SPECIFIC CRITERIA for CALIBRATION LABORATORIES IN ...€¦ · Doc. No: NABL 122-12 Specific Criteria for Calibration Laboratories in Mechanical Discipline –Pressure Balance/Dead

NATIONAL ACCREDITATION BOARD FOR TESTING AND CALIBRATION

LABORATORIES

NABL 122-12

SPECIFIC CRITERIA

for CALIBRATION LABORATORIES IN MECHANICAL DISCIPLINE :

Pressure Balance/Dead Weight Tester

MASTER COPY Reviewed by Approved by

Quality Officer

Director, NABL

ISSUE No. : 05 AMENDMENT No. : 00 ISSUE DATE: 12.08.2014 AMENDMENT DATE:

National Accreditation Board for Testing and Calibration Laboratories Doc. No: NABL 122-12 Specific Criteria for Calibration Laboratories in Mechanical Discipline –Pressure Balance/Dead Weight Tester Issue No: 05 Issue Date: 12.08.2014 Amend No: 00 Amend Date: - Page No: 1 of 19

AMENDMENT SHEET

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Sl. No. Contents Page No. 1 General Requirements

1.1 Scope 3 1.2 Calibration Measurement Capability(CMC) 3 1.3 Personnel, Qualification and Training 3-4 1.4 Accommodation and Environmental Conditions 4-5 1.5 Special Requirements of Laboratory 6 1.6 Safety Precautions 6 1.7 Other Important Points 6 1.8 Proficiency Testing 6 2 Specific Requirements – Calibration of Pressure Generating Device

2.1 Scope 7 2.2 National/ International Standards, References and Guidelines 7 2.3 Metrological Requirements 7-8 2.4 Terms and Definitions 8-9 2.5 Selection of Reference Equipment 9-10 2.6 Calibration Interval 10 2.7 Legal Aspects 10 2.8 Environmental Conditions 10-12 2.9 Calibration Methods 12-13

2.10 Calibration Procedure & Measurement Uncertainty (Method A) 13-15 2.11 Calibration Procedure & Measurement Uncertainty (Method B) 15-17 2.12 Evaluation of CMC 17 2.13 Sample Scope 18 2.14 Key Points 19 2.15 Important Technical Information 19


1. General Requirement

• The purpose of this document is to specify requirements with which a laboratory has to operate and demonstrate its competency to carry out calibration in accordance with ISO/IEC 17025:2005.

• To achieve uniformity between the laboratories, assessors and assessment process in terms of

maximum permissible error, CMC, measurement uncertainty etc in line with National/International standards.

• To achieve uniformity in selection of equipment’s, calibration methods, maintaining required

environmental conditions, personnel with relevant qualification and experience. 1.1 Scope

This specific criteria lays down the specific requirements in for calibration of Pressure balance/Dead weight Tester under mechanical discipline. This part of the document thus amplifies the specific requirements for calibration of Pressure balance/Dead weight Tester and supplements the requirements of ISO/IEC 17025:2005.

1.2 Calibration and Measurement Capability (CMC)

1.2.1 CMC is one the parameters that is used by NABL to define the scope of an accredited

calibration laboratory, the others being parameter/quantity measured, standard/master used, calibration method used and measurement range. The CMC is expressed as “the smallest uncertainty that a laboratory can achieve when calibrating the best existing device”. It is an expanded uncertainty estimated at a confidence level of approximately 95% corresponding to a coverage factor k=2.

1.2.2 For evaluation of CMC laboratories shall follow NABL 143 - Policy on Calibration and

Measurement Capability (CMC) and Uncertainty in Calibration. 1.3 Personnel, Qualification and Training

1.3.1 Technical Personnel: 1.3.1.1 Qualification required for carrying out calibration activity:

The following are the specific requirements. However, qualification and experience will not be the only criteria for the required activity. They have to prove their skill, knowledge and competency in their specific field of calibration activity.

a) B.E / B.Tech or M.Sc. (having Physics as one of the subject) degree with 3 months

experience in Basics of Pressure Metrology. b) B.Sc (with Physics as one of the subject) or Diploma with 6 months experience in

Basics of Pressure Metrology. c) ITI with 1 year of experience in Basics of Pressure Metrology.

1.3.1.2 Training and experience required:

a) Training may be external/ internal depending on the expertise available in the field. b) Training in calibration of Pressure Balance and in Uncertainty Measurements,

CMC including statistical analysis for Technical Manager.


c) Experience and competence in Pressure Metrology. d) Sufficient knowledge about handling of reference equipment, maintenance,

traceability, calibration procedure and effect of environmental conditions on the results of calibration.

e) During training calibration activity should be done under supervision.

