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INTRODUCTION TO METROLOGY

INTRODUCTION TO METROLOGY. DEFINITIONS. Metrology is the study of measurements Measurements are quantitative observations; numerical descriptions. OVERVIEW. This longer lecture explores general principles of metrology

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INTRODUCTION TO METROLOGY

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  1. INTRODUCTION TO METROLOGY

  2. DEFINITIONS • Metrology is the study of measurements • Measurements are quantitative observations; numerical descriptions

  3. OVERVIEW • This longer lecture explores general principles of metrology • Next 3 shorter lectures apply principles to specific measurements: weight, volume, pH • Later will talk about measuring light transmittance (spectrophotometry)

  4. WE WANT TO MAKE “GOOD” MEASUREMENTS • Making measurements is woven throughout daily life in a lab. • Often take measurements for granted, but measurements must be “good”. • What is a “good” measurement?

  5. EXAMPLE • A man weighs himself in the morning on his bathroom scale, 172 pounds. • Later, he weighs himself at the gym,173 pounds.

  6. QUESTIONS • How much does he really weigh?

  7. Do you trust one or other scale? Which one? Could both be wrong? Do you think he actually gained a pound?

  8. Are these “good measurements”?

  9. NOT SURE • We are not exactly certain of the man’s true weight because: • Maybe his weight really did change – always sample issues • Maybe one or both scales are wrong – always instrument issues

  10. DO WE REALLY CARE? • Do you care if he really gained a pound? • How many think “give or take” a pound is OK?

  11. ANOTHER EXAMPLE • Suppose a premature baby is weighed. The weight is recorded as 5 pounds 3 ounces and the baby is sent home. • Do we care if the scale is off by a pound?

  12. “GOOD” MEASUREMENTS • A “good” measurement is one that can be trusted when making decisions. • We just made judgments about scales. • We make this type of judgment routinely.

  13. IN THE LAB • Anyone who works in a lab makes judgments about whether measurements are “good enough” – • but often the judgments are made subconsciously • differently by different people • Want to make decisions • Conscious • Consistent

  14. QUALITY SYSTEMS • All laboratory quality systems are concerned with measurements • All want “good” measurements • Some language is quoted in your lab manual

  15. NEED • Awareness of issues so can make “good” measurements. • Language to discuss measurements. • Tools to evaluate measurements.

  16. METROLOGY VOCABULARY • Very precise science with imprecise vocabulary • (word “precise” has several precise meanings that are, without uncertainty, different) • Words have multiple meanings, but specific meanings

  17. VOCABULARY • Units of measurement • Standards • Calibration • Traceability • Tolerance • Accuracy • Precision • Errors • Uncertainty

  18. UNITS OF MEASUREMENT • Units define measurements • Example, gram is the unit for mass • What is the mass of a gram? How do we know?

  19. DEFINITIONS MADE BY AGREEMENT • Definitions of units are made by international agreements, SI system • Example, kilogram prototype in France • K10 and K20 at NIST

  20. EXTERNAL AUTHORITY • Measurements are always made in accordance with external authority • Early authority was Pharaoh’s arm length

  21. A standard is an external authority • Also, standard is a physical embodiment of a unit

  22. STANDARDS ARE: • Physical objects, the properties of which are known with sufficient accuracy to be used to evaluate other items.

  23. STANDARDS ARE AFFECTED BY THE ENVIRONMENT • Units are unaffected by the environment, but standards are • Example, Pharaoh’s arm length might change • Example, a ruler is a physical embodiment of centimeters • Can change with temperature • But cm doesn’t change

  24. STANDARDS ALSO ARE: • In chemical and biological assays, substances or solutions used to establish the response of an instrument or assay method to an analyte • See these in spectrophotometry labs

  25. STANDARDS ALSO ARE: • Documents established by consensus and approved by a recognized body that establish rules to make a process consistent • Example ISO 9000 • ASTM standard method calibrating micropipettor

  26. CALIBRATION IS: • Bringing a measuring system into accordance with external authority, using standards • For example, calibrating a balance • Use standards that have known masses • Relate response of balance to units of kg • Do this in lab

  27. PERFORMANCE VERIFICATION IS: • Check of the performance of an instrument or method without adjusting it.

  28. TOLERANCE IS: • Amount of error that is allowed in the calibration of a particular item. National and international standards specify tolerances.

  29. EXAMPLE • Standards for balance calibration can have slight variation from “true” value • Highest quality 100 g standards have a tolerance of + 2.5 mg • 99.99975-100.00025 g • Leads to uncertainty in all weight measurements

  30. TRACEABILITY IS: • The chain of calibrations, genealogy, that establishes the value of a standard or measurement • In the U.S. traceability for most physical and some chemical standards goes back to NIST

  31. TRACEABILITY • Note in this catalog example, “traceable to NIST”

  32. VOCABULARY • Standards • Calibration • Traceability • Tolerance • Play with these ideas in labs

  33. ACCURACY AND PRECISION ARE: • Accuracy is how close an individual value is to the true or accepted value • Precision is the consistency of a series of measurements

  34. EXPRESS ACCURACY % error = True value – measured value X 100% True value Will calculate this in volume lab

  35. EXPRESS PRECISION • Standard deviation • Expression of variability • Take the mean (average) • Calculate how much each measurement deviates from mean • Take an average of the deviation, so it is the average deviation from the mean • Try this in the volume lab

  36. ERROR IS: • Error is responsible for the difference between a measured value and the “true” value

  37. CATEGORIES OF ERRORS • Three types of error: • Gross • Random • Systematic

  38. GROSS ERROR • Blunders

  39. RANDOM ERROR • In U.S., weigh particular 10 g standard every day. They see: • 9.999590 g, 9.999601 g, 9.999592 g …. • What do you think about this?

  40. RANDOM ERROR • Variability • No one knows why • They correct for humidity, barometric pressure, temperature • Error that cannot be eliminated. Called “random error”

  41. RANDOM ERROR • Do you think that repeating the measurement over and over would allow us to be more certain of the “true” weight of this standard?

  42. RANDOM ERROR • Yes, because in the presence of only random error, the mean is more likely to be correct if repeat the measurement many times • Standard is probably really a bit light • Average of all the values is a good estimate of its true weight

  43. RANDOM ERROR AND ACCURACY • In presence of only random error, average value will tend to be correct • With only one or a few measurements, may or may not be accurate

  44. Mean Median Mode

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