Evaluation of interlaboratory performance through proficiency testing using pressure dial gauge in the hydraulic pressure measurement up to 70 MPa

Evaluation of Interlaboratory Performance through Proficiency Testing ........ 70 MPa

79

Evaluation of Interlaboratory Performance throughProficiency Testing using Pressure Dial Gauge in the

Hydraulic Pressure Measurement up to 70 MPa

SANJAY YADAV*, OM PRAKASH, V. K. GUPTA, B.V. KUMARASWAMY andA. K. BANDYOPADHYAY

National Physical Laboratory (NPLI), CSIRDr. K.S. Krishnan Road, New Delhi - 110 012, India

*e-mail : [email protected]

[Received : 14.03.2008 ; Revised : 21.05.2008 ; Accepted : 27.05.2008]

AbstractThe present paper reports the results of the proficiency testing (PT) accomplished for 17 laboratories,accredited by National Accreditation Board for Testing and Calibration of Laboratories (NABL). Themeasurements were performed in the pressure range 10-70 MPa using pressure dial gauge as anartifact. Only laboratories having best measurement capabilities 0.25 % or coarser than 0.25 % of full-scale pressure were included in this PT. The program started in May 2006 and completed duringOctober, 2007. The comparison was carried out at 10 arbitrarily chosen pressure points i.e. 10, 20, 30,40, 45, 50, 55, 60, 65 and 70 MPa. The results thus obtained show that out of the total 159 measurementresults, 135 (84.91 %) are found in good agreement with the results of the reference laboratory. Therelative deviations between laboratories values and reference values are well within 0.15 % for 75measurement points, 0.25% for 108 measurement points and 0.50% for 148 measurement points. Thedifference of the laboratories values with reference values are found almost well within the uncertaintyband of the reference values at 71.07 % measurement results, within their reported expanded uncertaintyband at 62.26% measurement results and within the combined expanded measurement uncertaintyband at 84.91 % measurement results. Overall, the results are considered to be reasonably good, beingthe first proficiency testing for most of the participating laboratories.

MAPAN - Journal of Metrology Society of India, Vol. 23, No. 2, 2008; pp. 79-99

© Metrology Society of India, All rights reserved.

1. Introduction

The physical quantity 'pressure' is one of the mostimportant derived parameters of the physico-mechanical measurement systems in many branchesof science and industry, particularly in technologicalprocesses. The measurement of pressure and vacuumare useful in different diversified areas of research &industry e.g., space research, atomic energy, defenceapplications, semiconductor industry, plasma physicsrelated applications, thin films deposition, powergeneration, X-rays/TV tube manufacturing,

assessment of health, optimization of domesticappliances, manufacturing of chemicals, pesticides,fertilizers, drugs and pharmaceuticals, synthesis ofsuper hard materials like diamonds, forging of coldand hot steels, ventilation, filtration and processcontrol in general, etc. [1-4].

As we move in to the new millennium, theglobalization and international competitiveness intrade and commerce and introduction of qualitysystem, the international trade will continue tosignificantly impact our economy. In compliance withthe recently updated quality standards, the

Sanjay Yadav et al.

80

maintenance of pressure measuring instruments inthe accredited laboratories to obtain optimumperformance requires the periodic calibration traceableto national standards.

To meet the requirements of ISO 17025 [5] andAPLAC MR001 [6], it is mandatory for all accreditingbodies to undertake proficiency testing programmesfor its accredited laboratories in conformity with ISO/IEC Guide 43 [7]. National Accreditation Board forTesting and Calibration Laboratories (NABL),Department of Science and Technology (DST),Government of India has assigned the task ofconducting Proficiency Testing (PT) programme toNational Physical Laboratory (NPL), New Delhi. Themain objectives of this PT programme are i) to assessthe status of analytical competence of participatinglaboratories in view of laboratory accreditation, ii) toidentify the serious constraints (random & systematic)in the working environment of laboratories iii) topromote the scientific and analytical competence ofthe concerned laboratories to the level of excellencefor better output and iv) to provide laboratories withan objective means of assessing and demonstratingthe reliability of data they are producing.

During last 5 years NABL has conducted severalproficiency testing experiments in pressure metrologyin the pressure range up to 70 MPa through NationalPhysical Laboratory (NPL), New Delhi. The first PT,designated as NABL-Pressure-PT001 was organizedfor seven laboratories having measurementcapabilities better than 0.05% of full scale pressureusing dead weight tester as an artifact [8]. The secondPT i.e. NABL-Pressure-PT002 was conducted foranother seven laboratories having measurementcapabilities coarser than 0.05% and better than 0.25% of full scale pressure using digital pressurecalibrator [9-10]. NABL-Pressure-PT003, the third PTincluded eleven laboratories having measurementcapabilities 0.25% or coarse than 0.25% of full scalepressure using pressure dial gauge as an artifact [11]and the NABL-Pressure-PT007 for fourteenlaboratories having measurement capabilities 0.25%or coarser than 0.25% of full scale pressure usingpressure dial gauge as an artifact [12].

In a series of these PT experiments, the present PTprogram, designated as NABL-Pressure-PT006, isrecently completed during October 2007. This PTprogramme was designed and organized in thehydraulic pressure region covering pressure range 10

- 70 MPa (100 to 700 bar) using the pressure dial gaugeas an artifact. The seventeen NABL accreditedpressure calibration laboratories, havingmeasurement capabilities 0.25 % or coarser than 0.25% of full-scale pressure, have been covered in this PT.This report summarizes the results of measurementsof these laboratories at 10 pressure points. Thecirculation of the artifact started in July 2006 andcompleted in July 2007 in a record time.

2. Methodology

The PT programme was designed as perguidelines stipulated in ISO/IEC 17025 [5], ISO/IECGuide 43 [7] and NABL-162 [13]. A high precisionBourdon tube pressure dial gauge, 15" dial mirror type,make Heise, USA, Sl. No. - CM42041 was used as anartifact. Selection of the measurement points is animportant aspect of the PT programme. The entirepressure range of 10 to 70 MPa was divided into 10arbitrarily chosen measurement points of 10, 20, 30,40, 45, 50, 55, 60, 65 and 70 MPa. All the participantswere advised to complete the measurements in twoweeks time and dispatch the artifact to nextparticipant within next one week. All the participantsperformed their measurements well in time. The wholecirculation programme was completed in two loops.Schematic diagram of the movement of the artifact isdepicted in Fig. 1.

