1 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology

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Introduction

Eccentricityeffects

INTIBuenos Aires

Oliver Mack:Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology

Materialeffects

Test Procedure

ConclusionOutlook

Temperaturegradients Oliver MackOliver Mack

Physikalisch-Technische Bundesanstalt (PTB)

Braunschweig

www.ptb.de

1 / 25Oliver Mack:

Recommendation For The Revision Of Test Procedures For Load Cells In Legal MetrologyINTI

Buenos Aires

Recommendation For The Revision Of Recommendation For The Revision Of Test Procedures For Load Cells In Test Procedures For Load Cells In

Legal MetrologyLegal Metrology

Conference “Load Cell Tests by OIML R60” Conference “Load Cell Tests by OIML R60”

Buenos Aires, June 7th to 11th, 2010

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Introduction

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Materialeffects

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ConclusionOutlook

Temperaturegradients

Introduction

Modules of a weighing instrument:• Indicator• Analog or digital data processing device• Weighing module• Terminal • Primary display• Load cell

Test procedure for load cells (LCs): • OIML Recommendation R60 (Edition 2000)• WELMEC Guide 2.4 (Edition 2001)

Aim of the presentation: • Overview of unconsidered abnormalities• Start a discussion for a revision of R60 and WELMEC 2.4

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Test procedures according to the recommendation OIML R60

m easuring tim e

load

5 m in 1 h 30 m in 30 m in

Accuracy tests (at 20°C, -10°C, 40°C, 20°C):Reproducibility, linearity, hysteresis, sensitivity

Creep forward and creep recovery (at 20°C, -10°C, 40°C):Creep effects and zero point return

Influence of humidity and barometric pressureRequire additional tests

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Classification:Class D (ordinary accuracy weighing instruments): up to 1000 vClass C (balance for trade purposes): up to 10000 vClass B (precision balance): up to 100000 vClass A (special accuracy weighing instrument): more than 50000 v

v maximum load

nmax

Verification interval v:

Classification according to the recommendation OIML R60

Maximum permissible error (mpe):

mpe pLC x 0.5v pLC x 1.0v pLC x 1.5v

class C 0v m

500v

500v m

2000v

2000v m

10000vpLC: apportionment factor, generally pLC = 0.7

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Abnormalitiies of LCs – uncovered acc. OIML R60 - Load transmission -

Single-Point LC

WELMEC 2.4: The load transmissions for beam LCs may be applied

Uncovered according OIML R60:

1. Eccentricity effects (covered acc. OIML R76)

2. The material of the force transmission

Beam LC with ball joint

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Mpe for class C3 and the LC-error expressed in verification Intervals v as a function of the load for a 8 kg-load cell

Abnormalitiies of LCs – uncovered acc. OIML R60 - Eccentricity effects -

Accoring to OIML R76 for weighing instruments the tests are carried out on 5 positions of the plattform

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Result:Depending on the position where the load is applied the mpe for the LC is signifiantly exceeded. But in combination with the modular concept it is not taken into account sufficiently.

Consequence:PTB carries out additional eccentricity tests according to OIML R76 if single point LCs are tested acc. to OIML R60.

Procedure not harmonised

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Abnormalitiies of LCs – uncovered acc. OIML R60 - Material of the load transmission -

Uncovered:

Neither OIML R60 nor WELMEC 2.4 regularize the kind of material used for the load transmission e.g. by a declaration of the manufacturer.

Single-Point LC

Example:

Single-point LC made of stainless steel with aload transmission made of the same material or aluminium.

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Requirements on load cells acc. OIML R60 - Temperature effect on MDLO -

Relevance:Temperature effect on the output signal under a minimum dead load Emin (e.g. Emin = 0 kg).

Criteria fulfilled:If MDLO of a LC over a defined temperature range does not vary by an amount greater than a minimum verification interval vmin for any change of ambient temperature of 5°C for class C.

Definition vmin:Smallest interval into which the LC measurement range can be divided and thus the smallest difference of mass which can be measured by the LC.

Criteria given by a ratio: Y = Emax / vmin

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Requirements on load cells acc. OIML R60 - Temperature effect on MDLO -

Temperature effect on MDLO in v for a class C3-LC and the mpe for Y = 20000 as a function of the ambient temperature

Stainless steel

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Abnormalitiies of LCs – uncovered acc. OIML R60 - Material of the load transmission -

Temperature effect on MDLO in v for a class C3-LC and the mpe for Y = 20000 as a function of the ambient temperature

Load transmission made of• Stainless steel: Y =

20000• Aluminium: Y = 4500

Aluminium

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Result:Even if the requirements for load transmission acc. WELMEC 2.4 are fulfilled, the geometry of the force introduction and the materials used for it strongly affect metrological characteristic of a LC.

Consequence:Concrete descriptions of the load transmission if necessary and a definition of applicable material should be discussed for a coming revisions of both OIML R60 and WELMEC 2.4.

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Abnormalitiies of LCs – uncovered acc. OIML R60 - Effects of temperature gradients -

Point of discussion:Makes this sense ?

Frame: No concrete temperature gradients (spatial and temporal) are specified in OIML R60.

Simply a temperature stability of 2°C within a accuracy test cycle has to be fulfilled.

PTB-Requirements: A temporal temperature stability of < 0.2°C / hA spatial temperature gradient of < 0.2°C / 10 cm

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Time stability of MDLO:

Time stability of MDLO and mpe in v for class C3 as well as the ambient temperature as a function of time


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Time stability of MDLO and mpe in v for class C3 as well as the ambient temperature as a function of time

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Experience of the current issues of OIML R60 and

WELMEC 2.4 are presented.

There are abnormalities of LCs which may strongly affect

the metrological characteristic of LCs but have not been

taken into account yet.

In particular the influence of force introduction, eccentricity

effects, temperature gradients, excitation voltage and

drying effects of the sealing are presented.

Need of further investigations

The effects are not considered in OIML R60 or WELMEC

2.4 and should be discussed in a coming revision.

Conclusion:Consequences for coming revisions

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Conclusion

Thank you for your attention




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1 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology