Characteristics of Measuring instruments
- 1. Characteristics of Measuring Instruments (Sub: Measurement and Metrology) Code: BMEC0003 Instructor Mr. Gaurav Bharadwaj Assistant Prof. Department of ME GLA University
- 2. Characteristics of Measuring instruments There are two types of characteristics of measuring instruments: 1. Static characteristics 2. Dynamic characteristics Static characteristics: • The static characteristics are defined for the instruments which measure quantities which do not vary with time. Dynamic characteristics : • These type of instruments are normally used for the measurement of quantities that fluctuate with time. First, we will discuss static characteristics.
- 3. Static characteristics Static characteristics are as follows: 1. Scale range 2. Scale span 3. Accuracy 4. Precision 5. Error 6. Calibration 7. Resolution 8. Threshold 9. Sensitivity 10. Repeatability 11. Reproducibility 12. Readability 13. Linearity 14. Drift 15. Hysteresis
- 4. Static characteristics 1. Scale range: • The minimum and maximum value of the scale of measuring instruments. Example: If a thermometer can read minimum value of 0 degree Celsius and maximum of 100 degree Celsius then range is 0 degree Celsius and 100 degree Celsius. 2. Span: • The difference between maximum value of scale and minimum value of scale. Example: Span of a scale having maximum value of 100 cm and minimum value of -10 cm = 100 – (-10) = 110 cm. 3. Accuracy: • Accuracy is the degree of agreement of the measured dimension with its true magnitude. • The maximum amount by which the result differs from the true value. • The nearness of the measured value to its true value. • Expressed as a percentage.
- 5. Static characteristics 4. Precision: •Precision is the degree of repetitiveness of the measuring process. • Precision is the repeatability of the measuring process. • Precision refers to the consistent reproducibility of a measurement. • If an instrument is not precise, it would give different results for the same dimension for repeated readings. • In most measurements, precision assumes more significance than accuracy.
- 6. Static characteristics Relation between cost and Accuracy: Difference between Precision and Accuracy (a) Precise but not accurate (b) Accurate but not precise (c) Precise and accurate (d) Not precise and not accurate
- 7. Static characteristics 5. Error: Difference between the indicated value and the true value of the quantity measured. E = Error Vm = Measured value Vt = True value E is also known as the absolute error % Error •known as relative error • Accuracy of an instrument can also be expressed as % error. • Accuracy of an instrument is always assessed in terms of error.
- 8. Static characteristics 6. Calibration: •The set of operations that establish the relationship between values indicated by instruments and corresponding values given by standards, under specific conditions. •It checks the accuracy of the instrument. In practice, calibration also includes repair of the device if it is out of calibration. 7. Resolution: •It is the minimum change or smallest increment in the measured value that can be detected with certainty by the instrument. • It can be least count of instrument. Example: Least count of Vernier caliper, micrometer, etc. 8. Threshold: •Threshold of a measuring instrument is the minimum value of input signal that is required to make a change or start from zero. •This is the minimum value below which no output change can be detected when the input is gradually increased from zero.
- 9. Static characteristics 9. Sensitivity: •It is the ratio of the magnitude of output signal to the magnitude of input signal. • It denotes the smallest change in the measured variable to which the instrument responds. Sensitivity=(Infinitesimal change of output signal)/(Infinitesimal change of input signal) • If a instrument is made of various part then sensitivity can be calculated as: 1. Part A is having sensitivity k1 2. Part B is having sensitivity k2 3. Part C is having sensitivity k3 K1=S2/S1, K2= S3/S2, K3 = S4/S3 Overall sensitivity K = K1× K2 × K3 = S4/S1
- 10. Static characteristics 10. Repeatability: • The ability of the measuring instrument to repeat the same results during the act of measurements for the same quantity is known as repeatability. • It is the closeness between successive measurements of the same quantity with the same instrument by the same operator over a short span of time, with same value of input under same operating conditions. 11. Reproducibility: • Variation that results when different conditions are used to make the measurements say, :Different operators–Different setups–Different units–Different environmental conditions–Different measurement systems. 12. Readability: •Readability refers to the ability of the user to read to the smallest unit on the measuring device using specified inspection procedures. •Line-graduated measuring devices that have very fine discrimination may not be very readable.
- 11. Static characteristics 13. Drift: • It is defined as the variation of output for a given input caused due to change in sensitivity of the instrument due to certain interfering inputs like temperature changes, component instabilities, etc. • Prime sources occur as chemical structural changes and changing mechanical stresses. • Drift is a complex phenomenon for which the observed effects are that the sensitivity and offset values vary. • It also can alter the accuracy of the instrument differently at the various amplitudes of the signal present. There are three types of drift: 1. Zero drift 2. Sensitivity drift 3. Combined drift
- 13. Static characteristics 15. Hysteresis: • Hysteresis is defined as the magnitude of error caused in the output for a given value of input, when this value is approached from opposite directions ; i.e. from ascending order & then descending order. • Causes are backlash, elastic deformations, magnetic characteristics, frictional effects (mainly). • Hysteresis can be eliminated by taking readings in both direction and then taking its arithmetic mean.
- 14. Dynamic characteristics There are the following types of dynamic characteristics: 1. Speed of response 2. Measuring lag 3. Fidelity 4. Overshoot 1. Speed of response: • It is defined as the rapidity with which a measurement system responds to changes in the measured quantity. 2. Measuring lag: • It is the retardation or delay in the response of a measurement system to changes in the measured quantity. 3. Fidelity: • Degree of closeness with which the system indicates or records the signal which is impressed upon it. • Ability of the system to reproduce the output in the same form as the input.
- 15. Dynamic characteristics 4. Overshoot: • Because of mass and inertia of moving part, the pointer of the instrument does not immediately come to rest in the final detected position. • The portion up to which pointer moves and then return back to final steady condition.
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