All the participants were advised to perform sixobservations at each pressure point, three each inincreasing and decreasing orders of pressures. Theywere also requested to report the temperatures atwhich the measurements were corrected. However,the laboratories were advised to correct the pressuremeasurements at 23º C. The characterization of theartifact was performed at the reference laboratory bydirect comparison method [1-4] against the nationalhydraulic secondary pressure standard, designatedas NPL200MPA, first at the start of the programmeduring May, 2006, second in the middle duringJanuary, 2007 and finally at the end of programmeduring September - October, 2007. The traceability ofthe NPL200MPA has been established as explainedelsewhere [14-16]. NPL200MPA has also been usedas NPL standard to calibrate the transfer standardsfor the recently concluded bilateral comparison withNIST, USA [17].

Before calibration, both the instrument i.e. theNPL200MPA and the artifact were leveled using

https://www.researchgate.net/publication/236004936_Characterisation_of_National_Hydraulic_Pressure_Standards_in_the_Pressure_Ranges_up_to_100_MPa_200_MPa_and_500_MPa?el=1_x_8&enrichId=rgreq-a5a3a51e-6120-4350-a22a-0185e6138cf5&enrichSource=Y292ZXJQYWdlOzIyODM3MTI5NTtBUzoxMDMzODQ5MjUzNDM3NDhAMTQwMTY2MDI5NTI4MQ==

https://www.researchgate.net/publication/236005018_Intercomparison_of_national_hydraulic_pressure_standards_up_to_500_MPa?el=1_x_8&enrichId=rgreq-a5a3a51e-6120-4350-a22a-0185e6138cf5&enrichSource=Y292ZXJQYWdlOzIyODM3MTI5NTtBUzoxMDMzODQ5MjUzNDM3NDhAMTQwMTY2MDI5NTI4MQ==

https://www.researchgate.net/publication/236004939_Re-establishment_of_Measurement_Uncertainty_in_Pressure_Measurement_through_In-House_Laboratory_Intercomparison_of_National_Hydraulic_Pressure_Standards_up_to_500_MPa?el=1_x_8&enrichId=rgreq-a5a3a51e-6120-4350-a22a-0185e6138cf5&enrichSource=Y292ZXJQYWdlOzIyODM3MTI5NTtBUzoxMDMzODQ5MjUzNDM3NDhAMTQwMTY2MDI5NTI4MQ==

https://www.researchgate.net/publication/236004995_Proficiency_testing_through_interlaboratory_comparison_in_the_pressure_range_up_to_70_MPa_using_pressure_dial_gauge_as_an_artifact?el=1_x_8&enrichId=rgreq-a5a3a51e-6120-4350-a22a-0185e6138cf5&enrichSource=Y292ZXJQYWdlOzIyODM3MTI5NTtBUzoxMDMzODQ5MjUzNDM3NDhAMTQwMTY2MDI5NTI4MQ==

https://www.researchgate.net/publication/236005079_Proficiency_Testing_Program_Under_NABL_in_the_Pressure_Range_7-70_MPa_Using_a_Dead_Weight_Tester?el=1_x_8&enrichId=rgreq-a5a3a51e-6120-4350-a22a-0185e6138cf5&enrichSource=Y292ZXJQYWdlOzIyODM3MTI5NTtBUzoxMDMzODQ5MjUzNDM3NDhAMTQwMTY2MDI5NTI4MQ==


81

leveling screws and sprit level. The necessary weightswere placed on the carrier of the NPL200MPA andadjusted as per the values of the pressure indicatedon the artifact. This was repeated several times so thatthe error due to this adjustment of the weights isminimized. Sufficient time, 10 minutes approximately,was provided between two successive observationsso that both the systems are in complete equilibrium.At this position, there was no pressure drop in theconnecting line and consequently no movement of thefluid. This procedure was repeated for all the 10pressure points i.e. 10, 20, 30, 40, 45, 50, 55, 60, 65 and70 MPa and observations were repeated six times,thrice in increasing order and thrice in decreasingorder, for each pressure point and the values ofmeasured pressure, their repeatability and expandeduncertainty were computed using computer softwaresdeveloped for this purpose [18-19]. The pressuremeasured by NPL200MPA was calculated using thefollowing equation;

i i NPL air mi

0 s n c p r

m .g (1 / ) CP pA (1 p )[1 ( )(T T )]∑ −ρ ρ + γ

= ±Δ+λ + α +α − [1]

where, mi, is the mass of the standard weight, gNPL

is the local acceleration of gravity, ρair is the density ofthe air at the temperature, barometric pressure andhumidity conditions prevailing in the laboratory, ρmiis the density of the ith weight of the standard, γ is thesurface tension of the pressure transmitting fluid, C isthe circumference of the standard piston where itemerges from the fluid, A0 is the effective area of thestandard piston-cylinder assembly at zero pressure,αc & αp are the thermal expansion coefficients of thestandard's piston and cylinder material, T is thetemperature of the standard's piston - cylinderassembly, Tr is the temperature at which A0 is referred,λs is the pressure distortion coefficient of the effectivearea for the standard and Δp = [(ρf - ρair).gNPL.H] is thehead correction in terms of pressure where H is thedifference in height between the reference levels of thestandard and the artifact and ρf is the density of thetransmitting fluid.

The details of the pressure measured (p) and theirmeasurement uncertainties are shown in Table 1 forall the three successive calibrations performed in May2006, January 2007 and September - October 2007.The reference values of pressure measured are thearithmetic mean of the data obtained during thesethree calibrations. The detailed uncertainty budgetthus prepared for the measurements performed on theartifact is shown in Table 2.

Fig. 1. Circulation and movement of the artifact during comparison. Period shown herein is the actual forwhich the artifact remained with the participating laboratory

Loop 1 (Started 20.07.2007) Loop 2 (Started 23.02.2007)

Loop 2(C

omp. 03.11.2007)

Electronics Test andDevelopment Centre Goa16-04-2007 to 03-05-2007

Loop 1 (Com

p. 09.11.2007)

https://www.researchgate.net/publication/236004937_Quality_of_measurements_a_software_for_estimation_of_measurement_uncertainty?el=1_x_8&enrichId=rgreq-a5a3a51e-6120-4350-a22a-0185e6138cf5&enrichSource=Y292ZXJQYWdlOzIyODM3MTI5NTtBUzoxMDMzODQ5MjUzNDM3NDhAMTQwMTY2MDI5NTI4MQ==

https://www.researchgate.net/publication/236005005_Computer_Software_for_Calibration_of_Industrial_and_Master_Simple_Reentrant_type_Piston_Gauges?el=1_x_8&enrichId=rgreq-a5a3a51e-6120-4350-a22a-0185e6138cf5&enrichSource=Y292ZXJQYWdlOzIyODM3MTI5NTtBUzoxMDMzODQ5MjUzNDM3NDhAMTQwMTY2MDI5NTI4MQ==

Sanjay Yadav et al.

82

Tabl

e 1

Det

ails

of m

etro

logi

cal c

hara

cter

istic

s of t

he a

rtif

act a

nd a

ssig

nmen

t of r

efer

ence

val

ues

109.

9763

29.

9838

010

.006

099.

9887

4-0

.012

41-0

.004

940.

0173

50.

0136

60.

0288

80.

0173

50.

0363

50.

0727

020

19.9

7498

19.9

9076

20.0

0007

19.9

8860

-0.0

1362

0.00

215

0.01

147

0.02

216

0.02

890

0.01

363

0.03

889

0.07

777

3029

.963

4429

.950

9329

.983

9129

.966

09-0

.002

65-0

.015

170.

0178

20.

0219

30.

0289

40.

0178

30.

0404

50.

0809

040

39.9

3430

39.9

4756

39.9

9509

39.9

5898

-0.0

2468

-0.0

1142

0.03

610

0.01

357

0.02

900

0.03

612

0.04

827

0.09

654

4544

.983

2745

.002

3444

.996

8544

.994

15-0

.010

880.

0081

90.

0026

90.

0350

10.

0290

30.

0108

90.

0467

60.

0935

350

49.8

7252

49.8

8077

49.9

7861

49.9

1063

-0.0

3812

-0.0

2987

0.06

798

0.03

120

0.02

907

0.06

798

0.08

025

0.16

050

5554

.889

1254

.905

7354

.975

2954

.923

38-0

.342

6-0

.017

650.

0519

10.

0260

30.

0291

20.

0519

00.

0649

60.

1299

160

60.0

2036

60.0

0703

59.9

4948

59.9

9229

0.02

807

0.01

474

-0.0

428

0.03

714

0.02

916

0.04

283

0.06

375

0.12

751

6564

.849

5964

.858

7264

.953

5664

.887

29-0

.037

70-0

.028

570.

0662

70.

0184

20.

0292

10.

0662

50.

0747

10.

1494

270

69.8

4850

69.8

4015

69.9

6512

69.8

8459

-0.0

3609

-0.0

4444

0.08

053

0.02

850

0.02

927

0.08

051

0.09

028

0.18

055

Nom

i-na

lPr

es-

sure

(MPa

)Pres

sure

(MPa

)p 1

May

2006

Pres

sure

(MPa

)p 2

Jan.

2007

Pres

sure

(MPa

)p 3

Sept

. - O

ct.

2007

Ave

rage

Pre

ssur

ep

(MPa

)Re

fere

nce

Val

ues

Dev

.p 1-p

(MPa

)

Dev

.p 2-p

(MPa

)

Dev

.p 3-p

(MPa

)

Unc

erta

inty

Thro

ugh

Type

AM

etho

d(M

Pa)

Unc

erta

inty

Thro

ugh

Type

BM

etho

d(M

Pa)

Unc

erta

inty

Thro

ugh

Stab

ility

of

the A

rtifa

ct(M

Pa)

Com

bine

dU

ncer

-ta

inty

uc(p

)(M

Pa)

Expa

nded

Unc

er-

tain

tyU

(p)

(MPa

)

Tabl

e 2

Unc

erta

inty

Bud

get o

f the

Art

ifac

t at M

axim

um P

ress

ure

of 7

0 M

Pa

Unc

erta

inty

of t

he S

tand

ard

u B170

0.00

49N

orm

al –

Typ

e B0.

0049

10.

0049

∞U

ncer

tain

ty d

ue to

Res

olut

ion

0.1

0.05

Rect

angu

lar –

Typ

e B

/ √3

0.02

891

0.02

89∞

of th

e Art

ifact

uB2

Repe

atab

ility

in th

e thr

ee C

alib

ratio

ns0.

0698

0.06

98N

orm

al –

Typ

e A /

√n

0.02

851

0.02

855

(Max

imum

) uA

1U

ncer

tain

ty d

ue to

Sta

bilit

y (M

axim

um0.

081

0.08

1N

orm

al –

Typ

e B

/ 1

0.08

11

0.08

1∞

Dev

iatio

n fr

om th

e Ref

eren

ce V

alue

) μB3

u c(p)

k =

10.

0905

6135

EXPA

ND

ED U

NC

ERTA

INTY

U(p

)k

= 2.

00.

181

The

expa

nded

unc

erta

inty

ass

ocia

ted

with

pre

ssur

e m

easu

rem

ents

is 0

.181

MPa

.

