of Pressure Measurement MAPAN -Standardisation Journal of Metrology Society of India, Vol. 26,Using No.2,Pressure 2011; pp.Balance 133-151as Transfer Standard
REPORT
Standardisation of Pressure Measurement Using Pressure Balance as Transfer Standard SANJAY YADAV*, V.K. GUPTA and A.K. BANDYOPADHYAY National Physical Laboratory (NPLI) Council of Scientific and Industrial Research (CSIR), New Delhi - 110 012, India * e-mail:
[email protected] [Received: 24.12.2010 ; Revised: 09.02.2011 ; Accepted: 11.02.2011]
Abstract This paper describes the results of the interlaboratory comparison for pressure measurements of 9 laboratories that are accredited by the National Accreditation Board for Testing and Calibration of Laboratories (NABL). The artifact used for the comparison was a pressure balance covering the pressure range (7 to 70) MPa. The primary objective of this comparison was to assess the laboratory's technical competence to perform measurements and also to assess the compatibility of the results submitted by the laboratories. The comparison began during March 2008 and ended during April 2010. For assigning the reference values, the pilot laboratory (NPLI) carried out 3 calibrations of the transfer standard; the first one at the beginning, the second at the middle and the last one at the end of the programme. The comparison was carried out at 10 pressure points i.e. (7, 10, 15, 20, 25, 30, 40, 50, 60 and 70) MPa throughout the entire pressure range of (7 to 70) MPa. The measurements were carried out by each laboratory with their own resources (personnel, calibration systems, environmental conditions in their installations). The deviations for each laboratory were compared against the reference values and the compatibility of the results was calculated using the normalized error value method. Out of the total 87 measurement results reported, 68 (78.2%) results are found in good agreement with the results of the reference laboratory. The normalized error (En) values of 5 laboratories out of the total 9 were found well within ± 1 over the entire pressure range. However, 2 other laboratories had shown good agreement with the reference values except one pressure point each. The En values of one of the participating laboratory were found beyond acceptable limits at all measurements points. Another laboratory had acceptable results only at 3 pressure points. The laboratories with unacceptable results have been advised to review their pressure measurement process. The deviations between laboratory values and of the reference values were found well within the uncertainty band of the reference values for 37% measurement results. The relative deviations for 82 measurement results were found well within 0.05%.
1.
Introduction
The quality assurance section of ISO/IEC 17025 stipulates the requirement for ensuring; i) that a single © Metrology Society of India, All rights reserved 2011.
analyst within a laboratory is able to consistently reproduce the same result on the same sample, ii) that the result produced by this analyst should reflect the result that would have come from any other analyst in the laboratory and iii) that any results from the
133
Sanjay Yadav, V.K. Gupta and A.K. Bandyopadhyay laboratory as a whole should reflect the results that are agreed upon by many other laboratories. It is due to this reason that the internal and external quality control (QC) and quality assurance (QA) clauses exist within ISO/IEC 17025. Although, the precise way of going about proving the consistency and the reliability of the results is not prescribed in ISO/IEC 17025, the accreditation bodies have built some prescriptive clauses into their requirements to try to facilitate meeting the requirements of ISO/IEC 17025 in an effective manner. An externally provided PT program is a useful tool in meeting the requirements of ISO/IEC 17025. However, participating in an external PT program will not necessarily mean that all quality assurance aspects have been met [1-3]. Proficiency testing is used by the accreditation body as part of the assessment processes, to evaluate the technical competence and ability of the laboratories to carry out tasks for which its accreditation has been applied for / granted. This test is a complement of the laboratory assessment carried out by technical experts in situ. It is also mandated by accreditation bodies that laboratories participate in the PT exercises for all types of analyses undertaken in that laboratory, when suitable exercises exist. To meet the requirements of MRA [1], ISO/IEC 17025 [2] and APLAC MR001 [3], the NABL has conducted several PT experiments in pressure metrology in the pressure range (7 to 70) MPa amongst the NABL accredited Indian pressure calibration laboratories in conformity with ISO/IEC Guide 43 [4] through the NMI of India i.e. NPLI, which also acted as a reference laboratory. In a series of 7 PTs organized, the 1st PT, designated as NABL-Pressure-PT001 was organized for 7 laboratories, having measurement capabilities better than 0.05% of full scale pressure using dead weight tester as an artifact [5]. The (2nd and 5th) PTs i.e. NABL-Pressure-PT002 and NABL-Pressure-PT005 were conducted for another 7 and 21 laboratories, having measurement capabilities coarser than 0.05 % and better than 0.25% of full scale pressure using digital pressure calibrator [6-7]. The (3rd and 6th ) PTs i.e. NABL-Pressure-PT003 and NABL-PressurePT006, included (11 and 17) laboratories, respectively having measurement capabilities 0.25% or coarser
134
than 0.25% of full scale pressure using pressure dial gauge as an artifact [8-9]. Similarly, another PT experiment i.e. NABL-Pressure-PT007 was carried out for 14 laboratories having measurement capabilities 0.25% or coarser than 0.25% of full scale pressure using pressure dial gauge as an artifact in the pressure range (6 to 60) MPa [10-11]. The final PT experiment, designated as NABLPressure-PT004, is recently completed during April 2010. This PT programme is designed and organized in the hydraulic pressure region covering pressure range (7 to 70) MPa (70 to 700) bar using the pressure balance as transfer standard. Initially, 10 NABL accredited pressure calibration laboratories participated and finally 9 laboratories submitted their results. 2.
Objectives
The main objectives and benefits of PT to participating laboratories are; i) the participating laboratory fulfills the requirements of ISO/IEC 17025 in the area of proficiency testing, from both interlaboratory and intra-laboratory standpoints, ii) The laboratory ensures the competence and capabilities of the staff involved in the measurements, iii) The laboratory collects the information that can assist in future planning for equipment upgradation and staff training and iv) identification of any difficulties with methodology, instrumentation, results and training needs. It also supplements laboratory's own quality control procedures by providing additional external audit and also provides objective evidences that a laboratory is competent enough and can achieve the level of uncertainty for which accreditation is granted. This exercise gives an opportunity to accredited calibration laboratory to demonstrate its technical competence of routine calibration services rendered to customers and to have the measurement traceability to the NMI. 3.
Methodology
The PT programme is designed as per guidelines stipulated in ISO/IEC 17025 [2], ISO/IEC Guide 43 [4] and NABL-162 [12]. A high precision pressure balance, Model No.- YW 1305, YANTRIKA, Sl. No.REB 095, make Ravika Engineers, New Delhi was used as transfer standard for this comparison. The detailed ‘Technical Protocol’ (TP) of the programme
Standardisation of Pressure Measurement Using Pressure Balance as Transfer Standard
March 24, 2008
NPL, New Delhi
February 23, 2010 Dec. 29, 2009
Measure Technique (MT), Chennai 24-03-2008 to 16-04-2009
MSME Testing Centre (RTC), New Delhi 29-12-2009 to 23-02-2010
Dott. Ing. Scandura Calibration & (SCANDURA) Chennnai 16-04-2008 to 25-04-2008
Electronics Regional Test Laborattory (ERTL), Delhi 25-11-2008 to 10-02-2009
Nagman Instrument and Electronics (P) Ltd., (NIEPL) 25-04-2008 to 10-05-2008
Bhart Heavy Electtricals Ltd. (BHEL), NOIDA 17-10-2008 to 25-11-2008
Regiech Calibration Pvt. Ltd. (RCPL), Chennai 10-05-2008 to 04-06-2008
National Cement and Building Materials (NCCBM), Ballabhgarh 12-08-2008 to 16-10-2010
Sushma Industries and Calibration Center (SICC), Bangalore 04-06-2008 to 07-07-2008
WIKA, Pune 09-07-2008 to 04-08-2008
Fig. 1. Circulation and movement of the artifact during comparison was prepared and circulated to all the participants containing the information about the artifact, calibration procedure, environmental conditions to be maintained, calculation of the results, procedure for reporting of the results etc. The entire measurement pressure range of (7 to 70) MPa was divided into 10 arbitrarily chosen measurement points of (7, 10, 15, 20, 25, 30, 40, 50, 60 and 70) MPa to accommodate at least 5 measurement points for each participant. The programme had run smoothly and almost all the participants performed their measurements well in time. The whole circulation programme was completed in two loops. After completion of the first loop, the artifact reached NPLI, New Delhi at the end of December, 2009. During its inspection and characterization, it was observed that the system had problems of leakage and some bad handling. The artifact was then sent for repair and overhauling to its manufacturer. This process of repair took almost 10 months time to start the second loop. Since, there was no problem with the piston and cylinder assembly of the transfer standard, the data taken
before and after the repair was well within the reported measurement uncertainty. It was recalibrated at NPLI, New Delhi and then dispatched for the circulation of the second loop. There was no technical problem, fault, snag or difficulty reported by any of the participants. Schematic diagram of the movement of the artifact is depicted in Fig. 1. 3.1 Characterisation of the Artifact and Assigning Reference Values The characterization of the artifact was performed thrice, first at the start of the programme during December, 2007, second during (March and April) 2009 and finally at the end of the programme during March 2010. The characterization starts with the calibration of individual mass values of the each dead weight and piston assembly of the artifact, collection of pressure data, determination of effective area as a function of applied pressure (Ap), zero pressure effective area (A0) and distortion coefficient (λ), stability of Ap and A0 during the whole period of PT programme and finally assigning the reference value with measurement uncertainty.
135
Sanjay Yadav, V.K. Gupta and A.K. Bandyopadhyay Calibration of the individual mass values of the each dead weight and piston assembly of the artifact was performed at Mass Metrology Section of NPLI, New Delhi against the national standards of mass and balances. After mass calibration, pressure characterization of the artifact was performed using the well-established and internationally accepted method of 'cross-floating' of pressure balances. The artifact was connected with national hydraulic secondary pressure standard, designated as NPL100MPN for cross floating measurements as discussed elsewhere [13-16]. In a cross-floating position, the two gauges were considered to be in balance when the sink rate of each of the piston was normal for that particular pressure. At this position, there was no pressure drop in the connecting line and consequently no movement of the fluid. This procedure was repeated for all the 10 pressure points i.e. (7, 10, 15, 20, 25, 30, 40, 50, 60 and 70) MPa and observations were repeated six times, three times each in increasing and decreasing orders of pressures, for each pressure point. The traceability of the NPL100MPN is established by cross-floating it against national primary pressure standard [17-20]. The NPL100MPN has also participated in several key comparison exercises, APMP.M.P. K7 [21], CCM P. K7 [22] and APMP-SIM.M.P. K7 [23]. The values of the pressure generated, the effective area, repeatability and the expanded uncertainty were computed using the computer softwares developed for this purpose [24-25]. The least square curves were fitted to know the most probable values of the zero pressure area (A0t) and the distortion coefficients (λt) alongwith their standard deviations [26]. The pressure measured by laboratory standard (LS) was calculated using the following equation; PLS =
