calibration of balance

Calibration of Balance

Sri Lanka

1

Nihal Gunasekara

Bangladesh BEST Programme

Types of Balances• Electromagnetic force balance (Top loading)• Single-pan, two-knife-edge balance• Two-pan, three-knife-edge balance• Triple beam balance• Weighing scale

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Analytical Balance

Balance with an enclosed weighing chamber and a resolution 0.01 or 0.1 mg

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Micro Balance

Balance with a resolution of 1 microgram


Micro Balance

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Semi Micro Balance

Balance with a resolution of 10 microgram

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Ultra (Super) Micro Balance

Balance with a resolution of 0.1 microgram


Resolution or Discrimination

The smallest change in mass which can be detect by the balance

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Sensitivity

This is a measure of the ability of the balance to detect changes in the load applied to it


Response time

Time taken for the weighing instrument to indicate the value of a change in load

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Stability

A measure of the time for which the reading on the balance remains unchanged


Correction

Correction = True Value - Reading

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Error

Error = Reading - True Value


Departure From Nominal Value

The amount by which the reading on an instrument depart from its nominal value

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Hysteresis

The indications of a measuring instrument when the same value of the quantity measured is reached by increasing or decreasing that quantity


Standby

Operating state of an electronic instrument where one part is switched off for energy saving reasons

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Accuracy class. (mass)

Legal metrology term. Classification of weights for calibration purposes by OIML R111-1 weights of Accuracy class E1, E2, F1, F2, M1, M2, M3 from 1 mg to 50 kg


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Different accuracy classes of mass Bangladesh BEST Programme

Repeatability

Closeness of the agreement between the results of successive measurements of the same measurement carried out under the same conditions of measurement.

Repeatability condition include :• the same observer• the same measuring instrument, used under the same

conditions• the same location• repetition over a short period of time

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Reproducibility Closeness of the agreement between the results of measurements of

the same measurement carried out under changed conditions of measurement.

A valid statement of reproducibility requires specification of the conditions change

The changed conditions may include – principle of measurement– method of measurement– observer – measuring instrument– reference standard– location– conditions of use– Time

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Influence Factors

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Air movements and heat sources

Support surfaces and vibration

Electrical and electromagnetic interference

Magnetic effects

Buoyancy effects

Influence factor

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Use of a high-mass support, set on a solid surface, for the weighing machine, independent of the adjacent flooring, Eg fixed to bedrockNote : low frequency, vibration is likely to be more serious in its effects than high (acoustic) frequenciesRubber pads used to damp high frequencies may pass low frequency vibration, and may cause tilting of the weighing machine

PrimaryStability of reading under constant load SecondaryAccelerometer measurements

Changes forces on the weighing machine due to acceleration

Various, Eg. Machinery, roads, railways, aircraft operator Vibration causes damages

Vibration Environmental

PrecautionsTest for effectEffectSourceParameterInfluencefactor


Influence factor

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Avoid direct sunlight and other connections to the external environment; avoid changes in heat and lighting input. Leave balance cases ajar when not in use. Avoid excessive handling of equipment during weighing. Allow weighted object to acclimatize

Temperature monitoring instruments

Temperature changes and differentials can change the weighing machine response, cause convection effects and change buoyancy

Sunlight, heating systems, operator’s body, greenhouse heating in the weighing chamber

Temperature: air, weight, weighing machine

Environmental



Influence factor

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Avoid liquid water in the working area unless rh is too low .Air conditioning, humidity controller

Rh monitoring instruments

Buoyancy, absorbed moisture, condensation

Air supply ,air conditioning, changes in activity levels

HumidityEnvironmental



Influence factor

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Avoid temperature gradients. draught free working space, enclose weighing

machines

Primary

Stability of reading under constant load Secondary

Enclose weighing machine and observe change of reading stability

Varies force on weighing machine

Air conditioning, wind, convection, operator movement

Airflow Environmental


Influence factor

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Avoid weights with residual magnetism (de-gauss) Test with compass or gaussmeter Limit magnetic permeability of weights weighed check for magnetic fields with gaussmeter Screen with mumetal

PrimaryUse a magnet to check for induced reading changesSecondaryMonitoring readings when weights are rotated and at varying heights above the weight receptor (place on/under non-magnetic mount)

