NMISA New Gas Flow Calibration Facility

Transcription

1 NMISA New Gas Flow Calibration Facility Speaker / Author: D. Jonker Co-author: E.P. Tarnow National Metrology Institute of South Africa (NMISA) Private Bag X34, Lynnwood Ridge, Pretoria, 0040, South Africa djonker@nmisa.org Phone: Fax: Abstract NMISA has recently purchased a new primary standard for gas flow calibrations to replace the obsolete Califlow instrument which has been in service. The main features of this new standard are its high accuracy, unequalled speed and convenience. This paper presents a discussion of the standard its design and operation. Before being put into operation to offer a calibration service to industry this primary standard and the calibration method were validated against a calibrated mass flow controller. The validation procedure and validation results are presented. Some theoretical aspects of gas flow; other laboratory equipment being used for gas flow measurements; different gas flow calibration measurement methods; analysis of measurement results and the calculation of measurement uncertainties are also discussed. 1. Introduction Flow is defined as the total volume of a fluid that flows past a fixed point in a given time. Flow measurement is widely used in industry as a means of process or quality control. Typical applications include environmental monitoring, industrial hygiene, manufacturing process control, and research and development. NMISA is mandated to provide South African industry with traceable measurements and for many years, gas flow traceability was provided through a Califlow primary gas flow standard. The Califlow has become obsolete since it is no longer supported by the manufacturer and has become unreliable. Therefore a new primary standard for gas flow measurement has been purchased to replace the Califlow instrument. 2. Overview of the NMISA Flow Laboratory The Gas Flow laboratory has now been positioned in the newly established Flow Section which consists of the Pressure, Viscosity and Gas Flow laboratories. The laboratory is staffed by one metrologist who is responsible for all gas flow calibrations. Currently, only gas flow calibration services are offered. The flow range covered is from 0,5 ml/min to ml/min. Nitrogen gas is used as the flow medium. Typical instruments received for calibration include mass flow controllers, mass flow meters, bubble flow meters and rota meters.

2 The gas flow laboratory is equipped with the following equipment: - Bios Met Lab ML-800 primary flow standard consisting of three flow cells, - Bios Integrator 110 Met Lab Command & Control Module (in the process of being purchased), - Bios Gas Flow Bench (in the process of being purchased), - Vane anemometer (currently out of service), - Bubble flow meter, - Environmental monitoring equipment (barometers; temperature- and humidity loggers). The NMISA Gas Flow laboratory is not yet SANAS (South African National Accreditation System) accredited. At the moment measurements are traceable to international standards held at NIST (National Institute of Science and Technology) which is the equivalent NMISA institute in the USA. NMISA is aiming to achieve SANAS accreditation in 2014 and in-house traceability shortly thereafter. 3. Standard flow rate versus volumetric flow rate Volumetric flow rate is defined as the actual volume flow of the gas exiting a flow meter. This is typically the flow rate as measured by primary standard flow meters such as piston provers and bubble flow meters. Mass flow meters on the other hand, measure the mass of the medium flowing past a point in a given time. Standard flow rate is defined as the equivalent flow rate of the gas if the temperature and pressure were at standard conditions. It is usually the most useful measure of gas flow because it defines the mass flow, number of molecules, and heat-carrying capacity of the gas. The term STP (Standard Temperature and Pressure) can be used to refer to a customer specific set of reference conditions, however if not specified by the customer, it specifically refers to a temperature of 273,15 K (0 C) and a pressure of 101,325 kpa (760 mmhg). Volumetric flow can be converted to Standard flow rate and vice versa as follows: Volumetric T flow Q T m std P P std m Where Q is the Standard flow rate, T m is the measured temperature of the gas in the flow tube, T std is the Standard temperature, P m is the gas pressure measured in the flow tube, P std is the Standard pressure [1] When standard flow rate values are reported on calibrations certificates, in order to differentiate them from volumetric flow rates (ccm cubic centimetres per minute or lpm litres per minute), the units are typically sccm (standard cubic centimetres per minute) or slpm (standard litres per minute).

