Unit-1
Introduction
Standards of Measurement are classified into the following categories:
(i) International Standards (ii) Primary Standards
(iii) Secondary Standards (iv) Working standards
The international standards are defined on the basis of international agreement. They represent the units of measurements which are closest to the possible accuracy attainable with present day technological and scientific methods .
International standards are checked and evaluated regularly against absolute measurements in terms of fundamental units.
The international standards are maintained at the International Bureau of Wieghts and Measures and are not available to ordinary user of measuring instruments for purpose of calibration or comparison.
Primary standards:
Primary standards are absolute standards of such high accuracy that can be used as the ultimate reference standards. These standards are maintained by national standards laboratories in different parts of the world. One of the main functions of primary standards is the verification and calibration of secondary standards.
The following points must be taken into consideration when a primary standard is built :
(i) The material should have long time stability
(ii) The temperature co-efficient of the material should be as small as possible.
(iii) The deterioration of materials caused by moisture and other environmental conditions should be eliminated as far as possible.
(iv) The machining of parts should be accurate
(v) The measurement of physical dimension on which the accuracy of the standard depends predominantly should be done with most sosphisticated techniques available.
(vi) The rigidity of the construction should be insured.
Secondary Standard:
The secondary standard are the basic reference standard used in industrial measurement laboratories. The responsibilty of maintainence and calibration of these standards lies with the particular industry involved. These standards are checked locally against reference standards available in the area.
Secondary standards are normally sent periodically to the national standards laboraties for calibration and comparison against primary standards. The secondary standards are sent back to the industry by national laboratories with ceritification as regards their measured vallues in terms of primary standards.
Working Standards
The working standards are the major tools of measurement laboratory. These standards are used to check and calibrate general laboratory instruments for their accuracy and performance.
For example a manufacturer of precision resistances may use a Standard Resistance in quality control department for checking the values of resistors that are being manufactured.
Key Take Aways:
Measurement Error also called Observational Error is the difference between a measured quantity and its true value. It includes random error naturally occurring errors that are to be expected with any experiment and systematic error caused by a mis-calibrated instrument that affects all measurements.
Errors may rise from different sources and are classified as:
- Gross Errors
- Systematic errors
- Random Errors
Gross Errors: This class of errors covers human mistakes in reading instruments and recording and calculating measurement results. The responsibility of the mistake normally lies with the experimenter. The experimenter may grossly misread the scale.
Example
Due to some reasons, he might have read the temperature as 31.5 while the actual reading may be 21.5. He may transpose the reading while recording. For example, he may read 25.8 as 25.8. If human beings are involved gross errors will be committed.
Systematic Errors
These types of errors are divided into three categories:
- Instrument errors
- Environmental errors
- Observational errors
Instrumental Errors
These errors arise due to three main reasons:
(i) Due to inherent shortcomings in the instrument
(ii) Due to misuse of the instrument
(iii) Due to loading effects of instruments.
- Inherent short comings of instrument because of their mechanical structure. They may be due to construction, calibration or operation of the instruments or measuring device.
While making precision measurements we recognize the possibility of such errors as it is often possible to eliminate them by using the following methods:
(1) The procedure of measurement must be carefully planned. Substitution methods or calibration against standards may be used for the purpose.
(2) Correct factors should be applied after determining the instrumental errors.
(3) The instruments may be re-calibrated carefully.
(2) Misuse of instrument:
A good instrument used in an unintelligent way may give erroneous results.
Examples which may be cited for this misuse of instrument may be failure to adjust the zero of instruments, poor initial adjustments using leads too high a resistance and so on.
(3) Loading effects: One of the most common errors committed by beginners is the improper use of an instrument for measurement work. For example, a well calibrated voltmeter may give a misleading voltage reading when connected across high resistance circuit. However, when the same voltmeter is connected in low resistance circuit it may give dependable reading. These examples illustrate that the voltmeter has loading effect on the circuit.
Environmental errors will happen due to the outside situation of the measuring instruments. These types of errors mostly happen due to the temperature result, force, moisture, dirt, vibration otherwise because of the electrostatic field or magnetic. The remedial measures used to remove these unwanted effects include the following.
