Evaluation of Load Cells for Legal Metrology

at NIST

Kevin Chesnutwood

Mechanical Engineer

Mass and Force Group

Physical Measurement Laboratory

• Objectives to cover briefly today:

– NIST force facilities – What is “type evaluation” testing and legal

metrology?– How NIST fits into type evaluation testing – Typical tests performed during a type evaluation

Certain commercial equipment, instruments, software, or materials may be identified in this paper. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

The Constitution of the United States

Sect. 8. The Congress shall have power ...

To coin money, regulate the value thereof, and of foreign coin

and fix the standard of weights and measures;

Higher Calling???

Leviticus 19:35 Do not use dishonest standards when measuring length, weight or quantity.

A false balance is an abomination to the LORD, But a just weight is His delight.

Proverbs 11:1

Unique Facilities at NIST Serve the Nation

• Mass Standards Facility– Houses U.S. National

Prototype Kilogram– Custom environment

with world-class measurement capability

• 4.44 MN Force Deadweight Machine – Largest in the world - most other

large force machines are traceable to this one

• Small Force Lab– World’s first laboratory

for the realization of traceable forces in the mN to pN ranges.

Force Metrology Laboratories Cross-Section Showing Six Deadweight Machines

NIST Engineering Mechanics Building (202)

Legal metrology is metrology which ensures the quality and credibility of measurements that are used directly in regulation and in areas of commerce. Legal metrology deals with traceability, but also with risks of misuse of the instruments, of tampering and of accidental influences on the measuring instrument. In many cases, laws or regulations govern the accuracy of these measurements as well as the conformity of the measuring instruments against national or international specifications.

Legal Metrology Defined(per BIPM website)

• Load cells can be sensitive to a number of parameters which contribute to the uncertainty associated with it’s use.

• Calibration characterizes the uncertainty of a device by using a specific set of parameters.

• The commercial weighing industry uses a classification or “type” system to limit these uncertainties by means of assigned tolerances.

• The testing that demonstrates whether or not a device meets these tolerances is called a “type evaluation.”

What is a “type evaluation?”

How is NIST involved?

– NIST was asked in 1989 by NCWM to be a technical resource to the NTEP program by:

– Being an independent third party

– Providing advice on requirements and test procedures

– Performing type evaluation tests• from 100 lbf to 120,000 lbf

– have since evaluated nearly 600 load cells for companies throughout the U.S. and the world

Type EvaluationTesting Protocols

• National Protocol– National Type

Evaluation Program (NTEP)

• NIST Handbook 44• National Conference

on Weights and Measures (NCWM) Publication 14 “Checklist for Load Cells”

• International Protocol– International

Organization of Legal Metrology (OIML)

• Recommendation 60 (R60)

Testing Procedures

• A minimum of 4 major tests are involved in a type evaluation.

• Determines the linearity, hysteresis, and repeatability characteristics of the load cell

• Load cell is mounted in an environmental chamber in the NIST dead weight machine.

• Load cell is cycled through 4 temperature conditions.

Xxxxxxxxxxxxxxxxx

Load Cell Error with Respect to Temperature (Load Cycle Test)

• Performed to determine deviation from linearity due to temperature change

– Forces applied incrementally in monotonically increasing order

– Subsequently removed in reverse order

– Each ascending/descending cycle is a run.

– 3 runs required for most tests done at NIST

– Repeated at each temperature condition

Load (lbf) Run 1 Run 2 Run 3 Average Output

10 0.05055 0.05054 0.05055 0.0505540 0.18657 0.18657 0.18657 0.1865765 0.29994 0.29990 0.29994 0.29993

105 0.48130 0.48129 0.48130 0.48130305 1.38822 1.38820 1.38821 1.38821405 1.84165 1.84168 1.84166 1.84166305 1.38824 1.38826 1.38825 1.38825105 0.48131 0.48132 0.48132 0.4813265 0.29994 0.29994 0.29994 0.2999440 0.18658 0.18658 0.18658 0.1865810 0.05054 0.05054 0.05054 0.05054

Output in mV/VInitial 20oC run

LOAD CELL ERRORS

-1.25

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

1.25

0 500 1000 1500 2000 2500 3000

Load (v )

Err

or

(v)

20.4 C (1)

LOAD CELL ERRORS

-1.25

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

1.25

0 500 1000 1500 2000 2500 3000

Load (v )

Err

or

(v)

20.4 C (1)

39.7 C

-9.1 C

20.3 C (2)

Temperature Effect on Minimum Dead Load Output (TEMDLO)

• Determines the amount of variation in the load cell output at minimum dead load (MDL) from one temperature condition to the next.

• Data extracted from the Load Cycle Test.• Amount of variation allowed is defined by a

tolerance that is dependent on the actual temperature change.

