Calibration Traceability Guidelines

Transcription

1 Calibration Traceability Guidelines Check the website of accrediting bodies for the accredited laboratories individual scope of accreditation ts.nist.gov/standards/accreditation/index.cfm The 1 st page of any ISO is useless after it has been determined that the laboratory is accredited. 1

2 Calibration Traceability Guidelines It is the scope of accreditation that determines the laboratory s capability. The cert should state the best measurement uncertainty for different ranges. On this cert the best measurement uncertainty is % for torque calibrations from N-m and % for force calibrations up to 120,000 LBF. 2

3 7 Essential Elements Chain of Measurement Traceability Measurement Uncertainty Measurement Assurance Realization of SI Units Periodic Recalibration Technical Competence 3

4 Measurement Calibration Hierarchy Reference Standard used in the calibration of equipment BIPM/SI National Metrology Institute (NMI) Primary Reference Laboratory Morehouse Instrument Company Accredited Calibration Service Supplier Instrument/Equipment Measurement Process: A. Reference Standard B. Measurand C. Measurement Result D. Measurement Traceability: 1. Unbroken Chain of Comparisons 2. Measurement Uncertainty 3. Documented Procedure 4. Technical Competence 5. Realization of SI Units 6. Periodic Recalibration 7. Measurement Assurance 4

5 Measurement Traceability Requirements ISO/IEC 17025, Clause Specific requirements Calibration laboratories When using external calibration services, traceability of measurement shall be assured by the use of calibration services from laboratories that can demonstrate competence, measurement capability and traceability... 5

6 Measurement Calibration Hierarchy Reference Standard used in the calibration of equipment Accredited Calibration Service Supplier Laboratory Instrument/Equipment Supplier Evaluation Measurement Process: A. Reference Standard B. Measurand C. Measurement Result D. Measurement Traceability: 1. Unbroken Chain of Comparisons 2. Measurement Uncertainty 3. Documented Procedure 4. Technical Competence 5. Realization of SI Units 6. Periodic Recalibration 7. Measurement Assurance Laboratory Measurement 6

7 Measurement Calibration Hierarchy Reference Standard Accredited Calibration Service Supplier Supplier Evaluation Measurement Process: A. Reference Standard B. Measurand C. Measurement Result D. Measurement Traceability: 1. Unbroken Chain of Comparisons 2. Measurement Uncertainty 3. Documented Procedure 4. Technical Competence 5. Realization of SI Units 6. Periodic Recalibration 7. Measurement Assurance Laboratory use of a calibrated Reference Standard 7

8 Measurement Uncertainty & the Measurement Hierarchy BIPM/SI Uncertainty National Metrology Institute (NMI) Primary Reference Laboratory Morehouse Instrument Company Accredited Calibration Service Supplier Working Standards Instrument/Equipment Measurement Traceability: 1. Unbroken Chain of Comparisons 2. Measurement Uncertainty 3. Documented Procedure 4. Technical Competence 5. Realization of SI Units 6. Periodic Recalibration 7. Measurement Assurance 8

9 Measurement Uncertainty BIPM/SI NMI Primary Standards Accredited Cal. Lab Working Standards Field Measurement All measurements are subject to uncertainty and a measured value is only complete if it is accompanied by a statement of the associated uncertainty. Uncertainty is a measure of how close a particular test result, the product of one laboratory, is to the true value. 9

10 Measurement Uncertainty 10,000 LBF LOAD CELL TYPE A UNCERTAINTY.5 LBF NMI Primary Standards Accredited Cal. Lab Working Standards Field Measurement NIST ( %) TOTAL TEST UNCERTAINTY LBF MOREHOUSE (0.001 %) TOTAL TEST UNCERTAINTY LBF SECONDARY STANDARDS (0.02 %) TOTAL TEST UNCERTAINTY LBF WORKING STANDARDS (0.1 %) TOTAL TEST UNCERTAINTY LBF FIELD MEASUREMENT (1 %) TOTAL TEST UNCERTAINTY LBF 10

11 Measurement Uncertainty BIPM/SI NMI Primary Standards Accredited Cal. Lab Working Standards Field Measurement The further away from calibration by primary standards the larger the Overall Uncertainty will become 11

12 Documented Procedure Documented Procedure Written description of a prescribed course of action or process. Per ANSI/ISO/IEC The laboratory shall use the appropriate methods and procedures for all tests and/or calibrations within its scope. 12

