IMPROVEMENT AND CONFIRMATION OF A NEW AUTOMATIC SYSTEM FOR AC-DC CALIBRATION AT NIS, EGYPT

Transcription

1 Simpoio de Merología al 7 de ocubre de 006 IMPROEMENT AND CONFIRMATION OF A NEW AUTOMATIC SYSTEM FOR AC-DC CALIBRATION AT NIS, EGYPT Mamdouh M. Halawa, Ahmed Huein Naional Iniue for Sandard (NIS Tera S., Giza, El-Ahram, Box: 136, Code: 111, Egyp Tel , Fax , Mamdouh_halawa@yahoo.com Abrac: The recen Improvemen in AC DC ranfer capabiliie a Naional Iniue for Sandard (NIS, Egyp, include an auomaed AC DC ranfer comparaor yem wih inernaional confirmaion of he reul hrough bilaeral inercomparion wih NPL, England. The yem can be ued o meaure addiional characeriic of hermoelemen and operae over a frequency range from 0 Hz o 100 khz for a volage level from 0.5 o 1 k. The reul of bilaeral inercomparion including he proficiency e of he new yem are repored. The expanded uncerainy aigned o he NIS working andard i given and i componen are briefly dicued. The reul how ha he agreemen beween he wo laboraorie i beer han he expeced uncerainie 1. INTRODUCTION AC volage and curren are mo accuraely meaured by uing hermal converer ha compare he heaing effec of he unknown alernaing ignal o hoe produced by a known DC ignal. The device ued o make hi comparion are hermal volage converer (TC and hermal curren converer (TCC [1]. They uually coni of a heaer rucure, which carrie alernaely he AC and DC ignal and one or more hermocouple paced along he heaer o monior i emperaure. By applying AC and boh polariie of DC in a imed equence, and meauring he hermocouple oupu, one can ue he convenional definiion of AC-DC difference,, a: ac dc dc (1 where dc i ha value of DC which, when applied wih poiive and negaive polariie, produce he ame mean repone a he rm AC quaniy ac. AC DC ranfer andard are mainained by Naional Iniue for Sandard (NIS, Egyp o provide a primary link beween AC and DC quaniie. Thoe andard are raceable o he Naional Iniue for Sandard and Technology (NIST, US. The need for lower uncerainy in calibraion of TC and TCC moivaed he NIS o upgrade i andard and faciliie. The auomaed yem decribed here overcome he deficiencie of he manual e mehod a NIS. Uing he conrol ofware, he operaor make he range and frequency elecion and receive an auomaed e repor a he end of each meauremen. The yem cover a frequency range from 0 Hz o 100 khz for he volage range from 0.5 o For all volage, he goal wa o reaemen of he oal meauremen uncerainie in hi area. In addiion o performing AC-DC difference e, he yem can be ued o calibrae AC volage ource or high eniiviy digial AC volmeer. The yem produce aiical informaion abou he AC-DC ranfer difference meauremen a well a ome novel meauremen uch a lineariy conan, ime conan, and repone ime. Thi uncerainy level obained by he new yem i ufficien o aify he preen requiremen a NIS.. SYSTEM HARDWARE The block diagram of he auomaic calibraion yem i hown in Fig. 1. The yem coni of a programmable ource, Waveek 9100, for boh AC and DC vol, wo TC (one andard, he oher he unknown, wo digial mulimeer (DMM, HP 3458A, wih reoluion of 10 n o monior he oupu of each TC, a PC wih a GP- IB conroller and a priner for prining he e repor. The meauring equence adoped in hi yem i AC, DC, DC-, AC []. The reading from DMM are ued o compue ome characeriic (lineariy, ime conan ec. and he AC-DC difference of he e device. The complee comparion procedure, including all daa and aiical analyi, i performed wihou any inerrupion. 1

