Establishment of the Gas Flow Calibration Facility at CSIR-NPL, India

Transcription

1 Establishment of the Gas Flow Calibration Facility at CSIR-PL, India Speaker: Shiv Kumar Jaiswal Fluid Flow Measurement Standards CSIR-ational Physical Laboratory (PLI) Dr. K.S. Krishnan Road ew Delhi India Phone: ; Fax: Authors: Shiv Kumar Jaiswal, I. S. Taak and Chatar Singh Abstract Recently Gas Flow Calibration System (GFCS) has been established in the Fluid Flow Measurement Standard group of PLI in the flow range from 10 sccm (standard cubic centimeter per minute) to 1000 slm (standard litre per minute) at 0 ºC and kpa. The expanded measurement uncertainty of the system in the above flow range is ±0.2 % at k=2. This GFCS is suitable for calibration of different types of flowmeters i.e. mass flow controllers, mass flow meters, rotameters, totalizer type meters, digital flow calibrators, compact provers, etc used in many research, development and industrial applications mainly process controls, environmental monitoring, pharmaceuticals and drugs manufacturing, health assessment etc. and also in many ABL accredited laboratories. Present paper describes the details of the GFCS architecture and calibration of a rotameter (i.e. volumetric flow rate device) as a case study. The uncertainty has been estimated as per ISO GUM document guidelines [1]. The standards used for calibration are traceable to appropriate ational standards. Index Terms Gas Flow Calibration System, uncertainty, national metrology institute (MI), air compressor, laminar flow element, sonic nozzle, volume flow rate, mass flow rate. 1. Introduction The CSIR-ational Physical Laboratory, India (PLI) is the national metrology institute (MI) of India and it is custodian of all physical parameters except ionizing radiation. The Fluid Flow Measurement Standard group of CSIR-ational Physical Laboratory, India (PLI) has water meter testing and calibration facilities. The group has testing facility up to 50 mm and calibration facility up to 200 mm. Presently, the group is providing testing services of the water flow meters as per IS 779, IS 6784 and ISO 4064 documentary standards to the various user organizations traceable to the ational Standard. The upgradation of old calibration facility using state-ofthe-art instrumentation and control has been planned to make it comparable to advanced MIs and provide traceable measurements to Indian users. As a follow up of the upgradation, a new prototype automatic water flow calibration standard (i.e. primary standard of water flow) of size D100 was designed and developed indigenously. The expanded uncertainty of this new system at 2000 kg collection mass is 0.03% at k=2. Keeping in view the requirement of in-house users such as Chemical Metrology group, Radio and Atmospheric Science Division, Solar Energy