13 Measurement Assurance Measurement Assurance -Practices put in place to monitor a testing or calibration process and to ensure the calibration status of equipment, reference standards or reference materials used in a measurement process. 13

14 Measurement Assurance Measurement Assurance is essential to holding together the entire chain of traceability as its purpose is to prevent incorrect results from being reported 14

15 Measurement Assurance Monitoring the measurement process Monitoring the: Calibration status of calibrated equipment Calibration status of calibrated Reference Standard(s) Calibration status of Reference Material(s) A combination of monitoring both the measurement process and/or the equipment or the reference(s) A means of strengthening measurement traceability A means of verifying the estimated measurement uncertainty 15

16 Measurement Assurance ISO/IEC 17025, Section 5 Technical Requirements When intermediate checks are needed to maintain the confidence in the calibration status of the equipment, these checks shall be carried out according to a defined procedure. 16

17 Measurement Assurance ISO/IEC 17025, Section 5 Technical Requirements Intermediate checks Checks needed to maintain confidence in the calibration status of reference, primary, transfer, or working standards and reference materials shall be carried out according to defined procedures and schedules. 17

18 Measurement Assurance ISO/IEC 17025, 5 Technical The laboratory shall have quality control procedures for monitoring the validity of tests and calibrations undertaken. The resulting data shall be recorded in such a way that trends are detectable and, where practicable, statistical techniques shall be applied to the reviewing of the results. This monitoring shall be planned and reviewed and may include, but not be limited to, the following: 18

19 Measurement Assurance ISO/IEC 17025, 5 Technical Quality control data shall be analyzed and, where they are found to be outside pre-defined criteria, planned action shall be taken to correct the problem and to prevent incorrect results from being reported. 19

20 Technical Competence Technical Competence - is the ability of a person to do their job comparatively and on the same level as other professionals in their field. Per ANSI/ISO/IEC The laboratory shall ensure the competence of all who operate specific equipment, perform tests and or calibrations, evaluate results, and sign test reports and calibration certificates. 20

21 Demonstrating Technical Competence 21 Proficiency testing determines the performance of individual laboratories for specific tests or measurements and is used to monitor laboratories continuing performance

22 Realization of SI Units Realization of SI Units Force is derived from SI units of Mass; length; and time. CIPM/BIPM defines 1N as the force required to accelerate one kg to one meter per second per second in a vacuum 22

23 Periodic Recalibration Per ANSI/NSCL Z540.3 Calibration intervals and/or process controls shall be established for measuring and test equipment that are included in the calibration system to monitor and maintain equipment performance to the stated application requirements. 23

24 Calibration Traceability The Torque Machine at Morehouse belonged to a NMI (National Physical Laboratory) Morehouse uses dead weight primary standards up to 120,000 LBF accurate to.002 % of applied force. 24

25 NIST 8-Step Process for Estimating and Reporting Measurement Uncertainty 1: Specify the measurement process 2: Identify uncertainty components 3: Quantify uncertainty components 4: Convert quantities to standard uncertainties 5: Calculate combined standard uncertainty 6: Expand the combined standard uncertainty by coverage factor (k) 7: Evaluate the expanded uncertainty 8: Report the uncertainty 25

26 Calibration Defined Calibration is the comparison of an unknown (typically referred to as the Unit Under Test or UUT) to a device known within a certain error(typically referred to as the Calibration Standard or Reference Standard) for the purpose of characterizing the unknown Operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication. Calibration Standards in regards to ASTM E74 are typically characterized as either Primary or Secondary Standards 26

27 Primary Force Standard (as defined by ASTM E74-13) Primary Force Standard a deadweight force applied directly without intervening mechanisms such as levers, hydraulic multipliers, or the like, whose mass has been determined by comparison with reference standards traceable to national standards of mass To be a classified as a primary standard the masses of the weights shall be determined within % of their values by comparison with reference standards traceable to national standards of mass 27

28 Primary Force Standard (as defined by ASTM E74-13) Require correction for the effects of Local Gravity Air Buoyancy Must be adjusted to within % or better (N.I.S.T weights are adjusted to within U = %, Morehouse U= %) Made from stable materials with good surface finish (Stainless Steel preferred material) 28

29 Secondary Force Standard (as defined by ASTM E74-13) Secondary Force Standard an instrument or mechanism, the calibration of which has been established by comparison with primary force standards. In order to perform calibrations in accordance with ASTM E74 your force standard must be calibrated with primary standards 29

