1. What are the basic performance characteristics of a system?
Ans:
STATIC CHARACTE RISTICS
The static characteristics of an instrument are, in general, considered for instruments which are used to measure an unvarying process condition. All the static performance characteristics are obtained by one form or another of a process called calibration. There are a number of related definitions (or characteristics), which are described below, such as accuracy% precision, repeatability, resolution, errors, sensitivity, etc.
l. Instrument: A device or mechanism used to determine the present value of the quantity under measurement.
2. Measurement: The process of determining the amount, degree, or capacity by comparison (direct or indirect) with the accepted standards of the system units being used.
3. Accuracy: The degree of exactness (closeness) of a measurement compared to the expected (desired) value. 4. Resolution: The smallest change in a measured variable to which an instrument will respond.
5. Precision: A measure of the consistency or repeatability of measurements, i.e. successive readings does not differ. (Precision is the consistency of the instrument output for a given value of input).
6. Expected value: The design value, i.e. the most probable value that calculations indicate one should expect to measure.
7. Error: The deviation of the true value from the desired value.
8. Sensitivity: The ratio of the change in output (response) of the instrument to a change of input or measured variable.
DYNAMIC CHARACTERISTICS
Instruments rarely respond instantaneously to changes in the measured variables. Instead, they exhibit slowness or sluggishness due to such things as mass, thermal capacitance, fluid capacitance or electric capacitance. In addition to this, pure delay in time is often encountered where the instrument waits for
some reaction to take place. Such industrial instruments are nearly always used for measuring quantities that fluctuate with time. Therefore, the dynamic and transient behavior of the instrument is as important as the static behavior.
The dynamic behavior of an instrument is determined by subjecting its primary element (sensing element) to some unknown and predetermined variations in the measured quantity. The three most common variations in the measured quantity are as follows:
l. Step change in which the primary element is subjected to an instantaneous and finite change in measured variable.
2. Linear change, in which the primary element is following a measured variable, changing linearly with time.
3, Sinusoidal change, in which the primary element follows a measured variable, the magnitude of which changes in accordance with a sinusoidal function of constant amplitude . The dynamic characteristics of an instrument are (i) speed of response, (ii) Fidelity, (iii) lag, and (iv) dynamic error.
(i) Speed of Response:
It is the rapidity with which an instrument responds to changes in the measured quantity.
(ii) Fidelity:
It is the degree to which an instrument indicates the changes in the measured variable without dynamic error (faithful reproduction).
(iii) Lag:
It is the retardation or delay in the response of an instrument to changes in the measured variable.
(iv) Dynamic Error:
It is the difference between the true values of a quantity changing with time and the value indicated by the instrument, if no static error is assumed.
When measurement problems are concerned with rapidly varying quantities, the dynamic relations between the instruments input and output are generally Defined by the use of differential equations
Ans:
STATIC CHARACTE RISTICS
The static characteristics of an instrument are, in general, considered for instruments which are used to measure an unvarying process condition. All the static performance characteristics are obtained by one form or another of a process called calibration. There are a number of related definitions (or characteristics), which are described below, such as accuracy% precision, repeatability, resolution, errors, sensitivity, etc.
l. Instrument: A device or mechanism used to determine the present value of the quantity under measurement.
2. Measurement: The process of determining the amount, degree, or capacity by comparison (direct or indirect) with the accepted standards of the system units being used.
3. Accuracy: The degree of exactness (closeness) of a measurement compared to the expected (desired) value. 4. Resolution: The smallest change in a measured variable to which an instrument will respond.
5. Precision: A measure of the consistency or repeatability of measurements, i.e. successive readings does not differ. (Precision is the consistency of the instrument output for a given value of input).
6. Expected value: The design value, i.e. the most probable value that calculations indicate one should expect to measure.
7. Error: The deviation of the true value from the desired value.
8. Sensitivity: The ratio of the change in output (response) of the instrument to a change of input or measured variable.
DYNAMIC CHARACTERISTICS
Instruments rarely respond instantaneously to changes in the measured variables. Instead, they exhibit slowness or sluggishness due to such things as mass, thermal capacitance, fluid capacitance or electric capacitance. In addition to this, pure delay in time is often encountered where the instrument waits for
some reaction to take place. Such industrial instruments are nearly always used for measuring quantities that fluctuate with time. Therefore, the dynamic and transient behavior of the instrument is as important as the static behavior.
The dynamic behavior of an instrument is determined by subjecting its primary element (sensing element) to some unknown and predetermined variations in the measured quantity. The three most common variations in the measured quantity are as follows:
l. Step change in which the primary element is subjected to an instantaneous and finite change in measured variable.
2. Linear change, in which the primary element is following a measured variable, changing linearly with time.
3, Sinusoidal change, in which the primary element follows a measured variable, the magnitude of which changes in accordance with a sinusoidal function of constant amplitude . The dynamic characteristics of an instrument are (i) speed of response, (ii) Fidelity, (iii) lag, and (iv) dynamic error.
(i) Speed of Response:
It is the rapidity with which an instrument responds to changes in the measured quantity.
(ii) Fidelity:
It is the degree to which an instrument indicates the changes in the measured variable without dynamic error (faithful reproduction).
(iii) Lag:
It is the retardation or delay in the response of an instrument to changes in the measured variable.
(iv) Dynamic Error:
It is the difference between the true values of a quantity changing with time and the value indicated by the instrument, if no static error is assumed.
When measurement problems are concerned with rapidly varying quantities, the dynamic relations between the instruments input and output are generally Defined by the use of differential equations
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