Sour

ce o

fU

ncer

tain

ty(X

i)

Estim

ates

(xI)

(MPa

)

Lim

its±Δ

x i(M

Pa)

Prob

abili

tyD

istr

ibut

ion

- Typ

e Aor

Typ

e B F

acto

r

Stan

dard

Unc

erta

inty

U(X

i) (M

Pa)

Sens

itivi

tyC

oeffi

cien

tU

ncer

tain

tyC

ontr

ibut

ion

u i(y) (

MPa

)

Deg

ree

of F

reed

om( F

)


83

The values of measured pressures p1, p2 and p3,are determined using Eq. 1 for three successivecalibrations, respectively. The arithmetic means ofthese pressure values (p1, p2 and p3) are the referencevalues of the pressure measured (p) for individualmeasurement points throughout the entire pressure

scale. In order to study the behaviour and stability ofthe artifact, the calibration factor (Cf) of the artifact isplotted as a function of measured pressure in Fig. 2(a)and the relative deviations of the measured pressuresp1, p2 and p3 from the re`ference values, p in Fig. 2(b).

Fig. 2(a). The Calibration factor (Cf) and its average values plotted as a function of appliedpressure p for all the three successive calibrations of the artifact

Fig. 2(b). Relative deviations (% of reading) of the measured pressures p1, p2 and p3 from the referencevalues p for all the three successive calibrations

Sanjay Yadav et al.

84

The calibration factor (Cf) is determined asfollows;

gf

S

pC

p= [2]

where, pg is the reading of the artifact and pS iscorresponding pressure measured by the standardduring calibration. The behaviour of the artifact wasnot found identical in all the three calibrations. It is

clearly evident from Fig. 2(a) that during the twocalibrations of May 2006 and September - October2007, the artifact behaved almost in a similar fashion.Although, the behaviour of the artifact was slightlydifferent during January 2007 but the values ofcalibration factor were found much closer to the unitywhich indicates a close agreement between gaugereading and the standard pressure

The relative deviations of the measured pressuresp1, p2 and p3 from the reference values, p are found

Fig. 2(c). Relative deviations (% of full scale pressure) of the measured pressures p1, p2 and p3from the reference values p for all the three successive calibrations

Fig. 3. Expanded measurement uncertainty U(p) as a function of measured pressure p

U(p) = (0.00171 p + 0.042) MPa

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0 10 20 30 40 50 60 70

Pressure (MPa)

Ex

pa

nd

ed

Un

ce

rta

inty

(MP

a)

Experimental U(p) Fitted U(p)


85

well below ± 0.17 % of reading [Fig. 2(b)] and ± 0.1 %of full scale [Fig. 2(c)]. Although, the deviations arewell within the manufacturer specifications of 0.1 %of span (full scale) except at one pressure point. Themaximum deviation of 0.17 % of reading is taken in toconsideration to estimate the expanded uncertaintyU(p) = (0.042 + 0.00171p) MPa. The expandeduncertainty U(p) as a function of measured pressurep is shown in Fig. 3. This concludes that the artifactremained stable during the whole PT programmewithin its estimated measurement uncertainty.

3. Experimental Setup and Calibration Procedure

Total 17 laboratories participated in the programexcluding the reference laboratory, NPLI, New Delhi.In order to maintain confidentiality in the results, eachparticipating laboratory was assigned a random codenumber and only these code numbers are used hereinthereafter in this PT report. Code number assigned tothe reference laboratory, NPLI, New Delhi, is '1' (Ref.).All the laboratories were advised to install theexperimental set-up as shown in Fig. 4.

All the laboratories were asked to use cleanmineral oil as pressure transmitting fluid in the oilreservoir. The calibration of the artifact starts withleak testing, zero adjustment and the selection of areference or datum level. For leak testing, they wererequested to pressurize both the standard and the

artifact up to 700 bar with the help of hydraulic screwpump and needle valves and wait for at least 10minutes and then release the pressure slowly to zero.Laboratories were asked to repeat this process at leastthree times to ensure that there are no leaks in thesystem. In this way compressibility of the transmittingoil, packing of the valves, pump plunger and O-ringseals are stabilized to reach an optimum level.

Zero adjustment of the artifact is then performedusing the zero adjustment knob of the artifact.Participating laboratories were also requested toensure zero adjustment of their standard (in case ofdigital pressure instrument or pressure dial gauge).In case zero adjustment knobs are not provided withtheir standards, they were asked to record the initialbias in the measurements and apply necessarycorrection at the appropriate level.

The selection of appropriate and precise referenceor datum plane is very important for applying thehydrostatic head correction. Usually, reference ordatum plane is marked on the standard or noted inthe operation manuals. If no such information isavailable, the centre point of the elastic element isconsidered the reference or datum plane.

Needle setting of the artifact is also one of theimportant points to be taken into consideration duringmeasurements. The normal practice is to check the

Fig. 4. Experimental setup for the measurement using pressure dial gauge as an artifact

Sanjay Yadav et al.

86

reflection of the needle from the mirror. In order tominimize the parallax error, the best position formeasurement would be when the reflected imagecoincides with real object i.e. needle in the presentcase. Laboratories were advised to follow the sameeye estimation uniformly for all the pressure points.

In this way, the system would be ready to performcalibration. The full scale pressure (measurementrange in the present case) of 700 bar is then dividedinto 10 pressure points of 100, 200, 300, 400, 450, 500,550, 600, 650 and 700 bar. The needle of the artifact isthen brought to first measurement point bypressurizing the system and the corresponding valueof the pressure measured by the standard is recordedafter applying all corrections i.e. temperaturecorrection, hydrostatic head correction and unitconversion. Laboratories were advised to record thecorrected pressure measured by the standard only inbar or MPa. Subsequently, the needle of the artifact isfixed to the next pressure point and the pressure

measured by the standard is recorded. This process isrepeated till the full-scale pressure of 700 bar isachieved. They were asked to maintain sufficient time(approximately 10 minutes) between two successiveobservations to allow the system to reach a state ofthermal equilibrium. It was also suggested to wait forat least 10 minutes after reaching full-scale pressurebefore the observations are repeated in the decreasingorder of pressure till the pressure reaches to zero.Laboratories were requested to record at least 20observations, 10 each in the order of increasing anddecreasing pressures, to perform one pressure cycleand then to repeat the measurements for at least 3pressure cycles to make the total number of 60observations. A layout diagram shown in Fig. 5presents the sequence of measurements to be taken.