30 Secondary Force Standard (as defined by ASTM E74-13) Secondary Force Standard Range of use limited by loading ranges established by the standard ASTM E74 Class AA Load Range for calibration of secondary standard load cells. This is found by multiplying the lower limit factor by 2000 (.05 %) 5:1 ratio ASTM E74 Class A Load Range for calibration of testing machine. This is found by multiplying the lower limit factor by 400 (.25 %) 4:1 ratio. Range of use cannot be less than the lowest applied force. Loading range cannot be less than 400 for Class A or 2000 for Class AA times the resolution. 30

31 Calibration Preparation - Stabilization Temperature Stabilization It is recommended that a device be kept in the area or lab where it is to be calibrated for the device to stabilize in the environment. A good rule of thumb is to allow 24 hours for temperature stabilization. Recommended Temperature is 23 degrees C Electrical Stabilization Depending on the equipment common practice is to allow minutes to warm up. Exercise the instrument to be calibrated. The instrument should be set up in the machine and exercised to the maximum force that is to be applied during the actual calibration. Typically we recommend 3-4 exercise cycles; most standards require a minimum of 2 exercise cycles. 31

32 Calibration In Accordance with ASTM E74 At least 30 force applications are required (we typically recommend 3 runs of 11 or 33 force applications) At least 10 must be different forces and each force must be applied at least twice. Either 15 forces applied twice for 30 force applications or 11 forces applied 3 times for 33 force applications. There should be at least one calibration force for each 10% interval throughout the loading range and if the instrument is to be used below 10% of its capacity a low force should be applied. This low force must be greater than the resolution of the device multiplied by 400 for Class A or 2000 for Class AA devices 32

33 Calibration Temperature ASTM E74 requires that the temperature be monitored during calibration as close to the device as possible and that the temperature change not exceed +/- 1 degree C during calibration. Temperature corrections must be applied to non-compensated devices. Deflection generally increases by % for each 1 degree C increase in temperature. If the calibration laboratory is not operating at 23 degrees C they should make corrections by correcting the applied force accordingly. 33

34 Calibration In Accordance with ASTM E74 Randomization of Loading Conditions Shift or rotate the UUT in the calibration machine before repeating any series of forces (suggestion is to rotate 0, 120 and 240 degrees) For Tension and Compression calibration, intersperse the loadings. Be sure to re-exercise the UUT prior to any change in setup. Zero Return during calibration - This is lab-dependent and it is recommended that no more than 5 forces be applied before return to zero. 34

35 Calibration In Accordance with ASTM E74 Deflection calculation Methods Method B Deflection readings should be calculated as the difference between readings at the applied force and the average or interpolated zero force readings before and after the applied force readings. Method A Deflection readings are calculated as the difference between the deflection at the applied force and the initial deflection at zero force. 35

36 Number of Calibration Values 30 +points reduces standard measurement error T distribution = sample vs mean, Z = population 36

37 Calibration In Accordance with ASTM E74 LOAD REVERSAL OR DESCENDING LOADING If a force measuring device is to be used to measure forces during decreasing load sequences, then it must be calibrated in this manner. Separate calibration curves can be used for Ascending values and Descending Values A combined curve may also be used though the STD DEV of the combined curve will be much higher than using separate curves. 37

38 Calibration In Accordance with ASTM E74 Criteria for Use of Higher Degree Curve Fits Resolution must exceed 50,000 counts An F distribution test is used to determine the appropriate best degree of fit (instructions for this test can be found in the Annex A1 of the ASTM E74 Standard) The Standard deviation for the established curve fit is calculated as before using all the individual deflection values 38

39 Calibration In Accordance with ASTM E74 Criteria for Lower Load Limit Uncertainty = 2.4 * STD DEV This corresponds to a 99 % Coverage Factor Based on Uncertainty or Resolution whichever is higher Class A 400 times the uncertainty or resolution Class AA 2000 times the uncertainty or resolution NOTE: Any instrument that is either modified or repaired should be recalibrated Recalibration is required for a permanent zero shift exceeding 1.0 % of full scale 39

40 Calibration In Accordance with ASTM E74 Recalibration Interval Secondary Standards should be calibrated or verified annually to ensure that they do not change more than.032 % over the loading range Instruments used as Class A devices (Typically used to calibrate testing machines) should be calibrated or verified annually to ensure that they do not change more than.16 % over the loading range. If the Calibration device is stable to within.16 % over the loading range then the calibration interval can be 2 years as long as the UUT continues to meet the stability criteria 40

