Quality Progress - February 2014 - (Page 44)

MEASURE FOR MEASURE BY DILIP SHAH The Right Number How many data points do you need for calibration? RECENTLY, I WAS asked the following calibration process consumes time, and of reference at which the measurement question: "How many data points should time is money. So, understandably, the was made. Other conclusions cannot be I measure when calibrating a voltmeter calibration supplier does not want to derived without prior history or perfor- with a range of 0 to 100 volts?" The make extra unnecessary measurements. mance data. person wanted to know the minimum But, the supplier also wants to ensure number of data points necessary to that an adequate number of measure- made at 30, 60 and 90 volts. As the figure calibrate the voltage range to ensure ments were made to ensure confidence in shows, there can be three separate measurement performance of the the calibration. inferences made about the linearity of Figure 2 shows three measurements the measurement (not including the best instrument was characterized correctly Data patterns fit line estimation). If the measurement It's important to remember that measure- range was nonlinear, it would complicate voltage. To calibrate a voltmeter, you ment equipment may behave in a linear interpretation of data further. need a traceable source device to gener- manner across its whole range, or it may ate a voltage the voltmeter can measure. behave nonlinearly. You may need to made at 20, 40, 60, 80 and 100 volts. Mak- Most measurement devices have a vari- extrapolate our measurement assump- ing four or five measurements across a able range of values they can measure. tions across the range, so it's crucial that range would be the best scenario to deter- Examples include digital multimeters, you know how the measurement device mine the confidence in calibration and to micrometers, dial indicators and weigh- behaves. Find this out by taking the interpolate the other values between the ing balances. The source device may be a adequate number of data points. Consider range measurements. fixed or a variable range device. Exam- the following measurement scenarios as ples of fixed source devices include gage examples. and with confidence. A voltmeter is a device that measures blocks, weights and fixed-value resisters. Figure 3 shows five measurements When the measurement range is nonlinear, it is important to consider the Figure 1 shows a single-point measure- extreme points in the range aside from In the question posed, a variable voltage ment made at the midpoint-at 50 volts the intermediate points. With the advent source device is required for calibration. on the 0 to 100 volt range. This provides of computer spreadsheet software, it is information about that one single point easy to chart data, draw trend lines and Single-point measurement made at the midpoint of 0 to 100 volt range / FIGURE 1 Nominal value 100 90 80 70 60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 110 Measured value 44 QP * www.qualityprogress.com Nominal Measured value value 0 10 20 30 40 50 49.55 60 70 80 90 100 Three measurements made at 30, 60 and 90 volts / FIGURE 2 100 90 80 70 60 50 40 30 20 10 0 Nominal value Taking measurements during the 0 10 20 30 40 50 60 70 80 90 100 110 Measured value Nominal Measured value value 0 10 20 30 29.93 40 50 60 65.03 70 80 90 89.83 100 http://www.qualityprogress.com

Table of Contents for the Digital Edition of Quality Progress - February 2014

Up Front
LogOn
Expert Answers
Keeping Current
Mr. Pareto Head
Learning From Experience
Choose Wisely
A Pathway for Patients
Hoist Help
3.4 per Million
Quality in the First Person
Measure for Measure
Career Corner
Statistics Roundtable
Standards Outlook
SPECIAL SECTION: Six Sigma Resource Guide
QP Toolbox
QP Reviews
Back to Basics

