, requirements of a unit under test and requirements of a client. One has to be aware of the problem of narrow limits on which decisions on conformance to a specification are based. Major error sources, contributing to the overall uncertainty of the final result-test report, could in principle be attributed to the reference standards or reference materials, methods and equipment used, environmental conditions as well as properties and conditions of the unit being tested and the operator. The above stated requirements will be elaborated in a test procedure for testing of safety of electrical household appliances. 2 Requirements of the test method The following contribution deals with test methods related to the safety of electrical household appliances. The paper describes the system that enables traceable mea-

2 Evaluation of measurement uncertainty in testing laboratories Computer with a data acquisition module Module for connection of several thermocouples Unit under test Black test corner with built-in thermocouples Figure 1. Suggested measurement set-up 80 mm 40 mm mm 66.6 mm 5mm in Cu 15 mm 7mm in wood Solder Disk of coope 1mm thick 25 mm wood Wire 1.5 mm Figure 2. Measurement plate with built-in thermocouples surements and evaluates measurement uncertainty in test results. The EU directives that refer to particular technical standards are: Low voltage directive 73/23/EEC (2(, Machinery directive 89/392/EEC and Construction product directive 89/106/EEC. The Standard that acknowledges the international level of danger protection and includes fundamental safety requirements of the above-mentioned directives is the Slovenian standard SIST EN , (3( (is resumed EN ). It includes general requirements for safety of household and similar electrical appliances. The European committee for standardisation in the field of electrical engineering CENELEC accepted the European standard. The paper deals with the Item 11 (Heating) of the Standard SIST EN It states that appliances and their surroundings shall not attain excessive temperatures in normal or abnormal use. Thus compliance is checked by determining the temperature rise under specified conditions in the so-called black test corner. Temperature rises other than those of windings are determined by means of fine-wire thermocouples positioned so that they have a minimum effect on the temperature of the unit under test. Thermocouples having wire of a diameter not more than 0,3 mm are considered to be fine-wire thermocouples. Thermocouples used for determining the temperature rise of a surface of walls, ceiling and floor are attached to the back of small blackened disks of copper or brass, 15 mm in diameter and 1 mm thick. The front of the disk is flushed with a surface of the board. Tested appliances are positioned so that thermocouples detect the highest temperatures. The ambient

3 288 Begeš, Drnovšek, Pušnik, Bojkovski temperature should be 20 o C ± 5 o C. If the temperature rise is close to the highest allowed level, the ambient temperature shall be 23 o C ± 2 o C. These are very general requirements for testing of excessive heating. The standard explicitly describes how appliances are installed in the black test corner, but does not state anything about measuring set-up and its maintenance as well as the measurement procedure. On the other hand, for testing laboratories it is very important that the measurements are traceable and reproducible in order to fulfil requirements of the ISO/IEC Standard. That is the main purpose of the suggested measurement set-up described in the paper. 3 Measurement set-up for testing of excessive heating The main guidance in the development of a measurement set-up for measuring heating of electrical appliances was consisted of testing and comparability of test results of various units under test. Therefore, the measurement uncertainty had to be evaluated and traceability assured with appropriate periodical calibration of the measurement setup. The black test corner was designed according to the requirements of the standard in order to enable the decision about appropriate functioning of a unit under test. Moreover, requirements for traceability were identified and implemented in the measurement set-up. The principal requirement was traceable temperature measurement. Due to the large number of thermocouples, which had to be calibrated, a special construction of the measuring plate was required. Figure 1 shows the suggested measurement set-up. The black test corner is formed of many individual interchangeable wooden block plates mounted in a rectangular frame. One of the plates is a measurement plate with 60 thermocouples and mounted according to particular test requirements. Figure 2 shows dimensions of the built-in thermocouples. The border thermocouples are built-in at half the distance (as it is between the thermocouples) from the edge of the measurement plate, thus enabling the plates to be combined in parallel. Thermocouples are made of the Fe-CuNi wire (type J) with a diameter of 0,2 mm and soldered to a copper disk. The diameter of the disk is 15 mm and its thickness is 1 mm. Thermocouples were fixed by using high temperature resistant silicon mastic. The question is what is the optimum distance between two built-in thermocouples? A theoretical approach to the estimation of the distance between thermocouples is shown with four neighbouring thermocouples depicted in Figure 3. The minimal distance (d min ) between two adjacent notes/thermocouples is defined according to relation (1) u TC <T GRAD, (1) where u TC is the uncertainty of a thermocouple and T GRAD is the maximum temperature deviation between two notes. The maximum distance (d max ) depends on requirements of the temperature heating related standard. Experimentally, the distance between the thermocouples was determined on the basis of measured temperature gradients. d d = f (technology, economy) 1 2 =f(u) M I N C d M A X 3 4 = f (standard Figure 3. Four neighbouring notes/thermocouples It is shown that the distance where the temperature gradient between two thermocouples lies within the measurement uncertainty of the system is less than one diameter of the copper disk. The largest distance between two thermocouples where temperature can still be measured is about 25 cm. This is one half of the distance where temperature gradient is about 60 K. According to the experimental results, the distance between two thermocouples should be from 1 cm to 25 cm. That is why a decision about the distance between two thermocouples is made on economical and technological basis. The price of the measurement system and existence of a sufficient data acquisition system are two major facts that should be taken into consideration. The described measurement system was designed on the basis of the above mentioned facts. 4 Uncertainty calculations, traceability and reporting of test results The basic requirements of the ISO/IEC standard is that any testing laboratory shall implement and apply a procedure for estimation of the measurement uncertainty. In estimation of the measurement uncertainty, all uncertainty components in a given situation shall be taken into account as foreseen by appropriate analysis. The nature of the test method may preclude a rigorous, metrologically and statistically valid calculation of measurement uncertainty. In such cases, the laboratory shall at least attempt to identify all the components of uncertainty and reasonably estimate them, and shall ensure that the form of reporting of the result does not give a wrong impression of the uncertainty. A reasonable estimation shall be based