Σi mi . gL (1 − ρair / ρmi ) + γ C A0 (1 + λ pn + λ ′pn2 )[1 + (α c + α p )(T − Tr )]
[1]
The term (l-ρair/ρmi) is the air buoyancy correction for weights, γC is the force exerted on the piston by surface tension of the transmitting fluid, [1 + (αc+ αp) (T -Tr)] is the thermal expansion correction factor, the term (1+λpn+ λ'pn2) describes the change of the effective area with pressure which is the most important correction term. The various terminology used in the equation are defined as follows;
136
mi mass of the ith weight combination (in kg) placed on the LS, gL value of local acceleration of gravity (in m/s2) in the measurement laboratory,
ρair density of the air (in kg/m3) at temperature, barometric pressure and humidity prevailing in the laboratory, ρmi density (in kg/m3) of the material of the weights, γ
surface tension (in N/m) of the pressure transmitting fluid used,
C circumference (in m) of the piston where it emerges from the fluid, A0 zero pressure effective area (in m2) of the LS,
αc &αp linear thermal expansion coefficients (in /0C) of the material of the cylinder and piston, respectively of the LS, T
measured temperature (in 0C) of the LS piston cylinder assembly,
Tr temperature (in 0C) at which A0 (zero pressure effective area) of LS is referred,
λ
First order pressure distortion coefficient (in per Pa) of the LS,
λ'
Second order pressure distortion coefficient (in per Pa2) of the LS, and
Δp is the head correction (in Pa) in terms of pressure. The head correction term is Δp = [(ρf - ρair). gL. H], where H is the difference in height (in m) between the reference levels of the two dead weight testers and (ρf) is the density (in kg/m3) of the pressure transmitting fluid used in the measurements.. The temperature corrected forces F (in N) acting on the artifact, referred as Test herein thereafter, is calculated using the expression; FTest =
Σmit .gL (1-ρair /ρmi )+γC t [1+(α ct +α pt )(Tt -Trt )]
[2]
Standardisation of Pressure Measurement Using Pressure Balance as Transfer Standard where mit
mass (in kg) of the ith weight combination placed on the artifact,
Ct
circumference (in m) of the piston of the Test,
effective area ( Apt ) and relative deviation (σi) of ( p1 ) (say for first calibration in 2007) in a particular calibration is determined by simple statistical computation as follows; n
∑ PLS
αct &αpt linear thermal expansion coefficients (in 0C) of the material of the cylinder and piston, respectively of the Test,
p1 =
Tt
measured temperature (in 0C) of the pistoncylinder assembly of the Test,
Ap1 =
Trt
temperature at which A0t (zero pressure effective area) of Test is to be calculated.
The effective area Ap (in m2) of the Test is then calculated by; Ap = FTest / PLS Σmit .gL (1 − ρair / ρmi ) + γ C t Ap = PLS [1 + (α ct + α pt )(Tt − Trt ]
1
n
σ1 =
p=
1
[6]
n
( ∆A
p1
− Ap1
)
[7]
Ap1
(p
1
+ p2 + p3 3
[4]
The detailed metrological coefficients and the associated uncertainties of the Test for all the successive calibrations performed in 2007, 2009 and 2010 are shown in Table 2 for assigning the reference values. The average values of measured pressure ( p 1 ) ,
∑ Apt
The reference values of the pressure measured
[3]
The data thus obtained is recorded at different pressure points and observations were repeated six times on each pressure point. The pressure measured by LS is then least square fitted against the effective area of Test to determine the value of A0t (zero pressure effective area) and λt (distortion coefficient) of the Test. The measurement uncertainties in both the ranges are computed as per guidelines available in the literature [25-29]. The characterized values of the pressure measured using Eq. (1), the effective area calculated using Eq. (4), and the deviations from the average values are plotted in Fig. 2 and data are given in Table 1. The measurement uncertainties of the A0t and λt are estimated from the standard deviations of the A0t and λt obtained from the least square fitting method. The uncertainty budget for a maximum pressure of 70 MPa is shown in Table 2. The denominator in Eq. (2) is included as a temperature correction for effective area of the artifact.
[5]
n
Ap =
(A
p1
) and
+ Ap2 + Ap3
effective
area
)
are then obtained by 3 arithmetic mean of the data obtained during these three calibrations. The relative deviation ( σ ) of reference measured pressure value (p) is computed by root sum square method as follows;
σ = σ 12 + σ 22 + σ 32
[8]
The effective area Ap of the Test is plotted as a
Fig. 2. The effective area Ap and relative deviations of Ap plotted as a function of applied pressure p for all the three successive calibrations 137
Sanjay Yadav, V.K. Gupta and A.K. Bandyopadhyay function of applied pressure (p) for all the successive calibrations alongwith their relative deviations of the effective area as shown in Fig. 2. The relative deviations of the effective area, calculated as ⎛ Ap − Ap1 , Ap2 , Ap3 ⎜ Ap ⎝
σA = ⎜ p
⎞ ⎟ are well within ± 25 × 10-6. ⎟ ⎠
The effective area Ap is also plotted as a function of applied pressure p. The values of the zero pressure effective area, A0t and the distortion coefficient, λt of the Test, determined through least square fitting are also shown in Table 1 for all the three calibrations. The deviation of average value of A0t are found to be well within 37 × 10-6. Thus the deviations of Ap and A0t are found well within the standard uncertainty of the artifact estimated as 72 × 10-6 and shown in Table 2 as detailed uncertainty budget. This suggests that the metrological parameters of the artifact remained stable and well within the estimated uncertainty budget of the artifact during the whole comparison period. Each, participating laboratory was assigned a random code number and only these code numbers are used in this paper. As per NABL policy, the details of these code numbers are not divulged herein. However, these code numbers were reported to NABL, separately. However, the Code number assigned to reference laboratory, NPLI, is '1'.