All the participants were advised to apply thetemperature and head corrections carefully beforesubmitting the results. They were requested to correct

Fig. 5. Sequence of measurements taken


87

the values of the measured pressure for 23 ºC usingthermal expansion coefficient of the piston - cylinderassembly (if dead weight tester is used as standard)or elastic element (if pressure dial gauge or digitalcalibrator is used as standard) using standardequations. They were also requested to evaluate theuncertainty associated with pressure measurementsas per ISO Guide to the Expression of Uncertainty inMeasurement / NABL Document 141 on uncertaintyfollowing Type A and Type B methods of evaluation[20-21]. Each participating laboratory was requestedto prepare an uncertainty budget at the maximumpressure, considering all Type A and Type Buncertainty components.

4. Reporting of Results and Data Analysis

Laboratories were also asked to submit copies ofthe calibration certificates for the referenceinstruments used in measurements, calculation sheetsfor determining the uncertainty in measurements andcalibration certificate as issued to the customer forsuch measurements. The values of measured pressure,acceleration of local gravity and reference temperatureand the measurement traceability, reported by theparticipants are shown in Table 3. The performanceof the laboratories has been assessed on the basis ofError Normalized (En) number of each measurement.The En values are estimated for each participant ateach pressure as per guidelines in the literature [7, 13,22-23];

{ } { }

'

n 2 2'

p pEU(p ) U(p)

−=

+ [3]

where p' is the participant's measured pressure value,p is the calculated reference value, U(p') is theparticipant's claimed expanded uncertainty at acoverage factor k = 2 and U(p) is the expandedmeasurement uncertainty of the reference value at acoverage factor k = 2. An En value ±1 indicatesagreement within the combined uncertainties for theresults to be internationally acceptable. An En numberbetween -1 and +1 indicates an acceptable degree ofcompatibility between the laboratory's result and thereference value when the quoted uncertainties aretaken into account. En number outside the -1 and +1range is unacceptable and requires immediateinvestigation and corrective action by the laboratoryconcerned.

5. Results and Discussion

Details of the values of measured pressure (p')and other metrological characteristics of thelaboratory standards reported by the participants areshown in Table 3 (for all the participants). The relativedeviations of measured pressure (p') of eachparticipant from reference value (p) are shown inTable 4. The graphs plotted for the results forindividual laboratories, are depicted as Figs. 6 -17.The summaries of the reported expandedmeasurement uncertainties, combined expandedmeasurement uncertainties and the normalized error(En) values, are shown in Tables 5 to 7, respectivelyfor entire pressure scale of 10 - 70 MPa.

In general, the performance of the laboratory isconsidered satisfactory if normalized error En is≤ ±1. The tabulated data shown in Tables 5 to 7reveals that there are total 159 measurement results.Measurement results of 9 laboratories (Code No. 2, 6,7, 10, 11, 12, 13, 17 and 18) out of total 17 laboratoriesare well within acceptable limits of normalized errorover the entire pressure range of 10 - 70 MPa. However,the measurement results of another 2 laboratorieswith Code No. 8 and 16 are also quite good having Envalues ≥ ±1 only at one pressure point. En values of135 measurement results out of total 159 are

≤ ±1,

which is 84.91 %. These results are acceptable. Envalues of remaining 6 laboratories referred by CodeNo. 3, 4, 5, 9, 14 and 15 are

≥

±1 for 2 or more than 2pressure points. The larger the absolute value of theEn number, the bigger the problem. An En

≥

±1 meansthat there is a significant bias in the laboratory's resultsand that the quoted value of its associated uncertaintydoes not adequately accommodate that bias, thereforefurther investigations are needed on the part of thelaboratory.

The graphical representations in Figs. 6-15 givethe agreement between participating laboratories andthe reference laboratory for individual pressure pointsof 10 MPa, 20 MPa, 30 MPa, 40 MPa, 45 MPa, 50 MPa,55 MPa, 60 MPa, 65 MPa and 70 MPa, respectively.The summary of the normalized error values (En) as afunction of applied pressure (p') is given in Table 7and its graphical representation is depicted in Fig.16. The relative deviations of measured pressure (p')from reference values (p) are shown in Fig. 17. Thedeviations lying within the uncertainty band of thereference laboratory is an indication of satisfactory

Sanjay Yadav et al.

88

Tabl

e 3

Det

ails

of t

he re

fere

nce

valu

es o

f mea

sure

d pr

essu

re (p

), pr

essu

re m

easu

red

by th

e pa

rtic

ipan

ts (p

'), re

fere

nce

tem

pera

ture

and

oth

erm

etro

logi

cal c

hara

cter

istic

s of t

he la

bora

tori

es st

anda

rds

12

34

56

78

910

1112

1314

1516

1718

*pn

(MPa

) p(M

Pa)

#p’ (

MPa

)

109.

9887

10.0

253

10.0

575

9.97

7410

.057

010

.000

09.

9830

10.0

500

9.92

6710

.027

010

.013

69.