1.3.2 Authorised Signatory:

1.3.2.1 Qualification required for interpretation of results and signing the calibration certificates:

The following are only guidelines. However, qualification and experience will not be the only criteria for the required activity. They have to prove their skill, knowledge and competency in analysis and interpretation of calibration results.

a) B.E / B.Tech or M.Sc. (with having Physics as one of the subject) degree with 6

months experience Pressure Metrology.

b) B.Sc. (with Physics as one of the subject) or Diploma with 1 year experience in Pressure Metrology.

1.3.2.2 Training and experience required:

a) Training may be external/ internal depending on the expertise available in the field. b) Training, Experience and Competence in Pressure balance/DWT calibration and

training in Uncertainty Measurements, CMC including statistical analysis for Technical Manager.

c) Sufficient knowledge and competence in effective implementation of ISO/IEC

17025, specific criteria and NABL guidelines. d) Competency in reviewing of results, giving opinion and interpretations. e) During training the relevant activity has to be done under supervision.

1.4 Accommodation and Environmental Conditions

A Laboratory may be offering calibration services under different categories: i. Permanent laboratory service ii. Onsite service

The above category of laboratories may provide following types of services.

a) Service that intended primarily for measurement standards, reference equipments which are further

used for calibration purposes or high accuracy measurements which requires high degree of accuracy and better CMC.

b) Service that intended primarily for calibration and adjustment of test, measurement and diagnostic equipments to use in such areas as product testing, manufacturing and servicing.


Accommodation and environmental conditions adversely affect the results of calibration and measurement accuracy unless they are controlled and monitored. Hence, they play a very important role.

The influencing parameters may be one or more of the following i. e. temperature, relative humidity, atmospheric pressure, vibration, acoustic noise, dust particle, air currents/draft, illumination(wherever applicable), voltage fluctuations, electrical earthing and direct sunlight etc., depending on the nature of calibration services provided. The variables described above can play a major factor on calibration results.

The main difference between the permanent laboratory, onsite and mobile calibration services has to do with environmental conditions only. Since the onsite calibration relies on where the service is provided, it affects the results of calibration (Refer NABL 130)

The laboratories are advised to follow the requirement of accommodation and environment depending on the types of services provided as recommended: • By the manufacturers of the reference equipment • By the manufacturers of the Unit under calibration • As specified in the National/ International Standards or guidelines followed for the calibration.

The environmental monitoring equipments used should also meet the requirement of manufacturers’ recommendations and specifications as per the relevant standards followed.

If, accommodation and environmental conditions are not specified either by manufacturer or by National/International standards / guidelines, the laboratory shall follow the below recommendations.

1.4.1 Vibration

The calibration area shall be free from vibrations generated by central air-conditioning plants, vehicular traffic and other sources to ensure consistent and uniform operational conditions. The laboratory shall take all special/ protective precautions like mounting of sensitive apparatus on vibration free tables and pillars etc., isolated from the floor, if necessary.

1.4.2 Acoustic Noise

Acoustic noise level in the laboratory shall be maintained to facilitate proper performance of calibration work. Noise level shall be maintained less than 60 dBA, wherever it affects adversely the required accuracy of measurement.

1.4.3 Illumination

The calibration area shall have adequate level of illumination. Where permissible, fluorescent lighting is preferred to avoid localized heating and temperature drift. The recommended level of illumination is 250-500 lux on the working table.

1.4.4 Environmental Conditions and Monitoring

The environmental conditions for the activity of the laboratory shall be such as not to adversely affect the required accuracy of measurement. Facilities shall be provided whenever necessary for recording temperature, pressure and humidity values prevailing during calibration. The atmospheric conditions maintained in the laboratory during calibration shall be reported in the calibration report/ certificate.


1.5 Special Requirements of Laboratory

1.5.1 The calibration laboratory shall make arrangements for regulated and uninterrupted power supply of proper rating. The recommended voltage regulation level is ± 2% or better, and Frequency variation ± 2.5Hz or better on the calibration bench.

1.5.2 The reference standards shall be maintained at temperatures specified for their maintenance on

order to ensure their conformance to the required level of operation. 1.5.3 The laboratory shall take adequate measures against dust and external air pressure.