Change of current in force compensation coil, direct attraction to/from surrounding structure Motion-dependent restoring forces

Constructional steelwork, ferromagnetic weight (s) electrical induction earth’s magnetic field, moving-coil meters

Magnetic Environmental


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RepeatabilityLoad cell hysteresis

RepeatabilityElectrical Interference

ZeroMains fluctuation

Sensitivity, Zero, RepeatabilityIncorrect horizontal support

RepeatabilityMechanical errors

Type of errorSource of error


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ACCURACY CLASS (OIML R111-1 :2004)

0.200.060.0200.0061 mg

0.200.060.0200.0060.0035 mg

0.250.080.0250.0080.00310 mg

0.40.120.040.0120.00450 mg

1.60.50.160.050.0160.005100 mg

103.01.00.30.100.030.0101 g

124.01.20.40.120.040.0122 g

165.01.60.50.160.050.0165 g

206.02.00.60.200.060.02010 g

301031.00.30.100.03050 g

501651.60.50.160.05100 g

100301031. 00.30.10200 g

50016050165.01.60.51 kg

100030010030103.01.02 kg

250080025080258.02.55 kg

5000160050016050165.010 kg

Class M3Class M2Class M1Class F2Class F1Class E2Class E1

Tolerance in mgNominal value

0.003


Lowest class of weights required for different types of weighing machine

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E2E2E2F2M1M2M21 kg to 30kg

E2E2F1/E2F2M1M1200g to 1 kg

E2F1F2M1M230kg to 100kg

E2F1F2M1M1Up to 200g

E2F1M1/F2M2Above 100 kg

0.01mg or less

0.1mg1mg10mg100mg1g10g100g

ResolutionCapacity


Calibration of Top Loading and Analytical Balances

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Before use a Balance…………• Keep the Balance power on appropriate time to warm up.• Clean the balance as best as no contamination on the balance pan.• If it is applicable, level the balance before use.• Don’t move the balance after the calibration.• Lint or any other soft gloves should be worn by the user.• A pair of forceps or a pair tweezers with thermal insulating ends should

be used to lift and place weights.

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Bangladesh BEST Pregramme

• Make sure with the soft placement of weights on the balance pan as level best of possible.

• Readings should be taken after the balance is stabilized well. • The place should be free from vibration.• Any other relevant precaution practiced by the user.

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Before use a Balance Bangladesh BEST Programme

Balance Calibration

The calibration of the electromagnetic force compensation balance consists of the following tests:

• Test 1 - Scale Value • Test 2 - Repeatability of Reading• Test 3 - Departure from Nominal Value• Test 4 - Off centre Loading• Test 5 – Hysteresis

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Scale Value

Internal Scale Adjustment –Internal Mass

External Scale Adjustment – External Mass

Procedure – Balance Operating Manual

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Repeatability of Reading Objective of Measurement : The repeatability test is carried out to determine the ability of the

balance to give under defined conditions of use closely similar responses for repeated measurements

The balance Standard Deviation is the “repeatability of measurement”

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Repeatability of Reading :• M 1 -Set the reading to zero without any mass on the pan• M 2 - Record the zero reading of the balance (z)• M 3 - Place the mass on the pan and record the reading

(r)• M 4 - Remove the mass from the pan• M 5 – Repeat M2 to M4 ten times• during the process the balance should not be zeroed

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Repeatability of Reading:Performed at three points : Near zero

Near half load

Near full load


DATA RECORDING Mass 1 : Mass 2: Mass 3:

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No Zero reading (z)

Mass reading (r)

Corr. reading (r-z)

Zero reading (z)

Mass reading (r)

Corr. reading (r-z)

Zero reading (z)

Mass reading (r )

Corr. reading (r-z)

1

2

3

4

5

6

7

8

9

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Repeatability of Reading

Mass 1: Mass 2: Mass 3 : Mean value :Std. Dev. ( σ ) : Max. Diff. : Uncer. Type A :

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Repeatability of Reading

0.20.10.1

0.0670.0520.052

0.05100200

Maximum Difference Between Successive

Readings (mg)

Standard Deviation of Reading (mg)

Load on Pan(g)

Repeat three different values of M ; near zero, half maximum and maximum load.