Quality Progress - February 2014

http://www.nxtbook.com/naylor/ASQM/ASQM0719
http://www.nxtbook.com/naylor/ASQM/ASQM0619
http://www.nxtbook.com/naylor/ASQM/ASQM0519
http://www.nxtbook.com/naylor/ASQM/ASQM0419
http://www.nxtbook.com/naylor/ASQM/ASQM0319
http://www.nxtbook.com/naylor/ASQM/ASQM0219
http://www.nxtbook.com/naylor/ASQM/ASQM0119
http://www.nxtbook.com/naylor/ASQM/ASQM1218
http://www.nxtbook.com/naylor/ASQM/ASQM1118
http://www.nxtbook.com/naylor/ASQM/ASQM1018
http://www.nxtbook.com/naylor/ASQM/ASQM0918
http://www.nxtbook.com/naylor/ASQM/ASQM0818
http://www.nxtbook.com/naylor/ASQM/ASQM0718
http://www.nxtbook.com/naylor/ASQM/ASQM0618
http://www.nxtbook.com/naylor/ASQM/ASQM0518
http://www.nxtbook.com/naylor/ASQM/ASQM0418
http://www.nxtbook.com/naylor/ASQM/ASQM0318
http://www.nxtbook.com/naylor/ASQM/ASQM0218
http://www.nxtbook.com/naylor/ASQM/ASQM0118
http://www.nxtbook.com/naylor/ASQM/ASQM1217
http://www.nxtbook.com/naylor/ASQM/ASQM1117
http://www.nxtbook.com/naylor/ASQM/ASQM1017
http://www.nxtbook.com/naylor/ASQM/ASQM0917
http://www.nxtbook.com/naylor/ASQM/ASQM0817
http://www.nxtbook.com/naylor/ASQM/ASQM0717
http://www.nxtbook.com/naylor/ASQM/ASQM0617
http://www.nxtbook.com/naylor/ASQM/ASQM0517
http://www.nxtbook.com/naylor/ASQM/ASQM0417
http://www.nxtbook.com/naylor/ASQM/ASQC12518
http://www.nxtbook.com/naylor/ASQM/ASQM0317
http://www.nxtbook.com/naylor/ASQM/ASQM0217
http://www.nxtbook.com/naylor/ASQM/ASQM0117
http://www.nxtbook.com/naylor/ASQM/ASQM1216
http://www.nxtbook.com/naylor/ASQM/ASQM1116
http://www.nxtbook.com/naylor/ASQM/ASQM1016
http://www.nxtbook.com/naylor/ASQM/ASAC0016
http://www.nxtbook.com/naylor/ASQM/ASQM0916
http://www.nxtbook.com/naylor/ASQM/ASQA0016
http://www.nxtbook.com/naylor/ASQM/ASQM0816
http://www.nxtbook.com/naylor/ASQM/ASQM0716
http://www.nxtbook.com/naylor/ASQM/ASQM0616
http://www.nxtbook.com/naylor/ASQM/ASQM0516
http://www.nxtbook.com/naylor/ASQM/ASQM0416
http://www.nxtbook.com/naylor/ASQM/ASQM0316
http://www.nxtbook.com/naylor/ASQM/ASQM0216
http://www.nxtbook.com/naylor/ASQM/ASQM0116
http://www.nxtbook.com/naylor/ASQM/ASQM1215
http://www.nxtbook.com/naylor/ASQM/ASQM1115
http://www.nxtbook.com/naylor/ASQM/ASQM1015
http://www.nxtbook.com/naylor/ASQM/ASQM0915
http://www.nxtbook.com/naylor/ASQM/ASQM0815
http://www.nxtbook.com/naylor/ASQM/ASQM0715
http://www.nxtbook.com/naylor/ASQM/ASQM0615
http://www.nxtbook.com/naylor/ASQM/ASQM0515
http://www.nxtbook.com/naylor/ASQM/ASQM0315
http://www.nxtbook.com/naylor/ASQM/ASQM0215
http://www.nxtbook.com/naylor/ASQM/ASQM0115
http://www.nxtbook.com/naylor/ASQM/ASQM1214
http://www.nxtbook.com/naylor/ASQM/ASQM1114
http://www.nxtbook.com/naylor/ASQM/ASQM1014
http://www.nxtbook.com/naylor/ASQM/ASQM0914
http://www.nxtbook.com/naylor/ASQM/ASQM0814
http://www.nxtbook.com/naylor/ASQM/ASQM0714
http://www.nxtbook.com/naylor/ASQM/ASQM0614
http://www.nxtbook.com/naylor/ASQM/ASQM0514
http://www.nxtbook.com/naylor/ASQM/ASQM0414
http://www.nxtbook.com/naylor/ASQM/ASQM0314
http://www.nxtbook.com/naylor/ASQM/ASQM0214
http://www.nxtbook.com/naylor/ASQM/ASQM0114
http://www.nxtbook.com/naylor/ASQM/ASQM1213
http://www.nxtbook.com/naylor/ASQM/ASQM1113
http://www.nxtbook.com/naylor/ASQM/ASQM1013
http://www.nxtbook.com/naylor/ASQM/ASQM0913
http://www.nxtbook.com/naylor/ASQM/ASQM0813
http://www.nxtbook.com/naylor/ASQM/ASQM0713
http://www.nxtbook.com/naylor/ASQM/ASQM0613
http://www.nxtbook.com/naylor/ASQM/ASQM0513
http://www.nxtbook.com/naylor/ASQM/ASQM0413
http://www.nxtbook.com/naylor/ASQM/ASQM0313
http://www.nxtbook.com/nxtbooks/naylor/ASQM0213
http://www.nxtbook.com/nxtbooks/naylor/ASQM0113
http://www.nxtbook.com/nxtbooks/naylor/ASQM1212
http://www.nxtbook.com/nxtbooks/naylor/ASQM1112
http://www.nxtbook.com/nxtbooks/naylor/ASQM1012
http://www.nxtbook.com/nxtbooks/naylor/ASQM0912
http://www.nxtbookMEDIA.com