3 4. NMISA Primary Standard for Gas Flow Calibrations The Bios Met Lab ML-800 was purchased as a primary standard for gas flow calibrations at NMISA. The ML-800 is a positive displacement primary piston prover and can be used for gas flow measurements in pressure or vacuum applications. The ML-800 complies with the requirements necessary for a primary standard, namely that flow measurements are made in terms of volume per unit time. This is accomplished by means of a piston moving in a cylinder of known cross-sectional area over a measured distance in a measured time. Volumetric or standardised flow readings can be displayed at the push of a button, the standardised values being calculated from the pressure, as measured by the built in barometer in the ML-800 and a reference temperature, as entered by the metrologist. Measurements can be performed manually (one at a time), or automatically. Up to 100 measurements can be performed in an averaging sequence. The ML-800 measures gas flow rates at the measured atmospheric pressure ± 7 mmhg. The ML-800 consists of two primary components a common base and selectable flow cells. The main computer and timing crystal are housed in the base component as well as a precision barometer. The various flow cells plug into the base to create a functional system the two components cannot operate independently. The flow cells consist of a borosilicate glass tube with a precision-machined piston. In addition, they also contain an integrated temperature sensor and barometric pressure transducer in the gas flow stream for instant conversion of the volumetric readings into standardised flow. Four interchangeable flow cell models are available:- - ML-800 Ultra Low Flow Cell: ML (in use at NMISA) Flow range: 0,5 to 50 sccm (ml/min) Accuracy: ± 0,25 % - ML-800 Low Flow Cell: ML (future purchase being considered) Flow range: 5 to 500 sccm (ml/min) Accuracy: ± 0,15 % - ML-800 Medium Flow Cell: ML (in use at NMISA) Flow range: 50 to sccm (ml/min) Accuracy: ± 0,15 % - ML-800 High Flow Cell: ML (in use at NMISA) Flow range: 500 to sccm (ml/min) Accuracy: ± 0,15 % The measurements performed with ML-800 are traceable to international standards. Before dispatch, the standard was calibrated by the calibration laboratory of the supplier, which is NVLAP (National Voluntary Laboratory Accreditation Programme in the USA) accredited. The Clock Period of the timing crystal and the barometer of the base unit were calibrated. The ultra-low flow cell was gravimetrically calibrated together with its temperature and

4 pressure sensors. The medium and high flow cells were dimensionally calibrated together with their temperature and pressure sensors. For each flow cell, temperature and pressure corrections were applied to the volumetric flow readings to obtain standardised flow readings. [2] 5. Validation of the Bios Met Lab ML-800 Primary Flow Standard [3] 5.1 Purpose of the validation The purpose of the validation measurements was to ensure that the flow standard was not damaged during shipping from the United States of America to South Africa. The validation measurements were also required to provide objective evidence to demonstrate the competence of the new flow metrologist in the operation of the ML-800 and the calibration of flow instrumentation. 5.2 Validation method The ML-800 was used to calibrate two mass flow controllers (S/N & ) of an API Dynamic Dilution Calibrator (Model: 700; Serial Number: 777). These mass flow controllers had been previously calibrated against the MKS Califlow primary gas flow standard, Type A200, serial number N previously used in the NMISA flow laboratory. Nitrogen gas was used as the flow medium. The ML-800 and the API Calibrator, which controlled the mass flow controllers being calibrated, were operated according to the procedures contained in their respective operating manuals. The two mass flow controllers were calibrated against the ML-800, one at a time. The inlet port of the mass flow controller being calibrated was connected to the nitrogen gas cylinder and the outlet port to the inlet port of the appropriate ML-800 flow cell. Connections to the mass flow controllers were made directly to the inlet and outlet ports and not to the ports at the rear of the calibrator. The reason for this was to ensure there was no gas diffusion/leakage possible via any other gas paths, and that the full gas stream passed through the mass flow controller being calibrated. Before any measurements were performed, the gas path was investigated for leaks by means of pressurising the system to 200 kpa and then using Snoop liquid leak detector fluid. Each mass flow controller was calibrated at twenty points and for each calibration point, ten measurements were performed and an average calculated. The measurement results were recorded manually on a worksheet. Ambient conditions were also recorded. The detailed measurement results and uncertainty calculations are presented in the appendixes. 6. Measurement results Table 1 below contains a summary of the measurement results and resulting calculated Normalised Error (E n ) values. See Appendix A for a sample of the detailed validation measurement raw data and Appendix B for a sample of the raw data uncertainty calculations.

