CREATION AND DEVELOPMENT OF THE NATIONAL HUMIDITY STANDARD OF GASES

Transcription

1 CREATION AND DEVELOPMENT OF THE NATIONAL HUMIDITY STANDARD OF GASES Efimia LUCHIAN, Constantin BORDIANU, Grigore BUZUC, Victoria CEBAN National Institute of Metrology, Chisinau, Republic of Molova tel.: (+373) Summary: Creation an research of national humiity stanar of gases unit is a part of the program of metrology infrastructure evelopment, approve by the Government of Molova. It is one of the prime steps in ensuring the uniformity of measurements in the country since in the Republic of Molova was a lack of stanar base in this fiel of measurements. As a result, economic agents were force to invest more in the measurement provie in other countries, which inevitably le to higher costs for maintaining quality of the omestic prouction. In this paper, the principle of the realization of humiity unit gases an further transfer to measuring instruments are escribe. For the qualitative implementation of the foregoing, the main components of the measurement uncertainty are presente. INTRODUCTION Humiity measurement is an important part of many fiels of national economy, such as: - woo inustry (monitoring of rying process of woo an woo proucts); - foo inustry; - agro-inustrial sector (in orer to know: the irrigation rational soil moisture, humiity of cereals for a better harvesting an storage timing, humiity of inustrial plants etc.) - trae activity (import, export); - biology an meicine (germination, fermentation, iagnosis, treatment); - pharmaceutical inustry [1]. In orer to reuce the negative impact of humiity on the economy sectors of Republic of Molova we nee to know humiity values with a high egree of certainty. This requires a national stanar for gases humiity which will be able to collect, preserve an transmit the measurement unit of humiity. Creation, research an evelopment of the national stanars is a primary function of the National Institute of Metrology (NIM). At the same time, it is responsible for the transmission of measurement unit towars the measuring instruments use by economic agents from Molova. Another very important aspect is to ensure the uniformity for gas humiity measurement respecting the traceability of the national measurement unit to primary international stanars, an finally, to SI units (K Pa) []. This is necessary in orer to give to the economic entities the opportunity to perform the calibration of measuring instruments in the Republic of Molova, an in such way to reuce costs. The hierarchy of humiity stanars is presente in Figure 1. Figure 1. The hierarchy of humiity stanars

2 NATIONAL HUMIDITY STANDARD OF GASES (NHSG) During the selection process of the proper equipment neee for creation of NHSG several important factors have been taken into account, such as: Measurement accuracy require by economic entities, in orer to satisfy the country's nees; Scientific an technical progress in the fiel; The practice of other countries in creating national stanars; Technical an economic arguments. Since Molova is party to various international professional organizations, incluing the Interstate Council for Stanarization, Metrology an Certification (EASC), the requirements specifie in the interstate ocument GOST State system for ensuring the uniformity of measurements. State verification scheule for means measuring humiity of gases [3] were taken into account. NIM is currently registere in the key comparison of the regional organization COOMET, entitle 544/RU/11 Regional comparison humiity stanars of gases. Dew/frost point temperature -50 C to +0 C [4]. As a result the following equipments have been purchase: 1. Humiity Generator, moel 500ST prouce by Thuner Scientific (Figure ), which is an automate system able to reprouce various values of humiity, in a wie range of temperatures, an will be use for the calibration of measuring means of both the relative humiity of the air, an temperature.. Dew point stanar hygrometer, moel Dew Point Mirror 373H, prouce by MBW, Switzerlan (Figure 3). Figure. Humiity generator, moel 500ST Figure 3. Dew point stanar hygrometer, moel Dew Point Mirror 373H Technical an metrological characteristics of Humiity generator, moel 500ST, an Dew point stanar hygrometer, moel Dew Point Mirror 373H, are given in Table 1 an Table, respectively. Table 1. Technical an metrological characteristics of Humiity generator, moel 500ST Relative humiity range 10 95% Relative humiity isplay resolution 0.0% Relative humiity uncertainty 0.5% RH Chamber temperature range C Temperature isplay resolution 0.0 C Chamber temperature uniformity 0.1 C Chamber temperature uncertainty 0.06 C Chamber pressure uncertainty 0.15% Hygrometer Moel 373H fulfills the best performance achieve in the humiity measurement. It is base on optical etection technology of the conensing moment on the coole mirror.

3 Table. Technical an metrological characteristics of Dew point stanar hygrometer, moel Dew Point Mirror 373H Dew point measuring range (-30 70) C Relative humiity measuring range % Dew point accuracy ± 0.1 C Dew point repoructibility ± 0.05 C Mirror cooling 3-stage Peltier thermoelectric Measuring uncertainity 0.04 C In orer to minimize the influence of the human factor an, to increase the accuracy of measurement, the laboratory is equippe with specialize software applications capable to minimize the operator's intervention. THE METHOD OF REALIZATION THE HUMIDITY UNIT OF GASES Realization of the gas humiity unit is base on the two-pressure metho of proucing known atmospheres of relative humiity an assumes that the water vapor pressure remains a fraction of the total pressure, known as Dalton's Law of Partial Pressure. Dalton's Law states that the pressure exerte by a mixture of gases in a given volume at some temperature is equal to the sum of the pressures which woul be exerte by each iniviual gas if it alone occupie the volume at the same temperature [5]. P = p 1 + p p n where: p 1, p, p n - partial pressures of each component. The two pressure metho (shown in elemental schematic form in figure 4) involves saturating air, or some other gas such as nitrogen, with water vapor at a given pressure an temperature. The saturate gas then flows through an expansion valve where it is isothermally reuce to chamber pressure. If the temperature of the gas is hel constant uring pressure reuction, the humiity, at chamber pressure, may then be approximate as the ratio of two absolute pressures. Pchamber RH %.100 Psaturator Humiity prouce in the test chamber of this system oes not epen on evices such as psychrometers, ew point hygrometers, or soli state sensors for the measurement of water vapor content. Humiity that is prouce is solely epenent on the measurement of absolute pressures an on the maintenance of isothermal conitions. Precision humiity generation is etermine by the accuracy of these pressure measurements an uniformity of temperature throughout the generating system [6]. Figure 4. Elemental schematic form of the two pressure metho ESTIMATION OF UNCERTAINTY OF MEASUREMENT Measurement uncertainty estimation is carrie out in accorance with SM SR ISO/CEI Guie 98-3:011 [7] an it is quite ifficult ue to both the large number of influential factors an complexity of etermining relations for humiity unit. For example, the mathematical moel for uncertainty estimation in the case of ew or frost point temperature measurements can be expresse by next equation: u ( t t ) t s t u ( ts ) P s t u ( Ps ) P c t u ( Pc ) s u ( ) u s ( t mas ) (1)

4 Dew/frost point can be etermine from the following relation: f t s, Ps Pc ew( t ) es ( ts ) s f t, Pc Ps () Where: f t s, P s, f t c, P c multiplication factors evaluate at saturation temperature an pressure, an chamber temperature an pressure, respectively; e s saturation vapor pressure; e w saturation vapor pressure over water; e g saturation vapor pressure over ice; η s saturation efficiency; t, t f, t c, t s ew/frost point temperature from chamber an saturator; P c, P s chamber an saturator pressures. In this case, uncertainty components relate to ew/frost point generation are ue to saturator an chamber pressures, saturator temperature an efficiency. For example, the uncertainty ue to the saturator temperature u(t s ) will be: uc ( ts ) u ( ts ) u ( ts rift ) u ( ts rez ) u r ( e ) u ( ts eff ) (5) δt s is the contribution given by the calibration of temperature transucer of the saturator: Uts ts (6) U ts expane uncertainty (for k = ), from the calibration certificate. Contribution given by the temporal rift of the temperature transucer of the saturator - δt s rift. The eviation of the inication as against to the last calibration is estimate from calibration history. This temporal rift ( ts ) cannot be use as a correction, but may be taken into account in the uncertainty evaluation. The stanar uncertainty of this rectangular istribution is: ts u ts rift (7) 3 Contribution given by the temperature transucer resolution δt s rez. The associate uncertainty of this rectangular istribution is: ts rez u ts rez (8) 3 The uncertainty ue to the equation for vapor pressure calculation u r (e). Pressures e s an e c represents the pressures of saturate vapors calculate, using the equation Hary [8], at saturator an chamber temperature, respectively. 6 i e s exp ai T a7 lnt (9) i0 where T = t represents the temperature in K; a i interpolation coefficients accoring to Hary relation. Ieally, in the generators base on "two pressure" principle, the saturator an chamber temperatures are ientic, an the saturation pressure ratio e s /e c = 1. However, in a real system, there is a little ifference between these two temperatures, an they shoul be measure. The associate uncertainty ue to calculation of vapor pressure using this relation is: u r ( e ) 0,005 %, in the temperature range between 0 C an 100 C, u r ( e ) (0,01 0,005 t ) %, in the temperature range between -100 C an 0 C. u(t s eff ) uncertainty ue to saturation efficiency. In relation () t s is ew temperature at pressure P s at saturator outlet. Actually the value is influence by the temperature of the flui, which in the pre-saturator (before the saturator inlet) is preconitione to a temperature higher than the ew point. The associate uncertainty ue to the efficiency of the saturator takes into account the ifference between the temperature of the flui t f an t s resulting by saturate air flow at temperatures higher (or lower) than the conensate. The humiity generators, base on the two pressures principle, are base on the complete saturation capacity of the gas with water vapor, uring its passage from the inlet to the outlet of the

5 saturator. Saturation efficiency is estimate to be 100 ± 0.35%. The stanar associate uncertainty of this triangular istribution is: 0,35 ut s eff (10) 6 At the same time, some components of the mathematical moel are epenent on other influencing sizes, fact that further complicates the estimation of measurement uncertainty. CONCLUSIONS Creation, research an evelopment of the national stanars is a primary function of any Institute of Metrology an at the same time, the primary conition for the full implementation of the unity of measurements in the country. The national stanar of gas humiity unit is intene for the realization, storage an transfer of the unit through working stanars to measuring instruments use in the national economy. As a result of participation in the international comparisons within regional organizations COOMET, the National Institute of Metrology from Republic of Molova will emonstrate its measurement capabilities. Thus, both the national stanar an humiity measurements provie in the Republic of Molova will be recognize at regional an international levels. This fact will help to remove technical barriers to trae encountere by economic agents from Molova. BIBLIOGRAPHY [1] Borianu Constantin, The importance of creation of national stanar unit for humiity, Metrologie, Vol.1 (7)/013, p 9-1. [] SM SR Ghi ISO/CEI 99:01 Vocabulary of Metrology. Basic an General Concepts an Associate Terms (VIM) [3] GOST State system for ensuring the uniformity of measurements. State verification scheule for means measuring humiity of gases [4] [5] The Rotronic Humiity Hanbook, 005 Rotronic Instrument Corp. [6] Thuner Scientific Corporation, Operation an maintenance manual for Series 500 benchtop two-pressure humiity generator, 63 Wyoming S.E. Albuquerque, New Mexico , 003, eition 4, p 106. [7] SM SR Ghi ISO/CEI 98:011 Uncertainty of measurement. Part 3. Guie to the expression of uncertainty in measurement [8] R. Hary, Papers an Abstracts of the Thir International Symposium on Humiity an Moisture, April 1998, vol. 1, pp. 14,.