41 ASTM E74 Calibration Data Analysis Deviations from the fitted curve These are the differences between the fitted curve and the observed values Standard Deviation is the square root of the sum of all the deviations squared/n-m-1 N = sample size, m = the degree of polynomial fit Calibration equation Deflection or Response = A0+A1(load)+A2(load)^2+ A5(load)^5 LLF is 2.4 times the standard deviation Class A range is 400 times the LLF. Class AA range is 2000 times the LLF. 41

42 Calibration In Accordance with ASTM E74 Substitution of Electronic Instruments The indicating device used in the original calibration and the device to be substituted shall have been calibrated and the measurement uncertainty determined The uncertainty of each device shall be less than 1/3 of the uncertainty for the force measurement system. Excitation amplitude, wave form, and frequency shall be maintained Cable substitutions should be verified with a transducer simulator 42

43 Calibration In Accordance with ASTM E74 Allow UUT to come to room temperature Warm up Instrumentation Select Test points Fixture UUT in Test Frame Exercise UUT 2-4 times Apply 1 st series of forces (Run1) Rotate the UUT 120 degrees if possible for run 2 Apply 2 nd series of forces (Run2) IF UUT IS COMPRESSION AND TENSION SWITCH TO OTHER MODE AFTER FINISHING RUN 2 AND EXERCISE AND REPEAT ABOVE STEPS Rotate the UUT another 120 degrees if possible for run 3 Apply 3rd series of forces (Run3) 43

44 Accuracy and Precision It is a common mistake to assume that an accurate device is precise or that a precise device is accurate. 44

45 Accuracy Accuracy is the function of bias and precision (estimate of a systematic measurement error) The Qualitative term for the extent of the approximation of the measurement result to a true value (DIN55350) Accuracy does not tell you about the quality of the instrument. 45

46 Bias Measurement bias (bias): estimate of a systematic measurement error (VIM 2.18) Systematic Measurement Error: component of measurement error that in replicate measurements remains constant or varies in a predictable manner (VIM 2.17) Measurement Error: measured quantity value minus a reference quantity value (VIM 2.16) 46

47 Bias 47

48 Bias one part of Accuracy Accuracy - Influenced by both the precision and bias in a measurement process Accuracy: Closeness of agreement between a measured quantity value and a true quantity value of a measurand. (VIM 2.13) Between an individual measurement and the true value Defined with reference to a true value which is unknown Qualitative term If using a conventional true value, the accuracy can be known 48

49 Precision Precision is a measure of spread Precision refers to the repeatability of measurement. 49

50 Precision Example Example: 500 LBF was applied 3 times using a dead weight primary standard to a load cell and the load cell indicator s recorded output was LB, LB, LB. Conclusion: The instrument is precise to +/-.01 LB when 500 LBF is applied. 50

51 Bias and Precision A load cell may not give you the same measurement result at different orientations. A load cell may repeat when a force is applied, but may not read close to the desired engineering unit s value. 51

52 Accuracy Example Example: A Force Gauge was calibrated against a dead weight primary standard and at LBF the recorded output on the indicator was LBF. This measurement was repeated and LBF was observed a second time. The instrument was re positioned and LBF was observed a third time. This force gauge was determined to be accurate to +.1 % of full scale or +.5 LBF. Note: From this example the resolution of the instrument may be.1 LBF or.5 LBF and although the resolution may be the same as the accuracy in this example they are different. 52

53 Resolution Resolution - the smallest amount of input signal change that the instrument can detect reliably Resolution - is the ability of the measurement system to detect and faithfully indicate small changes in the characteristic of the measurement result. 53

54 Uncertainty of Force Standards Regardless of the force facility to be used, it is important to evaluate the uncertainty of the system. This should include contributions from all influencing parameters (e.g. mass, alignment, and environmental factors). The factors or influences to be reflected in calculation of the uncertainty differ between standards as well as processes. 54

55 Accuracy vs. Uncertainty Accuracy determined via a calibration is not the same as uncertainty! an accurate measurement with a large uncertainty is possible. 55

56 A2LA Policy R205 A2LA POLICY R205: SPECIFIC REQUIREMENTS: CALIBRATION LABORATORY ACCREDITATION A2LA Policy R205 states Every measurement uncertainty shall take into consideration the following standard contributors, even in the cases where they are determined to be insignificant, and documentation of the consideration shall be made: 56