- The preparation should be finished to remain the situations as stable as achievable.
- By the instrument which is at no cost from these results.
- With these methods which remove the result of these troubles.
- By applying the computed modifications.
Observational Errors
The observational errors may occur due to the fault study of the instrument reading, and the sources of these errors are many. For instance, the indicator of a voltmeter retunes a little over the surface of the scale. As a result, a fault happens except the line of the image of the witness is accurately above the indicator. To reduce the parallax error extremely precise meters are offered with reflected scales.
Key Takeaways:
A measurement standard may also be said to store, embody, or otherwise provide a physical quantity that serves as the basis for the measurement of the quantity.
All static performance characteristics are obtained in one form or another by a process called static calibration.
The calibration of all instruments is important since it affords the opportunity to check the instrument against a known standard and subsequently to errors in accuracy. Calibration procedures involve a comparison of the particular instrument with either
1) Primary Standard
2) Secondary Standard
3) Instrument of known accuracy.
Key Take Away:
Calibration is a comparison between a known measurement (the standard) and the measurement using your instrument.
Accuracy: It is the closeness with which the instrument reading approaches the true value of the quantity being measured. Thus, accuracy of measurement means conformity to truth.
An example might be given as ±1.0 millivolt (mV) offset error, regardless of the range or gain settings. In contrast, gain errors do depend on the magnitude of the input signal and are expressed as a percentage of the reading, such as ±0.1%. Total accuracy is therefore equal to the sum of the two: ±(0.1% of input +1.0 mV). An example of this is illustrated in Table 1.
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Key Takeaways:
Accuracy of a measured value refers to how close a measurement is to the correct value.
Precision: It is a measure of reproducibility of the measurements that is given a fixed value of quantity precision is a measure of degree of agreement within a group of measurements. The term precise means clearly or sharply defined.
Key Takeaways:
The precision of a measurement system is referring to how close the agreement is between repeated measurements
Sensitivity is an absolute quantity, the smallest absolute amount of change that can be detected by a measurement. Consider a measurement device that has a ±1.0-volt input range and ±4 counts of noise, if the A/D converter resolution is 212 the peak-to-peak sensitivity will be ±4 counts × (2 ÷ 4096) or ±1.9 mV p-p. This will dictate how the sensor responds. For example, take a sensor that is rated for 1000 units with an output voltage of 0-1 volts (V). This means that at 1 volt the equivalent measurement is 1000 units or 1 mV equals one unit. However, the sensitivity is 1.9 mV p-p so it will take two units before the input detects a change.
Key Takeaways:
Sensitivity is an absolute quantity, the smallest absolute amount of change that can be detected by a measurement
If the input is slowly increased from some arbitrary input value it will again be found that output does not change until a certain increment is exceeded. This increment is called resolution or discrimination. Thus, the smallest increment in input which can be detected with certainty by an instrument is its resolution or discrimination. So resolution defines the smallest measurable input change while the threshold defines the smallest measurable input.
Example,
A moving coil voltmeter has uniform scale with 100 divisions, the full-scale reading is 200V and 1/10 of scale division can be estimated with a fair degree of certainty. Determine the resolution of the instrument in volt.
Solution:
1 scale division = 200/100 = 2V
Resolution = 1/10 scale division = 1/10 x 2 = 0.2 V
Key Takeaways:
Resolution (MSA) is the ability of the measurement system to detect and faithfully indicate small changes in the characteristic of the measurement result.
Noise level is measured in decibels (dB). The louder the noise, the higher the decibels. Decibels can be adjusted to human hearing. Noise level is thus described in decibels A (dBA). The effects of noise vary with the noise to which a person is exposed.
Key Takeaways:
Most environmental noises can be approximately described by one of several simple measures.
References:
- Elements of Electronic Instrumentation and Measurement – by Carr
- Basic Electrical, Electronics and Measurement Engineering Paperback – 1 January 2019 by U.A Bakshi and A.P Godse.
- Electrical and Electronic Measurement and Instrument Paperback R.K Rajput
- Electrical & Electronic Measurements B. P. Patil, Pooja Mogre (Bisen)
- Electronic Measurements and Instrumentation by A.K. Sawhney