2 Simpoio de Merología al 7 de ocubre de AC-DC DIFFERENCE TEST PROCEDURE The repone of he TC i defined by [3]: n E K ( Fig. 1: Block Diagram of he Auomaic Syem 3. SYSTEM SOFTWARE The ofware, wrien in C and uing he Lab Window program, provide flexibiliy for he yem. The eleced funcion i enered ino he program via he creen hown in Fig.. The operaor chooe eiher he AC-DC difference meauremen or he e of deermining he TC characeriic. The e frequencie and he nominal e volage are enered in he main creen. The remaining enrie of he meauremen proce e he device warming-up ime, he number of meauremen per cycle, he e equence ued in he deerminaion of he AC- DC difference and he averaging ime of he DMM. The e reul are prined and/or aved on dik. where E i he oupu EMF of he Thermal Elemen (TE of he TC, i he volage applied, K i a conan and varie omewha wih large change in heaer curren and n i uually 1.6 o 1.9 a raed heaer curren bu i i nearly conan over a narrow range where nearly equal AC and DC value are compared. The facor K and n are parameer characeriic of he paricular TC. The relaionhip beween a mall change in TE heaer volage ( and he correponding change in oupu ( E i expreed a: (3 E n. E The AC-DC difference, baically, i defined a [3]: a d, (4 d where d i he DC volage which when revered produce he ame mean oupu volage a a, i.e. d (5 Afer compleing he equence of four ep by applying ucceively AC, DC, DC- and AC volage o minimize he effec of drif in he TC oupu, he AC-DC difference of he e TC,, i evaluaed a: ( Ea E n Ed (6 d ( Ea Ed n E d Fig. : Te Parameer Selecion Screen where i he AC-DC ranfer difference of he andard TC, E a and E a are oupu elecro moive force (EMF of he andard and he unknown TC for AC e volage, repecively. The mean EMF value for forward and revere DC e volage are aken a E d and E d, repecively. A he preceding equaion how, he value of he exponen n for he TC mu be known. The exponen varie wih applied volage, and i variaion can be deermined by performing

3 Simpoio de Merología al 7 de ocubre de 006 n-e. Thi e will be dicued in he nex ecion. A e run coni of 0 deerminaion of AC-DC difference for he ame volage and frequency hen he average of he 0 deerminaion i calculaed and prined on he yem priner. An imporan addiional opion i involved in he AC- DC difference program o avoid applicaion of volage grea enough o damage he TE. The applied value mu be le han 110 percen of he raed volage of he TC. If he applied value i ouide hi condiion, he program hould o be abored immediaely. 5. OTHER SYSTEM CAPABILITIES 5.1 The n Te One imporan e ha i readily performed auomaically i ha of deermining a Thermal Elemen n characeriic. Typical reul for a 5 ma, Ballanine ype TC which wa eed wih a nominal 10 range reior are ploed in Fig. 3. The characeriic n i given by [3]: E / E n (7 / Where E i he meaured change in oupu EMF for mall change in he applied volage, i he nominal e volage for he TC and E i he meaured EMF a he nominal e volage n Te of 10 daa poin wa abou 1 hour. Each value of n i compued a he average of deerminaion (0.5 % and -0.5 % a any given volage. The compued equaion for n a a funcion of applied volage wa ploed. The n value a he raed volage of he 5 ma Ballanine ype 10 TC, for example, wa The value of n a he raed volage i eiher ored in he compuer daa file or i yped by he operaor ino he compuer program for he TE a he aring of an AC-DC difference meauremen. 