2 group, etc. and external users such as Pollution Control Boards, pharmaceutical industries, petrochemical industries, environmental monitoring equipment manufacturers, R&D laboratories, ABL accredited laboratories, gas flow meters (mass flow controllers, mass flow meters, rotameters, digital flow calibrators) manufacturers, aerospace industries, etc., the GFCS has been established recently at Fluid Flow Laboratory which is based on laminar flow elements and sonic nozzles. The flow range of laminar flow element is from 10 sccm to 50 slm with operating pressure of 100 kpa absolute to 525 kpa absolute. The flow range of sonic nozzle (also called critical flow nozzle) is from 10 slm to 1000 slm with operating pressure of 50 kpa absolute to 525 kpa absolute. The expanded uncertainty of the GFCS is ±0.2 % at k=2 in the flow range of 10 sccm to 1000 slm. This system is suitable for calibration of different types of flowmeters such as mass flow controllers, mass flow meters, rotameters, totalizer type meters, digital flow calibrators, compact provers, etc. The GFCS has capabilities to supports multi-gas calibration such as nitrogen, air, carbon dioxide, helium etc. In the present paper, the details of the system have been described highlighting calibration results of a rotameter as a case study. 2. Details of Architecture and Methodology of the GFCS The system is essentially consisted of laminar flow elements, sonic nozzles, high precision flow display device named as mass flow terminal, molstic for mounting laminar flow elements, sonic nozzles and device under calibrations along with associated instrumentations such as barometric pressure monitor and digital thermometer (PT-100 sensor with indicator). There are four numbers of laminar flow elements (i.e. 100 sccm, 1 slm, 10 slm and 30 slm) which cover the flow range of 10 sccm to 50 slm with operating pressure of 100 kpa absolute to 525 kpa absolute. The lower flow of 0.8 sccm is also achievable with increased uncertainty. There are two numbers of sonic nozzles having flow coefficients of 200 sccm/kpa and 2 slm/kpa which cover the flow range of 10 slm to 1000 slm with operating pressure of 50 kpa absolute to 525 kpa absolute which uses vacuum pump for realizing the flow below 30% of full scale (FS) rating of sonic nozzle. The laminar flow elements and sonic nozzles are known as molblocs in Fluke* terminology and represented as molbloc-l and molboc-s respectively. Each molbloc has two numbers of built-in PT-100 sensors and upstream and downstream pressure taps. It has one EEPROM chip which stores information like flow range, calibration coefficients, different types of calibration, etc. The output of laminar and sonic nozzle is read by mass flow terminal. The mass flow terminal is a high precision display device which consists of two nos. of high precision absolute pressure sensors (reference pressure transducers) for reading the upstream and downstream pressure of laminar flow elements and sonic nozzles. The terminal has one number of PT-100 sensor indicator for gas temperature measurement. The pressure, temperature and IST gas reference property data are used to realize the flow. The mass flow terminal is essentially a flow computer which displays the flow in different units such as sccm, slm, vlm, uccm, pccm, mg/s, etc. Due to use of absolute pressure sensors, the accuracy of differential pressure measurement has increased resulting into improvement of flow accuracy. The molbox terminal has features for leak testing in the flow line and over pressure protection system [2-3]. Figure 1 shows the photographs of the GFCS which also includes air/ gas supply source. * Commercial equipment and materials are identified in order to adequately specify certain procedures. In no case does such identification imply recommendation or endorsement by PL-India nor does it imply that the material or equipment identified are necessarily the best available for the purpose.

3 (a) Photograph showing system architecture and calibration set up of a rotameter (b) Air Receiver of 1000 L capacity (c) Air compressor system and gas cylinders Figure 1. Photograph of Gas Flow Calibration System

4 2.1. Realization of Flow in Laminar Flow Element The mass flow rate of a compressible fluid in laminar flow element is realized using Poiseulle s Law [3] as follows: where, q m = mass flow rate (kg/s) P 1 = upstream absolute pressure (Pa) P 2 = downstream absolute pressure (Pa) P = (P 1 + P 2 )/2= average of upstream and downstream pressures (Pa) T = absolute temperature of gas (K) = gas density under P, T conditions (kg/m 3 ) P,T q m P 1 P 2 P, T P, T 6 L 3 R h R= flow passage radius (m) h = gap between piston and cylinder (m) η = dynamic gas viscosity under P, T conditions (Pa.s) L = length of laminar flow (m) (1) The gas density P,T is calculated by the following equation: where, ρ = standard gas density P = standard pressure ( kpa) T = standard temperature ( K) Z = gas compressibility factor under standard conditions = gas compressibility factor under P, T conditions Z P, T P, T P T P Z T Z P, T (2) By defining: C D R h 3 6 L and which becomes C G (i.e. experimentally determined geometrical constant) by calibration, the equation (1) becomes (3) q m P P 1 P 2 T Z P, T P, T T P Z C G (4)