1.6 Safety Precautions

1.6.1 Relevant fire extinguishing equipment for possible fire hazards, shall be available in the corridors or convenient places in the laboratory. Adequate safety measures against electrical, chemical fire hazards must be available at the work place. Laboratory rooms/ areas where highly inflammable

1.7 Other Important Points

1.7.1 Entry to the Calibration Area: As far as possible, only the staff engaged in the calibration activity shall be permitted entry inside the calibration area.

1.7.2 Space in Calibration Area: The calibration Laboratory shall ensure adequate space for

calibration activity without adversely affecting the results. 1.8 Proficiency Testing

To give further assurance to the accuracy or Uncertainty of measurements, a laboratory will be required to participate, from time to time, in Proficiency Testing Program. The laboratory shall remain prepared to participate in the Proficiency Testing Program through inter-laboratory, inter-comparison schemes wherever it is technically feasible. (Ref. NABL 162, 163 and 164 for further details)


2. Specific Requirements – Calibration of Pressure Generating Device 2.1 Scope: Calibration of Pressure Generating Device (Dead Weight Tester)

Requirements for the calibration of Pressure Generating Devices with following details:

Sl. No Description Relevant Standard/

Guidelines Permanent Facility Onsite Calibration

1 Hydraulic Dead Weight Testers/Pressure Balance EURAMET

cg-3, Version 1.0 (03/2011)

√ X

2 Pneumatic Dead Weight Testers/Pressure Balance √ X

3 Vacuum/Absolute Dead Weight Testers/ Pressure Balance √ X

Important Note: This technical requirement is based on the above mentioned guideline. Lab may

follow any relevant standard, however care shall be taken to follow the requirements in totality.

2.2 National/ International Standards, References and Guidelines

• EURAMET cg-3, Version 1.0 (03/2011) –“Calibration of pressure balances”(Guidelines).

• OIML R 110, 1994(E)- “Pressure balances”

• OIML R-111-1,edition 2004 Metrological and technical requirement of weights

• OIML D28 2004: Conventional value of the result of weighing in air

2.3 Metrological Requirement

• Unit of measurement of pressure is pascal, Pa SI Unit of measurement of pressure is Pascal, (Pa)

• Pressure gauges, vacuum gauges, Pressure-Vacuum gauges are to be calibrated in Pa, kPa, MPa, GPa, as per SI units. However, Units like bar and mbar, may also be used.

• All weights shall be traceable in SI Units for deriving Pressure in Pascal or bar.

• For Each weight, the expanded uncertainty, U, for k=2, of the true mass.

• Preferably all weights shall be equivalent or better than F2

standard as per OIML R-111-1.

• 'g' value shall be known with sufficient accuracy either by Geological Survey of India or any other relevant source for finer CMC.

• Laboratory may also calculate 'g' value knowing latitude and height as per the formula. However,

same shall be validated (as per 2.8.2.1 & 2.8.2.2).

• A suitable air buoyancy correction shall be applied if the weight are calibrated either by conventional basis or by true mass basis.

• Knowing the true mass, piston cylinder area value and 'g' value, Pressure value will be

determined after applying buoyancy correction.

• When the masses are submitted to vacuum, the balance measures an absolute pressure. The residual pressure in the bell jar around the masses creates a force in opposition to the measured


pressure. The residual pressure has to be measured and added to the pressure created by the masses.

• When overall masses are submitted to the atmosphere which also applies to the top of the piston,

the balance measures the gauge pressure.

In some cases, an adopter allows the reversal of the piston cylinder mounting, the balance then measures the negative gauge pressure (below atmospheric pressure) and generates an upward force opposed to the gravitational force.

• Accuracy Class

Pressure balances are classified into the following accuracy classes 0.005, 0.01, 0.02, 0.05, 0.1, 0.2. The accuracy class of a pressure balances shall be determined by calibration.

2.4 Terms and Definitions

Dead Weight Pressure Tester (Also called as Dead weight piston gauge or Pressure balance)

• One of the fundamental method – Force/ unit area of the piston. Consists of an accurately machined piston of known weight, which is inserted into a closed fitting cylinder (clearance between the piston and cylinder will be few microns), both of known cross sectional area. Weights of known mass loaded on one end the piston and the fluid pressure applied to the other end of the piston until enough force is developed to lift the piston-weight combination, When piston is floating freely within the cylinder (between limit stops), piston is in equilibrium with the unknown system pressure. So the applied pressure is equal to the ratio of force due to the weight-piston and the area of cross section of the piston cylinder.

Effective Area

• The area determined for a given piston-cylinder assembly which is used in conversion equation for

the calculation of the measured pressure.