3.2 Experimental Setup and Calibration Procedure All the laboratories were advised to install the experimental set-up as shown in Fig. 3. Usually, laboratory standard (LS) and the artifact were leveled using the leveling screws and the sprit level. The necessary weights were placed on the carrier of the NPL100MPN and adjusted as per the values of the pressure generated by the artifact. This is repeated several times so that the error due to the adjustment of the weights is minimized. Sufficient time, approximately 30 min, was provided between two successive observations so that both the systems are in complete equilibrium. Participants were instructed to place the necessary weights on the carrier of the LS so that the values of the pressure generated by piston plus carrier plus weights loaded are equal to the measurement points equalizing the pressure with pressure generated by the artifact. This process is called Cross Floating of pressure balances [13-15, 25]. During the cross floating, both the gauges are connected directly. The pressure balances are considered to be in balance when sink rate of each of the piston is normal for that particular pressure. At this position, there was no pressure drop in the connecting line and consequently no movement of the fluid. About 10 min time was provided between two successive observations to allow the system to return to equilibrium and 5 min time was sufficient to repeat the observations. A waiting time of 10 min was given
Fig. 3. Experimental setup for the measurement using dead weight tester as an artifact
138
6.9952
9.996106
14.99747
19.99894
25.00061
30.00248
40.00619
50.01019
60.0144
70.01883
7
10
15
20
25
30
40
50
60
70
Ap 1
2007
70.01943
60.01464
50.01032
40.00613
30.00205
25.00062
19.99902
14.9971
9.995766
6.995155
x 10-6
Ap 2
Δ Ap 2 − Ap 2
2009
9.786138
9.786275
9.786456
9.786682
9.786944
9.78711
9.787301
9.78753
9.78759
9.787874
Ap3 mm2
0.67
(λt -) x (10 MPa)
6
0.53
-2.6181 0.198
0.052
6
λt (10 MPa) 6 u(λt) x (10 MPa)
9.787952
9.78831088
70.02114
60.01638
50.01169
40.0072
30.00307
25.00111
19.99924
14.9975
9.996156
6.995234
p3 (MPa)
-3.1523
19.67
18.32
17.10
16.03
15.15
14.79
14.49
14.25
14.07
14.00
σ2 −
-1.9482
9.786302
9.786472
9.786635
9.786856
9.787178
9.787198
9.787308
9.787692
9.787874
9.787876
Ap2 mm2
7.65 36.6
7.99
7.77
7.58
7.42
7.29
7.24
7.20
7.16
7.14
7.13
p2 (Mpa)
9.78767634
ΔAp1 − Ap1
x 10-6
σ1 −
2010
2.0 28.3
9.786309
9.786437
9.786589
9.786778
9.786975
9.787148
9.787284
9.787375
9.787469
9.787747
Ap1 mm2
2009 Ap3
ΔAp3 − Ap3
10.70
10.26
9.88
9.55
9.29
9.18
9.10
9.03
8.98
8.96
x 10-6
σ3 −
Characerisations of the artifact performed during
(A0t-)/·106 λt x (106 MPa)
A0t / mm A0t / mm2 6 u(A0t)/A0t·10
2
Parameter
p1 (MPa)
Nominal Pressure MPa
2007 p=
9.78764 9.78753 9.78730 9.78715 9.78703 9.78677 9.78656 9.78639 9.78625
9.99601 14.99736 19.99907 25.00078 30.00253 40.00651 50.01073 60.01514 70.01980
0.14
0.084
-2.7538
3.24
9.7878715
2010
9.78783
)
24
22
21
20
19
19
19
18
18
18
x 10-6
+ Ap2 + Ap3 σ = σ21 + σ 22 + σ23 3
6.99520
1
(Ap mm2
AP =
Mpa
3
(p1 + p2 + p3 )
Reference Values Assigned
Table 1 Details of metrological characteristics of the Artifact and assignment of reference values (All the values reported here are at gNPL = 9.7912393 m/s2 and reference temperature of Tr = 23 °C)
Standardisation of Pressure Measurement Using Pressure Balance as Transfer Standard
139
Sanjay Yadav, V.K. Gupta and A.K. Bandyopadhyay Table 2 Uncertainty Budget of the Artifact Used at a Maximum Pressure of 70 MPa and at Trt = 23 °C Parameter
Value
Type
Sensitivity Coefficient Parameter
Value
Standard Uncertainty
Relative Uncertainty (x 10-6)
0.00142904
Type-A
(1/p)
0.014281401
5.83E-04
8.33
69.994137
Type-B
(1/ mit )
0.014286911
2.18E-04
3.11
9.7912393
Type-B
(1/gL )
0.102132117
1.01E-05
1.03
ρair (kg/m )
1.15951717
Type-B
(1/ρmi)
0.000125
3.52E-03
0.44
ρmi(kg/m3 )
8000
Type-B
(ρair /ρmi )
1.81175E-08
4.56E+01
0.83
ρ (N/m)
0.0312
Type-B
(Ct/ mit.gL)
1.6182E-05
3.12E-03
0.05
Ct (m)
0.01109
Type-B
(γ/mit .gL )
4.55256E-05
5.00E-06
0.00
A0t (m )
9.7878715
Type-A
(1/ A0t )
102167.2587
3.17E-05
3.24
αpt(/0C)
0.0000045
Type-B
(Tm-Tr)
0.13
4.50E-07
0.06
αct(/ C)
0.0000045
Type-B
(Tt-Trt)
0.13
4.50E-07
0.06
Tt-Trt (0C)
0.13
Type-B
(αpt +αct)
9.000000E-06
1.20E-01
1.08
λt(MPa-1)
-2.7538E-06
Type-A
(p)
70.02114
8.39E-08
5.88
p (MPa)
70.02114
Type-B
(λt )
-2.7538E-06
4.90E-03
-0.01
Repeatability (MPa ) mit (kg) gL (m/s2) 3
2
0
2
The total relative standard uncertainty evaluated through Type A method = (root mean square of sum of squares all the uncertainty components evaluated through Type A method and stability of zero pressure effective area A0t) The total relative standard uncertainty evaluated through Type B method = (root mean square of sum of squares all the uncertainty components evaluated through Type B method)
13.7 x 10-6
3.58 x 10-6
The relative standard uncertainty of the standard used in the measurements = 70.0 x 10-6 The combined relative standard uncertainty (at k = 1) = 72.0 x 10-6
140
Standardisation of Pressure Measurement Using Pressure Balance as Transfer Standard after taking the reading at maximum pressure range i.e. 70 MPa to start the observations in decreasing pressure order of pressure. This procedure was repeated for 10 pressure points (7, 10, 15, 20, 25, 30, 40, 50, 60 and 70) MPa, selected for the present comparison and observations were repeated six times (3 times each in increasing and decreasing order) for each pressure point and the values of the pressure generated, the repeatability and the expanded uncertainty were computed. 4.
Data Compilation and Analysis
4.1 Gravity Correction The measured pressure values reported by the laboratories are corrected for gNPL = 9.7912393 m/s2 (acceleration of gravity at NPLI, New Delhi) using the following relationship; [9]
where pcorr and prep are the values of the corrected and the reported pressure, respectively and gLAB is the value of the acceleration of gravity reported by the laboratory. 4.2 Temperature Correction The measured pressure values reported by the laboratories were also corrected for the temperature at 23 0C using the following formula; p' = [pcorr / {1+(α'p+α'c)*(23 - TLAB)}]
4.3 Estimation of Normalized Error (En) The measurement performance of the laboratory was assessed on the basis of normalized error (En) number of each measurement. The En values are estimated for each participant at each pressure using the equation [4-5, 30]; En Value =
p '− p R
{U( p ')} + {U( p )}
2
2
[11]
R
All the laboratories were advised to submit their measurement results within a month after completion of the measurements. Laboratories were also asked to submit the copies of the calibration certificates for the LSs used in the measurements, calculation sheet for determining the uncertainty in measurements and the calibration certificate issued to the customer for such measurements. The values of the measured pressure, acceleration of local gravity and the reference temperature reported by the participants are shown in Table 3. The following corrections were applied before compiling and comparing the results;
pcorr = prep * (gNPL / gLAB)
reported by the laboratory.
[10]
where, p' is the final corrected pressure, α'p, α'c and TLAB are the thermal expansion coefficients of piston, thermal expansion coefficients of cylinder and the reference laboratory temperature, respectively,
where p' is the participant's measured pressure value, p R is the reference pressure value, U(p') is the participant's claimed expanded uncertainty at a coverage factor k = 2 and U(pR) is the expanded measurement uncertainty of the reference value at a coverage factor k = 2. 5. Results Details of the values of measured pressure (prep) and other metrological characteristics of the laboratories standards reported by the participants are shown in Table 3. However, the details of the corrected pressure (p') for gravity (gNPL) and at 23 0C are shown in Table 4, the deviations from the reference value (pR) in (kPa) in Table 5. The measured pressure values, the associated uncertainties and calculated En values for individual pressure points are depicted in Figs. 4 (a)-(k). In general, the performance of the laboratory is considered satisfactory if normalized error En is inside ± 1. The plots shown in Figs. 4 (a) to (k) reveal that there were total 87 measurement results. Measurement results of 5 laboratories out of total 9 laboratories were found well within acceptable limits of the normalized error over the entire pressure range of (7 to70) MPa. Results of the two other laboratories were also within acceptable limit except one pressure points of 40 MPa. The En values of 68 measurement results out of the total 87 were found well within ± 1, which is 78.2%. These results are acceptable. The En values of the laboratory referred as Code No. 6, were > 1 at all the measurement points. An En value greater than unity means that there is a significant bias in the laboratory's results and that the quoted value of its associated uncertainty does not adequately accommodate that
141
Sanjay Yadav, V.K. Gupta and A.K. Bandyopadhyay
Table 3 Details of the reference values measured pressure (prep ) reported by the participants and other metrological characteristics of the laboratories standards Nominal pressure prep (MPa)
Laboratory code 1
2
3
7 10 15 20 25 30 40 50 60 70
6.995196 9.996010 14.99736 19.99907 25.00078 30.00253 40.00651 50.01073 60.01514 70.01980
g LAB (m/s 2 )
9.7912393 9.78269777 9.78244
TLAB (°C) 23 A0 mm2 9.805937 λ x 10-6 0.822 / MPa αp x 10-6 4.55 / °C αc x 10-6 4.55 / °C Trans- Sebacate mitting Fluid Used Trace- NPLI-H1 ability CCM. P. K7 APMP. M.P.K7
7.007740 10.013790 15.02425 20.03436 25.04505 30.05539 40.07749 50.09954 60.12077 70.14357
6.989308 9.988209 14.98599 19.98388 24.98173 29.98007 39.97616 49.97274 59.97089 69.96813
4 6.991900 9.988600 14.98890 19.98750 24.99090 29.99000 39.99080 49.99040 59.98490 69.97160
5 7.006520 10.012280 15.02132 20.03044 25.04017 30.04992 40.06945 50.08887 60.10919 70.12950
9.7823794 9.80665
6
7
8
9
6.997410 9.994660 14.99075 19.98668 24.98295 29.98000 39.97528 49.96934 59.96754 69.96917
6.989166 9.986768 14.98347 19.98010 24.97728 29.97385 39.96857 49.96401 59.95960 69.95400
6.993000 9.993300 14.99390 19.99360 24.99650 29.99650 39.99630
9.780352
9.7836664
9.7912393 9.7909591
9.7914503
6.993290 9.995280 14.99259 19.99289 24.99253 29.99118 39.99024 49.99395 59.99573 69.99858
10 6.995277 9.995614 15.00062 20.00160 25.00335 30.00248 40.00755 50.01311 60.01926 70.02413
23 4.032 1.4
23 10.01621 2.06
23 23 23 4.0332310 9.817711 4.03181 2.0 1.02 1.2
23 4.03440 3.0
23 -
23 4.02896 3.43
23 4.02896 8.22
5.0
4.5
5.0
4.5
5.0
11.0
-
4.9
4.5
11.5
4.5
11.5
4.5
11.5
11.0
-
11.5
4.5
VG22
Sebacate Shell Tellus 731 NIM, UKAS, China U.K.
