Basic Concepts: DefinitionsBasic Concepts: Definitions

• Readability (R): indicates the closeness with which Readability (R): indicates the closeness with which the scale of the instrument may be read.the scale of the instrument may be read.

Readability < ReadabilityReadability < Readability

• Least count (LC): indicates the smallest difference Least count (LC): indicates the smallest difference between two indications that can be detected on between two indications that can be detected on the instrument scale.the instrument scale.

37º 37.5º

37.7 37.8 LC = 0.1

Basic Concepts: DefinitionsBasic Concepts: Definitions

- Both the “readability” and the “least count” - Both the “readability” and the “least count” depend on: scale length, spacing graduation, and depend on: scale length, spacing graduation, and size of pointer.size of pointer.

- These two definitions have no meaning with These two definitions have no meaning with Digital instruments.Digital instruments.

• Sensitivity (S): the ratio of the linear movement of Sensitivity (S): the ratio of the linear movement of the pointer on an analog instrument to the change the pointer on an analog instrument to the change in the measured variable causing this motion. in the measured variable causing this motion.

- Higher S means better instrument. - Higher S means better instrument.

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• Hysteresis Error (HE): related to the difference in Hysteresis Error (HE): related to the difference in readings depending whether the value of the readings depending whether the value of the measured quantity is approached from above or measured quantity is approached from above or below. below.

- H.E. may the result of: friction, magnetic field, - H.E. may the result of: friction, magnetic field, elastic deformation, thermal effects,… elastic deformation, thermal effects,…

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• Accuracy (A): “the closeness of the agreement between the Accuracy (A): “the closeness of the agreement between the result of a measurement and a true value of the measured.” It result of a measurement and a true value of the measured.” It indicates the deviation of a reading from a known point.indicates the deviation of a reading from a known point.

- Usually expressed as a percentage of a full scale. Usually expressed as a percentage of a full scale.

- Precision (P): indicates the ability of an instrument to Precision (P): indicates the ability of an instrument to reproduce a certain reading with a given accuracy.reproduce a certain reading with a given accuracy.

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• Error: the deviation of an instrument reading from a Error: the deviation of an instrument reading from a known known valuevalue..

• Uncertainty: “a parameter, associated with the result of a Uncertainty: “a parameter, associated with the result of a measurement, that characterizes the dispersion of values that measurement, that characterizes the dispersion of values that could reasonably be attributed to the measured.” Or: the could reasonably be attributed to the measured.” Or: the range of instrument readings where the range of instrument readings where the known valueknown value is not is not available. available.

• Calibration: a comparison of a particular instrument with Calibration: a comparison of a particular instrument with

another higher accuracy instrument, or with a known input another higher accuracy instrument, or with a known input source.source.

Dimensions and unitsDimensions and units• Fundamental dimensions:Fundamental dimensions:- L = lengthL = length- M = massM = mass- F = ForceF = Force = time= time- T = temperatureT = temperature

• All other physical quantities used may be All other physical quantities used may be expressed in terms of these fundamental expressed in terms of these fundamental dimensions.dimensions.

Dimensions and unitsDimensions and unitsQuantity Unit Symbol

Basic unitsLength meter mMass kilogram kgTime second sElectric current ampere ATemperature Kelvin KLuminous intensity

candela cd

Supplemental unitsPlane angle radian radSolid angle steradian sr

See table 2.6 for derived SI units

Basic and supplement SI units

The generalized measurement The generalized measurement systemsystem

- Most measurement systems may be divided Most measurement systems may be divided into three parts:into three parts:

1.1. A detector-transducer stage: detects the A detector-transducer stage: detects the physical variable and perform either a physical variable and perform either a mechanical or electrical transformation to mechanical or electrical transformation to convert the signal into a more usable form.convert the signal into a more usable form.

2.2. Intermediate stage: modifies the direct signal Intermediate stage: modifies the direct signal by amplification, filtering, or other means so by amplification, filtering, or other means so that desirable output is available.that desirable output is available.

3.3. Final or terminating stage: indicate, record, or Final or terminating stage: indicate, record, or control the measured variable. control the measured variable.

The generalized measurement system; The generalized measurement system; Burdon-tube pressure gageBurdon-tube pressure gage

Detector-transducer stage

Indicator stage

Modifying stage

System ResponseSystem Response

• ……

Experimental PlanningExperimental Planning

1.1. - Establish the need for the experiment.- Establish the need for the experiment. - Establish the budgetary, manpower, time,…- Establish the budgetary, manpower, time,…2.2. Plan your experiment: objectives, variables to be Plan your experiment: objectives, variables to be

measured, the required accuracy, no. of measured, the required accuracy, no. of measurements, possible error sources, …measurements, possible error sources, …

3.3. Select instrumentationsSelect instrumentations4.4. Conduct a primary experiment and analyze the dataConduct a primary experiment and analyze the data5.5. Modify in accordance with step 4Modify in accordance with step 46.6. Conduct the bulk of experimental data and the Conduct the bulk of experimental data and the

resultsresults7.7. Organize, discuss, and state the resultsOrganize, discuss, and state the results