5 Thus, for laminar flow element, the flow is proportional to the differential pressure (P 1 -P 2 ), created across the laminar flow gap Realization of Flow in Sonic ozzle The sonic nozzles also known as critical flow venturies (CFV), measure the flow using a geometrically specified flow path where the flow rate is the maximum based on the upstream conditions of the gas. As long as the ratio of downstream pressure to upstream pressure is low enough, the flow is choked, i.e. the speed of the gas is limited to approximate local sonic velocity. The ratio of downstream pressure to upstream pressure is called back pressure ratio (BPR). The sonic nozzle is under choked condition if the value of BPR is less than 0.5. Once this condition is met, the mass flow rate of a compressible fluid in sonic nozzle is realized using the following formula [3]: q m A * C C R R M T 0 P 0 (5) where q m = mass flow rate (kg/s) A* = throat area (m 2 ) C = discharge coefficient C R = critical flow coefficient P 0 = absolute stagnation pressure (Pa) R = universal gas constant (J/kg-mole-K) M = molecular mass (kg/kg-mole) T 0 = Absolute stagnation temperature (K) In a sonic nozzle, the flow is increased or decreased by changing the upstream pressure to the nozzle using either a variable pressure regulator or a variable restriction. 3. Calibration Set-up and Method Figure 2 shows the schematic diagram of the calibration of a rotameters using molbloc. The flow can be also controlled by using a mass flow controller or flow control knob of rotameters, if the rotameter has provision for it. In calibration, the comparison method is used where volume flow rate read by standard meter (i.e. molbloc and mass flow terminal) is compared against volume flow rate indicated by meter under calibration (MUC). The volume flow rates are referenced to standard temperature (25 ºC) and standard barometric pressure ( kpa) conditions in our laboratory. The percentage error is estimated from the following formula: (Q vmuc Q vstd ) Percentage Error (% e) = (6) Q vstd

6 where, Q vmuc = the flow indicated by the MUC under normal temperature T and normal pressure P conditions Q vmuc = the flow indicated by the MUC corrected to standard temperature T S and standard Pressure P S conditions Q vstd = the flow measured by reference standard under standard temperature T S and standard pressure P S conditions Q vmuc is calculated from the following equation: P Ts Q vmuc = Q vmuc (7) P S T Flow In Pressure Regulator Molbloc eedle Valve Rotameter Flow Out Molbox1+ Terminal Barometer Thermometer Legend: Pressure cable Temperature & EEPROM cable Figure 2. Schematic diagram of a rotameter calibration using molboc. 4. Uncertainty Analysis 4.1. Mathematical Model The mathematical model in calibration of rotameter is as follows: e = [Q vmuc + δq A + δq R + δq ZD ] [Q vstd + δq vstd ] (8) P Ts and Q vmuc = Q vmuc (9) P S T where; e = error of measurement (absolute unit) Q vmuc = volume flow from the meter under calibration (MUC) corrected to standard pressure and temperature condition (absolute unit) δq A = error due repeatability in measurement of MUC (absolute unit)

7 δq R = error contribution due to resolution of MUC (absolute unit) δq ZD = error contribution due to zero drift of MUC (absolute unit) Q vstd = volume flow from the reference standard (absolute unit) δq vstd = error contribution from the reference standard (absolute unit) Q vmuc = volume flow indicated by the MUC under normal temperature T and normal pressure P (absolute unit) 4.2. Uncertainty Equation Assuming all the contributory factors as uncorrelated quantities, the combined uncertainty u c in volume flow rate is given by; u c 2 = (c 1 u 1 ) 2 +(c 2 u 2 ) 2 +(c 3 u 3 ) 2 + (c 4 u 4 ) 2 +(c 5 u 5 ) 2 + (c 6 u 6 ) 2 (10) where, u 1 (δq A ) = uncertainty due to repeatability in measurement of MUC u 2 (δq R ) = uncertainty due to resolution of MUC u 3 (δq ZD ) = uncertainty due to zero drift of MUC u 4 (δq STD ) = uncertainty from calibration certificate of the reference standard u 5 (P) = uncertainty due to barometric pressure monitor u 6 (T) = uncertainty due to thermometer (i.e. PT-100 sensor with indicator) The sensitivity coefficients of various contributions as c 1, c 2, c 3, c 4, c 5 and c 6 are estimated using; c 1 = e/ δq A =1, c 2 = e/ δq R =1, c 3 = e/ δq ZD =1, c 4 = e/ δq STD =1; c 5 = Q vmuc / P = Ts Q vmuc / (Ps T); c 6 = Q vmuc / T = -(P Ts Q vmuc )/ (Ps T 2 ) Since there are product components apart from additive components for uncertainty, therefore, it is easier to estimate the uncertainty in relative form using below given equation [4] and sensitivity coefficients are 1: [u c (e)/e] 2 = [u 1 (δq A )/Q VMUC ] 2 + [u 2 (δq R )/Q VMUC ] 2 + [u 3 (δq ZD )/Q VMUC ] 2 + [u 4 (δq VSTD )/Q VSTD ] 2 + [u 5 (P)/P] 2 + [u 6 (T)/T] 2 (11) The rotameter of 5 to 55 L/min flow range was calibrated using laminar flow standard. Table 1 shows the data of calibration of rotameter at a particular flow rate (i.e. 50 L/min). Type A uncertainty was determined from those data. Table 2 shows the uncertainty budget of rotameter calibration at 50 L/min. Depending on MUC and customer requirement, the % error of reading or FS may be reported.