Sebacate ISO VG22
ST55
SAE20W/40
Spinesstic Sebacate 22
NAVLAP, UKAS, USA U.K.
DKD, NPLI, Germany India
DHBudenberg, UK
NPLI, India
NPLI India
Table 4 Details of the corrected pressure (p') for gNPL and at 23 °C p' (MPa) 7 10 15 20 25 30 40 50 60 70
142
1 6.99520 9.99601 14.99736 19.99907 25.00078 30.00253 40.00651 50.01073 60.01514 70.01980
2 6.99673 9.99805 15.00064 20.00288 25.00569 30.00816 40.01451 50.0 60.02629 70.03334
3 6.99559 9.99719 14.99947 19.99907 25.00420 30.00704 40.01212 50.01769 60.02483 70.03106
4 6.99826 9.99768 15.00252 20.00288 25.01362 30.01726 40.02715 50.03584 60.03943 70.03520
Laboratory code 5 6 6.99551 9.99655 14.99771 20.00185 25.00082 30.00270 40.00648 50.01016 60.01473 70.01929
7.00520 10.00579 15.00744 20.00567 25.01076 30.01337 40.01978 50.02496 60.03429 70.04706
7
8
6.99458 9.99450 14.99507 19.99896 24.99661 29.99705 39.99950 50.00268 60.00601 70.00815
6.99300 9.99330 14.99390 20.00893 24.99650 29.99650 39.99630 -
9 6.99349 9.99557 14.99302 19.99557 24.99325 29.99204 39.99138 49.99538 59.99745 70.00058
10 6.99513 9.99540 15.00030 19.99360 25.00281 30.00183 40.00668 50.01203 60.01797 70.02262
Standardisation of Pressure Measurement Using Pressure Balance as Transfer Standard
(a)
(b)
(c)
Fig. 4
(Fig. 4 continued to next page)
143
Sanjay Yadav, V.K. Gupta and A.K. Bandyopadhyay
(d)
(e)
(f)
Fig. 4
144
(Fig. 4 continued to next page)
Standardisation of Pressure Measurement Using Pressure Balance as Transfer Standard
(g)
(h)
(i)
Fig. 4
(Fig. 4 continued to next page)
145
Sanjay Yadav, V.K. Gupta and A.K. Bandyopadhyay
(j)
(k)
Fig. 4 (a) to (k) represent the plots at (7, 10, 15, 20, 25, 30, 40, 50, 60 and 70) MPa, respectively. Black diamonds indicate the deviation of the measured pressure (p') by the laboratory from the reference value (pR). The estimated expanded measurement uncertainty (at k = 2) of the laboratory is depicted as error bars across the central solid line. The gap between two horizontal dotted lines shows the expanded measurement uncertainty band of the pR. Fig. 4 (k) showing the summary of the normalized error value (En) as a function of measured pressure (p') for each laboratory. The gap between two horizontal dotted lines shows the acceptable limit of the normalized error value
146
Standardisation of Pressure Measurement Using Pressure Balance as Transfer Standard
bias and need further investigations at the part of the laboratory. The larger the absolute value of the En number, the bigger the problem. The graphical representations in Figs. 4 (a) to (k) give the agreement between the participating laboratories and the reference laboratory. The results lying within the uncertainty band of the reference laboratory is an indication of the satisfactory results without any bias in the measurement. It is clearly evident from these plots that the deviations between the laboratories values and the reference values are well within the uncertainty bands of the
reference values for 68 measurements points. The bias in the measurements may be due to the errors in the measuring instrument or the estimation / measurement of local acceleration of gravity, the values of thermal expansion coefficients of piston and cylinder materials reported and used by the laboratory and under the estimation of the measurement uncertainty. The management of Laboratory with Code No.-6 is required to rectify the problems by a review of their uncertainty calculations and other systematic affects as mentioned above.
Table 5 Deviations (in kPa) of the measured pressure of each participating laboratory from reference values (p) p' (MPa) 7 10 15 20 25 30 40 50 60 70
1 -
2 1.53 2.04 3.28 3.81 4.91 5.63 8.00 10.08 11.15 13.54
3 0.40 1.18 2.11 2.79 3.41 4.50 5.61 6.96 9.69 11.26
4 3.06 1.67 5.17 6.60 12.84 14.73 20.65 25.11 24.29 15.40
Laboratory Code 5 6 0.31 0.54 0.36 -0.10 0.04 0.16 -0.02 -0.57 -0.41 -0.51
10.00 9.78 10.08 9.86 9.98 10.84 13.27 14.23 19.15 27.26
7 -0.62 -1.51 -2.29 -3.50 -4.17 -5.48 -7.00 -8.05 -9.13 -11.65
8 -2.20 -2.71 -3.46 -5.47 -4.28 -6.03 -10.21 -
9 -1.71 -0.44 -4.34 -5.60 -7.54 -10.49 -15.12 -15.35 -17.69 -19.22
10 -0.07 -0.61 2.94 2.10 2.03 -0.70 0.18 1.30 2.83 2.82
Fig. 5 (a). Deviations (in kPa) of the measured pressure (p') by each laboratory from the reference value (pR). The gap between two horizontal dotted lines shows the expanded uncertainty band of the reference values
147
Sanjay Yadav, V.K. Gupta and A.K. Bandyopadhyay
Fig. 5(b). Relative deviations (%) of the measured pressure (p') by each laboratory from the reference value (pR). The gap between two horizontal dotted lines shows the expanded uncertainty band of the reference values
Fig. 6: Estimated expanded measurement uncertainties U(p') and U(pR) reported by each laboratory The deviations of the measured pressure (p') by each laboratory from the reference value (pR) are shown in Figs. 5(a) in kPa and 5(b) in relative terms of percentage. It is clear from the plots that deviations are well within 0.15% for 34 measurement points and within 0.05% for 82 measurement points. This suggests that deviations are well within 0.05% for 94% results. Since this comparison was organized basically 148
for those laboratories having measurement capabilities of 0.05% or better, such results are quite encouraging. It is also worth mentioning here that 37 % measurement results are well within the uncertainty band of the reference values, specially for the laboratories with Code Nos. - 3, 5 and 10. These laboratories deserve appreciation.
