week 1a what is a measurement (1)

1What is a Measurement?

Encyclopedia Encarta

In classical physics and engineering, measurement

generally refers to the process of estimating or

determining the ratio of a magnitude of a

quantitative property or relation to a unit of the

same type of quantitative property or relation.

Process of measurement involves the comparison

of physical quantities of objects or phenomena

What is a Measurement? (contd) Wikipedia

Measurement is the estimation or determination of extent,

dimension or capacity, usually in relation to some standard

or unit of measurement.

2

Comparison to a Standard

(Metrology)

Metrology is the study of measurement.

-In general, a metric is a scale of measurement defined in terms of a standard: i.e. in terms of well-defined unit.

- If one says I am 5, that person is indicating a measurement without

supplying an applicable standard.

-They could mean I am 5 years old or I am 5 feet high.

-Measurements are at best ambiguous, or at worst, meaningless, with out units!

3

Units of Measure What is a Unit of Measure?

-Act of measuring involves comparing the magnitude

of a quantity possessed by an object with a

standard unit by using an instrument

under controlled conditions.

-Examples of measuring instruments include:

Thermometer (Deg.)

Current Meter (Amps)

Pressure Sensor (psi)

What are

These gages

Reading?

Without

Prior

Knowledge

Of units

we have

No idea!4

Units of Measure (contd)

Same quantity,

.. Different units

5

Units of Measure (contd) Systems of Units

Imperial (English)

Before SI units were widely adopted around the world, the British

systems of English units and later Imperial units were used in Britain,

the Commonwealth and the United States. The system came to be known

as U.S. customary units in the United States.

Sometimes called foot-pound-second systems after

the Imperial units for distance, weight (mass), and time.

6

Units of Measure (contd) Systems of Units

Metric (MKS)

The metric system is a decimalised system of measurement based on the

Meter (M), kilogram (K), and second (S).

The main advantage of the metric system is that is has a single base unit for

each physical quantity. All other units are powers of ten or multiples of ten

of this base unit.

Unit conversions are always simple because they will be in the ratio of ten,

one hundred, one thousand, etc.

Also referred to as Systeme International (SI) Units

7

Fundamental Units of

Measure

A system of measurement is a set of units which can be used

to specify anything which can be measured. Some quantities

are designated as fundamental units meaning all other needed

units can be derived from them.

Historically a wide range of units were used for the same

quantity; for example, in several cultural settings, length was

measured in inches, feet, yards, fathoms, rods, chains, furlongs,

miles, nautical miles, leagues, with conversion factors which

are not simple powers of ten or even always simple fractions.

8


Measure (contd)

This disagreement of units had serious military, cultural, and

Fiscal impacts and eventually the British Royal Society headed by

Michael Faraday adopted 3 fundamental Units,

distance (ft), weight (lb), and time (sec).

Later (1824) it was determined to be more

fundamental to substitute Mass (slug) for weight

(lb) as a fundamental unit of measure

F ma1lbf 1slug ftsec2

9


Measure (contd) In the 19th century, science developments showed that either

electric charge or electric current must be added to complete

the minimum set of fundamental quantities.

Mesures usuelles (French for customary measurements)

were a system of measurement introduced to act as

compromise between metric system and traditional

measurements.

This system of measures would eventually lead to the

Evolution of the modern SI system of measurements

10


Measure, SI system The SI system is founded on 8 fundamental units. All other

Units can be derived from these quantities.

11

Derived Units of Measure, SI

system Derived units are

algebraic combinations

of the eight base units

with some of the

combinations being

assigned special names

and symbols.

12

Derived Units of Measure, SI

system (contd)

13

Conversion of Units of Measure

Although the Imperial System of units is gradually being

Replaced by SI system, these units are still in common use

Amongst U.S. defense contractors , and NASA!

This use of the Imperial system is especially prevalent

For mechanical units like distance, force, moments of

inertia, pressure, and volume.

Accurate conversion from one system to another is

Essential!

14

Conversion of Units of

Measure (contd)Not Important?!

The Mars Climate Orbiter (1998) was

destroyed when a navigation error caused the

spacecraft to miss its intended 150 km

altitude above Mars during orbit insertion.

Instead the spacecraft entered the Martian

atmosphere at about 57 km altitude.