4 Evaluation of measurement uncertainty in testing laboratories 289 on knowledge of the performance of the method and shall make use of previous experience and validation data. In general, a complete measurement uncertainty of the measured quantity u c depends on uncertainty contributions related to all influential parameters, u 1, u 2,..., u N. T u T L T Tí M A X = f(standard) =f( u ) M A X T L u c = f(u 1,u 2,..., u N ), (2) Typically, not all are equally relevant. There are two major ones used for calculation of the measurement uncertainty. The measurement uncertainty of an indirectly measured value u c (y), where quantities are not correlated, is calculated as the square root of the sum of the squares of below specified products of factors u c (y) = [c 1 u(x 1 )] 2 +[c 2 u(x 2 )] [c N u(x N )] 2, (3) where c 1 = y,c 2 = y,..., c N = y (4) x 1 x 2 x N and u 1 (x 2 ),..., u N (x N ) are the measurement uncertainty of influenceing values. The measurement uncertainty of a directly measured value, where quantities are not correlated, is calculated as the square root of the sum of the squares of all relevant uncertainty contributions. u c (y) = [u 1 (y)] 2 +[u 2 (y)] [u N (y)] 2 (5) What does the calculated uncertainty of a measurement system mean in practical work? In order to get the highest allowed temperature heating (T max) measured during testing in a production line, it is important to know that the highest allowed measured value of temperature heating (T max ) should be reduced by measurement uncertainty of the measurement system (u TL ). The above facts are represented in Figure 4. In our case, the excessive temperature is the final test result of a unit under test. The measurement uncertainty of the excessive temperature u(t) depends on many factors such as: calibration method and equipment for calibration of thermocouples, data acquisition module, thermocouple type, temperature deviation of the thermocouples, colour of the test corner, distance between two neighbouring thermocouples, ambient temperature, contribution of the tested unit and operator. Identification of all uncertainty components and their reasonable estimation shall ensure that the form of reported results does not give a wrong impression of the uncertainty. This is the reason why three major uncertainty contributions were estimated on the basis of experience. They are presented below. T =f(room temp.) Figure 4. Test results of heating of a kitchen-range Before thermocouples were built into the wooden plate, they had been calibrated in the accredited laboratory. Calibration was performed at 20 o C, 40 o C, 60 o C, 80 o C, 100 o C, 150 o C and 180 o C. As the thermocouples were connected to the data acquisition system (DAS), the whole measurement system was calibrated. All plates of the test corner were inserted in an aluminium guidance being easily reconfigured. The DAS was connected to a computer module of the National Instruments AT-MIO- 64E. This module has 64 single-ended or 32 differential terminals, a temperature sensor for cold-junction compensation and is plugged in a personal computer. The program package used was the LabVIEW. The main program offers saving and printing of measured data in a graphical or numerical form. Two sub-programs perform scooping of temperature and measuring temperature of one measuring plate. The measurement uncertainty of the system (u s ) has three main contributions. The uncertainty due to the computer module resolution (u cm =0,4 o C). The uncertainties of auxiliary measuring instruments, temperature calibration baths and the standard thermometer used in the calibration (u ci =0,04 o C), and the uncertainty of a thermocouple with the worst standard deviation (2s th =0,69 o C). u s = u 2 cm + u 2 ci +2s2 th = (0, ) 2 4o C +(0, 04 o C) 2 +(0, 69 o C) 2 =0, 73 o C 3 (6) Judging from measurement uncertainty of thermocouples, the uncertainty of the whole system is 0,8 o C. This is the worst case uncertainty found with the worst thermocouple exhibiting the maximum standard deviation of 0,69 o C. 0 t