5 Table 1. Summary Validation Measurement Results Calibration Point Ref Val (L/min) Lab Val (L/min) Lab Val Ref Val (L/min) Ref Val Unc (%) Lab Val Unc (%) Calculated E n value 1 0,805 0,815 0,010 0,64 1,27 0,01 2 1,870 1,910 0,040 0,64 0,19 0,06 3 2,932 2,985 0,053 0,64 0,18 0,08 4 3,995 4,058 0,063 0,64 0,18 0,09 5 5,077 5,136 0,059 0,64 0,16 0,09 6 6,156 6,223 0,067 0,64 0,16 0,10 7 7,248 7,304 0,056 0,64 0,16 0,08 8 8,336 8,396 0,060 0,64 0,16 0,09 9 9,433 9,489 0,056 0,64 0,16 0, ,52 10,58 0,06 0,64 0,16 0, ,62 11,68 0,06 0,64 0,16 0, ,74 12,78 0,04 0,64 0,16 0, ,85 13,89 0,04 0,64 0,16 0, ,95 15,01 0,06 0,64 0,16 0, ,07 16,12 0,05 0,64 0,16 0, ,19 17,25 0,06 0,64 0,15 0, ,30 18,37 0,07 0,64 0,15 0, ,41 19,52 0,11 0,64 0,15 0, ,47 20,65 0,18 0,64 0,16 0, ,51 21,80 0,29 0,64 0,16 0,44 Measurement results are only presented for one mass flow controller (S/N ). The measurement results for the other mass flow controller were obtained and analysed in exactly the same way. 7. Discussion of the measurement results Refer to Appendix A A detailed description of the results as they appear in the spread sheet printout follows hereunder.

6 Nominal Flow Rate (l/min) (highlighted in yellow) The API calibrator mass flow controller flow reading as calibrated previously against the MKS Califlow instrument. It is the standardised flow reading at 760 mmhg and 25 C. This flow rate value is used as the reference value. Uncertainty of reference values (%) = 0,64 (highlighted in orange) The uncertainty associated with all the reference values. Mass Flow Controller Reading 0 C The ML-800 flow reading standardised to 760 mmhg and 0 C. Temperature ( C) This value is the temperature reading from the ML-800. This value is a measure of the temperature of the gas stream in the flow cell. Pressure (mbar) This value is the pressure reading from the ML-800 and is a measure of the pressure of the gas stream in the flow cell. Mass Flow Controller Reading 25 C This value is a calculated value. The flow values of the Mass Flow Controller, controlled by the API Calibrator and ML-800, differed significantly. The reason for the differences was that the flow rates displayed by the API calibrator were standardised to 760 mmhg and 25 C whereas the ML-800 displayed readings were standardised to 760 mmhg and 0 C. The difference in the standardised temperatures caused the difference in the standardised flow readings. For direct comparison purposes therefore, a calculation was necessary to convert the ML-800 readings (ST: 0 C) to ML-800 readings (ST: 25 C) according to the following formula: 273,15 25 ML 800( ST : 25C) ML 800( ST : 0C) 273,15 The values highlighted in green are the calculated averages of the ten calculated measurements (760 mmhg and 25 C) for each point. These values are the laboratory values. The values highlighted in blue at the bottom of each measurement block, Unc % x.xxx, indicate the measurement uncertainty calculated for each point. The column, Lab Val Ref Val (l/min), indicates that there is good agreement between the values of the two calibrations of the instruments.

7 Time This indicates the time period over which the ten measurements were performed. Calculated Normalised Error (E n ) values The E n value is calculated for each point. The method used for the evaluation of the measurement results was to calculate the error, E n, normalised with respect to the UoM using the following formula: E n Labvalue Re ference value U 2 LAB U 2 REF Where: Lab value is the Mass Flow Controller Reading 25 C Reference value is the Nominal Flow Rate (l/min) U lab is the value highlighted in blue at the bottom of each measurement block, Unc % x.xxx U ref is the Uncertainty of reference values (%) = 0,64 (highlighted in orange) To ensure that the error is within the laboratory s uncertainty of measurement (UoM), the E n value should be between the limits of ±1. Therefore values of E n < 1 indicated that the measurement results obtained during calibration of the mass flow controller using the ML-800 were equivalent to those previously obtained during the calibration against the MKS Califlow. This validated both that the ML- 800 had travelled well after it calibration from the USA and that the Flow Laboratory metrologist was competent to perform calibrations of mass flow controllers using the ML Uncertainty budget Uncertainties are calculated according to the Guide to the Expression of Uncertainty in Measurement. The following sources of uncertainty were considered: - Flow cell accuracy Since the calibration certificates for the Flow Cells provided evidence that their accuracy was well within the manufacturer s accuracy specification, the manufacturer s accuracy specification was accepted as the uncertainty estimate. It was treated as a normal distribution variable, at k = 2 and therefore divided by two.

