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1. Metrology
Adapted from Introduction to Metrology from the Madison Area Technical College,
Biotechnology Project (Lisa Seidman)
http://biotech.matcmadison.edu/resources/methods/measurement/measure.htm

2. What is metrology?
 The study of measurements
 Measurements are quantitative
observations; numerical descriptions
 Measurements are part of the daily routine in
a biotech lab
 Measurements are expected to be “good”

3. What is a “good”
measurement?
 If you weigh at home and then at the
doctor’s office and get a different weight,
which is correct?
 Did your weight change (sample issue)?
 Is one or both scales wrong (instrument
issue)?
 How do you know which of these is correct?

4. What is a “good”
measurement?
 A “good” measurement is one that can be
trusted when making decisions
 Decisions are made daily on whether
measurements are good enough, but they
are made subconsciously and often by
different people
 Decisions need to be conscious and
consistent.

5. Metrology Vocabulary
 Unit of measurement
 Accuracy
 Precision
 Standards
 Calibration
 Verification
 Traceability
 Tolerance
 Errors
 Uncertainty

6. Question
Would you rather me give you the one worth
one or worth five?

7. Units
 Units define measurements
 Units give the numbers value
 Definition set by international SI system

8. Accuracy vs Precision
 Accuracy is how close an individual value is to the
true or accepted value
 Precision is the consistency of a series of
measurements
From Basic Laboratory
Methods for
Biotechnology:
Textbook and
Laboratory Reference,
Seidman and Moore,
2000

9. Measurements can be:
 Accurate and precise (best)
 Accurate and imprecise (user error)
 Inaccurate but precise (instrument error)
 Inaccurate and imprecise

10. Expressions
 Accuracy
 % error = True value – measured value X 100%
True value
Precision
 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

11. Recording measured values
 Record measured values (or large counts)
with correct number of significant figures
 Don’t add extra zeros; don’t drop ones that
are significant
 With digital reading, record exactly what it
says; assume the last value is estimated
 With analog values, record all measured
values plus one that is estimated

12. Significant Figures
 The digits 1 - 9 always count. (51 has 2)
 Zeroes between the digits 1 - 9 always
count. (501 has 3)
 Zeroes in the beginning of a number never
count. (0.00501 only has 3)
 Zeroes at the end of a number count only
if there is a written decimal point. (5010
has 3, 501.0 has 4)

13. Rounding
 Greater than or equal to 5 then round up
 Less than 5 then round down
 When adding or subtracting, the number of
decimal places in the result equals the
smallest number of decimal places in the
input numbers.
 When multiplying or dividing, the number of
significant figures in the result equals the
smallest number of significant figures in the
input numbers.

14. Scientific Notation
 The coefficient must be greater than or
equal to 1 and less than 10.
 The base must be 10.
 The exponent must show the number of
decimal places that the decimal needs to be
moved to change the number to standard
notation. A negative exponent means that
the decimal is moved to the left when
changing to standard notation.

15. Standards
 Measurements made in accordance with
an external authority
 A standard is an external authority
 They are physical objects, the properties
of which are known with sufficient
accuracy to be used to evaluate other
items
 Units are unaffected by the environment,
but standards are
 Also solutions or documents

16. Calibration
 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 your balance to units of kg

17. Verification
 Check of the performance of an instrument
or method without adjusting it.

18. Tolerance
 Amount of error that is allowed in the
calibration of a particular item. National and
international standards specify tolerances.

19. 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

20. Traceability
 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(National Institute
of Standards and Technology)

21. Error
 Error is responsible for the difference
between a measured value and the “true”
value
 Three types of error:
 Gross (blunders)
 Random
 Systematic

22. Random Erros
 Random errors are errors that cannot be
eliminated. They are variability and no one
knows why. Maybe humidity, pressure, etc.
This is why we take several measurements
and average them to get best estimate of
true value
 Random error leads to loss of precision

23. Systemic Error
 Defined as measurements that are
consistently too high or too low, bias
 Many causes, contaminated solutions,
malfunctioning instruments, temperature
fluctuations, etc., etc.
 Technician controls sources of systematic
error and should try to eliminate them, if
possible
 Impacts accuracy so try not to repeat them

24. Uncertainty
 Estimate of the inaccuracy of a
measurement that includes both the random
and systematic components.
 Errors lead to uncertainty in measurements
 Can never know the exact, “true” value for
any measurement.
 Idea of a “true” value is abstract – never
knowable.
 In practice, get close enough

25. Which ruler gives the length of
the arrow with the most
certainty?