Calibration Scenario
• The Unit Under Test Torque Wrench is a Type II class A tool (adjustable click type) and has a full scale of 350 N•m. • It has a setting dial resolution of 2 N•m • We are to calibrate it according to ISO 6789 which requires a calibration point at full scale (100 % of range) viz. at 350 N•m and the estimation of the measurement uncertainty at this point
GUM Steps
Model the measurement Identify and quantify the sources of uncertainty Categorize as type A or type B Manipulate appropriately to obtain • Standard uncertainties, u(xi) • Sensitivity coefficients, ci • Uncertainty contributor, u(yi) • Combine to obtain combined standard uncertainty, uc(y) • Expand to obtain an expanded uncertainty, U, at an appropriate level of confidence • Report the result • • • •
Quantifying the sources of uncertainty (2)
• SCAL
• This is the uncertainty due to the accuracy of the Reference Standard Torque Transducer, which is not perfect • Corrections must first be applied, or the uncertainty increased, to take the error into account (largest error on values either side of the calibration point was +1,0 N•m) • The Reference Standard Torque Transducer used has a full scale of 700 N•m and was calibrated in 100 N•m steps (See calibration certificate) • Therefore we will have to use the polynomial equation to determine the actual torque generated by the UUT at 350 N•m since it is a measurement point in between 300 N•m and 400 N•m. • Since we have to interpolate a value we will use the largest reported uncertainty from the calibration certificate for the values on either side of the calibration point which is ± 1 N•m.
Quantifying the sources of uncertainty (3)
• SCAL • Since we will be using the polynomial to interpolate a value at 350 N•m, we DO NOT need to correct for the + 1 N•m error at 399,8 N•m. • Therefore total uncertainty for the “accuracy” of the Reference Standard Torque Transducer is ±1 N•m • This is treated as normal at 95,45% Level of Confidence • The divisor is the coverage factor k which for 95,45% LOC is 2 • The degrees of freedom are always ∞ or 100 % Reliable due to the source of traceability being accredited and reputable.
Quantifying the sources of uncertainty (4)
• SRES • This is due to the resolution of the Reference Standard Torque Transducer Readout Unit • We must first determine the “effective resolution” • The least significant digit displayed is 0,1 N•m • Resolution is always treated as a Rectangular distribution source of uncertainty and this is the range. • The semi-range is therefore (0,1 N•m/2)=0,05 N•m • The divisor is √3 • The degrees of freedom are always ∞ or 100 % Reliability
Quantifying the sources of uncertainty (5)
• SCF • Polynomial Equation Coefficients Table from the Calibration Certificate
POLYNOMIAL EQUATION POLYNOMIAL COEFFICIENTS a b c d Standard Error of the polynomial curve fit for a Level of Confidence of 68,27% and 4 degrees of freedom NORMAL FUNCTION 2,71846 x10 9,99825 x10 -7,89039 x10 5,16535 x10
-2 -1 -6 -9
Quantifying the sources of uncertainty (7)
• URES • This is due to the resolution of the UUT Torque Wrench scale. (How it influences the setting of the wrench to a specified torque) • Typically this would be the smallest graduation on the UUT setting dial which for this UUT is 2 N•m • This is the range of the rectangular distribution • Therefore the semi-range is (2 N•m/2)=1 N•m • The divisor for Rectangular Distributed uncertainty contributors is √3 • The degrees of freedom for resolution is always ∞ or 100 % Reliability
Quantifying the sources of uncertainty (9)
• UREP • Treating as “Repeatability” (as per ISO 6789) • We use the ESDM • ESDM = ESD/SQRT (n) = 0,98/sqrt (5) = 0,436348 N•m • Treating as “Reproducibility” (preferred option but contrary to ISO 6789) • We use the ESD • ESD = 0,98 N•m
GUM Steps
Model the measurement Identify and quantify the sources of uncertainty Categorize as type A or type B Manipulate appropriately to obtain • Standard uncertainties, u(xi) • Sensitivity coefficients, ci • Uncertainty contributor, u(yi) • Combine to obtain combined standard uncertainty, uc(y) • Expand to obtain an expanded uncertainty, U, at an appropriate level of confidence • Report the result • • • •
Categorize as type A or type B
• • • • • SCAL - type B, not statistically determined SRES - type B, not statistically determined SCF - type A, statistically determined (standard deviation) URES - type B, not statistically determined UREP - type A, statistically determined (standard deviation)
GUM Steps
• • • • Model the measurement Identify and quantify the sources of uncertainty Categorize as type A or type B Manipulate appropriately to obtain • Standard uncertainties, u(xi) • Sensitivity coefficients, ci • Uncertainty contributor, u(yi) • Combine to obtain combined standard uncertainty, uc(y) • Expand to obtain an expanded uncertainty, U, at an appropriate level of confidence • Report the result
GUM Steps
Model the measurement Identify and quantify the sources of uncertainty Categorize as type A or type B Manipulate appropriately to obtain • Standard uncertainties, u(xi) • Sensitivity coefficients, ci • Uncertainty contributor, u(yi) • Combine to obtain combined standard uncertainty, uc(y) • Expand to obtain an expanded uncertainty, U, at an appropriate level of confidence • Report the result • • • •
Conclusions
• Both methods in this case prove that the UUT is well within the allowable ± 4% of Maximum (± 14 N•m) • Using the ESDM (In accordance with ISO 6789) results in the smallest uncertainty (unrealistic??) • Using the ESD (contrary to ISO 6789) results in the largest uncertainty (realistic??) • Always use the polynomial for calibrations using the laboratory Reference Standard Torque Transducer • This will correct for any error on the Reference Standard eliminating the need to apply corrections • This will solve the problem of the “Applied Torque” not being exactly at the nominal values