8 - Temperature No uncertainty contribution for temperature was considered since the ML-800 sensitivity to temperature was unknown. However, since the ML-800 was used in a laboratory environment within that specified by the manufacturer for the ML-800 to remain within its accuracy specification, the effect of temperature was assumed to be negligible. - Pressure No uncertainty contribution for pressure was considered since the ML-800 sensitivity to pressure was unknown. The ML-800 compensated for atmospheric pressure, as measured by its internal barometer. - Repeatability of measurements (ESDM) The Experimental Standard Deviation of the Mean was used as the uncertainty estimate for the variation of repeated measurements, since the measurements were performed under repeatability conditions (conditions remained the same) and the calculated mean value of ten independent flow measurements were reported. The degrees of freedom were the number of measurements minus one, which for ten measurements was nine. - Resolution of standard (Bios ML-800) The displayed resolution of the ML-800 was dependent on which flow rate was being measured. Therefore the uncertainty estimate was accepted as one least significant digit of the displayed flow. Since this was the full range of a rectangular distributed input variable, it was divided by two and then further divided by the square root of three. It was assigned infinite degrees of freedom. - Resolution of the API Calibrator The displayed resolution of the API Calibrator was dependent on which mass flow controller was being calibrated. Therefore the uncertainty estimate was accepted as one least significant digit of the displayed flow. Since this was the full range of a rectangular distributed input variable, it was divided by two and then further divided by the square root of three. It was assigned infinite degrees of freedom. In all cases, the uncertainty estimates were converted into relative values in percentage, before being entered into the uncertainty budget. Measurement uncertainties were calculated for each measurement point (see Appendix B). The uncertainty calculations show that apart from the flow cell s uncertainty, the repeatability uncertainty contributor proved to be the most significant.

9 9. The way forward The first priority is to achieve SANAS accreditation for gas flow calibrations in The second priority is to decrease current client calibration turnaround times by equipping the laboratory with the necessary instrumentation, tools, tubing and fittings necessary for the calibration of the wide variety of flow instrumentation being received for calibration. References 1. TSI, Application Note Flow-004, December Bios Met Lab Series ML-800 User Manual, 2011, Bios International Corporation, MK01-33 Rev J. 3. D. Jonker, NMISA , Validation Report of the Bios Met Lab ML-800 Primary Flow Standard, March 2013.

10 APPENDIX A VALIDATION MEASUREMENTS Bios ML-800 Validation Measurements Ambient temperature: 20 C ± 2 C UUT: Dynamic Dilution Calibrator Ambient humidity: 50 %rh ± 20 %rh Manufacturer: API Ambient pressure: 850 mbar ± 100 mbar Model: 700 Standard pressure = Pa = mbar = 760 mmhg Serial Number: 777 Standard temperature = 25 C MFC -S/N: (MFC1) Volumetric flow = standardised flow * (std pressure/meas pressure) * (meas temp/std temp) Date: 2013/01/30 Standardised flow = volumetric flow * (meas pressure/std pressure) * (std temp/meas temp) std temp (K) = C = K Readout: Bios ML-800 Primary Flow Standard Std Pres temp (K) = C Serial Number: Std Temp 25 Bios ML-800 readings = standardised flow Bios Flow Cell: ML (S/N: ) Uncertainty of reference values (%) = ) Dil Driv 250 Nominal Flow Rate (l/min) AVE STDEV ESDM ESDM % Lab Val - Ref Val Mass Flow Controller Reading 0 C (l/min) Temperature ( C) Pressure (mbar) Mass Flow Controller Reading 25 C Time 9h10 9h25 Unc (%) = En= ) Dil Driv 500 Nominal Flow Rate (l/min) AVE Mass Flow Controller Reading 0 C Temperature ( C) Pressure (mbar) Mass Flow Controller Reading 25 C Time 9h30 9h38 Unc (%) = En= ) Dil Driv 750 Nominal Flow Rate (l/min) AVE Mass Flow Controller Reading 0 C Temperature ( C) Pressure (mbar) Mass Flow Controller Reading 25 C Time 9h40 9h50 Unc (%) = En=

11 APPENDIX B - UNCERTAINTY CALCULATIONS Calibration of a MFC (Calibrator) Manufacturer: API Model: 700 Serial number: 777 & (MFC1) Certificate number: Validation Calibration medium: Nitrogen gas MFC Flow Range L/min L/min Bios flow cell ML ML REF STD: Value U(Xi) Unit Probability Distr Divisor Factor Sensitivity coeff Ci ui(k=1) (%) Reliability % D.o.f. (D.o.f)eff Value U(Xi) Unit Probability Distr Divisor Factor Sensitivity coeff Ci ui(k=1) (%) Reliability % D.o.f. (D.o.f)eff Bios ML-800 flow cell % Normal k = E % Normal k = E-08 Temperature Normal k = E Normal k = E+00 Pressure Normal k = E Normal k = E+00 ESDM % Normal k = E % Normal k = E-06 Resolution % Rectangular E % Rectangular E-14 UUT: ESDM Normal k = E+00 Normal k = E+00 Resolution % Rectangular E % Rectangular E E E+00 uc(k=1) (%): Eff deg of freedom E E-06 t(eff d.o.f.): U(k=2) (%):