Standardisation of Pressure Measurement Using Pressure Balance as Transfer Standard
Fig. 7. Combined expanded measurement uncertainty Uc(pR) estimated for each laboratory in this comparison The expanded measurement uncertainty U(p') reported by each laboratory and the combined expanded uncertainty U c ( p ) =
{U( p ')} + {U( p )}
2
2
R
estimated in this comparison are shown in Figs. 6 and 7, respectively. Some of the laboratories reported better measurement uncertainties then the measurement uncertainties of the reference laboratory, U(pR) 6. Discussion and Suggested Corective Actions Although all the participating laboratories were asked to submit the copy of the formal calibration certificate issued to the customer and traceability certificates of their standards, only 4 laboratories have submitted the copies of the formal calibration certificates of the dead weight tester (artifact, in the present case) while traceability certificates were submitted by only 5 laboratories. The seriousness to follow the instructions given in TP was found lacking as only 3 laboratories with Code numbers 3, 4 and 6 submitted both of these required documents. Laboratories with code numbers 2 and 10 even did not bother to submit any of such formal certificates. The certificates thus obtained were examined and found adequate except that there was little uniformity in the calibration certificates issued by the
participants, especially in reporting the measurement results. Most of the laboratories submitted their measurement results in time. All the laboratories prepared their 'Uncertainty Budget' as per instructions given in the TP. Some of the laboratories reported better measurement uncertainties then the measurement uncertainties of the reference laboratory. As mentioned in Section 5, the En numbers greater than unity require investigations and corrective action by the participating laboratory. The laboratory's management needs to ensure that the problem is rectified and procedures are put in place to prevent a recurrence. Laboratories with Code Nos. 4 and 6 need to review their results and take appropriate corrective actions. These laboratories are advised to improve their calibration facilities / modify the measurement method. 7. Conclusion An interlaboratory comparison programme (proficiency testing) is carried out in the pressure range 7 - 70 MPa using dead weight tester as an artifact. Total number of 9 NABL accredited laboratories participated in this programme. The comparison was performed at 10 pressure points selected arbitrarily throughout the entire pressure range. The proficiency testing concludes that out of 149
Sanjay Yadav, V.K. Gupta and A.K. Bandyopadhyay the total 87 measurement results reported here in this report, 68 (78.2%) are in agreement with the reference laboratory. The results of 5 laboratories out of the total 9 are acceptable and well within their reported expanded uncertainty at a coverage factor k = 2 throughout the entire pressure range. Two other laboratories also have acceptable results except one pressure point. These results are quite encouraging in the country. However, after taking the appropriate corrective actions by the participant laboratories, we expect that the performance of the laboratories with Code Nos. - 4 and 6 would certainly improve. These laboratories may reassess their measurement capabilities by participating in the future PT programme in this area.
Comparison: Part - 1: Development and Operation of Proficiency Testing Schemes. Part - 2: Selection and use of Proficiency Testing Schemes by Laboratory Accreditation Bodies, ISO / IEC 43: (1997). [5]
S. Yadav and A.K. Bandyopadhyay, Proficiency Testing Program Under NABL in the Pressure Range 7-70 MPa Using a Dead Weight Tester, Med. J. Meas. Contrl., 1 (2005) 138-151.
[6]
S. Yadav and A.K. Bandyopadhyay, Proficiency Testing (PT) Program Under NABL in the Pressure Range 7 - 70 MPa, Metrology and Measurement Systems, XII (2005) 323-340.
[7]
S. Yadav, V.K. Gupta and A.K. Bandyopadhyay, Standardization of Pressure Calibration (7-70 MPa) using Digital Pressure Calibrator, J. Sci. and Indusl. Res., 69 (2010) 27-33.
[8]
S. Yadav, V.K. Gupta, O. Prakash and A.K. Bandyopadhyay, Proficiency Testing Through Interlaboratory Comparison in the Pressure Range 7-70 MPa Using Pressure Dial Gauge as an Artifact, J. Sci. and Indusl. Res., 64 (2005) 722-740.
[9]
S. Yadav, V.K. Gupta, O. Prakash and A.K. Bandyopadhyay, Evaluation of Interlaboratory Performance through Proficiency Testing Using Pressure Dial Gauge in the Hydraulic Pressure Measurement up to 70 MPa", Mapan-Journal of Metrology Society of India, 23 (2008) 79-99.
[10]
S. Yadav and A.K. Bandyopadhyay, External Proficiency Testing in the Pressure Range up to 60 MPa", Measure, USA, 4 (2009) 42-51.
[11]
S. Yadav and A.K. Bandyopadhyay, Evaluation of Laboratory Performance through Interlaboratory Comparison, Mapan-Journal of Metrology Society India, 24 (2009) 125-138.