5 290 Begeš, Drnovšek, Pušnik, Bojkovski Measurement data, abnormal use of the kitchen-range TEMPERATURE RISE in K Place (X, Y) of temperature rise Left wall Right wall Ceilling NM NM NM NM NM NM NM NM NM NM Floor Table 1. Temperature rise of the kitchen-range (numerical values)

6 Evaluation of measurement uncertainty in testing laboratories 291 Figure 5. Test results of heating of a kitchen-range 5 Measurement of heating of a kitchen-range Special requirements for kitchen ranges (SIST EN ), [3], were taken into consideration. The maximum normal temperature rise in a black test corner shall not exceed the specified temperature rise of 65 K in a normal use and 150 K in a specified abnormal use. In a normal use, the appliance is placed as near as possible to the sidewalls in the test corner. The appliance is operated in accordance with conditions of adequate heat discharge, with all heating elements that can be switched on at the same time under normal conditions of use in the circuit. The supply voltage is such that the total input of the appliance is 1.15 times the rated input. If the appliance is provided with means limiting the total input, the test is made with whichever heating unit, or combination of heating units, as may be selected by this means, imposes the most severe conditions. Hob elements are operated in accordance with conditions of adequate heat discharge with their control devices adjusted to the highest setting until the water boils. The control devices are then adjusted so as to maintain the water gently boiling. Ovens provided with a thermostat are operated so that the mean temperature over a cycle of the thermostat, in the centre of the useful oven space, is maintained at 240 o C ± 4 o C. If this condition cannot be obtained, the control is adjusted to the highest setting. The appliance is operated for 60 minutes, unless steady conditions are obtained before. In an abnormal use, the appliance is placed as near as possible to the sidewalls in the test corner. It is tested separately with the switched oven and separately with only that heating unit operating, which imposes the most sever conditions. Its control device is adjusted to the highest setting. The supply voltage is such that the total input of the appliance is 0.85 times the rated input, 1.24 times the rated input and 1.15 times the rated input with a shortcircuited oven thermostat. The appliance is tested until

