The Process of Measurement
Introduction:
The process or act of measurement consists of obtaining a quantitative comparison between a pre-defined standard and a measurand. The act of measurement produces the result.
Legal or recognized agency for standards
National Institute of Standards and Technology (NIST) National Bureau of Standards (NBS) International Organization for Standards (ISO) American National Standards Institute (ANSI)
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The significance of mechanical measurement Three elements of complexity in mechanical design
1. Experience element
Is based on previous exposure to similar systems and an engineers common sense
2. Rational element
Relies on quantitative engineering principles, laws of physics, and so on
3. Experimental element
Based on measurement of various quantities
Measurement
provides a comparison between what was intended and what was actually achieved. Is a fundamental element of any control process. Must be reliable for being useful Having incorrect info is potentially more damaging than having no info. 3
Fundamental Methods of Measurement Two basic methods
1. Direct comparison 2. Indirect comparison through the use of calibrated system
Direct Comparison
with either primary or secondary standard
Using Calibrated System
Use of some form of transducing device coupled to a chain of connecting apparatus, the measuring system The chain of devices converts the input into an analogous form which is processed and out is presented in the form of a known function of the original input.
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The Generalized Measuring System
Most measuring systems fall in three stages:
Stage-1 Stage-2 Stage-3 A detection-transduction, or sensortransducer stage An intermediate stage, or signalconditioning stage A terminating, or readout-recording stage
Standard & Dimensional Units of Measurement
MEASUREMENT SYSTEMS
The General Measurement Systemhas three basic constituents
1) The sensing element
Also called transducer Produces a signal which is related to the quantity being measured Takes info about the thing being measured and changes it into some other form which enables rest of the measurement 7 system to give a value to it.
2) The signal converter
Takes signal from sensing element and converts it suitable to display partComposed of three sub-elements
a) A signal conditionerconverts the signal from sensing element into a physical form suitable for the display b) A signal processorimproves the quality of the signal, e.g., amplifies it c) A signal transmitterconvey the signal some distance to the display
3) The display element
Displays the output from the measuring system, e.g. a pointer moving across the scale
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System Transfer Function (Gain)
Transfer function G = o/i
Where o = output i = input
o 1 o 2 G= = x x i i 1 2
G = G1 x G2 x G3
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Types of Input Quantities
Time Dependence
1. 2. Staticconstant in time Dynamicvarying in time
a) Steady-state periodic b) Non-repetitive or transient
i. ii. Single pulse or aperiodic Continuing or random
Analog and Digital Signals
Analog
E.g. speed of an automobileA sensor that responds to velocity will produce an output signal having a time variation analogous to the time change in the autos speed. A digital signal is a set of discrete numbers, each corresponding to the value of the analog signal at a single specific instant of timeStrings of 1s and 0s with each string held in a separate memory register.
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Digital
Reporting Results
1) Executive Summary
Directed towards the key features of the work: what was done and what was concluded
2) Lab note or Technical memo
Written to be read by someone thoroughly familiar with the project, such as an immediate supervisor etc.
3) Full technical report
Tells the complete story to one who is interested in the subject but has not been in direct contact with the specific work, e.g., top official of a lrge company or a review committee of a sponsoring agency.
4) Progress repot
One of the several possible interim reports describing current status of an ongoing project
5) Technical paper
A brief summary of a project, the extent of which must be tailored to fit either a time allotment ata a meeting or space in 19 a publication.
Intelligent Instruments
Intelligentmeans a microprocessor or computer is included in the system
Measurement is made and then further processing occurs and data is interpreted. Can make decisions based on measurement made earlier, carryout calculations on data, manipulate information and initiate actions based upon the results obtained.
Dumbmeans when no microprocessor
Only gives a measure of quantity. The user has to process and interpret the result
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Uncertainty; Accuracy of Results
Errora difference between measured result and the true value If we estimate a likely upper bound on the error, that bound is called the uncertainty. Two types of error:
Bias, or Systematic error (e.g., zero offset) Precision or random error (e.g., a scale error)
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Calibration
The process of checking a measurement system against a standard when the transducer is in a defined environment. Single point calibration
If output is exactly proportional to the input (output = constant x input)
Multipoint calibration
When the output is not simply proportionala number of different input values are applied.
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The basic standards from which all others derive are the primary standards. Seven such standards and two supplementary are:
1) 2) 3) 4) 5) 6) 7) Mass (kg) Length (meter) Time (second) Current (Ampere) Temperature (Kelvin) Luminous intensity (Candela) Amount of substance (mole) The two supplementary are
1. Plane angle (radian) 2. Solid angle (steradian)
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