8 Observation o. Table 1. Data of rotameter calibration at 50 L/min. MUC Flow C Ref. Flow C Error (MUC-STD) C FS Error % Average FS error= % Standard deviation (σ) = % Type A uncertainty = σ/ n = % Quantity X i Table 2. Uncertainty budget of rotameter calibration at 50 L/min. Estimate x i Limits ±Δ x i (%) Probability distribution Type A/B Repeatability of MUC (δq A ) 50 L/min - ormal, Type A Resolution of 1 L/min Rectangular, MUC (δq R ) Zero drift of 0 L/min 0 Rectangular, MUC (δq ZD ) Standard ormal, (δq STD ) L/min Barometer 100 kpa 0.01 ormal, Thermometer 25 C 0.04 ormal, Error (e) FS % Combined uncertainty u c (e) Expanded uncertainty U Relative standard uncertainty ±u i (%) Sensitivity coefficient c i Relative uncertainty contribution u i (y)= c i. u i (%) Degree of freedom ν i At coverage factor k= % (FS) Results and Future work In the present paper, the main features of GFCS have been reported. The calibration of rotameter using GFCS has also been presented along with detailed estimation of measurement uncertainty. In a typical calibration of rotameter, the uncertainty of measurement is found to be ±1.077 % at k=2. Similarly, if some improved artifact such as digital flow calibrators which have much better resolution and uncertainty is chosen, the estimated uncertainty would have been much improved.

9 In case of present rotameter calibration, resolution of the MUC is a dominant factor but digital gauges have better resolution. Our GFCS is a secondary standard but its uncertainty is comparable to primary standards like piston provers and bell provers. The advantages with piston provers and bell provers are that their traceability can be established in the fundamental units of length and time due to being primary device but same is not applicable to laminar flow elements and sonic nozzles. The laminar flow elements and sonic nozzles are made traceable by calibrating them against primary standards such as piston prover, bell prover, gravimetric system and PVTt standards. The uncertainties of gravimetric systems and PVTt standards are better than piston prover and bell prover. Keeping in view the better uncertainty of PVTt standard, it is being planned to develop PVTt standard at PLI in technical collaboration with advanced MIs like IST, USA and CMS, Taiwan so that the traceability of our gas flow calibration system may be established at in-house. Acknowledgement: The authors are thankful to Dr. A. Sen Gupta, Director, CSIR-ational Physical Laboratory, ew Delhi and Dr. V.. Ojha, Head, Apex Level Standards and Industrial Metrology (ALSIM) division for their constant encouragement. References: 1. JCGM 100, Evaluation of measurement data Guide to the expression of uncertainty in measurement, Fluke User manual for Molbox1+ Terminal and associated literature for molblocs. 3. Michael Bair, Technical ote 2011T06A: Uncertainty analysis for flow measured by molboc-l and molbloc-s mass flow transfer standards, 17 Feb 2003, Revised 18 May D. Kruh, Assessment of uncertainty in calibration of a gas mass flowmeter, Accred Qual Assur, vol. 5, Springer-Verlag, , 2000.