57 These uncertainty contributors are : Repeatability (Type A) Resolution Reproducibility Reference Standard Uncertainty Reference Standard Stability Environmental Factors 57

58 Uncertainty a measurement is not very meaningful unless there is some way of estimating the associated uncertainty good analyses should include uncertainty estimates 58

59 Uncertainty Per section of the GUM The word uncertainty means doubt, and thus in the broadest sense Uncertainty of measurement means doubt the validity of the result of a measurement. 59

60 Uncertainty Uncertainty is a measure of how close a particular test result, the product of one laboratory, is to the true value. There is no assurance that any laboratory using the same test method will have the same accuracy; some will be better and some worse. Specifications of apparatus and materials in test methods attempt to control uncertainty but cannot guarantee a value. 60

61 Uncertainty In Metrology, two common types of uncertainty evaluations are Type A and Type B. 61

62 Uncertainty Type A Uncertainty - The method of evaluation of uncertainty by the statistical analysis of a series of observations. Repeatability condition of measurement, Intermediate precision condition of measurement, Reproducibility condition of measurement 62

63 Uncertainty ASTM E2428 (TORQUE) and ASTME74 (FORCE) calibration test for the reproducibility and repeatability condition of measurement and is an example of Type A Uncertainties. The term used in these standards is Lower Limit Factor which applies a coverage factor of 2.4 If the equipment used to perform the test has a relatively low overall uncertainty then a large percentage of the TTU (Total Test Uncertainty) will be quantified with reproducibility and repeatability 63

64 Uncertainty Type A Example A series of measurements are taken to determine the Type A uncertainty of the measurement

65 Uncertainty Type B Uncertainty - method of evaluation of uncertainty by means other than the statistical analysis of a series of observations. Type B uncertainty - Evaluation based on information associated with a quantity value of a certified reference material, - obtained from a calibration certificate or manufacture s specifications, obtained from the accuracy class of a verified measuring instrument, obtained from limits deduced from a test or experiment. Examples include torque or load cell temperature effect, drift, resolution, etc. 65

66 Uncertainty Type B evaluation method: The method of evaluation of uncertainty of measurement by means other than statistical analysis of a series of observations. Examples: Based on specification History of parameter Other knowledge or test of the process parameter 66

67 Uncertainty Type B Examples The temperature effect on force or torque cell output is +/-.004 % per degrees Celsius The specification of the torque arm is +/ inches Bending, cross-force, cosine error, etc. 67

68 Uncertainty So if we are dealing with A component of Uncertainty it is either Type A or Type B, If we have both Type A and Type B uncertainties and RSS (root sum square) then this becomes a Combined Standard Uncertainty and if we apply a coverage factor to the Combined Uncertainty this becomes our Overall Uncertainty also known as Expanded Uncertainty. We will go over an expanded uncertainty analysis in the next section. 68

69 Uncertainty Determining the Uncertainty of a Measurement (UOM) is different from the practice of Determining the Expected Performance of a Device. Determining the Expected Performance of a Device (which includes the De-Rating Specifications associated with its performance and which are NOT part of the Measurement Uncertainty Analysis performed/calculated to determine the UOM any more than the Base EP (Expected Performance) of the device is included in the UOM). In other words the manufacturer s specification is not to be used in place of the uncertainty. 69

70 Uncertainty Example The next example will deal with an example derived from an uncertainty analysis that is in the ASTM E74-13a standard as an appendix. Repeatability, Reproducibility, and Resolution are all accounted for in the ASTM E74 uncertainty or LLF (Lower Limit Factor). 70

71 Uncertainty Example We will gather some necessary information and run through a sample expanded uncertainty calibration. We will need the following: 1. Calibration Report for the Device 2. The uncertainty of the instrument(s) that were used to perform the calibration 3. Calibration History (if available) 4. Manufacturer s Specification Sheet 5. Error Sources, if known 6. Dissemination Error, if known 71

72 Performance & Uncertainty for Calibration performed by Morehouse Type A Uncertainty To do a type A uncertainty analysis, information will be needed from the calibration report. The ASTM E74 LLF or Uncertainty from the report should be entered into the spreadsheet. In ASTM E74-13a, Uncertainty has been changed to LLF (Lower Limit Factor). 72

