Coverage factor in calibration certificate for a multimeter

A

akmal122g

#1
Hello. I am want to issue a calibration certificate for a multimeter. I am not sure about which coverage factor to state in the certificate. I used 95% confidence level in my calibration.

For resistance my coverage factor is 2.776, for voltage the factor is 2.571 and for current the factor is 3.182. I have seen many calibration certificates that state only one coverage factor. Do I need to state all these coverage factors in the calibration certificate or choose the smallest factor. Can someone advice me?
 
Elsmar Forum Sponsor

dgriffith

Quite Involved in Discussions
#2
You only need one if all measurement uncertainty quantities are indeed at that level. However, not sure where you got the posted factors. Here is a generalization of how it should go:
All measurement and influence quantities are reduced to their standard uncertainties, U.
They are then added together in quadrature (root sum square): this becomes the combined standard uncertainty, Uc.
The result is then multiplied by the coverage factor, in most cases 2. This becomes the expanded uncertainty of 2Uc with a coverage factor of approximately 95% (k=2).
 
Last edited:

dgriffith

Quite Involved in Discussions
#3
. . . All measurement and influence quantities are reduced to their standard uncertainties, U.
When I say this I mean each element that will be considered in the uncertainty budget already has an assumed distribution. The distribution will determine what divisor to use when reducing to the standard uncertainty; *this is how you are able to combine different distribution types. The standard uncertainties are squared, summed and rooted for the combined standard uncertainty.

*This should be done for each test point in each measurement type: resistance, voltage, current, etc. Since all types will be accomplished in the same or similar manner, all expanded uncertainties will be at the same coverage, so only one statement should be needed for the cert.
 
Last edited:
A

akmal122g

#4
Hello. I got the coverage factors by calculating the the Effective Degree of Freedom from Welcsh-Satterthwaite Formula and applying the result to Student's T-distribution Table to get the coverage factor.

For resistance my coverage factor is 2.776 at 95% confidence level because the result for Effective Degree of Freedom is 4.

For voltage my coverage factor is 2.571 at 95% confidence level because the result for Effective Degree of Freedom is 5 and for current is 3.182 because the Effective Degree of Freedom is 3.
 

dgriffith

Quite Involved in Discussions
#5
Hello. I got the coverage factors by calculating the the Effective Degree of Freedom from Welcsh-Satterthwaite Formula and applying the result to Student's T-distribution Table to get the coverage factor.

For resistance my coverage factor is 2.776 at 95% confidence level because the result for Effective Degree of Freedom is 4.

For voltage my coverage factor is 2.571 at 95% confidence level because the result for Effective Degree of Freedom is 5 and for current is 3.182 because the Effective Degree of Freedom is 3.
Goodness knows I'm no expert, which is why I paid handsomely for software written by an expert to get expert results--well, I like the illusion of it, anyway . . .
Short version: You do not want those factors. You only want the sample standard deviations.

Long-winded version: You have taken a sample of size X to get a measurement average, 3,4 or 5 and so on. The df is n-1 for a sample average, so a sample of 5 has a df of 4. If I had samples describing a linear curve, I might have 10 points, and since the curve fit requires zero and slope, there are n-2 df; df=8, and so on. I'm not sure we have to get Mr.'s Welch and Satterthwaite out of bed for this. Somebody tell me if this is incorrect.

Your t-table will give the factors you have for the df you stated. Those factors multiplied by the standard deviation of the sample will give you the error limits for the sample average at the stated confidence, and perhaps you could use this if you were to compare different samples with each other. That and all the other characterizations like kurtosis and halitosis and skewness and so on just describe the quality of the sample or some-such. But I think this is the wrong path for us.

The standard deviation of the sample is the standard uncertainty we want. This is the value we will use to combine with the others. Remember, we are combining standard uncertainties. Then I think we will use W-S to estimate the combined df from all of the budget elements.
 
Thread starter Similar threads Forum Replies Date
D Influence of the Coverage Factor - Worksheets based on the Average and Range Method Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 7
A Coverage and differences: EN 60601-1:2006+A12:2014 Vs AAMI/IEC 60601-1:2005+AMD1:2012 IEC 60601 - Medical Electrical Equipment Safety Standards Series 2
R Impact of an MDSAP certificate with all countries coverage Other Medical Device Regulations World-Wide 8
A Internal Audit - Coverage for ISO 9001:2015 Internal Auditing 4
B ISMS Certified - Any requirement for Insurance Coverage for Infrastructure Assets? IEC 27001 - Information Security Management Systems (ISMS) 4
K ISO9001:2008 Internal Audit Period Coverage Internal Auditing 18
J Code Coverage in VHDL and Verilog for Medical Devices IEC 62304 - Medical Device Software Life Cycle Processes 17
V Coverage of Process Failures in dFMEA (Scope/Boundaries for Inclusion) & vice versa Pharmaceuticals (21 CFR Part 210, 21 CFR Part 211 and related Regulations) 1
DanteCaspian Operator Break Coverage - Custom injection mould shop Manufacturing and Related Processes 9
G Coverage of ISO 14001 within an organization - Implementation ISO 14001:2015 Specific Discussions 5
N Painting experts: Coverage area for each litre of paint and thinner mix Manufacturing and Related Processes 5
C ISO 14001 Coverage and exclusion - Testing of electonics devices ISO 14001:2015 Specific Discussions 2
B Opinions on the best conference to go to in terms of ISO 14001 coverage ISO 14001:2015 Specific Discussions 7
E 60601-1 - Tilt testing - Tensile safety factor IEC 60601 - Medical Electrical Equipment Safety Standards Series 1
S Minitab - Factor Analysis: Label on second series of data for Biplot Using Minitab Software 4
S Is Human Factor Testing required as part of Design Control Validation? 21 CFR Part 820 - US FDA Quality System Regulations (QSR) 3
V Using K-Factor(Tolerance Interval) Analysis for Design Verification Statistical Analysis Tools, Techniques and SPC 3
C FOS (Factor of Safety) Medical Device Other Medical Device and Orthopedic Related Topics 4
E IEC 60601-1 Tensile Load Safety Factor IEC 60601 - Medical Electrical Equipment Safety Standards Series 1
E GR&R ANOVA 3rd Factor for Fixture Nests Using Minitab Software 3
N 3 factor - 3 level experiment with DoE and Minitab Using Minitab Software 10
S Human Factor Summative Study Class III Device RA Checklist wanted Human Factors and Ergonomics in Engineering 2
M DOE: Significant factor or not Using Minitab Software 2
F Can be several measurement in a repl considered as nested factor in Minitab analysis Using Minitab Software 14
M Taguchi Analysis of Discrete Factor Using Minitab Software 10
N Minimum Design factor for CE Marking for machinery CE Marking (Conformité Européene) / CB Scheme 2
G Setting up a 3 Factor, 2 Level,2 response DOE in Minitab Using Minitab Software 7
Z A DOE in Minitab - 3 factors and 3 levels for each factor Using Minitab Software 12
R How do I extend Correction factor of Calibration cert to compute instrument accuracy Measurement Uncertainty (MU) 1
B DOE Question: 3 factors with 2 levels and 1 factor with 3 levels Six Sigma 2
Z Help with running 3 factor DOE factor optimization Using Minitab Software 2
P Help Setting Up and Analyzing 3 Factor 2 Level Full Factorial Design for DOE Using Minitab Software 3
C 3 Factor DOE (Design of Experiments) Template Document Control Systems, Procedures, Forms and Templates 0
P How to analyze DOE: 2 Factor, 4 Level Experiment Six Sigma 7
J DOE Analysis Experiment, 5 factors, 4 factors having 3 levels, and 1 factor having 5 Using Minitab Software 38
M Multi-factor Anova with Minitab Using Minitab Software 1
K Human Factor Assessment for IRB (Institutional Review Board) Approval? EU Medical Device Regulations 7
I Multiple Comparisons for a Random Factor Using Minitab Software 2
L DOE with 2 factors (3,4 levels) and optimized factor Using Minitab Software 12
P Multi-factor Anova with Minitab Using Minitab Software 11
R What is a Z Value/Factor and where is it most useful? Statistical Analysis Tools, Techniques and SPC 4
T 4 Factor Optimization using Minitab Using Minitab Software 7
I How GUM Uncertainty is calculated and where I locate the multiplication factor b Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 1
L Filled examples of a Factor Tree (PDCA-FTA) Analysis or a Training Document wanted Problem Solving, Root Cause Fault and Failure Analysis 2
N Test for Equal Variances - Two factor data set - Minitab Using Minitab Software 3
S Surface Finish - Conversion factor to transform Rz Jis in Rz Inspection, Prints (Drawings), Testing, Sampling and Related Topics 6
T Setting up a Standard Experimental Plan for Food where Factor Levels are Different Statistical Analysis Tools, Techniques and SPC 2
C DoE and ANOVA - 4 factor and 3 levels Taguchi design with 2 replicates Using Minitab Software 14
C Design of Experiments or Factor analysis using existing data Quality Tools, Improvement and Analysis 7
Y Acceptable Fatigue Safety Factor for Testing Medical Device ISO 13485:2016 - Medical Device Quality Management Systems 2

Similar threads

Top Bottom