APPLICATION OF MASS MEASUREMENT TECHNIQUE IN CALIBRATION AND CONTROL FOR GAS CYLINDERS

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1 Mesureent of Mss, Force nd Torque (APMF 203) Interntionl Journl of Modern Physics: Conference Series Vol. 24 (203) (7 pges) The Authors DOI: 0.42/S APPLICATION OF MASS MEASUREMENT TECHNIQUE IN CALIBRATION AND CONTROL FOR GAS CYLINDERS Int. J. Mod. Phys. Conf. Ser Downloded fro YI-CHING LIN *, CHIN-FEN TUAN Nno & Mechnicl Mesureent Lbortory, Center for Mesureent Stndrds, Industril Technology Reserch Institute, Hsinchu, Tiwn 300, R.O.C. * frnceslu@itri.org.tw This pper dels with esureent nd control of the stndrd gs cylinder. In CMS (Center of Mesureent Stndrd), the ss esureent technology is used to clibrte the concentrtion of stndrd gs cylinders. This esureent ethod is bsed on the ss chnge of gs cylinder before nd fter filling. By using blnce nd clculting ir buoyncy the ss difference of gs cylinder before nd fter filling could be obtined. In order to understnd stbility of the gs cylinder, we use sttisticl control techniques to onitor the chnge of concentrtion of the stndrd gs cylinders, to tke iedite esures when the concentrtion of the stndrd gs cylinders is chnged. Keywords: Gs filling; grvietric ethod; sttisticl control.. Introduction Gs cylinders nd gs indictors (nd gs detectors) in Industry cn be trced bck to the stndrd gs cylinder of the Gs Mesureent Syste of CMS (Center of Mesureent Stndrd), nd the stndrd gs cylinder cn be trced bck to the ss stndrd, s shown in Fig.. Prototype Kilogr No.78 (kg) Stndrd Weight Stndrd Gs Cylinder Gs Cylinder, Gs detectors Mss Mesureent Syste in CMS Gs Mesureent Syste in CMS Gs Mesureent Syste in CMS Industry All kinds of pure gs, liquid gs, cheicl substnces, ixtures design nd instlltion of industril, seiconductor, edicl nd vriety of specil gs cylinder configurtion nd the gs test. Fig.. The schetic digr for trcebility chin of gs esureent. This is n Open Access rticle published by World Scientific Publishing Copny. It is distributed under the ters of the Cretive Coons Attribution 3.0 (CC-BY) License. Further distribution of this work is peritted, provided the originl work is properly cited

2 Y.-C. Lin & C. F. Tun Int. J. Mod. Phys. Conf. Ser Downloded fro The volue clibrtion of stndrd gs cylinder use grvietric ethod to perfor the esureent. Its principle is bsed on the chrcteristic of ss chnge fter the gs cylinder filling gs ixture, in order to estite the olr concentrtion of ech coponent []. The esureent procedure of the grvietric ethod is dopting the substitution weighing ethod to get ss differences of gs cylinder before nd fter filling. During weighing, the ss difference between sple nd reference cylinder becoe iniized by dding dditionl weights in the Gs Mesureent Syste of CMS. An utotic turntble device of utotic weighing ws equipped to reduce hun error of the esureent nd then the volue obtined fro ss ws evluted to coplete the clibrtion. Due to the effect of teperture nd huidity, the gs cylinders re esily unstble even in vlid period. So in the long-ter use, the concentrtion of gs cylinder filling could be influenced by environent or indequte opertion. In order to control the concentrtion chnges of gs cylinder, the check schee should be estblished in ddition to regulrly clibrtion. This pper lso describes the checking echniss of Gs Mesureent Syste in CMS. 2. Clibrtion of the stndrd gs cylinders 2.. Mesureent equipent In CMS, the weighing syste of The Gs Mesureent Syste is designed to use n utotic turntble weighing syste, s shown in Fig. 2. It ws put into n ntisttic crylic shield. The pn of weighing syste cn crry four cylinders with two-stge drops, the first stge rpid decline to will sve esureent tie, the second stge slow down when gs cylinder is close to the blnce pn. The tie intervl between weighing is kept constnt. gs cylinder Turntble Blnce Grnite ltfor Fig.. Autotic turntble of weighing syste