8930

10.0

900

10.1

450

9.97

8010

.178

9.96

2275

10.0

052

2019

.988

620

.039

20.1

070

19.9

915

20.0

690

20.0

000

19.9

830

20.0

300

19.9

567

20.0

760

20.0

471

19.9

360

20.0

830

20.0

690

19.9

140

20.0

202

19.9

8387

19.9

987

3029

.966

130

.063

730

.079

729

.938

630

.062

030

.000

029

.985

030

.030

029

.974

729

.966

030

.019

029

.916

030

.093

029

.955

29.8

990

29.9

385

29.9

602

29.9

9340

39.9

590

40.0

887

40.1

487

40.0

521

40.1

660

40.0

000

40.0

010

40.1

000

39.8

487

39.9

580

40.0

334

39.9

960

40.0

860

40.1

7239

.810

039

.97

39.9

6951

40.0

115

4544

.994

1545

.110

3345

.103

45.0

0893

45.1

2844

.993

44.9

9844

.97

44.9

1115

44.9

3945

.039

8644

.935

45.0

4145

.074

44.7

990

44.8

788

44.9

5071

44.9

868

5049

.910

650

.131

550

.095

250

.058

450

.116

049

.999

049

.987

049

.950

049

.914

549

.894

050

.029

749

.939

050

.043

050

.029

49.8

540

49.8

623

49.9

3196

49.9

892

5554

.923

455

.153

055

.153

655

.055

055

.074

055

.003

054

.988

054

.940

054

.843

654

.912

055

.038

054

.898

055

.001

054

.963

54.8

270

54.8

154

.918

0554

.970

360

59.9

923

-60

.102

060

.046

860

.160

059

.996

059

.983

059

.940

059

.810

859

.979

060

.037

759

.860

060

.006

059

.974

59.7

740

59.8

645

59.9

1085

59.9

852

6564

.887

3-

65.0

953

65.0

694

65.1

160

65.0

060

64.9

880

64.8

700

--

65.0

371

64.7

790

64.9

780

65.1

9664

.796

064

.799

564

.918

3764

.967

370

69.8

846

-70

.092

070

.085

1-

69.9

660

-69

.970

0-

-70

.024

7-

70.0

330

-69

.718

070

.002

869

.932

4469

.963

g(m

/s2 )

9.79

1239

3-

9.78

759

9.78

799.

9993

47-

9.78

7177

9.88

029.

7909

591

9.78

2939

9.78

0352

9.78

389.

784

9.78

807

9.78

553

9.78

8184

39.

7911

1017

9.78

584

Ref

.Tem

p.23

23+2

23.6

-24.

820

2323

23+1

2323

.523

.423

+125

+423

+123

+124

23+1

2023

+2 (0 C

)

Trac

-N

PLI-H

1,R

TC,

Mea

sure

NV

LAP,

NPL

,EM

C,

NPL

,N

agm

an,

-M

easu

reU

KA

S,ID

EMI,

ETD

C,

Mea

sure

IDEM

I,ER

TL,

NPL

,ID

EMI,

eabi

lity

NIS

T,N

ewTe

chni

quw

USA

New

Kol

katta

New

Che

nnai

Tech

niqu

wU

KM

umba

iC

henn

ai T

ech.

Mum

bai

Mum

bai

New

Mum

bai

USA

Del

hiC

henn

aiD

elhi

Del

hiC

henn

ai C

henn

aiD

elhi

* Nom

inal

Pre

ssur

e

#Pr

esur

e V

alue

s fo

r La

bora

tory

cod

e 2

to 1

8.

Labo

rato

ry C

ode


89

Tabl

e 4

Rel

ativ

e de

viat

ions

(in

%) o

f the

mea

sure

d pr

essu

re (p

') fr

om re

fere

nce

valu

es (p

)

p (M

Pa)

12

34

56

78

910

1112

1314

1516

1718

10-

0.37

0.69

-0.1

10.

680.

11-0

.06

0.61

-0.6

20.

380.

25-0

.96

1.01

1.56

-0.1

11.

89-0

.26

0.16

20-

0.25

0.59

0.01

0.40

0.06

-0.0

30.

21-0

.16

0.44

0.29

-0.2

60.

470.

40-0

.37

0.16

-0.0

20.

0530

-0.

330.

38-0

.09

0.32

0.11

0.06

0.21

0.03

0.00

0.18

-0.1

70.

42-0

.04

-0.2

2-0

.09

-0.0

20.

0940

-0.

320.

470.

230.

520.

100.

110.

35-0

.28

0.00

0.19

0.09

0.32

0.53

-0.3

70.

030.

030.

1345

-0.

260.

240.

030.

300.

000.

01-0

.05

-0.1

8-0

.12

0.10

-0.1

30.

100.

18-0

.43

-0.2

6-0

.10

-0.0

250

-0.

440.

370.

300.

410.

180.

150.

080.

01-0

.03

0.24

0.06

0.27

0.24

-0.1

1-0

.10

0.04

0.16

55-

0.42

0.42

0.24

0.27

0.14

0.12

0.03

-0.1

5-0

.02

0.21

-0.0

50.

140.

07-0

.18

-0.2

1-0

.01

0.09

60-

-0.

180.

090.

280.

01-0

.02

-0.0

9-0

.30

-0.0

20.

08-0

.22

0.02

-0.0

3-0

.36

-0.2

1-0

.14

-0.0

165

--

0.32

0.28

0.35

0.18

0.16

-0.0

3-

-0.

23-0

.17

0.14

0.48

-0.1

4-0

.14

0.05

0.12

70-

-0.

300.

29-

0.12

-0.

12-

-0.

20-

0.21

--0

.24

0.17

0.07

0.11

Labo

rato

ry C

ode

Tabl

e 5

Sum

mar

y of

the

expa

nded

unc

erta

inty

(M

Pa) e

stim

ated

and

repo

rted

by

the

part

icip

ants

p (M

Pa)

12

34

56

78

910

1112

1314

1516

1718

100.

0727

0.26

620.

0606

0.05

770.

069

0.01

200.

1279

0.07

250.

0966

0.12

360.

0650

0.10

560.

1233

0.10

540.

0080

0.06

820.

0587

60.

062

200.

0778

0.26

640.

0634

0.07

150.

065

0.01

200.

1262

0.07

250.

0483

0.12

680.

0660

0.11

560.

1047

0.09

680.

0320

0.11

940.

0580

40.

0730

0.08

090.

2662

0.06

240.

0578

0.08

20.

0120

0.12

370.

0725

0.04

140.

1332

0.06

500.

1154

0.13

030.

1026

0.03

300.

095

0.05

850.

076

400.

0965

0.26

620.

0642

0.07

150.

067

0.01

200.

1221

0.07

250.

0487

0.12

640.

0660

0.12

440.

1095

0.09

680.

0080

0.08

480.

0587

0.08

445

0.09

350.

2662

0.06

560.

0578

0.07

30.

0146

0.12

310.

0725

0.04

120.

1196

0.07

000.

1428

0.10

400.

0961

0.01

000.

0854

0.05

876

0.08

250

0.16

050.

2668

0.06

980.

0631

0.07

10.

0146

0.13

430.