Departure from Nominal Value:

Objective of Measurement :

“The departure from nominal value measurements are carried out to determine the deviation of the balance reading from the expected value”

This is the Correction to Balance Reading

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1 Divide the range into at least 10 equally spaced steps 2 The masses corresponding to M, 2M, 3M, 4M,

5M, 6M etc. until the capacity of balance is reached 3 In addition choose four more test points

corresponding to 20 %, 40 %,60%and 80 % of the first step for single range.

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Departure from Nominal Value: 1 - Read the zero z1

2 - Place the weight on the pan &record reading r1

3 – Remove the weight momentarily and place it back on the pan and note the new reading r2

4 - Remove the weight and read the zero z2

5 – Repeat 1 to 4 for weights of masses 2M,3M,4M,5M etc until the capacity of balance is reached

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Departure from Nominal Value: • Average zero reading zi = (z1 + z2 )/ 2

• Average mass reading ri = (r1 + r2 )/ 2

• Difference di = ri – zi

• Correction ci = Mi – di

• Mi is the conventional value of the mass37


Correction to Balance Reading

Pan Load Conv. Value Mi

Reading Mean Difference ri - zi

Correction Mi - di

M- 100 100.02 0.00100.000.00100.02

0.00 100.01

100.01 0.01

2M

3M

4M

5M

6M

7M

8M9M

10M



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0.00.0-0.10.1

-0.00.00.0-0.1-0.1-0.1-0.1-0.2-0.3-0.3-0.3

After the Calibration

Before the Calibration

0.0960.0970.0970.098

0.0990.0980.0980.0990.0990.0990.100.100.100.110.11

--------

0.2----

0.4--

0.6--

0.4--

0.5--

481216

20406080

100120140160180200220

Expanded Uncertainty k=2+(mg)

Correction(mg) Reading(g)


Off Centre Loading:

Objective of Measurement : “The off center loading is carried out to find the

variation in the displayed weighted as the object is moved to various positions on the weighing pan”

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Bangladesh Best Programme

Off Centre Loading

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This test to find the variation of the reading for several place of the balance pan. Test should be done for rectangular type balance pans and as well as for circular type balance pans.

A single weight should be used

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1

52

3

2

4

5

1


Off Centre Loading:

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Center (g)100.0000-100.0000=0.0000Left (g)

100.0001-100.0000=0.0001

Back (g)100.0000-100.0000=0.0000

Right (g) 99.9999-100.0000= -0.0001

Front (g)100.0001-100.0000=0.0001

Max Difference =0.0002 g

Place a mass on the centre of the pan and move it successively to the front rear, left and right positions ( position being 2/3 of the distance between the centre and the lip of the pan) on the pan recording the reading each time


Off Centre Loading:

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0.0002- 0.00010.00010.00000.00010.0000

Maximum Difference (g)

Right(g)Left(g)Back(g)Front(g)Centre(g)

Mass of 100g was moved to various positions on the pan. The balance readings obtained relative to the centre reading

Place a mass on the centre of the pan and move it successively to the front rear, left and right positions ( position being 2/3 of the distance between the centre and the lip of the pan) on the pan recording the reading each time


Hysteresis:

Objective of Measurement : “The hysteresis measurements are performed to determine the

difference in the indication at the same load , depending upon the direction of approach to that load ie increasing or decreasing the load”

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Hysteresis

Perform the test at one point about mid range• Zero the balance and record the reading z1 • Place a mass M1 on the plan and record the reading r1

• Add extra mass M2 to bring the balance close to full range• Remove the extra mass M2 and record the reading with the

mass M1 still on the pan• Remove the mass M1 and record the reading r2

• Repeat above steps three times• Take the average of three readings

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Hysteresis Mass M = g

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Pan Load Reading 1 Reading 2 Reading 3

Zero ( z1 )

M ( m1 )

M + M1

M ( m2 )

Zero ( z2 )

m1 - m2

z1 - z2

Hysteresis:

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0.1100

0.00

Hysteresis Error(mg)Pan Load (g)


Hysteresis:

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Mass Value VS Display Value

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100

Mass Value

Disp

lay

Valu

e

Increasing

Decreasing

Hysteresis error


Calibration of Single Pan two Knife Edge Balances

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Balance Calibration

The calibration of the single pan two knife edge balance consists of the following tests:

• Test 1 - Repeatability of Reading• Test 2 - Departure from Nominal Value• Test 3 – Built-in-Mass• Test 4 - Off centre Loading• Test 5 – Hysterisis

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Balance Calibration

The calibration of the single pan two knife edge balance consists of the following tests:

• Test 1 - Repeatability of Reading- same as electronic balance

• Test 2 - Departure from Nominal Value- same as electronic balance

• Test 3 – Built-in-Mass• Test 4 - Off centre Loading- same as electronic balance

• Test 5 – Hysteresis- same as electronic balance

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Balance Calibration The procedure to determine the Built-in- Mass• With all dials set to the zero and no load on the pan,

release the balance and record the reading z1

• Set the "1" on the dial to be tested, place a calibrated mass, release the pan and record the reading m1a

• Remove the mass , replace it and record the reading m1b

• Return the dial to zero, remove the mass from the pan and record the zero z2

• Repeat the above steps for all dial settings of the balance (i.e. 2 , 3, 10 , 20 ,…………..90 , 100 , 200 )

• Follow the same procedure used for linearity test to determine the correction

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Calibration of Triple Beam Balances

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Balance Calibration

The calibration of triple beam balances consists of the following tests:

• Test 1 - Repeatability of Reading - same as electronic balance

• Test 2 - Departure from Nominal Value (Linearity Test) -

same as electronic balance

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Triple Beam Balance

Before the calibration• Clean the weighing pan and other parts of the balance

taking due care• Level the balance• Exercise the balance by making several weighing

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Triple Beam BalanceRepeatability of readings:• Set the reading to zero without any mass on the pan• Record the zero reading zi of the balance without any mass

on the pan • Place the mass M on the pan and record the mass reading

mi

• Remove the mass M from the pan• Repeat the above steps ten times (The balance should not

be zeroed during the series of readings)• Repeat for this for near zero, half load and near maximum

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Triple Beam Balance

• Select a minimum of 10 equally spaced points ( M , 2M ,…..10M ) on the measuring range in such a way they cover the entire range or the part of the measuring range to be calibrated. In addition four test points corresponding to 20%,40%,60% and 80 % of the first step

• Record the zero reading z1

• Set the slider scale to the first step on the measuring range (M) , place the calibrating mass(es) on the pan until the balance comes to equilibrium and record the mass reading mi

• Remove the mass(es), place it back on the pan and record the mass reading mi2

• Remove the mass(es) and record the zero reading zi+1

• Repeat the above steps for the remaining points ( 2M , 3M ,….)

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Weighing Scale (digital or analogue)

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The calibration of weighing scales consists of the following tests:Test 1 - Repeatability of Reading - same as electronic balance

Test 2 - Departure from Nominal Value (Linearity Test) - same as electronic balance

Calibration Results of a Set of Weights –Class F1

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Nominal Value Conventional Value Uncertainty k=2 Max. Perm. Error

1 mg 1 mg – 0.001 mg 0.006 mg ±0.02 mg

2 mg 2 mg + 0.011 mg 0.006 mg ±0.02 mg

5 mg 5 mg+0.002 mg 0.006 mg ±0.02 mg

20 mg 20 mg – 0.005 mg 0.01 mg ±0.03 mg

50 mg 50 mg + 0.009 mg 0.012 mg ±0.04 mg

100 mg 100 mg + 0.011 mg 0.015 mg ±0.05 mg

500 mg 500 mg + 0.033 mg 0.025 mg ±0.08 mg

1 g 1 g – 0.028 mg 0.03 mg ±0.1 mg

2 g 2 g + 0.022 mg 0.04 mg ±0.12 mg

20 g 20 g + 0.009 mg 0.08 mg ±0.25 mg

50 g 50 g – 0.12 mg 0.1 mg ±0.3 mg

100 g 100 g – 0.22 mg 0.15 mg ±0.5 mg

200 g 200 g + 0.04 mg 0.3 mg ±1 mg

Issued by an accredited laboratory (DKD)

Thank you

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calibration of balance

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