Free Rotation Time of the Piston • The time during which the piston rotates freely after manual spinning to a specified rotation rate,

until it stops at 20% of pressure value by loading related quality of masses.

Rate of Fall of Piston • The speed of fall of the piston at its operating level at 100% pressure value by loading related

quantity of masses.

Upper limit

of the measuring range (MPa)

Free Rotation Time (min)

for accuracy class

0.005 0.02 0.02 0.05 0.1 0.2

from 0.1 to 6 included 4 4 3 2 2 2

over 6 to 500 included 6 6 5 3 3 3


Pressure medium

in clearance

Upper limit of the measuring

range (MPa)

Maximum piston fall rate (mm/min) for accuracy class

0.005 0.01 0.02 0.05 0.2 0.1 gas 0.1 to 1 included 1 1 1 2 2 – gas more than 1 2 2 2 3 3 –

liquid 0.6 to 6 included from 0.4 0.4 0.4 1 2 3 liquid 6 to 500 included 1.5 1.5 1.5 1.5 3 3

Datum Levels • Operating level of the piston: level of the piston , with respect to a clearly defined part of the

support column or the base of a pressure balance.

• Pressure reference level: The vertical level, with respect to a clearly defined part of the support column or the base of the pressure balance, to which a measured pressure is related when the piston is at a specific operating level.

Cross Float Sensitivity • For a pressure balance tested by comparison against a standard pressure balance, minimum change

in the load that result in a detectable change in the equilibrium of both the tested and standard pressure balances.

Gauge Pressure • Gauge pressure is zero reference at ambient pressure which is equal to absolute pressure minus

atmospheric pressure.

Absolute Pressure • Absolute pressure is zero reference against a perfect vacuum. It is equal to gauge pressure+

atmospheric pressure.

Differential Pressure • It is the difference in pressure between two points.

2.5 Selection of Reference Equipment

Reference Standard Pressure generating system

2.5.1 Hydraulic Dead Weight Tester Accessories: 2.5.1.1 Proper connecting pipes and adopters. 2.5.1.2 Float level indicators for both reference and DUC. 2.5.1.3 Suitable arrangements to measure temperatures both of the PCU using

thermometers/PRTs in close proximity to PCUs. 2.5.1.4 Preferably constant volume valve or equivalent device can be used.


2.5.2 Pneumatic Dead Weight Tester

Accessories: 2.5.2.1 Proper connecting pipes and adopters.

2.5.2.2 Float level indicators for both reference and DUC.

2.5.2.3 Suitable arrangement to measure temperatures of both the PCUs using thermometers /

PRTs in close proximity to PCUs. 2.5.2.4 Vacuum pump (if negative calibration is done).

2.5.2.5 Gas supply.

2.5.3 Absolute Pressure Dead Weight Tester (for Vacuum and Absolute Pressure)

Accessories: 2.5.3.1 Proper connecting pipes and adopters.

2.5.3.2 Vacuum generating pump.

2.5.3.3 Float level indicators for both reference and DUC.

2.5.3.4 Suitable arrangement to measure temperatures of both the PCUs using thermometers /

PRTs in close proximity to PCUs.

2.5.3.5 Gas supply

Note: The reference balance should have better or at least equal uncertainty or the pressure balance under calibration

2.6 Calibration interval

Note: Calibration interval for Dead Weight Tester will be given for 5 years if DWT is calibrated by area method and temperature to be maintained as per the requirement of the standard.

In all other conditions validity for DWT will be for 2 years.

2.7 Legal Aspects Calibration of Pressure Dead weight testers done by any accredited laboratories is meant for scientific

and industrial purpose only. However, if used for commercial trading, additional recognition/ approval shall be complied as required by Dept. of Legal metrology, Regulatory bodies, etc. This should be clearly mentioned in the calibration certificate issued to the customer.

2.8 Environmental Conditions

The ambient temperature shall be between 15 0C to 25 0

C. The temperature shall not vary more than ± 1°C / hour during measurement.

Reference Equipment Recommended interval Dead Weight Tester 2 years


• It is preferable to maintain Relative humidity 60% RH ± 20% RH.

• ‘g’ value shall be known to sufficient accuracy.

2.8.1 Recommended Environment Monitoring Equipments at Calibration Laboratory:

• Temperature with a resolution of 0.1°C • Humidity with a resolution of 1% RH • Barometer with a resolution of 1 mbar

However, laboratory shall evaluate the requirement of accuracy, resolution and Uncertainty depending on the CMC aimed at.