The spacecraft was destroyed by

atmospheric stresses and friction at

this low altitude. 15



A review board found that thruster impulse data

were calculated on the ground in Imperial

units (pound-seconds) and reported that

way to the navigation team, who were

expecting the data in metric units

(newton-seconds)

Anticipating a different set of units, systems

aboard the spacecraft were not able to

reconcile the two systems of measurement,

resulting in the navigation error and loss of spacecraft!16



This calculation just saved $300 million dollars!

1 ntsec

1 kgmsec2

sec 1 kgm

sec

1

14.5939

slugkg

3.28083 ftm

1 lbfslug ft

sec2

3.28083

14.5939

kgmsec

slug

kg

ft

mlbf

slug ft

sec2

0.22481 lbf sec

17


Measure (contd)Careful with English units!

Pounds-mass (lbm) is not a fundamental unit of measurement!

Metric

English -- pounds mass (lbm) and pounds force (lbf) do not cancel

F ma (1)nt kg msec2

F m

gca gc 32.174 lbm ft

lbf sec2

(1)lbf 32.174 lbm ftsec2

18

Dimensional Analysis Most physical quantities can be expressed in terms

of combinations of five basic dimensions. These are

mass (M), distance (D, L), time (t), electrical current (I),

and temperature (T)

Dimensions aren't the same as units. I.e. the

physical quantity, speed, may be measured in units of

meters per second, knots ; but regardless

of the units used, speed is always a distance

divided a time, so we say that the dimensions

of speed are distance divided by time, or instantaneously dD/dt.

19

Dimensional Analysis (contd)In same manner, dimensions of area are D2 . area can

always be calculated as a distance in one direction times a

Perpendicular direction

. area of a circle --> r2 is really a result of the integral

Acircle 1

2r rd

0

2

r2

22 r2

[1/2 Length of arc] x [height of triangle]

20

Dimensional Analysis (contd)

Simple Dimensional Analysis Example

21

Dimensional Analysis (contd)More Complex Dimensional Analysis Example

22

Dimensional Analysis (contd)In algebraic expression, additive terms must have same dimensions.

--> each term on the left-hand side of an equation must have the same dimensions as each term on the right-hand side.

"a" must have the same dimensions as the product "bc", and "(1/2)xy"

must also have the same dimensions as "a" and "bc".

Equation is dimensionally correct when terms have consistent dimensionality

Dimensional analysis is a valuable tool for validating the correctness

of an algebraic derivation i.e. finding algebra errors

a b c 1

2x y

23

Dimensional Analysis (contd)

Source: http://www.physics.uoguelph.ca/tutorials/dimanaly/dimanaly_ans7.html, Cited 12-22-06

More complex examples

24

The Measurement Process:

Comparison to a Standard

Direct Comparison to a Standard

Length of a bar

Use a carpenters Rule

25

StandardsAny time you measure anything, you are comparing it to something whose

value you think you know. You assume your ruler is 1 ft long. But who says

what a foot is?

A combination of several international agencies are responsible for maintaining the

primary standard measures of various quantities. The standard kilogram and the

standard second are maintained by the French. Others are kept elsewhere. It

extremely important that these standards do not change with time, even over

hundreds of years.

The National Institute of Standards and Technology in Maryland is responsible for

keeping standards for the US.

http://www.nist.gov/public_affairs/standards.htm

26

IST-F1 Cesium Fountain

Atomic Clock Primary Time and Frequency Standard for the United States

27

Hierarchy of Standards

The hierarchy of measurement standards starts from the

international standard at the apex, which is known with the highest

precision and goes all the way down to working standards.

International measurement standards are standards recognized by an

international agreement to serve internationally as the basis for

assigning values to other standards of the quantity concerned.

The oldest standard in use today is the International Prototype of

the Kilogram, kept at the Bureau International des Poids et Mesures

(BIPM) in Sevres.

28

Hierarchy of Standards (contd)

These primary standards cant be

passed around to any entity that

wants to take some measurements

if we expect them to maintain their

values, so secondary standards are

kept which may be somewhat less

accurate, but much more accessible.

These are calibrated against the

primary standards. In this manner, a

hierarchy of standards exist.

29

Hierarchy of Standards (contd)

A primary standard is designated or widely acknowledged as having the

highest metrological qualities and whose value is accepted without

reference to other standards of the same quantity.

Secondary standards are standards whose value is assigned by comparison

to a primary standard of the same quantity. Primary standards are usually used

to calibrate secondary standards. A working standard is a standard that is used

routinely to calibrate or check material measures, measuring instruments or

reference materials. A working standard is usually calibrated with reference

to a secondary standard, and may be used to ensure that routine measurements

are being carried out correctly - a check standard.

