Recommended Guide for Determining and Reporting Uncertainties for Balances and Scales Val Miller NIST Office of Weights and Measures

Why Is A Guide Needed? AIAG/QS9000 requirements for accreditation FDA requirements for traceability Quality system requirements (ISO/IEC 17025) for reporting uncertainties Service providers with no experience or education on reporting uncertainties Important part of traceability No documented guidelines for weighing instruments A specific application of the GUM to weighing instruments is needed!

Specific Resources Currently Available Uncertainty Documents Guide to the Expression of Uncertainty in Measurement (GUM), NIST Technical Note 1297, and NCSLI Recommended Practice RP-12 Eurachem: QUAM UKAS: LAB 12, M3003 Balance Procedures NIST HB 44, HB 112 NCWM Pub 14, ASTM Standard Practices (E , E , E ), and Reference papers

Factors Considered in Current Scale/Balance Procedures

Typical Understanding of Calibration Process Ensure that the indication is within the allowable error Apply suitable mass standards

Process for Uncertainty Determination Eight step process: 1: Specify the calibration process 2: Identify and characterize uncertainty sources 3: Quantify uncertainty estimates 4: Convert factors to standard uncertainties 5: Calculate combined standard uncertainty 6: Calculate expanded uncertainty 7: Evaluate the expanded uncertainty 8: Report the uncertainty

Step 1: Specify the calibration process? Weighing system indication equals the applied mass ± uncertainty Uncertainty equals: Where: u s = uncertainty of mass standard s p = process standard deviation u (x) = other uncertainty components to be included

Truck Scale Calibration

Step 2: Identify and characterize uncertainty sources Uncertainty Design InstallationStaff & Procedures Standards Facility Environment/Location Method of Use Capacity Accuracy Sensitivity Readability Draft Shield Off center loading sensitivity Design Experience of installer Repeatability Foundation/ Supports Experience Attitude Procedure selection Frequency of test Training Auto zero tracking CLC Reported Uncertainty/ Tolerance Substitution load Handling/ condition Storage Internal standards Use of error weights Temperature stability Product contamination Snow Wind Drafts Vibration RFI/EMI Static Rain Range of use Type product weighed Gross/net weight Auto zero-tracking on/off Repair Shock loading Dynamic vs static weighing Factors affecting Uncertainty Sample Cause and Effect Diagram

Significant and uncorrectable factors Uncertainty Design InstallationStaff & Procedures Standards Facility Environment/Location Method of Use Repeatability Reported Uncertainty/ Tolerance Substitution load Temperature stability Wind Factors affecting Uncertainty Sample Cause and Effect Diagram

Sample uncertainty budget at kg

Step 3: Quantify uncertainty estimates Units must be made to match –Temperature in °C –Wind in km/hr –Uncertainties of mass standards in kg –Repeatability in kg Conduct research to determine impact –e.g. manufacturer’s data says temperature coefficient is 10 x /°C

Sample uncertainty budget at kg

Step 4: Convert factors to standard uncertainties How are the principle uncertainty contributors included in the uncertainty? Repeatability/ Reproducibility Standard mass uncertainty or tolerance Substitution loads Wind effects Temperature effects Treated as a Normal, or Uniform distribution depending on method used for estimation and results of other tests k=1 calibration uncertainty if corrected values are used, or treat as Uniform distribution if nominal mass values are used Treated as Uniform distribution depending on other tests Treated as Uniform distribution

Repeatability estimations Estimate s p as: or d=1 scale division Calculate from Measured Data 2. Estimate based on scale division (d) Preferred (discrimination test required)

Uncertainties of the standards Measured mass values of standards use the k=1 calibration uncertainty Nominal mass values of the standards use

Sample uncertainty budget at kg

Step 5: Calculate combined standard uncertainty Combine one standard deviation equivalents into the combined standard uncertainty (u c )

Sample uncertainty budget at kg

Step 6: Calculate expanded uncertainty Multiply by the appropriate coverage factor ‘k’ U=k*u c –The GUM states that k will typically be between 2 and 3, –NIST Tech Note 1297, Appendix C, by NIST convention k=2 for a confidence interval of approximately 95 %. Using a coverage factor other than k=2 requires explanation and a description of how k was chosen and why.

Sample uncertainty budget at kg

Step 7: Evaluate the expanded uncertainty Does the expanded uncertainty make sense? (Are there any calculation errors?) Does it seem appropriate for the device tested? Does it meet the requirements of the user’s process?

What steps can be taken to reduce the uncertainty? Perform discrimination test Use standards with smaller uncertainty Eliminate or minimize significant measurement influences Perform multiple measurements to obtain standard deviation of weighing device Use check standards to determine the standard deviation of the process over time

Sample uncertainty budget at kg U = 6.7 kg

Step 8: Report the uncertainty Uncertainty components must be identified and a rationale given for inclusion so that an individual not participating in the calibration will be able to understand how the uncertainty was calculated, what was included and why.

Summary Uncertainty calculations require: Defined Process Understanding of uncertainty contributors Documented method for evaluation Proper presentation The Guide provides guidance needed for correctly calculating balance and scale uncertainties.

Where will the Recommended Guide for Determining and Reporting Uncertainties for Balances and Scales be found? Download from NIST Office of Weights and Measures internet web page ( by Fall 2002 (NIST IR 6855, “Overview of the Recommended Guide for Determining and Reporting Uncertainties for Balances and Scales” available now) Soon available in hardcopy form as a NIST Recommended Practice Guide Available to NCSLI, NCWM and others for distribution

Additional Concerns User requirements and prerequisites for quality weighing Calibration uncertainty is only part of scale and balance uncertainty issue Scale and balance testing procedures require updates to include uncertainty issue. Real need is in “production” process vs. calibration/service process Use check standards in the routine measuring process!