7 292 Begeš, Drnovšek, Pušnik, Bojkovski steady conditions are achieved. Graphical results of heating of the appliance in an abnormal use are shown in Figure 5. Numerical results are not presented as clearly as graphical results. Figure 5 shows the heating of a side-wall, a back wall and a bottom of the appliance, in a grey scale. The grey scale represents temperature rises from 0 K to 150 K. Every coloured square in Figure 4 represents the temperature rise measured with a thermocouple. The system enables determination of the highest temperature and the place of its appearance. Table 1 presents numerical values for the temperature rise of each thermocouple at the end of the measurement. From Table 1 it is possible to read the position and the value of the temperature rise of the black test corner. Of course, the graphical presentation is more evident than the numerical. If a value from Table 1 is compared with the dimensions of the measuring plates (Figure 4), it is easy to determine the position of a particular thermocouple and its temperature rise. The abbreviation NM means not measured. 6 Maintenance and improvements Easy maintenance of the measurement system is very important. A modular structure of measuring plates enables a relatively simple recalibration of thermocouples in a climatic chamber. The chamber has a defined uncertainty rate because it is evaluated according to the accredited procedure, [5], thus assuring the traceability of recalibration. In this way the recalibration does not invalidate the performance of built-in thermocouples, but increases the total uncertainty of the complete system. Namely, the temperature uncertainty of a typical climatic chamber (0,6 o C) is 150 times worse than the uncertainty of a temperature calibration bath with the equalising block (0,004 o C), [6]. The used chamber has the volume of 1 m 3. This is enough for simple placement of the measurement plates into the chamber in order to perform the calibration. In this way the recalibration of the thermocouples and the traceability of the measurement system are assured. Improvements are possible in the field of a new approach to the highest automation of the measurement. The calibration can be performed after thermocouples are built into wooden plates. Thus damaging of thermocouples is avoided. Improved automation requires only one data acquisition module and a switch module for scanning measurements on many measuring plates. A reliable data acquisition module for controlling many measuring plates is under investigation. 7 Conclusion Testing in the black test corner is important due to the fact that low voltage appliances that do not have to be electrically tested can cause fire too. The paper describes an evaluation procedure for the measurement uncertainty in testing laboratories and implementation of a computer along with measurement software as needed for measurements of the temperature rises in the black test corner. The temperature is measured with calibrated thermocouples connected to a data acquisition module, which is installed in the computer. Since testing of excessive heating requires temperature measurements by built-in thermocouples, a special construction of the black test corner is required so as to enable calibration of thermocouples and have traceable temperature measurements and reliable test results. At the time being, laboratories worldwide use different methods of testing as being applicable for testing of particular appliances only. For an independent testing laboratory it is important that its measurements are independent of the size of tested appliances. A testing device, the so called black test corner, can be considered as a complete measurement system that can be regularly recalibrated and thus providing traceable and reproducible measurements. It is important to include the measurement uncertainty into the test result, enabling the producer to have it taken into account in the production process. Appliances that are made to operate at safety margins are not considered as quality appliances because already a minor interruption or unexpected action can cause a great damage. Damages caused by a bought appliance are not excused by consumers. When some standard specifications cannot be satisfied as being too rigorous or when the available measurement technology is not developed enough, recommendations proposed in particular standards should be considered, too. 8 References [1] IEC/ISO 17025, First edition [2] Low Voltage Directive 73/23/EEC. [3] SIST EN , SIST EN [4] European Accreditation, Publication reference EAL-R2, Expression of the uncertainty of measurement in calibration, Edition 1, April [5] I. Pušnik, J. Bojkovski, J. Drnovšek, Quality Manual; Procedure for evaluation of climatic chambers, LMK, FE, Ljubljana, [6] J. Drnovšek, J. Bojkovski, I. Pušnik, Analysis and Modelling of Equalising Blocks, Instr. Science & Technology, Vol. 26, No. 5, pp , Gaber Begeš was born in Jesenice, in He received his B.Sc. degree in Electrical Engineering in 1999 from the Faculty of Electrical Engineering of the University of Ljubljana. In September 1999 he became a Research Assistant at the Laboratory of Metrology and Quality at the same faculty, where he is presently a postgraduate student and is working towards his M.Sc. His current research interests are testing of electrical household appliances, automation of measuring systems

8 in testing laboratories, and implementation of requirements of ISO/IEC standard in testing laboratories. Jovan Bojkovski was born in Belgrade, in He received his B.Sc. degree in Electrical Engineering in 1994 from the Faculty of Electrical Engineering of the University of Ljubljana. In October 1992 he became a Research Staff Member at the Laboratory of Metrology and Quality at the same faculty, where he is presently a postgraduate student and is working towards his Ph.D. His current research interests are temperature calibration, International Temperature Scale 1990, thermometers, relative humidity, automation of the calibration laboratory and on-site calibrations. Janko Drnovšek was born in Ljubljana, in He received his M.Sc. degree from the Imperial College of Science and Technology, London, the UK, in 1979, and his Dr. Sc. from the Faculty of Electrical Engineering of the University of Ljubljana, in He joined the Faculty of Electrical Engineering in Between 1982 and 1986 he was the project leader for laboratory measuring systems at Iskra Kibernetika, Kranj. At the Faculty of Electrical Engineering he is a Professor and the Head of the Laboratory of Metrology and Quality, which is an accredited calibration laboratory for temperature and relative humidity. Dr. Drnovšek is President of the Slovenian Society for Process Control and Measurement, President of Metrology Council at the Standards and Metrology Institute of the Republic of Slovenia, Deputy Chairmen of TC-11 in the International Measurement Confederation IMEKO, and the Slovenian delegate to the EUROMET. Igor Pušnik was born in Ljubljana, in He received his B.Sc. and M.Sc. degrees in Electrical Engineering from the Faculty of Electrical Engineering of the University of Ljubljana in 1995 and 1997, respectively. In 1995 he became a Research Staff Member at the Laboratory of Metrology and Quality at the same faculty, where he is presently a Senior Lecturer and is working towards his Ph.D. His current research interests are temperature calibration, International Temperature Scale 1990, radiation thermometry, relative humidity, on-site calibrations and quality systems.