73 Uncertainty Example Type A and B uncertainty analysis COMPANY LOAD CELL MANUFACTURER SAMPLE MOREHOUSE ENTER LOAD CELL S/N P-7768 YOUR CAPACITY LBF CALIBRATION ASTM E74 Uncertainty for K= LBF INFORMATION THE LOWEST FORCE AT WHICH THE SECONDARY STANDARD WILL BE USED 1000 LBF IN PRIMARY FORCE CALIBRATION STANDARD UNCERTAINTY K= % HIGHLIGHTED PRIMARY ELECTRICAL CALIBRATION STANDARD UNCERTAINTY K=1 (IF APPLICABLE) COLUMNS CAL DATE 10/27/2010 Using the Excel sheet available at content/uploads/2012/05/type-a-and-b-uncertainty-analysis- ASTM-E74-1.xls We will enter the load cell S/N, Capacity, ASTM Uncertainty which was.237 LBF, Lowest force the standard will be used, and the Uncertainty of the standard used to perform the calibration at k=1. 73

74 Uncertainty Example Type A and B uncertainty analysis COMPANY LOAD CELL MANUFACTURER SAMPLE MOREHOUSE ENTER LOAD CELL S/N P-7768 YOUR CAPACITY LBF CALIBRATION ASTM E74 Uncertainty for K= LBF INFORMATION THE LOWEST FORCE AT WHICH THE SECONDARY STANDARD WILL BE USED 1000 LBF IN PRIMARY FORCE CALIBRATION STANDARD UNCERTAINTY K= % HIGHLIGHTED PRIMARY ELECTRICAL CALIBRATION STANDARD UNCERTAINTY K=1 (IF APPLICABLE) COLUMNS CAL DATE 10/27/2010 The uncertainty of the standard or standards that were used to perform the calibration should be found somewhere on the certificate of calibration with a coverage factor. This coverage factor is typically 2, so you will need to reduce this to k=1. 74

75 Uncertainty Example Type A Uncertainty % Uncertainty Description Uncertainty Distribution Divisor Standard Uncertainty Squared ASTM E74 Uncertainty % at the lowest calibration force to be used % normal E E-09 Combined Type A Uncertainty 9.88E E-09 The information from the ASTM E74 report is your Type A uncertainty. To get this, we are dividing the uncertainty or LLF by the lowest force this instrument is going to be used at. Then we divide by 2.4 the ASTM E74 coverage factor to reduce this to the uncertainty or LLF at k=1 (above). 75

76 Uncertainty Example Type B Uncertainty % Uncertainty Description Uncertainty Distribution Divisor Standard Uncertainty Squared PRIMARY FORCE CALIBRATION STANDARD UNCERTAINTY 0.001% rectangular E E-11 PRIMARY ELECTRICAL CALIBRATION STANDARD UNCERTAINTY (IF APPLICABLE) 0.000% rectangular E E+00 STABILITY OF THE SECONDARY FORCE STANDARD OVER TIME 0.005% rectangular E E-10 CREEP ERROR FOUND ON LOAD CELL SPEC SHEET 0.002% rectangular E E-11 MISALIGNMENT ERROR (SEE ASTM E1012) AND/OR SIDE LOAD SENSITIVITY FROM LOAD CELL SPEC SHEET 0.005% rectangular E E-10 DISSEMINATION ERROR (FOR CALIBRATION LABORATORIES) 0.005% rectangular E E-10 TEMPERATURE ERROR +/- FROM LOAD CELL SPEC SHEET % rectangular E E-11 Combined Type B Uncertainty 5.18E E-09 Next, we will be looking at the type B uncertainty. If the system was calibrated in accordance with ASTM E74 and consisted of a meter and load cell, then we will treat the ASTM E74 uncertainty or LLF as a system uncertainty, and there would not be a need to look at the Electrical Calibration Standard Uncertainty. 76

77 Uncertainty Example PRIMARY FORCE CALIBRATION STANDARD UNCERTAINTY 0.001% rectangular E E-11 PRIMARY ELECTRICAL CALIBRATION STANDARD UNCERTAINTY (IF APPLICABLE) 0.000% rectangular E E+00 The first type B uncertainty component to examine is the uncertainty of the standards used to perform the calibration. The can usually be found on the calibration laboratory s scope of accreditation. In this example, the load cell was sent in with an indicator, so we will only consider the Primary Force Standard Uncertainty, which was dead weight with an uncertainty of 0.001% for K=1. 77