A reference standard is a standard generally having the highest

metrological quality available at a given location or in a given organization

from which the measurements made at that location are derived.

Calibration laboratories maintain reference standards for calibrating their

working standards. 30

The Measurement Process:

Using Calibrated System

Using a Calibrated System

Length of a Big Snake

-- Naturalists measure length of the animal using in a string following a imaginary

middle line of the body from head to tail.

-- then the length of the string is measured by laying it on a ruler

-- allows recording of the actual length of the animal regardless

of its position and without having to stretch the snake.

Hold still will ya!

31

Calibration Example (2)

-10

-5

0

5

10

-2

-1.5

-1

-0.5

0

0.5

1

-0.3 -0.2 -0.1 0 0.1 0.2 0.3

P [

kP

a]

% fs erro

rVolts

32

Calibration Example (3)Thermal Anemometry relies on property many materials change resistance

with temperature. A hot-wire anemometer is a device that heats a wire by

pumping current through it and keeps its resistance (and thus its temperature)

constant. When air blow on the wire the current required to keep the wire hot

goes up. . instrument is sensitive to velocity. Requires calibration against

a known velocity.

0

10

20

30

40

50

60

-6

-4

-2

0

2

4

6

-3 -2 -1 0 1 2 3

Ve

locity[m

/s]

%e

rror

Volts

A1, V1, P1

A2, V2, P2

A1 is 100 times larger than A2, so V12 is

negligible compared to V22. So,

Bernoulli says that

P1

V1

2

2P2

V2

2

2

V2 2 P1 P2 /

Hot wire

33

Sensors/Transducers (1)

A sensor is something that is sensitive to some phenomenon that we

are interested in. It needs to respond to the phenomenon is some way

that we can see or measure.

Examples: Mercury thermometer

Wind Sock

Thermocouple

A transducer is a sensor tied to stuff (very often electronics) that

makes the output of the sensor readable. Most transducers output a

voltage or a current. typically this is an element of a sensor

34

Generalized Measurement

System

35

QuickTime and aTIFF (LZW) decompressor

are needed to see this picture.

QuickTime and aTIFF (LZW) decompressor

are needed to see this picture.

Pin outs

Scope to monitor analog

Out from PPT

PPT

Terminal Block

For RS-485 Bus

Honeywell PPT Interface Lab Engineering Development Unit

PC

Running Labview

7.1

Example I: Simple Lab

Measurement System

Sea level Systems:

USB to RS-422, RS-485 Serial Interface Adapter

36

Example II: Aircraft Airspeed

Sensing System

Transducers are

Sub-elements of sensor

37

Example III: Complex Sensor

System

38

Transduction Transducers convert the physical phenomenon being sensed

Into an alternative signal that can be more easily sensed

Pressure inputElectrical output

39


40


41

Fiber Optic Sensors

Source: http://www.bluerr.com/papers/Overview_of_FOS2.pdf

42

Fiber Optic Sensors (contd)


43


Fiber Optic Sensor Examples


44


Fiber Optic Sensor Examples


45

Fiber Optic Measurement

System

Source: http://www.bluerr.com/papers/Overview_of_FOS2.pdf46

Very Complex Example:

Remote Sensing Mission

Source: Sellers, Understanding Space 47

Remote Sensing

Payloads Look at Target -- move sensor to point at subject

See the Target -- collect EM radiation from subject

Conversion -- Transform sensed EM radiation to useable data

Processing -- analyze data to produce useable information

48

Telescopic Optical Sensing Systems

All remote sensors are basically one of two variations on a

Telescope Reflecting telescope (Hale (Mt. Palomar), Radar,

Radio telescopes, DSS)

Refracting telescope (very cumbersome and expensive)

Objective lens

Eyepiece

Eyepiece

Primary

Mirror

49

Telescopic Optical Sensors (contd)

Catadioptric Telescope (hybrid)

Convex lens

Convex Mirror

Secondary Mirror i.e Hubble Space Telescope

50

Hubble Space Telescope

2.5 m

Catadioptric

Design

2.4 m

51

week 1a what is a measurement (1)

Documents

imperial units

disagreement of units

customary units

set of units

needed units

withoutprior knowledgeof

wide range of units

standardor unit of measurement