1. Mechanical Measurements and Metrology
Subject Code: 10ME32B
Prepared By: Nikita
Department: Aeronautical Engineering
September 10, 2014

2. UNIT- V
MEASUREMENTS
September 10, 2014

3. MEASUREMENTS
"MEASUREMENT“ is the determination of the size or magnitude of something "Or" The comparison of unknown quantity with some standard quantity of the same rates is known as measurement
September 10, 2014

4. Units and Standards Units of Measurement: C.G.S. System of Units
Centimeter – Gram – Second system of unit
M.K.S. System of Units:
Meter – kilogram – second system of units
International System (SI) of Units:
the meter (m), kilogram (k), second (s), and ampere (A) of the MKSA system and, in addition, the Kelvin (K) and the candela (cd) as the units of temperature and luminous
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5. Simple Instrument Model
If the signal from Sensor output is small, it is needed to be amplified. In many cases it is also necessary for the instrument to provide a digital signal output for connection with a computer-based data acquisition systems.
Physical Process
Output
Physical Measurement Variable
Analog Signal Variable
SENSOR X S
Measurand
AMPLIFIER
A/D Converter
Digital Signal Variable
Computer
Memory
If the signal from Sensor output is small, it is needed to be amplified. In many cases it is also necessary for the instrument to provide a digital signal output for connection with a computer-based data acquisition systems.
September 10, 2014

6. Pressure-Type Air Gage
September 10, 2014
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7. Static Performance of Instrument
SYSTEMATIC CHARACTERISTICS
Range
Span
Linearity
Sensitivity
Environmental effects
Hysteresis
Resolution
Death space
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8. Range
The input range defines the minimum and maximum value of the variable to measure.
The output rage defines the minimum and maximum value of the signal given by the transducer.
Assume a temperature transducer which temperature range is from 100°C to 250°C
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9. Span
The input span is the maximum change of the input and the output span is the maximum change of the output.
Input span:
Output span:
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10. Linearity
It is normally desirable that the output reading of an instrument is linearly proportional to the quantity being measured.
An instrument is considered if the relationship between output an input can be fitted in a line.
Omax
Omin
Imin
Imax
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11. Terminology in instrumentation
Precision  Degree of repetitiveness. If an instrument is not precise it will give different results for the same dimension for the repeated readings.
Accuracy  The maximum amount by which the result differ from true value(ie) Closeness to true value
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12. Terminology in instrumentation
Threshold  The minimum value of input signal that is required to make a change or start from zero.
Hysteresis  All the energy put into the stressed component when loaded is not recovered upon unloading. so the output of measurement partially depends on input called Hysteresis.
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13. Terminology in instrumentation
Calibration
is the process of establishing the relationship between a measuring device and the units of measure. This is done by comparing a devise or the output of an instrument to a standard having known measurement characteristics.
Sensitivity
It is ratio between output signal to input signal
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14. Terminology in instrumentation
Readability is a measure of an instrument's ability to display incremental changes in its output value.
True size  Theoretical size of a dimension which is free from errors.
Actual size  size obtained through measurement with permissible error
September 10, 2014

15. Terminology in instrumentation
Repeatability is the variation in measurements taken by a single person or instrument on the same item and under the same conditions. A measurement may be said to be repeatable when this variation is smaller than some agreed limit.
Reproducibility is one of the main principles of the scientific method, and refers to the ability of a test or experiment to be accurately reproduced, or replicated, by someone else working independently.
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16. Methods of measurement. 1. Direct Method 2. Indirect Method
3. Comparison Method
4. Coincidence Method.
Classification of measuring instruments.
  1. Angle measuring instruments
2. Length measuring instruments
3. Instruments for surface finish
4. Instruments for deviations.
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17. Sources of error Controllable Errors-
Calibration Errors ,ambient Conditions , Stylus pressure, avoidable errors
Random Errors
These occur randomly and the specific causes of such errors cannot be determined, but likely sources of this type of error are small variations in the position of setting standards and workpiece, slight displacement of lever joints in the measuring joints in the measuring instrument,
September 10, 2014

18. Parallax Error :
On most dials the indicating finger or pointer lies in a plane parallel to the scale but displaced a small distance away to allow free movement of the pointer. It is then essential to observe the pointer along a line normal to the scale otherwise a reading error will occur.
September 10, 2014