78 Uncertainty Example STABILITY OF THE SECONDARY FORCE STANDARD OVER TIME 0.005% rectangular E E-10 This can be determined by comparing the previous calibration with the current calibration. The # to be used should be the number of the lowest calibration force that will be used for calibration. If you do not have any previous calibration data, then the suggestion would be to contact the manufacturer or use a conservative number based on similar systems. 78

79 Uncertainty Example Calibration History can be found from taking the difference from one calibration to the next, or looking at several calibrations, if available. In this example, I would opt to use the % change of %. This is the highest % change throughout the loading range. 79

80 Uncertainty Example A less conservative way to approach change from previous would be to take the Standard Deviation of all of the change from previous numbers. 80

81 UNCERTAINTY EXAMPLE CREEP ERROR FOUND ON LOAD CELL SPEC SHEET 0.002% rectangular E E-11 Creep error This can usually be found on the manufacturer s spec sheet, and is usually % reading for 20 minutes. Since we typically hold the force for around 30 seconds when performing the calibration, the creep error is much lower. If the end user replicates holding the force for 30 seconds, then the creep error of the system should be better than %. A creep test can be performed and is included in the new ASTM revision for those using method A. 81

82 Uncertainty Example MISALIGNMENT ERROR (SEE ASTM E1012) AND/OR SIDE LOAD SENSITIVITY FROM LOAD CELL SPEC SHEET 0.005% rectangular E E-10 A well aligned calibration machine may demonstrate bending less than 2 %. The % can usually be found on the load cell spec sheet under Side Load Sensitivity. Note: If using a Morehouse UCM and Morehouse Ultra Precision Load Cell the Morehouse press will transfer the force applied to the load cell at an angle of no more than 1/16 th inch measured off centerline of the load cell. (This number is usually 0.05 % *.0625 =.003%) 82

83 Uncertainty Example DISSEMINATION ERROR (FOR CALIBRATION LABORATORIES) 0.005% rectangular E E-10 Dissemination Error Assuming we have compared results with primary standards accurate to % of applied force, and we achieved actual measurement results comparing 2 standards, each calibrated with primary standards against one another that suggested our measurements to be within %, we will use this number for Dissemination Error. 83

84 Uncertainty Example TEMPERATURE ERROR +/- FROM LOAD CELL SPEC SHEET % rectangular E E-11 Temperature Error - This is found from the load cell spec sheet. It is usually in terms of % of reading/100 per degree F or C. This number should then be multiplied by the maximum temperature difference from the temperature at which the calibration was performed. If the manufacturer s spec sheet suggests.0015 % per degree C and you are operating within +/- 1 degree, then use this number. If you are +/- 2 Degrees C, then use.003 %. 84

85 Type A and B uncertainty analysis COMPANY LOAD CELL MANUFACTURER SAMPLE MOREHOUSE ENTER LOAD CELL S/N P-7768 YOUR CAPACITY LBF CALIBRATION ASTM E74 Uncertainty for K= LBF INFORMATION THE LOWEST FORCE AT WHICH THE SECONDARY STANDARD WILL BE USED 1000 LBF IN PRIMARY FORCE CALIBRATION STANDARD UNCERTAINTY K= % HIGHLIGHTED PRIMARY ELECTRICAL CALIBRATION STANDARD UNCERTAINTY K=1 (IF APPLICABLE) COLUMNS CAL DATE 10/27/2010 CALCULATED VALUES LOAD CELL UNCERTAINTY IN % FOR FULL SCALE K= % LOAD CELL UNCERTAINTY IN % FOR FULL SCALE K= % LOAD CELL UNCERTAINTY IN % FOR LOWEST FORCE APPLIED K = % LOAD CELL UNCERTAINTY IN % FOR LOWEST FORCE APPLIED K = % Type A Uncertainty % Uncertainty Description Uncertainty Distribution Divisor Standard Uncertainty Squared ASTM E74 Uncertainty % at the lowest calibration force to be used % normal E E-09 Combined Type A Uncertainty 9.88E E-09 Type B Uncertainty % Uncertainty Description Uncertainty Distribution Divisor Standard Uncertainty Squared PRIMARY FORCE CALIBRATION STANDARD UNCERTAINTY 0.001% rectangular E E-11 PRIMARY ELECTRICAL CALIBRATION STANDARD UNCERTAINTY (IF APPLICABLE) 0.000% rectangular E E+00 STABILITY OF THE SECONDARY FORCE STANDARD OVER TIME 0.005% rectangular E E-10 CREEP ERROR FOUND ON LOAD CELL SPEC SHEET 0.002% rectangular E E-11 MISALIGNMENT ERROR (SEE ASTM E1012) AND/OR SIDE LOAD SENSITIVITY FROM LOAD CELL SPEC SHEET 0.005% rectangular E E-10 DISSEMINATION ERROR (FOR CALIBRATION LABORATORIES) 0.005% rectangular E E-10 TEMPERATURE ERROR +/- FROM LOAD CELL SPEC SHEET % rectangular E E-11 Combined Type B Uncertainty 5.18E E-09 Review of everything we entered 85