Load (lbf) Run 1 Run 2 Run 3 20oCAVERAGE

10 0.05055 0.05054 0.05055 0.0505540 0.18657 0.18657 0.1865765 0.29994 0.29990 0.29994

105 0.48130 0.48129 0.48130305 1.38822 1.38820 1.38821405 1.84165 1.84168 1.84166305 1.38824 1.38826 1.38825105 0.48131 0.48132 0.4813265 0.29994 0.29994 0.2999440 0.18658 0.18658 0.1865810 0.05054 0.05054 0.05054

TEMDLO Data Taken from LCT Tests

0.0320

0.0322

0.0324

0.0326

0.0328

0.0330

0.0332

0.0334

0.0336

0 10 20 30 40

Time (h)

MD

L r

esp

on

se (

mV

/V)

Minimum Dead Load Response through Temperature Cycles

Initial

room

cold

hot

Finalroom

Repeatability

• Determines the maximum difference in the output of the load cell between runs at each applied force within a temperature condition

• Calculated independently at each temperature condition

• Maximum difference between any of the readings between runs is compared to a specified tolerance

Load (lbf) Run 1 Run 2 Run 3 20oCMaximum Difference

10 0.05055 0.05054 0.05055 0.0000140 0.18657 0.18653 0.18657 0.0000465 0.29994 0.29990 0.29994 0.00004

105 0.48131 0.48129 0.48130 0.00002305 1.38822 1.38820 1.38821 0.00002405 1.84165 1.84168 1.84166 0.00003305 1.38824 1.38826 1.38825 0.00002105 0.48131 0.48132 0.48132 0.0000165 0.29995 0.29994 0.29993 0.0000240 0.18658 0.18659 0.18658 0.0000110 0.05054 0.05054 0.05052 0.00002

Repeatability Sample

Creep Test• Determines how the output of the load cell

varies while under a constant applied force.• Completed independently from the LCT

tests and is completed at 3 temperature conditions

• Reference reading taken at a specified time after load is applied

• Subsequent readings are then compared to the reference reading and the maximum difference must fall below the specified tolerance.

CREEP 39.9 oC

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0 5 10 15 20 25 30 35

Time (min)

MD

L R

esp

on

se (

v)

creep

Creep Recovery

• Now a required test in NTEP• Always has been useful as backup

material because it creates a curve of similar shape and magnitude as creep

• Takes readings at the same time intervals as the Creep test except that the clock starts whenever the cell is unloaded

• multiply the results by -1.0 to allow curves to be plotted in the same quadrant.

CREEP 39.9 oC

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0 5 10 15 20 25 30 35

Time (min)

MD

L R

esp

on

se (

v)

creeprecovery

Effects of Barometric Pressure

• Determines changes in the zero load output of the load cell due to atmospheric pressure changes.

• Pressure sensitivity calculated and compared to the specified tolerance

• Pressure sensitivity also used to apply a correction factor to the LCT and Creep test data – ensures that all parts of the temperature cycle

were effectively evaluated at the same barometric pressure

Classifications and Tolerances

• Tolerances are based on several parameters chosen by the manufacturer

– Number of divisions in the measuring range, n

– Minimum verification division or interval, vmin

– Accuracy Class

Classifications and Tolerances

• Once the manufacturer chooses the number of divisions, the verification interval, v, can be calculated.

• v = (Load cell capacity)/(number of divisions)

• Tolerances are specified as a function of v units

For example a 5000 lbf load cell evaluated at 10000 divisions: v = 5000/10000 = 0.5

Minimum Verification Division Vmin

• Chosen by the manufacturer• Required to be less than or equal to 1 v• Used in calculations for TEMDLO and

Pressure tests

• The lower the value of Vmin, the tighter the tolerances the cell must meet

Accuracy Classes

• NTEP Protocol– Class III Single

– Class III Multiple

– Class IIIL Single

– Class IIIL Multiple

• single or multiple chosen by how the cell will ultimately be used in the field.

• OIML Protocol– Class A– Class B– Class C– Class D

Intercomparisons

• Last one completed between NIST and 3 other National Measurement Institutes.

• Mutual Recognition is a big push behind intercomparison work.

• MRA exists for Canada• NIST/NCWM only signed on as utilizing

member of the OIML MAA

LOAD CELL ERRORS 20 oC

-1.25

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

2.50

2.75

0 500 1000 1500 2000 2500 3000

Load (v )

Err

or

(v)

LAB A

NIST (1)

LAB B

LAB C

NIST (2)

LOAD CELL ERRORS 40 oC

-1.25

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

2.50

2.75

0 500 1000 1500 2000 2500 3000

Load (v )

Erro

r (v)

LAB A

NIST (1)

LAB B

LAB C

NIST (2)

LOAD CELL ERRORS -10 oC

-1.25

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

2.50

2.75

0 500 1000 1500 2000 2500 3000

Load (v )

Erro

r (v)

LAB A

NIST (1)

LAB B

LAB C

NIST (2)

Conclusions

• NIST fulfills mission of helping U.S. Industry by:– being a liaison between U.S. companies and

national and international legal metrology bodies– independent third party to evaluate load cells to

ensure compliance with legal metrology bodies– perform intercomparisons for past and future

mutual recognition arrangements– customer access to the top of the traceability

chain