Introduction to Engineering Metrology :
Hello mechons, in this post we are going to explain to you the basics of Engineering Metrology(i.e. Introduction to Metrology) in which we talk about Metrology with its definition also with its type along with we will also explain what is measurement and types of Measurements.
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What is Metrology?
Metrology is defined as the Science of Measurement. Concerned knowledge in measurement and includes both theoretical and practical problems related to measurement. It is used to enforce, validation, and verification to predefined standards. Metrology is a term which is derived from the Greek word "Metrologia" which means measure.
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Types of Metrology
Metrology is into three types based on the purpose of using. They are
- Scientific Metrology
- Industrial Metrology
- Legal Metrology
Scientific Metrology
It deals with the organization and development of measurement standards and with their maintenance at the highest level. They are creating some standard to measure the object and that is taken as reference.
Industrial Metrology
It deals with the to validate enough function of measuring instruments which used in the Industry and Production and Testing Process. It is vital to work with quality in Industrial activities. Already we had a reference by using that reference we try to check whether it is the correct one or not.
Legal Metrology
It is related to the accuracy of measurement where these have an influence on the transparency of economical transaction, safety, and Health. We are using this measurement in daily life but it is not taken into account. Its parameter is to regulate, advise, supervise, and control the manufacturing and calibration of measuring instruments.
Measurement
Let's continue with, What is measurement?
Measurement is a Process or act of obtaining a quantitative comparison between a predefined standard and an unknown magnitude. For example, we have a one-liter vessel. We want to know how much water containing another vessel. For that, we have to compare with a known quantity with the unknown quantity, that process known as Measurement.
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Methods of Measurements
Basically, the measurements are classified into two types
- Direct method
- Indirect method
Direct Method of Measurement
The value of quality is obtained directly by comparing the known standard.
There are no mathematical calculations to get a result.
Eg. Measuring Height by using scale or tape. This method is not accurate since it has human interactions.
Indirect Method of Measurement
In this method, parameters are measured directly by direct measurement, and the value is determined by some mathematical relationship.
Eg. Measurement of Velocity by measuring distance traveled and time is taken to travel that distance.
Levels of Measurement
- Primary Measurement
- Secondary Measurement
- Tertiary Measurement
Primary Measurement:
In this measurement, we need not do any conversion (translation) to get the required quantity. It is get by direct observation.
Eg. Matching of two heights, it means determining the height of two objects with a centimeter or meter rod.
Secondary Measurement:
A secondary measurement we need only one conversion (translation) to get the required quantity.
Eg. Measure the Pressure of gas may not be observable.
Frist the Pressure is converted into length. A length scale or a standard that is calibrated in length units equivalent to known changes in pressure.
Tertiary Measurement
A Tertiary measurement involves two translations.
The measurement of the temperature of an object by Thermocouple.
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If you to know about the standards of measuring instruments, read our article about "Calibration of measuring instruments", where you will get all details about Calibration process, standards of measuring instruments and so on.
Metrological Terminologies
Accuracy
The measuring instrument gives a value close to the true value of the measured quantity.
Accuracy is determined as the maximum amount by which the result differs from the true value.
Precision
Precision is the capacity of a measuring instrument to give the same reading when repetitively measuring the same quantity under prescribed conditions.
Precision is not close to the true value.
ACCURACY | PRECISION |
Accuracy is a measure of correctness. | Precision is a measure of how much-defined information is given. |
The value is correct and free from error. | Exactness is applied |
Accuracy can be improved | Precision cannot be improved |
Depends on the person measuring | Depends on the tool measuring |
Accuracy = Mean value - True Value | Precision = Individual value - Arithmetic value |
If Accuracy means the following results may be obtained
| If Precision means the following results may be obtained
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| (High accuracy & low precision) |
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| (High accuracy & high precision) |
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| (Low accuracy & high precision) |
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| (Low accuracy & low precision) |
Sensitivity
The ratio of the magnitude of the output signal to the input signal or If we change the smallest value in the measured value, the instrument will respond that is known as Sensitivity.
It is represented by the slope of the calibration curve if the coordinates are expressed in actual units.
Repeatability
Repeatability is the precision of a set of measurements taken over a short tie interval.
Condition:
Where test results are obtained independently using the same methods, items, place, operators equipment with a short interval of time.
Reproducibility
Reproducibility is the precision of a set of measurements.
Condition:
Where test results obtained using the same method and items but in different places, operators, and equipment.
Readability
It refers to the ease with which readings of a measuring instrument can be read.
The measuring device can give value in the number form. Readability improved by magnified glass fine and widely spaced graduation.
Response time
The time elapses after a sudden change of the measured quantity until the Instrument gives an indication.
Resolution
The smallest variation of a measured quantity which changes the indication of the measuring instrument.
Stability
The ability of the measuring instrument that constantly maintains the metrological characteristics with time.
Calibration
The set of operations that create the relationship between values indicated by instruments and corresponding values given by standards under specified conditions.
Confirmation
The set of operations required to validate an object of measuring equipment is in a state of compliance with requirements for its intended use.
Correction
It is defined by systematic error. Systematic error cannot find exactly sp correction is subjected to uncertainty.
Range
The difference between the largest and smallest readings of the instrument able to measure is called the range of Instrument. It is represented as lower and upper values.
Span
Span is the algebraic difference between the upper and lower range values of the instrument.
If the highest point of calibration is Smax units and Lowest is Smin units then the range can be calculated as follows
Span= (Smax -Smin ) units
Drift
A slow change of metrological characteristics of measuring instruments.
Drift occurs very slowly and can be checked by periodic inspection and maintenance.
Causes of Drift:
- Temperature Variation.
- Wear and tear.
- Stray electric and Magnetic fields
- High mechanical stress created in some parts of instruments and systems
- Mechanical vibrations
Classification of drift
- Zero drift
- Span drift
- Zonal drift
If the whole of instrument calibration gradually shifts over by the same amount.
It may be a permanent set or slippage and can be corrected by shifting the pointer position.
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| ( Fig. Zero drift) |
Span drift:
If the calibration from zero upward changes proportionally, it is called span or Sensitivity drift.
It may be due to change in spring gradient etc.
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| (Fig: Span drift) |
Zonal drift:
When the drift occurs only over a portion of the span of the instrument it is called zonal drift.
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| (Fig: Combined drift) |
The difference between the instrument measured value and the true value of that object.
Types of Errors
- Systematic Errors
- Random errors
- Gross errors
Systematic errors are reproducible
inaccuracies that are consistently in the
same direction.
Random errors:
Random errors occur due to random and unpredictable variations in experimental conditions like temperature, Pressure, etc.,
Gross errors:
Gross errors occur due to the carelessness of an observer is known as gross errors.
Hysteresis
- Hysteresis is the phenomenon defined as the difference outputs while loading and unloading.
- Hysteresis takes place due to the fact that all the energy put into the stressed part when loading is not recoverable while unloading.
- The output at particular input while increasing and decreasing varies because of internal friction or hysteresis damping.
Linearity
- Linearity is defined as the ability of an instrument to produce its input linearly.
- The closeness of the calibration curve to a specified straight line.
Conclusion
That's all about the Introduction to Metrology, I hope you understand the definitions, types, and levels of Measurement and Metrology.
If you have any queries or doubts or need improvements in this article put that in the below-mentioned comments section we will reply and rectify as fast as possible.
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