Static characteristics - Mechanical Measurements
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The document outlines the static characteristics of measurement instruments, emphasizing their quality, scale range, readability, and the effects of environmental conditions like drift. It also explains concepts such as repeatability, reproducibility, accuracy, precision, threshold, resolution, dead time, dead zone, linearity, hysteresis, loading effect, static sensitivity, and noise. These characteristics are crucial for understanding the performance and reliability of measurement instruments.
Static characteristics - Mechanical Measurements
- 1. Static Characteristics - Ameya P. Nijasure napradeep@mes.ac.in
- 2. ➔ Criterias for which an instrument gives a meaningful description. ➔ Static characteristic reflects into Quality of measurement. ➔ Criterias that are constant or very low with respect to time i.e static. Static Characteristics
- 3. ➔ Scale range of an instrument is defined as the difference between largest & the smallest reading of an instruments. ➔ Scale range of an instrument is mentioned as Xmax to Xmin or sometimes just Xmax Scale span = Xmax - Xmin E.g - thermometer having calibration -40℃ to 400℃ then Range is -40℃ to 400℃ or 400℃ only & Span is 400-(-40) = 440℃. Scale Range & Scale Span
- 4. ➔ Readability of an instrument indicates closeness with which scale of an analog instrument can be read. ➔ Only applicable for analog type of instruments. ➔ Scale readability depends on a. Number of graduations b. Spacing of graduations c. Size of pointer d. Parallax effect e. Ability to read (Observer) Readability
- 5. ➔ An environmental condition affects output of instrument is known as drift. ➔ Standard ambient conditions are defined (Specifications) Drift Types of drift A. Zero Drift Calibration gradually shifts due to slipping, zero setting etc. A. Span / Sensitivity Drift Proportional change in characteristic A. Zone drift Occurs only over a portion of span
- 6. ➔ Repeatability describes the closeness of output readings, when the same input is applied repetitively for short period of time with same measurement condition, same instrument/observer, same location and same conditions of use maintained throughout. ➔ Reproducibility describes the closeness of output readings, for same input when there is change in method of measurement, observer, location, conditions of use & time of measurement. Repeatability & Reproducibility
- 7. ➔ Accuracy is closeness with which an instrument reading approaches the true value. ➔ It is conformity to truth. ➔ Accuracy can be specified as a. Point Accuracy b. Accuracy as “Percentage of scale range” c. Accuracy as “Percentage of True value” ➔ Precision is a measure of reproducibility of measurements. ➔ Degree of agreement within the group of measurements. Accuracy & precision
- 8. ➔ If the instrument input is increased very gradually form zero there will be some minimum value below which no output change can be detected. This minimum value defines threshold of an instrument ➔ It is smallest measurable input. ➔ If input is increased from some arbitrary non zero input value, output does not change at all until certain increment exceeds. This increment is called resolution of an instrument. ➔ It is smallest measurable input change. Threshold & Resolution
- 9. ➔ Dead time is defined as the time required by the measurement system to begin to respond to a change in measurand. ➔ Dead zone is defined as largest change of input quantity for which there is no output for an instrument. Dead time & dead zone
- 10. ➔ Output is linearly proportional to input. ➔ Convenient to calculate output depending upon a constant factor. ➔ Simplifies design and analysis of control system. Linearity
- 11. ➔ Hysteresis is non coincidence of loading & unloading curve. ➔ As energy in loading is not recoverable in unloading - no process is reversible. Hysteresis
- 12. ➔ The incapability of the system to faithfully measure, record, or control the input signal (measurand) in undistorted form is called the loading effect. ➔ Distortion due to reduction is magnitude, waveform distortion, phase shift or combination of all. Loading Effect
- 13. ➔ Static sensitivity of an instrument is a ration of the magnitude of output quantity (response) to the magnitude of input (quantity being measured) ➔ Unit depends on types of input & output ➔ Sensitivity of instrument should be high. Static Sensitivity
- 14. ➔ Noise is a signal that does not convey any useful information. ➔ Can be external or generated internal in the system ➔ Signal to noise ratio : ratio of desired signal to unwanted noise ➔ Sources of noise a. Generated noise - Internal to system b. Conducted noise - Externally generated c. Radiated noise - Electric/ magnetic disturbances Noise
- 15. Thank you….!! - Ameya P. Nijasure napradeep@mes.ac.in