0725

0.04

130.

1238

0.06

900.

1134

0.11

730.

0966

0.04

600.

0702

0.05

884

0.08

855

0.12

990.

2660

0.06

960.

0716

0.08

10.

0172

0.13

620.

0725

0.04

750.

1370

0.07

100.

1134

0.10

950.

1029

0.00

800.

0714

0.05

888

0.09

260

0.12

75-

0.06

980.

0716

0.06

70.

0172

0.13

950.

0725

0.09

560.

1256

0.07

000.

1054

0.12

510.

1040

0.03

000.

0698

0.05

850.

165

0.14

94-

0.07

120.

0580

0.05

90.

0146

0.15

140.

0725

--

0.07

100.

1064

0.11

690.

1058

0.04

700.

0888

0.05

904

0.10

270

0.18

06-

0.07

320.

0606

-0.

0146

-0.

0725

--

0.06

80-

0.16

16-

0.03

200.

080.

0589

40.

108

Labo

rato

ry C

ode

Sanjay Yadav et al.

90

Tabl

e 6

Sum

mar

y of

the

com

bine

d ex

pand

ed u

ncer

tain

ty (M

Pa) e

stim

ated

dur

ing

com

pari

son

p (M

Pa)

12

34

56

78

910

1112

1314

1516

1718

100.

0727

0.27

590.

0946

0.09

280.

1000

0.07

370.

1472

0.10

260.

1209

0.14

340.

0975

0.12

820.

1431

0.12

800.

0731

0.09

970.

0935

0.09

5520

0.07

780.

2775

0.10

030.

1056

0.10

110.

0787

0.14

830.

1063

0.09

150.

1487

0.10

200.

1393

0.13

050.

1241

0.08

410.

1425

0.09

700.

1046

300.

0809

0.27

820.

1022

0.09

940.

1149

0.08

180.

1478

0.10

860.

0909

0.15

580.

1038

0.14

090.

1534

0.13

060.

0874

0.12

480.

0998

0.11

1040

0.09

650.

2832

0.11

590.

1201

0.11

770.

0973

0.15

570.

1207

0.10

810.

1591

0.11

690.

1575

0.14

600.

1367

0.09

690.

1285

0.11

300.

1280

450.

0935

0.28

220.

1142

0.11

000.

1184

0.09

470.

1546

0.11

830.

1022

0.15

180.

1168

0.17

070.

1398

0.13

410.

0941

0.12

670.

1105

0.12

4450

0.16

050.

3114

0.17

500.

1725

0.17

570.

1612

0.20

930.

1761

0.16

570.

2027

0.17

470.

1965

0.19

880.

1873

0.16

700.

1752

0.17

090.

1830

550.

1299

0.29

600.

1474

0.14

830.

1533

0.13

100.

1882

0.14

880.

1383

0.18

880.

1480

0.17

240.

1699

0.16

570.

1302

0.14

820.

1426

0.15

9260

0.12

75-

0.14

540.

1462

0.14

400.

1287

0.18

900.

1467

0.15

940.

1790

0.14

550.

1654

0.17

870.

1646

0.13

100.

1454

0.14

030.

1620

650.

1494

-0.

1655

0.16

030.

1605

0.15

010.

2127

0.16

61-

-0.

1654

0.18

340.

1897

0.18

310.

1566

0.17

380.

1607

0.18

0970

0.18

06-

0.19

480.

1904

-0.

1811

-0.

1945

--

0.19

29-

0.24

23-

0.18

340.

1975

0.18

990.

2104

Labo

rato

ry C

ode

Tabl

e 7

Sum

mar

y of

the

norm

aliz

ed e

rror

(En)

of e

ach

part

icip

ant

p (M

Pa)

23

45

67

89

1011

1213

1415

1617

18

100.

130.

73-0

.12

0.68

0.15

-0.0

40.

60-0

.51

0.27

0.25

-0.7

50.

711.

22-0

.15

1.90

-0.2

80.

1720

0.18

1.18

0.03

0.80

0.14

-0.0

40.

39-0

.35

0.59

0.57

-0.3

80.

720.

65-0

.89

0.22

-0.0

50.

1030

0.35

1.11

-0.2

80.

830.

410.

130.

590.

090.

000.

51-0

.36

0.83

-0.0

8-0

.77

-0.2

2-0

.06

0.24

400.

461.

640.

771.

760.

420.

271.

17-1

.02

-0.0

10.

640.

240.

871.

56-1

.54

0.09

0.09

0.41

450.

410.

950.

131.

13-0

.01

0.02

-0.2

0-0

.81

-0.3

60.

39-0

.35

0.34

0.60

-2.0

7-0

.91

-0.3

9-0

.06

500.

711.

050.

861.

170.

550.

360.

220.

02-0

.08

0.68

0.14

0.67

0.63

-0.3

4-0

.28

0.12

0.43

550.

781.

560.

890.

980.

610.

340.

11-0

.58

-0.0

60.

77-0

.15

0.46

0.24

-0.7

4-0

.76

-0.0

40.

2960

-0.

750.

371.

160.

03-0

.05

-0.3

6-1

.14

-0.0

70.

31-0

.80

0.08

-0.1

1-1

.67

-0.8

8-0

.58

-0.0

465

-1.

261.

141.

420.

790.

47-0

.10

--

0.91

-0.5

90.

481.

69-0

.58

-0.5

10.

190.

4470

-1.

061.

05-

0.45

-0.

44-

-0.

73-

0.61

--0

.91

0.60

0.25

0.37

Labo

rato

ry C

ode


91

Fig. 6. Black points indicate the deviation of the measured pressure (p') by the laboratory from the referencevalue (p) and error bars show the estimated reported expanded measurement uncertainty U(p'). The gap

between two horizontal dotted lines shows the expanded uncertainty band of the reference value.


between two horizontal dotted lines shows the expanded uncertainty band of the reference value.

Ref.

Ref.

10 MPa

20 MPa

Sanjay Yadav et al.

92


between two horizontal dotted lines shows the expanded uncertainty band of the reference value



Ref.

Ref.