86 Uncertainty Example FOR K= 1 UC= SQUARE ROOT OF TOTAL COMBINED TYPE A AND B % FOR K=2 U = K * UC (UNCERTAINTY % AT THE LOWEST FORCE TO BE APPLIED) 0.022% LBF Summary UC AT CAPACITY * 2 ( (UNCERTAINTY % AT INSTRUMENT CAPACITY) 0.011% LBF After all the data has been entered, the Big U or Expanded Uncertainty for this 10K load cell that had an ASTM E74 uncertainty or LLF of.237 LBF (K=2.4) is now LBF for K=2 at full scale capacity. 86

87 Uncertainty Example This example is just a guideline for calculating expanded uncertainty. The actual uncertainty components in your system may vary. In addition to this example, there is also an Uncertainty example for torque in the next section. 87

88 Uncertainty Example Using handouts and the material provided in class we will fill out an excel spreadsheet. 88

89 Uncertainty -SINGLE RUN CALIBRATION 89

90 Uncertainty -SINGLE RUN CALIBRATION APPLIED LBF FORCE MEASURED LBF INSTRUMENT ERROR in LBF Calculated Residual % Error % % % % % % % % % % Uncertainty %

91 Types of Load Cells Column Load Cell (Single-Column or High- Stress Load Cells) Multi-Column Load Cell S-Beam or S-Type Shear Web 91

92 Column Load Cell The spring element is intended for axial loading, and typically has a minimum of four strain gauges, two in the longitudinal direction, and two oriented transversally to sense the Poisson strain. 92

93 Column Load Cell Column load cells have a reputation for inherent non-linearity. This deviation from linear behavior is commonly ascribed to the change in the crosssectional area of the column (due to Poisson s ratio), which occurs with deformation under load. Column cells also exhibit a larger creep characteristic than other cells and often do not return to zero as well as other cells. The advantage of a column cell is that at higher capacities, its physical size and weight will make the cell more portable. It is not uncommon to have a 1,000,000 LBF column cell weigh less than 100 lbs. 93

94 Multi - Column Load Cell In this type of design, the load is carried by four or more small columns, each with its own complement of strain gauges. The corresponding gauges from all of the columns are connected in a series in the appropriate bridge arms

95 Multi - Column Load Cell 95

96 Multi - Column Load Cell Multi-Column load cells can be more compact than high-stress column cells and offer improved discrimination against the effects of off-axis load components. These cells typically have less creep and have better zero returns than single-column cells. In many cases, a properly designed shear-web spring element can offer greater output, better linearity, lower hysteresis, and faster response. 96

97 S-beam Load Cell This type of design is often used in weighing applications. There are four gauges placed inside the beam. In general, linearity will be enhanced by minimizing the ratio of deflection (at rated load) to the length of the sensing beam, thus minimizing the change in shape of the element. These cells are very sensitive to off-axis loading and ideally suited for scales or tension applications. Ideal for measuring small forces (under 50 LBF) when physical weights cannot be used. 97

98 Shear Web Load Cell This type of load cell is typically the most accurate when installed on a tapered base with an integral threaded rod installed. These cells typically have very low creep and are not as sensitive to offaxis loading as the other cells discussed. These cells would be the recommended choice for force applications from 100 LBF through 100,000 LBF. After 100,000 LBF, the weight of the cell makes it very difficult to use as a field standard. A 100,000 LBF Shear Web cell weighs approximately 57 lbs and a 200,000 LBF shear web cell weighs over 140 lbs. 98

99 Shear Web Load Cell If these cells are used without a base or without an integral top adapter, there may be significant errors associated with various loading conditions. 99