2.8.2 Effect of Gravity ''g'' on Calibration

• It is very important to establish the gravitational value of the laboratory since it is one of the major quantity during realization of force. The effect of not doing this could be a variation in force produced by the weight perhaps 0.5% of the force. It is therefore recommended that, the Pressure calibration laboratory establishes local value of gravity (g) and use weights that have been calibrated at that gravitational constant.

2.8.2.1 Effect of gravity 'g' on calibration, when Dead Weight Testers are used

For measurement uncertainty of applied force, ‘g’ value shall be known. For realization of applied force more than 0.01%, ‘g’ value shall be calculated using the formula given in below For better than 0.005%, ‘g’ value shall be measured by appropriate authority.

2.8.2.2 Validation of local ‘g’ and its Uncertainty

Formula for calculation of Acceleration due to gravity. An approximate value for g

, at given latitude and height above sea level, may be calculated from the formula:

9.780 g = 7 (1 + Α sin2 L - B sin2 2L) - 3.086 × 10-6 H m·s-2

Where, A = 0.005 302 4, B = 0.000 005 8, L = latitude, H

= height in meter above sea level.

2.8.2.3 To validate this calculated ‘g’ value the simple steps given below can be followed:

• Find out the actual ‘g’ value of NMI from the certificate issued by them or by any other source.

• Find out the actual ‘g’ value of NMI from the certificate issued by them or by any

other source.

• From the web search engine maps click on the location of NMI, find out latitude and height above sea level. (you can know the ‘g’ value).


• Calculate the ‘g’ value using the above formula with these latitude and height. The difference between the calculated value of ‘g’ and the actual value of the NMI should be within 20 to 30 ppm.

• Now, go to the web search engine maps and click on location of the lab and find out the latitude and height of the place as per web search engine (you can know the ‘g’ value also).

• Calculate the ‘g’ value for this latitude and height. The value obtained should be

within 20 to 30 ppm.

• Then this value can be taken as ‘g’ value of the lab and uncertainty of ‘g’ can be assumed to be within ± 50 ppm.

2.8.3 Effect of Temperature

Piston cylinder of the pressure balance is temperature sensitive and must, therefore, be corrected to a common temperature datum. Variation in the indicated pressure resulting from changes in temperature arises from the change in effective area of the piston due to the expansion or contraction caused by temperature changes. The solution is a straight forward application of the thermal coefficients of the material of the piston and cylinder. Hence the effect of temperature is critical in pressure realization and correction for the area is to be done.

2.8.4 Estimation of Air Density

Air density should be known to sufficient accuracy depending on the required uncertainty of the applied force by measuring temperature, RH & barometric pressure.

(Approximation formula as per OIML R-111-1: 2004 pg. No. 76)

(E3.1 OIML)

Where, Pressure (P) in mbar, temperature (t) inº C and humidity (h) in % Equation(E-3.1) has a relative uncertainty of 2 X 10-4

in the range 900hPa < p<1100hPa, 10 º C<t<30 º C and rh< 80%

2.9 Calibration Methods

Cross float method applies to both hydraulically and pneumatically operated balances. In both cases the method is a comparative one. Consisting of comparing the balance to be calibrated and the standard instrument both are subjected to same pressure and same environmental conditions is also called as cross floating method.

2.9.1 Method A – Pressure Generated Method

The scope of this method is to determine the bias error and the repeatability of the Calibrated pressure balance. This is done by determining the generated pressure corresponding to well-identified weights and effective area of the reference standard. In this method the weighing of the masses of the instrument under calibration is optional. This method is not employed where the smallest uncertainty is required.

ρa = 273.15+t 0.34848p -0.009*h*e^(0.061*t)


2.9.2 Method B - Effective Area Determination Method

This method determines the mass of the piston, weights of the balance and effective area of the Piston cylinder assembly. This method is employed where the smallest uncertainty of better than 0.01% is required.

The scope of this method is to determine:

2.9.2.1 The value of the mass of all the weights, including the piston of the pressure balance,

if removable.

2.9.2.2 The effective area Ap referred to 20 °C or another reference temperature tr of the piston-cylinder assembly of the pressure balance as a function of pressure. At high pressure, this area can be expressed from the effective area at null pressure A0

and the pressure distortion coefficient.

2.9.2.3 Repeatability as a function of the measured pressure. 2.10 Calibration Procedure and Measurement Uncertainty

Method A (Pressure Generation Method)

2.10.1 For cross floating the reference and test dead weight testers (pressure balance) are to be connected with suitable connections so that, there is no leakage (either hydraulic or pneumatic)

2.10.2 Three series of increasing and decreasing pressures are applied from 10 % to 100% of the

calibration range.

