INTRODUCTION TO METROLOGY. [email protected] DEFINITIONS Metrology is the study of...
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Transcript of INTRODUCTION TO METROLOGY. [email protected] DEFINITIONS Metrology is the study of...
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
Next 3 shorter lectures apply principles to specific measurements: weight, volume, pH
Later will talk about measuring light transmittance (spectrophotometry)
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?
EXAMPLE
A man weighs himself in the morning on his bathroom scale, 172 pounds.
Later, he weighs himself at the gym,173 pounds.
Do you trust one or other scale? Which one? Could both be wrong? Do you think he actually gained a pound?
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
DO WE REALLY CARE?
Do you care if he really gained a pound?
How many think “give or take” a pound is OK?
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?
“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.
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
QUALITY SYSTEMS
All laboratory quality systems are concerned with measurements
All want “good” measurements
Some language is quoted in your lab manual
NEED
Awareness of issues so can make “good” measurements.
Language to discuss measurements.
Tools to evaluate measurements.
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
VOCABULARY
Units of measurement
Standards Calibration Traceability Tolerance
Accuracy Precision Errors Uncertainty
UNITS OF MEASUREMENT
Units define measurements Example, gram is the unit for mass What is the mass of a gram? How do we
know?
DEFINITIONS MADE BY AGREEMENT Definitions of units are made by international
agreements, SI system Example, kilogram prototype in France K10 and K20 at NIST
EXTERNAL AUTHORITY
Measurements are always made in accordance with external authority
Early authority was Pharaoh’s arm length
A standard is an external authority Also, standard is a physical embodiment
of a unit
STANDARDS ARE:
Physical objects, the properties of which are known with sufficient accuracy to be used to evaluate other items.
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
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
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
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
PERFORMANCE VERIFICATION IS: Check of the performance of an instrument or
method without adjusting it.
TOLERANCE IS:
Amount of error that is allowed in the calibration of a particular item. National and international standards specify tolerances.
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
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
TRACEABILITY
Note in this catalog example, “traceable to NIST”
VOCABULARY
Standards Calibration Traceability Tolerance Play with these ideas in labs
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
EXPRESS ACCURACY
% error = True value – measured value X 100%
True value
Will calculate this in volume lab
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
ERROR IS:
Error is responsible for the difference between a measured value and the “true” value
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?
RANDOM ERROR
Variability No one knows why They correct for humidity, barometric
pressure, temperature Error that cannot be eliminated. Called
“random error”
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?
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
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
MeanMedianMode
THERE IS ALWAYS RANDOM ERROR If can’t see it, system isn’t sensitive enough
Less sensitive balance: 10.00 g,
10.00 g, 10.00 g
Versus 9.999600 g…
SO…
Can we ever be positive of true weight of that standard?
No There is uncertainty in every weight
measurement
RELATIONSHIP RANDOM ERROR AND PRECISION Random error –
Leads to a loss of precision
SYSTEMATIC ERROR
Defined as measurements that are consistently too high or too low, bias
Many causes, contaminated solutions, malfunctioning instruments, temperature fluctuations, etc., etc.
SYSTEMATIC ERROR
Technician controls sources of systematic error and should try to eliminate them, if possible Temperature effects Humidity effects Calibration of instruments Etc.
SYSTEMATIC ERROR
Systematic error – Does impact accuracy
Repeating measurements with systematic error does not improve the accuracy of the measurements
ANOTHER DEFINTION OF ERROR IS: Error = is the difference between the measured
value and the “true” value due to any cause
Absolute error = “True” value - measured value
Percent error is:“True” value - measured value (100 %)
“True” value
ERRORS AND UNCERTAINTY
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
UNCERTAINTY IS:
Estimate of the inaccuracy of a measurement that includes both the random and systematic components.
UNCERTAINTY ALSO IS:
An estimate of the range within which the true value for a measurement lies, with a given probability level.
UNCERTAINTY
Not surprisingly, it is difficult to state, with certainty, how much uncertainty there is in a measurement value.
But that doesn’t keep metrologists from trying …
METROLOGISTS
Metrologists try to figure out all the possible sources of uncertainty and estimate their magnitude
One or another factor may be more significant. For example, when measuring very short lengths with micrometers, care a lot about repeatability. But, with measurements of longer lengths, temperature effects are far more important
REPORT VALUES
Metrologists come up with a value for uncertainty
You may see this in catalogues or specifications Example:
measured value + an estimate of uncertainty
UNCERTAINTY ESTIMATES
Details are not important to us now
But principle is: any measurement, need to know where the important sources of error might be
SIGNIFICANT FIGURES
One cause of uncertainty in all measurements is that the value for the measurement can only read to a certain number of places
This type of uncertainty. It is called “resolution error”. (It is often evaluated using Type B methods.)
SIIGNIFICANT FIGURE CONVENTIONS Significant figure conventions are used to
record the values from measurements Expression of uncertainty Also apply to very large counted values
Do not apply to “exact” values Counts where are certain of value Conversion factors
ROUNDING CONVENTIONS
Combine numbers in calculations Confusing Look up rules when they need them
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 Discussed in Laboratory Exercise 1
ROUNDING
A Biotechnology company specifies that the level of RNA impurities in a certain product must be less than or equal to 0.02%. If the level of RNA in a particular lot is 0.024%, does that lot meet the specifications?
The specification is set at the hundredth decimal place. Therefore, the result is rounded to that place when it is reported. The result rounded is therefore 0.02%, and it meets the specification.
GOOD WEB SITE FOR SIGNIFICANT FIGURES http://antoine.frostburg.edu/cgi-bin/senese/
tutorials/sigfig/index.cgi
Match these descriptions with the 4 distributions in the figure:
Good precision, poor accuracy
Good accuracy, poor precision
Good accuracy, good precision
Poor accuracy, poor precision
THERMOMETERS
Look at the values for the thermometers on the board.
Significant figure conventions can guide us in how to record the value that we read off any measuring instrument.
With these thermometers, correct number of sig figs is _______.
THERMOMETERS
Were they accurate? How could we figure out the “true” value for
the temperature?
REPEATING MEASUREMENTS
Would repeating measurements with these thermometers, assuming we did not calibrate them, improve our ability to trust them?
Is their error an example of random or systematic error?
CALIBRATION
Calibration of the thermometers could lead to increased accuracy
This is a type of systematic error In the presence of systematic error, repeating
the measurement will not improve its accuracy
TOLERANCE
Here is a catalog description of mercury thermometers.
Are these thermometers out of the range for which their tolerance is specified?
PRECISION
Were they precise? How could precision be measured?
Would calibration help to make them more precise?
RETURN TO OUR ORIGINAL TYPE OF QUESTION Are our temperature measurements “good”
measurements? How do you make that judgment? Can we trust them?
THERMOMETERS – GOOD ENOUGH? Are times that we need to be very close in
temperature measurements. For example PCR is fairly picky.
Other times we can be pretty far off and process will still work.
EXPLORE SOME OF THESE IDEAS In lab:
Calibrate instruments Use standards Check performance of micropipettes Record measurement values Calculate per cent errors Calculate repeatability