3 Appliction of Mss Mesureent Technique in Clibrtion nd Control for Gs Cylinders 2.2. Mesureent eqution Int. J. Mod. Phys. Conf. Ser Downloded fro The gs cylinders (X) to be clibrted could be trced bck to the stndrd cylinder (R) fro Mesureent Syste of CMS, nd the stndrd cylinder (R), cn be trced bck to the stndrd weights fro CMS. The utotic turntble weighing syste fro Gs Mesureent Syste of CMS is used during the esureent, nd Eq. () fors the bsic odel of the esureent (the ir buoyncy is negligible): where ( R R_dj VR-dj) ( X X_dj VX_dj) + ρ + ρ = δ () R:The ss of the reference stndrd cylinder :The ss of the cylinder to be clibrted X R_dj :The ss of the djustent weight for the stnder cylinder V R-dj :The volue of the djustent weight for the stnder cylinder X_dj:The ss of the djustent weight for the cylinder to be clibrted V X_dj :The volue of the djustent weight for the cylinder to be clibrted ρ :Air density δ:corrected reding of the blnces. R_dj nd X_dj re the djustent weights. The ss differences could be obtined by rerrnging Eq. (), s shown below: ( ) = X R = R_dj X_dj + ρ VX_dj VR_dj δ. (2) The reding of the blnces is corrected by using the sensitivity weight (the noinl vlue is g). So the corrected reding of the blnces, δ, is obtined by using the sensitivity weight. is the ss difference before nd fter the cylinder ws filled with gs. ρ V V is the ir buoyncy correction, in order to clculte the reference ir ( X_dj R_dj) density ccording to CIPM 8-9 forul [3], the ir teperture, T, pressure, P, the reltive huidity, H, nd the concentrtion of CO 2 hve to be known, s shown in Eq. (3) M nd frction, Z is the copressibility fctor. PM M ρ = XV ZRT M M V re the oleculr weight of ir nd wter, 2.3. Uncertinty nlysis V. (3) X V is the wter vpor olr The uncertinties of the esureent Syste re: the uncertinty of repeted esuring the ss of cylinders, the uncertinty brought bout by the blnce, the uncertinty of stnder weights, nd the uncertinty cused by ir buoyncy

4 Y.-C. Lin & C. F. Tun Int. J. Mod. Phys. Conf. Ser Downloded fro Air buoyncy effect in the esureent process, re "the vritions of environentl conditions in the lbortory" nd "the volue differences (including cylinders nd djustent weights)". In this syste, the differences of the volue is resulted fro the differences in the volue of cylinders (the volue difference including the volue occupied by teril nd internl volue of the cylinder is bout up to 0.2 L for 6 L cylinder cpcity) nd weights. We will use the djustent weights to djust the difference of the ss between the sple nd the reference cylinder to be less thn g. In this cse, the ethod used for the volue of code cused by ir buoyncy effects need to be included in the clcultion. After filling, the sources of vritions of the gs cylinder volue include ir buoyncy effects nd gs-filled proportionl to the pressure. According to ISO 642 Annex exple cylinders for 5 L cpcity ssessent, when the pressure ws incresed to 50 br cylinder cused by chnge in volue is pproxitely 0.02 L. Therefore, tke the ir density extrees (extree vlues) into ccount, this effect results in n increse cused by weighing pproxitely 22.9 g ~ 24.8 g, with n verge of 23.8 g, its stndrd uncertinty is u exp = 23.8 / 3 = 3.7g. 3. The sttisticl control of stndrd gs cylinder The gs cylinders y be unstble during its usge. In order to control the chnge of the gs cylinder, in ddition to clibrte regulrly, we designed qulity control procedure. The interedite control checks re used between two clibrtions for the stndrd gs cylinder. The check stndrd of gs cylinder ws used in the control procedure to onitor long-ter observing of the stndrd gs cylinder nd to understnd the rndo vrition of environent, equipent nd gs cylinders. Using the sttisticl control techniques, the control liits nd control chrt fro esureent dt of historicl vlues were clculted to check the stbility of the gs cylinder. 3.. Check gs cylinder Gs concentrtion esureent use gs chrotogrphy (GC) with therl conductivity detector (TCD) or fle ioniztion detector (FID). The redings of esureent (B) re electronic signl by GC. The coprison ethod is used to esure the cylinder concentrtion, lternting interchnge the stndrd gs cylinders (R) nd the check gs cylinders (S), the concentrtion rtio (r) of gs cylinder re obtined, r = B S / B R. Both check nd stndrd gs cylinders re the se concentrtion, coposition nd teril. Mesured t different ties, the two gs cylinders re plced in the se environents, the esureent results of concentrtion rtio (r) should not be chnge. Three coprisons is perfored during ech esureent, the esureent verge of concentrtion rtio vlues is:

5 Appliction of Mss Mesureent Technique in Clibrtion nd Control for Gs Cylinders r r + r + r = (4) B Si, i,2,3 ri = = (5) B B Si :An electronic signl of the check gs cylinder by GC B Ri :An electronic signl of the stndrd gs cylinder by GC. The stndrd devition of single esureent dt is: Ri Int. J. Mod. Phys. Conf. Ser Downloded fro n s( r) = ( ri r), n= 3. (6) n i= If there re esureent dt, r,..., r i, collected fro historicl dt, the stndrd devition of esureent dt is: where nd the pooled stndrd devition is: 3.2. Control chrt s( r) = ( r i r), (7) S r p i= r + r + r 2 3 = (8) = i= 2 s ( r) i. (9) The esureent dt fro historicl, r,..., r i, should be norl distribution of rndo vrition. Using two kinds of control chrt to onitor the vrition of esureent dt. They re concentrtion rtio of control chrt nd stndrd devition of control chrt. Its control liits re clculted s follows: The control liits for the concentrtion rtio of gs cylinder re: Upper wrning liit = r+ 2 s( r) (0) Lower wrning liit = r 2 s( r) () Upper control liit = r+ 3 s( r) (2) Lower control liit = r 3 s( r). (3)

6 Y.-C. Lin & C. F. Tun Int. J. Mod. Phys. Conf. Ser Downloded fro The control liits for stndrd devition re: Upper wrning liit = 2S p (4) Upper control liit = 3S p. (5) The upper nd lower control liits re two horizontl lines drwn in the control chrt. A siple decision rule is when dt points fll outside of these lines, it indictes tht it is sttisticlly likely there is proble with the gs cylinder. If dt points fll in between control liits nd wrning liits, cre should be tken when using the cylinder nd the gs cylinder needs to be checked. For exple, the control chrt for concentrtion rtio of C 3 H 8 stndrd gs cylinder is shown in Fig.. In this chrt, solid lines re upper nd lower control liits, dotted lines re upper nd lower wrning liits, before 5 points re the historicl dt fro esureent, then = 5. The nuber 9 nd 20 dt points fll outside the control liits (lbeled s " " point in Fig. ), which indictes tht the concentrtion of gs cylinder y hd non-rndo chnge, which en tht the process is probbly out of control nd tht n investigtion is wrrnted to find nd eliinte the cuse or cuses. The nubers 2 nd 22 dt points fll in between control liits nd wrning liits (lbeled s "*" point in Fig. ), the gs cylinders ust intensive cre to void instbility. vlue vlue Nuber of esureent dt Upper control liit Upper wrning liit Lower wrning liit Lower control liit Fig.. This is the control chrt for concentrtion rtio of C 3 H 8 stndrd gs cylinder. 4. Result nd Discussion To estite the ss of the filled gs, the ss difference of the cylinder before nd fter the filling is clculted. Since the chnge of the cylinder volue hs been considered in ech esureent, the effect of this chnge cn be eliinted by the use of reltive weighing (i.e., using the se reference cylinder). The uncertinty is iniized by stbilizing the ir density round the syste. For exple, if there is ±

7 Appliction of Mss Mesureent Technique in Clibrtion nd Control for Gs Cylinders Int. J. Mod. Phys. Conf. Ser Downloded fro uncertinty in the teperture, nd ± 5 hp uncertinty in the ir pressure, nd the huidity could be controlled within ± 0% RH, then the xiu uncertinty of ir density is less thn g/l. There is no gurntee tht the cylinders will not chnge during usge, so the stndrd gs cylinders should be clibrted periodiclly. Since the interedite control checks cn understnd the chnge of gs cylinder nd to deterine the tie of clibrtion cycle. If the concentrtion of gs cylinder is very stble during long tie usge, then the clibrtion period could be prolonged ccordingly. The control dt could be nlyzed using control chrts nd sttisticl ethods. The "in control" iplies tht ll points re between the upper nd lower control liits nd they for rndo pttern. If there is periodic chnge in the control chrts, or the chnges re directed long one direction, to show non-norl distribution curve. The cuses of the exception include chnges in gs cylinders, opertors ight hve de istkes, the environent ight be unstble or the nlyzing instruent could be lfunctioned. Iproveents should be de ccordingly. The sttus of the cylinders could be onitored nd iproved by using sttisticl control ethods. To use the control chrt correctly, the clibrtion qulity could be gurnteed nd iproved. References. ISO 642 Gs nlysis -Preprtion of clibrtion gs ixtures - Grvietric ethod (200). 2. ISO/IEC Guide 98-3:2008, Uncertinty of esureent Prt 3: Guide to the expression of uncertinty in esureent (GUM:995). 3. R.S. Dvis, Eqution for the Deterintion of the Density of Moist Air (98/9), Metrologi, 992, 29, Stndrd Test Method for Anlysis of Nturl Gs by Gs Chrotogrphy, ASTM D945-03,