30 MPa

40 MPa


93

Fig. 10. Black points indicate the deviation of the measured pressure (p') by the laboratory from thereference value (p) and error bars show the estimated reported expanded measurement uncertainty U(p').The gap between two horizontal dotted lines shows the expanded uncertainty band of the reference value



Ref.

Ref.

45 MPa

50 MPa

Sanjay Yadav et al.

94

Fig. 12. Black points indicate the deviation of the measured pressure (p') by the laboratory from the referencevalue p) and error bars show the estimated reported expanded measurement uncertainty U(p'). The gap


Fig. 13. Black points indicate the deviation of the measured pressure (p') by the laboratory from the referencevalue (p) and error bars show the estimated reported expanded measurement uncertainty at k = 2. The gap


Ref.

Ref.

55 MPa

60 MPa


95



Fig. 15. Black points indicate the deviation of the measured pressure (p') by the laboratory from the referencevalue (p) and error bars show the estimated reported expanded measurement uncertainty at k = 2. The gap


Ref.

Ref.

65 MPa

70 MPa

Sanjay Yadav et al.

96

Fig. 16. 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

Fig. 17. Relative deviations of the measured pressure (p') by each laboratory from the reference value (p).The gap between two horizontal dark black solid lines represents deviations falling within ± 0.15 %,dim black lines represents deviations falling within ± 0.25 % and dotted lines represents deviations

falling within ± 0.5 %

results without any bias in the measurements. It isclearly evident from Fig. 17 that the relative deviationsbetween laboratories values and reference values arewell within the 0.15 % for 75 measurement points,0.25% for 108 measurement points and 0.50% for 148

measurement points.

Further, the difference between the pressurevalues reported by the participating laboratories andthe reference laboratory fall within the uncertainty


97

band of reference laboratory at 113 measurementresults i.e. 71.07 %, within their reported estimatedexpanded uncertainty band at 99 measurementsresults i.e. 62.26 % and within the combined estimatedexpanded uncertainty band of this PT experiment at135 measurement results (84.91 %). This clearly showsthe under-estimation of measurement uncertainty bymost of the laboratories. Most of the participatinglaboratories reported better measurement uncertaintythan the reference laboratory for 120 measurementresults. The main reasons for bias in the measurementsmay be due to errors in measuring instruments or inestimation/measurement of local acceleration ofgravity, the error in applying the temperature and headcorrections and the under-estimation of measurementuncertainty. Laboratories would be able to rectify theproblems by a review of their uncertainty calculationsand other systematic effects as mentioned above.

Although all the participating laboratories wereasked to submit the copy of the formal calibrationcertificate issued to the customer and traceabilitycertificates of their standards, only 12 laboratorieshave submitted the copies of formal calibrationcertificates of the pressure dial gauge (artifact, in thepresent case) while traceability certificates weresubmitted by only 10 laboratories. Laboratories withcode numbers 2, 9, 14, 15 and 16 have not submittedsuch formal certificates. Certificates thus examinedare adequate except that there is little uniformity inthe calibration certificates issued by the participants,especially in reporting the measurement results. Thereare few common typo errors in most of the certificatesexcept the laboratories with code numbers 11, 13 and17, for example, capital 'K' for coverage factor and inpressure unit as 'Kg/cm2'. Although, these are minorerrors but should have been avoided.

As mentioned above, En numbers greater thanunity require investigations and corrective action bythe participating laboratory. The laboratory'smanagement needs to ensure that the problem isrectified and procedures are put in place to prevent arecurrence. Laboratories with Code No. 3, 4, 5, 9, 14and 15 have been asked to review the results and takeappropriate corrective actions. All these laboratorieshave been asked to improve their calibration facilitiesand to modify the measurement method and toestimate the measurement uncertainties properly.Laboratories with Code No. 8 and 16 may also review

their results at their respective unacceptablemeasurement point (one each).

6. Conclusion

This interlaboratory comparison programme(proficiency testing) is carried out in the pressurerange 10 - 70 MPa using pressure dial gauge as anartifact. Total number of seventeen laboratoriesparticipated in this programme. The comparison wasperformed at 10 pressure points selected throughoutthe entire pressure range. The proficiency testingconcludes that out of the total 159 measurementresults reported here in this report, 135 (84.91 %) arein agreement with the reference laboratory. The Envalues of 9 laboratories are within acceptable limitsthrough out the entire pressure scale. However, the Envalues of 2 other laboratories are also quite acceptableexcept only one pressure point each. The En values ofthe remaining 6 laboratories are found beyond theacceptable limit for 2 or more pressure points. Thedifference between laboratories values and referencevalues of 113 measurement points (71.07 %) are wellwithin the uncertainty bands of the reference values.Total 99 measurements results i.e. 62.26 % fall withintheir reported expanded uncertainty band. However,84.91 % measurement results are found well withincombined estimated expanded measurementuncertainty band. Since some of the laboratories haveunder-estimated their measurement uncertainties,15.09 % measurement results are found to be out ofthe combined uncertainty band during thiscomparison. Overall, the results are considered to bereasonably good being the first proficiency testing formost of the participating laboratories.

Acknowledgement

We are grateful to Dr. Vikram Kumar, Director,National Physical Laboratory, New Delhi andDr. Hari Gopal, Director, National AccreditationBoard for Testing & Calibration Laboratories, NewDelhi for their support and encouragementthroughout this program. We are also thankful toNABL-NPL PT-Coordinators, Dr. P.C. Kothari, Dr. K.KJain, Mr. A.K. Saxena and Dr. Naveen Garg for theirconstant co-operation and time to time suggestionsand discussions, which were very helpful during thecourse of this comparison. Thanks are also due to allthe seventeen accredited laboratories participating in

Sanjay Yadav et al.

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this interlaboratory comparison exercise. Withouttheir active support and co-operation this PTprogramme would have not been completed in time.We would also like to acknowledge the help of thesecretariat of NABL for their administrative help.

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Documents

Evaluation of interlaboratory performance through proficiency testing using pressure dial gauge in the hydraulic pressure measurement up to 70 MPa