Conference Centre, Centurion August 1st Denis LOUVEL, MT

Calibration of Weighing Instruments

Conference Centre,

Centurion

August 1st

Denis LOUVEL, MT-SI

[email protected]

Measurement Uncertainty

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Agenda 2

For internal use - Confidential

1 What is the most important?

2 Risk management

3 Uncertainties related to a weighing

4 Physics behind a weighing result

5 Safe Weighing Range

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Product or balance?

What is the most important the product or the balance?

Do you need a compliant product or a compliant balance?

3


Product or balance?


Do you calibrate the balance for the balance?

Do you calibrate the balance for the product?

4


Product or balance?


Are you sure that the content of your current calibration certificate helps?

5


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Agenda 9



2 Risk management




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Risk management

Preferential regime?

We can assess the balance uncertainty since more than 20 years.

Nevertheless, the classic verification for a balance is to compare the error against the

maximum permissible error:

Error Maximum permissible error (MPE)

This assessment is the same as requested by the Legal Metrology

For other measuring instruments (e.g. thermometers) the uncertainty is always taken into

account in assessing compliance of the instrument:

Error + Uncertainty Maximum permissible error (MPE)

It seems that balances are entitled to a preferential regime and can be assessed according

to outdated criteria


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Risk management

Decide what is the best for you

Knowing that a balance has an error smaller than the MPE, that's good.

Knowing the measurement uncertainty is much better.

Ignore uncertainty when you weight the product is risky.


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- MPE + MPE

Target

Measured value

Measured value

1

2

3 - U + U

Target

Error

Target

Error

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Agenda 15



2 Risk management




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Uncertainties related to a weighing

3 uncertainties, not only 1

3 uncertainties are involved when you are weighing your product:

- The uncertainty of the error of indication

- The uncertainty of the balance

- The uncertainty of the product weighed on the balance


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Uncertainty of the error of indication

All these properties contribute to the uncertainty of the error of indication

In the first level, parameters below are taken into account for the determination of the

uncertainty of the indication error U(EI):

- Repeatability of the weighing

- Resolution of the balance (at zero and on load)

- Load eccentricity

- Ambient temperature during calibration (sensitivity coefficient of the balance to

temperature)

- Standards weights (uncertainty + durability)

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Repeatability

Eccentricity

Bal

ance

reso

lutio

n

At

zero

On

load

Sta

ndar

ds

wei

ghts

Calibration

DurabilityA

mbient

temperatureBalance

Coeff.Uncertainty of the

indication error

u(EI)

Temperature

deviation

The accreditation domain

The balance has not only 1 uncertainty, but one per load

The accreditation covers only the uncertainty of the error of indication


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- MPEbal + MPEbal

MeasureTarget

EI

U(EI)Accreditation

coverage

Uncertainty of the Balance

All these properties contribute to the balance uncertainty

A second level is necessary to take into account how the balance is used, installed, set:

- the use or not of the internal adjustment device

- the correction or not of the errors of indication EI

- the temperature range in the room where the balance is used

- the eccentricity

With these situations, we can define the balance uncertainty from which the user obtains

the uncertainty for each weighing he carries out.

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Repeatability

Eccentricity

Bal

ance

reso

lutio

n

At zero

On load

Air

dens

ity

Error

uncertainty

Error

modelingTem

perature

Balance

Coeff.

Temperature

deviation

Indi

catio

ner

ror

Greatest

error

Error

durability

Balance

Uncertainty

U(IP)


No information for u(IP)

The accreditation covers only the uncertainty of the error of indication

The accreditation never covers the uncertainty of the balance


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- MPEbal + MPEbal

MeasureTarget

EI

U(EI)

u(IP)

Accreditation

coverage

Uncertainty of the weighing of the final product

All these properties contribute to the balance uncertainty

A last level is necessary to finally know the influence of the balance on the product:

- the uncertainty of the balance

- the correction or not of the buoyancy effect

With this information, we can finally assess the product conformity.

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No information

The accreditation covers the uncertainty of the error of indication

The accreditation never covers the uncertainty of the balance

The accreditation doesn't help you to calculate the buoyancy effect (ncpa) and the

uncertainty of the weighing result, U(M)


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- MPEbal + MPEbal

MeasureTarget

EI

U(EI)

u(IP)

uncpa

U(M)

Accreditation

coverage

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Agenda 24



2 Risk management




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Physics Behind A Weighing Result

The balance is adjusted / verified / calibrated with weights and provides

indications as if the weighed object has a density identical to the weights

All balance users know the buoyancy effect (Archimedes' principle), but none of them

applies a correction.

They all think that the weight displayed by the balance is the true mass of the object and

no correction is necessary.

Unfortunately, this is totally wrong and this can question the quality of the weighing result.

When balance is adjusted with standard weights, both equilibriums carried out to weigh an

object (at zero and on load), leads to the following relation:

𝑀 × (1 – 𝑎/𝑟) = (𝑥– 𝐸𝐼) × (1– 𝑎/𝑟0)

Users of balances do not apply a correction related to the buoyancy effect.

The correction is not necessary as long as it doesn't jeopardize the compliance of the

product.

Before to decide to neglect the correction, it is necessary to know it.

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Air Buoyancy Effect

It may be neglected

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The relative air buoyancy decreases when the density of the object increases.

Close to 8,000 kg/m3, it becomes null and becomes negative for greater densities.

According your weighing requirement, decide if a correction is necessary or not.

0,001%

0,010%

0,100%

1,000%

10,000%

100 kg/m³ 1 000 kg/m³ 10 000 kg/m³

Buoyancy - Poussée aérostatiqueRelative bias

Relative Uncertainty

Relative bias + Relative uncertainty

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Agenda 28



2 Risk management




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Calibrate the process, not the balance

Contact Microsep about the safe weighing range of your balance

The uncertainty of the balance should be used at least to define the safe weighing range

Make sure that your balance is suitable for your process

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Weighing Range [kg] Balance capacity

Mea

sure

men

t Unc

erta

inty

(%

)

Smallest Net WeightMinimum

Weight

Determined

Safe Weighing Range

Relative Measurement Uncertainty [%]Weighing Tolerance : 1 %

Minimum weight determined x Safety factor required

Thank you for your attention

Visit us on stand to see our

solutions to evaluate the Risk

Management with Minimum Weight

Documents

Conference Centre, Centurion August 1st Denis LOUVEL, MT