2.10.3 The serial numbers of weights placed on both reference and the device under calibrations are noted along with the temperature of both the piston cylinder unit.

2.10.4 Since both the pistons are balanced, the pressure generated will be equal. The generated

pressure with reference to the reference is calculated as both mass and area are known.

2.10.5 Equation to calculate the generated Pressure for a Hydraulic DWT using true mass

pe[Σ

= ιmi ∗(1−ρa/ρm

(+) ρ)] ∗g+ σc

f

(A-1) .g.∆h

A0(1+ λ∗ p)*[1+( αp+αc)*(t-tr)]

p Pressure in pascal (at a calibration step) e = g = Local acceleration due to gravity in m/sρ

2 a Density of Air in kg/m =

ρ

3 m Density of mass in kg/m =

ρ

3 m1 Density of mass of PCU in kg/m =

A

3

0 Effective area of the Piston Cylinder Assembly (PCU) at reference temperature and zero pressure in m= 2

λ =

Pressure distortion co-efficient of Piston Cylinder Assembly (PCU) in /pa α= Linear thermal expansion coefficient of the piston in/ °C σ = Surface tension of the oil in N/m c = Circumference of emergent piston in m ρ Density of Hydraulic Fluid in kg/mf = 3


∆h = Difference between height h1 of the reference level of the balance and the height

h2 of the balance under calibration: ∆h = h1-h2 in m [∆h

is +ve if, the unit under test is below the reference PCU level and -ve if, it is above the reference PCU level]

Note 1: In case of Pressure generated from a pneumatic dead weight tester, Fluid head correction factor (ρf

.g.∆h) may be considered negligible.

Note2: In case of Pressure generated from a pneumatic dead weight tester, pressure distortion coefficient (λ) may be considered negligible if not reported in the certificate.

2.10.5.1 Equation to calculate the generated Pressure for a Hydraulic DWT using

conventional mass

Where, mci

ρ

= individual conventional mass value of each applied weight on the piston

0a = 1.2 kg/m3

ρ, conventional value of air density

0 = 8000 kg/m3

Other quantities are as referred above. , conventional value of mass density

2.10.5.2 Equation to calculate the generated absolute Pressure for a Pneumatic

DWT using true mass

pabs[Σ = ιmi ∗(1− ρa/ρm +µ )] ∗g

(A-3) A0*[1+( αp+αc)*(t-tr)]

Where, pabsm

is the absolute pressure measured at the bottom of the piston i

µ = = individual true mass value of each applied weight on the piston,

Other quantities are as referred above.

is the residual pressure surrounding the weights

2.10.6 Measurement Uncertainty

2.10.6.1 Estimation of type A Uncertainty:

Standard deviation of Repeatability of the balance for estimated pressure.

2.10.6.2 Estimation of type B Uncertainty (Method A):

Components associated with applied pressure

a) Uncertainties associated with Mass of Reference Dead Weight Tester(u1

• Mass placed on the carrier [including piston] )

• Mass instability in time • Cross floating sensitivity in terms of masses • Local acceleration due to gravity ''g''

p = Applied Nominal Pressure in pa

Pe[Σ

= ιmci ∗(1−(ρ0a/ρ0) + (ρ0a - ρa)/ρm (+) ρ)] ∗g+ σc

f (A-2) .g.∆h A0(1+ λ∗ p)*[1+( αp+αc)*(t-tr)]


• Air Density • Mass Density

b) Uncertainties associated with Effective area of PCU at Atmospheric Pressure and Reference Temperature (u2) • Piston Circumference • Surface Tension of the fluid (for hydraulic only) • Drift in effective area • Pressure distortion coefficient • Thermal expansion coefficient of piston • Thermal expansion coefficient of cylinder

c) Other associated Uncertainties (u3)

• Variation of Temperature of Reference PCU during calibration • Head Correction (Level) • Fluid Density(for hydraulic only) • Verticality of Piston

Total B type uncertainty

uB = √ (u12+ u2

2+ u32

)

2.10.6.3 Combined Uncertainty for Gauge Calibration

uc = √ (uA2+ uB

2

)

2.10.6.4 Expanded Uncertainty U = k x uc

Where, k= coverage factor corresponding to the effective degree of freedom

2.10.6.5 Reporting of Results (Method A)

• The certificate of calibration should be issued with Pressure values converted to standard ‘g’ 9.80665m/s2

and reference temperature of 20 °C (if or otherwise not specified ‘g’ value and temperature value on the dead weight tester or requested by the user).