Acknowledgement We are grateful to Prof. R.C. Budhani, Director, National Physical Laboratory, New Delhi and Dr. Anil Relia, Director, National Accreditation Board for Testing & Calibration Laboratories, New Delhi for their support and encouragement throughout this programme. The PT programme has been conducted as per NPL-NABL MoU. We are also thankful to Dr. K.P. Chaudhary, Programme Coordinator, NPL-NABL PT Programme for his constant co-operation and time to time suggestions and discussions, which were very helpful during course of this comparison. Thanks are also due to all the nine accredited laboratories participating in this interlaboratory comparison exercise. Without their active support and cooperation this PT programme would have not been completed in time. We would also like to acknowledge the help of the secretariat of NABL for their administrative help. References [1]
www.bipm.org/en/cipm-mra/ mra_main_text.html
[12]
[2]
General Requirements for the Competence of Testing and Calibration Laboratories", ISO / IEC 17025: (2005).
Guidelines for Proficiency Testing Program for Testing and Calibration Laboratories, NABL Doc. 162, (2001).
[13]
Procedures for Establishing and Maintaining the APLAC Mutual Recognition Arrangement Amongst Accreditation Bodies, APLAC MRA001, Issue No. 13, (2007).
F. Pavese and G. Molinar, Modern Gas Based Temperature and Pressure Measurements, Plenum Press, New York, (1992).
[14]
S. Yadav, A.K. Bandyopadhyay, N. Dilawar and A.C. Gupta, Intercomparison of National Hydraulic Pressure Standards up to 500 MPa, Measurement + Control, UK, 35 (2002) 47-51.
[3]
[4]
150
Proficiency Testing by Interlaboratory
Standardisation of Pressure Measurement Using Pressure Balance as Transfer Standard
[15]
[16]
[17]
S. Yadav, R. Agarwal, D.R. Sharma, A.K. Bandyopadhyay and A.C. Gupta, Modern Instrumentation Techniques in Pressure Metrology under Static Conditions, MapanJournal of Metrology Society of India, 18 (2003) 57-82. A.K. Bandyopadhyay, S. Yadav and N. Dilawar, Current Status of Pressure Standards at NPLI and our Experiences with the Key Comparison Data Base (KCDB), Mapan-Journal of Metrology Society of India, 21 (2006) 127-145. S. Yadav, A.K. Bandyopadhyay and A.C. Gupta, Characterisation of National Hydraulic Pressure Standards in the Pressure Ranges up to 100 MPa, 200 MPa and 500 MPa, Callab: The International J. Metrology, (2003).
[18]
A.K. Bandyopadhyay and A.C. Gupta, Realization of a National Practical Pressure Scale for Pressure up to 500 MPa, Metrologia, 36 (1999) 681-688.
[19]
S.Yadav, O. Prakash, V.K. Gupta and A.K. Bandyopadhyay, The Effect of PressureTransmitting Fluids in the Characterization of a Controlled Clearance Piston Gauge up to 1 GPa, Metrologia, 44 (2007) 222-233.
[20]
S. Dogra, S. Yadav and A.K. Bandyopadhyay, Computer simulation of a 1.0 GPa PistonCylinder Assembly Using Finite Element Analysis (FEA), Measurement, 43 (2010) 13451354.
[21]
T. Kobata, A.K. Bandyopadhyay, K. Moore, A.E. Eltawil Alaaeldin, S.Y. Woo, T.K. Chan, W. Jian, J. Man, N.N. Con, C.S. Fatt, W. Permana, M. Aldammad, W. Sabuga, T. Changpan, C.C. Hung and Z. Pengcheng, Final Report on Key Comparison APMP.M.P-K7 in Hydraulic Gauge Pressure from 10 MPa to 100 MPa", Metrologia, 42 (2005) 07006.
[22]
W. Sabuga, M. Bergoglio, T. Rabault, B. Waller, J.C. Torres, D.A. Olson, A. Agarwal, T. Kobata and A.K. Bandyopadhyay, Final Report on Key
Comparison CCM.P-K7 in the Range 10 MPa to 100 MPa of Hydraulic Gauge Pressure, Metrologia, 42 (2005) 07005. [23]
R.G. Driver, D.A. Olson, S. Yadav and A.K. Bandyopadhyay, Final Report on APMP.SIM.M.P-K7: Bilateral Comparison Between NIST (USA) and NPLI (India) in the Hydraulic Pressure Region 40 MPa to 200 MPa", Metrologia, 43 (2006) 07003.
[24]
S. Yadav, D.A. Vijayakumar and A.C. Gupta, Computer Software for Calibration of Industrial and Master Simple / Reentrant Type Piston Gauges", Mapan-Journal of Metrology Society of India, 12 (1997) 101-104.
[25]
S. Yadav, Characterization of Dead Weight Testers and Computation of Associated Uncertainties: A Case Study of Contemporary Techniques, Metrology and Measurement Systems, XIV (2007) 453-469.
[26]
S. Yadav, B.V. Kumaraswamy, V.K. Gupta and A.K. Bandyopadhyay, Least Squares Best Fit Line Method for the Evaluation of Measurement Uncertainty with Electromechanical Transducers (EMT) with Electrical Outputs (EO)", MAPAN-Journal of Metrology Society of India, 25 (2010) 97-106.
[27]
S. Yadav, V.K. Gupta and A.K. Bandyopadhyay, Investigations on Measurement Uncertainty and Stability of Pressure Dial Gauges and Transducers", Measurement Science Review, 14 (2010) 130-135.
[28]
Guide to the Expression of Uncertainty in Measurement", ISO Document - ISO/TAG/WG 3: (1995) (E).
[29]
Guidelines for Estimation and Expression of Uncertainty in Measurement", NABL Doc. 141, (2000).
[30]
H.S. Nielsen, Determining Consensus Values in Interlaboratory Comparisons and Proficiency Testing, Proceedings NSCL Conference, Tampa, Florida, USA, (2003) pp. 1-16.
151