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1 / 24
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
2 / 24
Introduction
Eccentricityeffects
INTIBuenos Aires
Oliver Mack:Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology
Materialeffects
Test Procedure
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
5 / 24
Introduction
Eccentricityeffects
INTIBuenos Aires
Oliver Mack:Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology
Materialeffects
Test Procedure
ConclusionOutlook
Temperaturegradients
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
6 / 24
Introduction
Eccentricityeffects
INTIBuenos Aires
Oliver Mack:Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology
Materialeffects
Test Procedure
ConclusionOutlook
Temperaturegradients
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
12 / 24
Introduction
Eccentricityeffects
INTIBuenos Aires
Oliver Mack:Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology
Materialeffects
Test Procedure
ConclusionOutlook
Temperaturegradients
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
13 / 24
Introduction
Eccentricityeffects
INTIBuenos Aires
Oliver Mack:Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology
Materialeffects
Test Procedure
ConclusionOutlook
Temperaturegradients
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
14 / 24
Introduction
Eccentricityeffects
INTIBuenos Aires
Oliver Mack:Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology
Materialeffects
Test Procedure
ConclusionOutlook
Temperaturegradients
Abnormalitiies of LCs – uncovered acc. OIML R60 - Eccentricity effects -
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
15 / 24
Introduction
Eccentricityeffects
INTIBuenos Aires
Oliver Mack:Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology
Materialeffects
Test Procedure
ConclusionOutlook
Temperaturegradients
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.
16 / 24
Introduction
Eccentricityeffects
INTIBuenos Aires
Oliver Mack:Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology
Materialeffects
Test Procedure
ConclusionOutlook
Temperaturegradients
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
17 / 24
Introduction
Eccentricityeffects
INTIBuenos Aires
Oliver Mack:Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology
Materialeffects
Test Procedure
ConclusionOutlook
Temperaturegradients
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
18 / 24
Introduction
Eccentricityeffects
INTIBuenos Aires
Oliver Mack:Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology
Materialeffects
Test Procedure
ConclusionOutlook
Temperaturegradients
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
19 / 24
Introduction
Eccentricityeffects
INTIBuenos Aires
Oliver Mack:Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology
Materialeffects
Test Procedure
ConclusionOutlook
Temperaturegradients
Abnormalitiies of LCs – uncovered acc. OIML R60 - Eccentricity effects -
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.
20 / 24
Introduction
Eccentricityeffects
INTIBuenos Aires
Oliver Mack:Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology
Materialeffects
Test Procedure
ConclusionOutlook
Temperaturegradients
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
21 / 24
Introduction
Eccentricityeffects
INTIBuenos Aires
Oliver Mack:Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology
Materialeffects
Test Procedure
ConclusionOutlook
Temperaturegradients
Abnormalitiies of LCs – uncovered acc. OIML R60 - Effects of temperature gradients -
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
Time stability of MDLO:
22 / 24
Introduction
Eccentricityeffects
INTIBuenos Aires
Oliver Mack:Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology
Materialeffects
Test Procedure
ConclusionOutlook
Temperaturegradients
Abnormalitiies of LCs – uncovered acc. OIML R60 - Effects of temperature gradients -
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
23 / 24
Introduction
Eccentricityeffects
INTIBuenos Aires
Oliver Mack:Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology
Materialeffects
Test Procedure
ConclusionOutlook
Temperaturegradients
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
24 / 24
Introduction
Eccentricityeffects
INTIBuenos Aires
Oliver Mack:Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology
Materialeffects
Test Procedure
ConclusionOutlook
Temperaturegradients
Conclusion
Thank you for your attention
Thank you for your attention
Thank you for your attention
Thank you for your attention