• The following equation should be given to enable the customer to convert the pressure to the local ‘g’ value ( at the lab) and the reference temperature

pe p

= s * gL * [1+(t-20)*α] 9.80665

Where,’t’ is the temperature during calibration and gravity 'gL

'

local acceleration due to gravity (g’ value of the lab) and 'α 'is the linear thermal expansion coefficient of the Piston / °C.

2.11 Calibration Procedure and Measurement Uncertainty

Method –B (Effective Area Method)


2.11.1 Pressure generated by the reference dead weight tester is calculated as its area and mass applied is known at each point of calibration [using the equation A-1, A-2, A-3 depending on the case).

pe[Σ

= ιmi ∗(1−ρa/ρm

(+) ρ)] ∗g+ σc

f

(A-1) .g.∆h

A0(1+ λ∗ p)*[1+( αp+αc)*(t-tr)]

2.11.1.1 Now the Effective area of the device under calibration, at the calibration temperature and the generated pressure from the reference is calculated using the below given equation

Where, A2t is the effective area in m2

m of the Test PCU at reference temperature

2 is the mass applied on the Test PCU and ρmi2

p

density of the mass of test PCU

1i

is the calculated pressure at reference temperature at a calibration point

By plotting a graph of applied pressure against the calculated area for each calibration point, we can calculate the effective area A0

at zero pressure for the test PCU and the pressure distortion constant along with their uncertainties.

2.11.2 Estimation of Uncertainty for Method B (Area Method)

p Pressure in Pascal ( for each calibration point) e = g = Local acceleration due to gravity in m/sρ

2 a Density of Air in kg/m =

ρ

3 m Density of mass in kg/m =

ρ

3 m1 Density of mass of PCU in kg/m =

A

3

0 Effective area of the Piston Cylinder Assembly (PCU) at reference temperature and zero pressure in m= 2

λ =

Pressure distortion co-efficient of Piston Cylinder Assembly (PCU) in /pa α= Linear thermal expansion coefficient of the piston in/ °C σ = Surface tension of the oil in N/m c = Circumference of emergent piston in m ρ Density of Hydraulic Fluid in kg/mf = p =

3 Applied Nominal Pressure in pa

∆h =Difference between height h1 of the reference level of the balance and the height h2 of the balance under calibration: ∆h = h1-h2 in m [∆h

is +ve if, the unit under test is below the reference PCU level and -ve if, it is above the reference PCU level]

Note 1: In case of Pressure generated from a pneumatic dead weight tester, Fluid head correction factor (ρf

.g.∆h) may be considered negligible.

Note-2: In case of Pressure generated from a pneumatic dead weight tester, pressure distortion coefficient (λ) may be considered negligible if not reported in the certificate. Note 3: Two to three series of measurement for five to seven points equally divided through the range should be carried out.

A2t

[Σ =

ιmi2 ∗(1−ρa/ρmi2

(B-1)

)] ∗g

P1i* [1+( αp+αc)*(t-tr)]


2.11.2.1 Estimation of type A Uncertainty (uA

components):

Repeatability of the balance, estimated as a function of pressure from the values of the standard deviation of the effective area as expressed in the table in 6.3.3. Alternatively, the type A uncertainty of pressure can be presented by an equation based on the variances and covariance of A0

and λ.

2.11.2.2 Estimation of type B Uncertainty (uB

components):

• Uncertainty of the reference pressure; • Uncertainty of the masses; • Uncertainty due to the temperature of the balance; • Uncertainty due to the thermal expansion coefficient of the piston-cylinder

assembly; • Uncertainty due to the air buoyancy; • Uncertainty due to the head correction; • Uncertainty due to the surface tension of the pressure-transmitting fluid; • Uncertainty due to tilt (negligible if perpendicularity was duly checked); • Uncertainty due to spin rate and/or direction, if applicable; • Uncertainty of the residual pressure (absolute mode only).

2.11.2.3 Reporting of Results (Method B)

• A certificate of individual weights along with their uncertainty either for true

mass or for conventional as per the customer’s requirement shall be issued. • The calibration certificate shall be issued with the effective area at zero

pressure (A0

) and pressure distortion coefficient with their uncertainties.

• Calibration certificate should report- values same as given in Clause 2.10.6.5 and overall expanded uncertainty for the device.

Note: For Area/Mass method scope should also be in % of reading (Pressure).

2.12 Evaluation of CMC

2.12.1 Refer NABL 143 for CMC evaluation. 2.12.2 CMC value is not the same as expanded uncertainty reported in the calibration

Certificate/Report. CMC values exclude the uncertainties which are attributed to the UUT (Unit under test/calibration).

2.12.3 For the purpose of CMC evaluation the following components should be considered.

• Uncertainty of the applied pressure.

• Repeatability of the artifact preferably from three series of measurement for five to seven

points equally divided through the range should be carried out


2.13 Sample Scope

An illustrative example: Correct presentation of scope

Laboratory: XYZ Date(s) of Visit:

Discipline: Mechanical

Sl Parameter*/ Device

under calibration

Master equipment

used

Range(s) of measurement

Calibration and Measurement Capability ** Remarks+/ Method used

Claimed by Laboratory

Observed by Assessor

Recommended by Assessor

1

2

Dead

Weight Tester

based on area

method

Dead Weight Tester

based on pressure method

Dead Weight Tester with known area and mass of

piston cylinder unit and mass sets required

for generating pressure

With known local g value

Dead Weight Tester

with known local g value

having uncertainty

0.006%

6 MPa to 100 MPa

6 MPa to 100 MPa

Uncertainty of the effective

area = 50 ppm

Uncertainty of mass

calibration equivalent to F1 standard

0.009 %rdg

Uncertainty

of the effective area = 60

ppm

Uncertainty of mass


0.01%rdg

Uncertainty of the effective

area = 60 ppm

Uncertainty of mass


0.01% rdg

Determination

of effective area by Cross Float principle

as per EURAMET

cg-3

As per OIML R 111-1

By Cross Float principle

comparison method

* Only for Electro-technical discipline; scope shall be recommended parameter wise (where applicable) and the ranges may be mentioned frequency wise.

** NABL 143 shall be referred for the recommendation of CMC + Remarks shall also include whether the same scope is applicable for site calibration as well. NABL 130 shall be

referred while recommending the scope for site calibration.

Signature, Date & Name of Lab Representative

Signature, Date & Name of Assessor(s)

Signature, Date & Name of Lead Assessor


2.14 Key Points

• The cutoff CMC for the applied pressure is 0.067 % rdg or better. Beyond this accreditation cannot be granted

• Demonstration of any CMC values doesn't automatically qualify for granting accreditation until the

lab satisfies the stipulated requirement given in this document. 2.15 Important Technical Information

2.15.1 If the reference Dead weight tester is calibrated at a different location or for standard 'g' 9.80665 m/s2

. The lab has to apply correction and correct it to local 'g' before deriving pressure.

2.15.2 Effect of Air Buoyancy

2.15.2.1 The internationally accepted method of measuring a weight is to compare an unknown weight with a standard weight of known density by weighing in air. Now weighing in air creates a problem, as objects of different density are buoyed up by the surrounding air, much the same as heavy objects become lighter in water.

2.15.2.2 For example, if a one litre bottle is lowered by a string into water, it will feel

lighter as it is submerged. In this case, the bottle is almost fully buoyed up by the water. As we have fully submerged in air, what effect can the air have? Well, if a 1 kg of Brass weight (density 8400 kg/m3) is made to balance with a 1 kg stainless steel standard weight ( density 8000 kg/m3) today, then tomorrow when there is change in atmosphere like, air pressure and temperature, the same weights will no longer balance. This is because, the brass and stainless steel were buoyed up in the air by differing amount, as the buoyancy is proportional to air density. By convention it is taken that the standard density for reference standard weights shall be 8000 kg/m3 at a standard air density of 1.2 kg/m3 and all other

weights are compared to this. This is called conventional mass of weights and is different to its true mass by small amount. [Unfortunately, this means that, the conventional mass of weights with a density other than 8000 kg/m3

, will vary daily with air density changes] As the conventional mass values change when there is change in air density during calibration, buoyancy corrections can be made if the air density is measured each time the weight is used and if required, but this can be a lengthy task.

National Accreditation Board for Testing and Calibration Laboratories NABL House

Plot No. 45, Sector- 44, Gurgaon – 122002, Haryana

Tel.: +91-124 4679700 Fax: +91-124 4679799

Website: www.nabl-india.org

Documents

SPECIFIC CRITERIA for CALIBRATION LABORATORIES IN ...€¦ · Doc. No: NABL 122-12 Specific Criteria for Calibration Laboratories in Mechanical Discipline –Pressure Balance/Dead