5. DC Reveral Difference (DCRD The DC reveral difference (DCRD of he TE could readily meaured and i defined a he difference in applied value of boh polariie of DC volage required o produce an equal oupu EMF of he e TE [4]. The value of DCRD wa acually meaured a: ( E E DCRD (8 n ( E E where E and E - are he TE oupu EMF wih equal DC and DC- volage applied, and n wa given a in (7. The program wa deigned o deermine he DCRD a a funcion of he applied volage. Typical reul for a 5 ma, Ballanine ype TC which wa eed wih a nominal 100 range reior are ploed in Fig. 4 where he volage wa changed from 70 % o 105 % of i raed value in 5 % incremen. DC Reveral Difference n alue y x Inpu olage ( Reveral Error (ppm y x x % Applied olage (v Fig. 3: n Te for a 10 TC Fig. 4: 100 DC Reveral Difference The value of wa programmed o be ± 0.5 percen of nominal e volage. Sufficien ime wa allowed afer each volage change ( 40 econd for TE o reach i final EMF value. According o he reul hown in Fig 4, he TC wa eed from 70 o 110 percen of he raed volage. The oal elaped ime o obain he eigh 5.3 Seady Sae Time of Thermal Elemen The eady ae ime of he TE wa meaured by applying he full raed DC and AC volage uddenly and monioring he variaion of he oupu EMF during wo minue. I wa found ha, when DC and AC volage (50 Hz are applied, he oupu EMF of he TC require abou 3 and 36 3

4 Simpoio de Merología al 7 de ocubre de 006 repecively o reach i raed value and o op he drif approximaely. 6. UNCERTAINTY STATEMENT Manual calibraion yem ha been in operaion for many year a NIS, and i uncerainy ha been documened [5]. For example, he beexpanded uncerainy of he manual operaion wa abou 30 ppm. However, hi value wa reduced by raio of abou 1:3 afer uing he new auomaed yem. In addiion o he calibraion of he reference uni, common ource of error in hi ype of meauremen are: he abiliy of calibraor and mulimeer, elf-heaing, ambien emperaure effec and drif effec [, 4, 5, 6]. The conribuion of noie o yem uncerainy can be reduced by proper grounding and hielding of he yem. The uncerainie were calculaed in accordance wih Naional Iniue of Sandard and Technology (NIST requiremen [6], which divide he uncerainy aigned o he meauremen ino Type A uncerainie (hoe evaluaed by aiical mean and Type B uncerainie (hoe evaluaed by oher mean and hen combine hee uncerainie in a form of roo-um of quare (RSS. For AC-DC meauremen, he Type B uncerainie are generally dominaing [7]. The mahemaical model o evaluae he uncerainy can be wrien a: or ( E E ( E E f ( (9 a d a d N Ed N Ed (10 The equaion for he propagaion uncerainy can be expreed a follow. u ( [ ] u ( u ( [ ] [ ] (11 The ource of relaive uncerainy are he ype A uncerainy of. For 0 ime of each meauring poin, he andard uncerainy (U A i calculaed by Equaion (8. σ u A (1 0 where σ i he andard deviaion of he 0 reading. For example, he compued ype A of AC-DC calibraion by uing hi new auomaic yem wa 3.5 ppm for 10 a 50 Hz. Type B ha many conribuion uch a he conribuion due o he T-connecor, he andard (calibraed TC and he ued DMM. The uncerainy deerminaion for he andard TC i no reaed in hi paper. Uing uncerainy aemen from commercially calibraed TC, he expanded uncerainy of 10 a 50 Hz (k, for 95 % confidence level i abulaed in Table 1. Table 1: Uncerainy budge of 10 a 50 Hz Source of Uncerainy Probabiliy Diribuion Uncerainy Conribuion ± ppm Calibraion Cerificae (Type B 5 Tee Connecor Recangular (Type B 0.63 Room Temp. Change Recangular (Type B 0.1 Repeaabiliy (for 0 ime (Type A 3.5 Combined Sandard 6.1 Uncerainy (k 1 7. CONFIRMATION OF THE SYSTEM RESULTS To confirm he reul of he new auomaed yem, a e of 0.5, 3 and 10 TC were en o NPL, UK, for calibraion. Two of hee range were re-calibraed a NIS, Egyp, by uing he new auomaed yem. The reul of he comparion are abulaed in Table. For example, he viual repreenaive of he relaive error and i aociaed expanded uncerainie for he range of 3 a 50 Hz i ploed again he reul of NPL a hown in Fig. 5. AC-DC Difference, ppm Com parion of 3 a 50 Hz NPL NIS Fig. 5: Comparion of 3 a 50 Hz. 4

5 Simpoio de Merología al 7 de ocubre de 006 Table : Reul of comparion beween NIS and NPL NPL Reul NIS Reul Range AC-DC Difference, ppm Uncerainy, ± ppm AC-DC Difference, ppm 3 (50 Hz (1 khz (0 khz (50 Hz (0 khz (100 khz Uncerainy, ± ppm Proficiency e (Efficiency e i defined a he deerminaion of he laboraory performance by mean of comparing and evaluaing calibraion or e on he ame or imilar iem or maerial by wo or more laboraorie in accordance wih predeermined condiion. The error normalized wa ued for he reamen of he daa according o he following proficiency equaion [9]. L R E n (13 U L U R where: E n normalized error of he applican laboraory. L ν he value a meaured by he applican lab. R ν he value a meaured by he reference lab. U Lν he uncerainy of he applican lab. U Rν he uncerainy of he reference lab. To pa he proficiency e, he value of E n hould o be le han one (i.e. En < 1 [9]. Table 3 li he reul of hi e uing NPL a a reference lab. The reul pa he efficiency e a all he meauring poin. Table 3: Summary of he efficiency e reul Range Frequency E n value 3 50 Hz 0.57 '' 1 khz 0. '' 0 khz Hz khz 0.04 '' 100 kh CONCLUSION Thi paper decribe NIS effor on improving i AC-DC ranfer comparaor yem and give a relaed uncerainy budge. I i uiable o uppor AC-DC difference deerminaion a ppm uncerainy level. To confirm he efficiency of hi yem, bilaeral inercomparion beween NIS, Egyp and NPL, England ha been performed uing he reul of AC-DC difference of HOLT 3 and 10 a differen frequencie. The calculaed reul how a good agreemen for he majoriy of hee funcion. ACKNOWLEDGMENT The auhor wih o acknowledge Joeph Kinard and Thoma Lipe, from NIST, U.S. Through a projec beween NIST and NIS, he eenial componen of hi yem were inalled. Their helpful uggeion and exered effor uppored hi work. REFERENCES [l] J. R. Kinard, J. R. Haing, T. E. Lipe, and C. B. Childer, AC-DC Difference Calibraion, NIST, Special Publicaion 50-7, May [] J. L. YING, S. W. CHUA and. K. S. TAN "Auomaic Calibraion Syem for AC/DC Tranfer Sandard", NCSL Workhop & Sympoium [3] Earl S. William The Pracical Ue of AC-DC Tranfer Inrumen, NBS TECHNICAL NOTE 1166, Ocober 198. [4] E. S. Williama AND j. r. Kinard, Adual-channel Auomaed Comparaor for AC-DC Difference Meauremen, IEEE Tran. Inrum. Mea., IM-34, pp , July [5] Mamdouh. M. Halawa Improved Applicaion of Thermal Tranfer Sandard Inrumen For he Highly Accurae Meauremen of A.C. Curren. Ph. D. Thei, Ain Sham Univeriy, 00. [6] B. N. Taylor and C. E. Kuya, Guideline for evaluaing and expreing he uncerainy of NIST meauremen reul, NIST Tech. Noe 197, U.S. Governmen Prining Office, [7] Jon M. Crickenberger Calibraion Laboraorie Technical Guide. NIST HANDBOOK 150-, Aug., [8] Thoma E. Lipe, A Reevaluaion of he NIST Low- Frequency Sandard for AC-DC Difference in he olage Range , IEEE Tran. Inrum. Mea., vol. 45, No. 6, DEC [9] Jame L. Cigler and anda R. Whie Procedure and General Requiremen, NIST HANDBOOK 150, March