To choose the instrument, most suited to a particular measurement application, we have to know the system characteristics and to have a clear understanding of all the parameters involved in defining the characteristics of the measurement device.
The performance characteristics of instruments and measurement systems can be divided into two distinct categories:
i)Static characteristics
ii)Dynamic characteristics
In this article we will be discussed about static characteristics. and Dynamic characteristics in the next article.
The performance characteristics of instruments and measurement systems can be divided into two distinct categories:
i)Static characteristics
ii)Dynamic characteristics
In this article we will be discussed about static characteristics. and Dynamic characteristics in the next article.
Static characteristics:
Applications involve the measurement of quantities that are either constant or vary only quite slowly with time. Under these circumstances it is possible to define a set of criteria that gives a meaningful description of quality of measurement without interfering with dynamic descriptions that involve the use of differential equations.These criteria is called static characteristics.
Thus the static characteristics of a measurement system are those which must be considered when the system or instrument is used under a condition not varying with time.
(Or)
(Or)
(Simply)
The set of criteria defined for the instruments, which are used to measure the quantities which are slowly varying with time or mostly constant, i.e., do not vary with time, is called Static Characteristics.
The various static characteristics are:
1. Accuracy
2. Precision
3. Sensitivity
4. Linearity
5. Reproducibility
6. Repeatability
7. Resolution
8. Threshold
9. Drift
10. Stability
11. Tolerance
12. Range or span
13. Hysteresis
14. Bias
15. Dead zone
16. Static error
17. Backlash
18. Conformance
19. Distortion
20. Noise
1. Accuracy: It is the One of the most important Characteristic of the instrument.
Accuracy is the degree of closeness with which the reading approaches the true value of the quantity to be measured.
(or)
Accuracy of a measurement describes how close the measurement approaches the true value of the process variable.
Thus means The accuracy of a measurement indicates the nearness to the actual/true value of the quantity.Accuracy can be specified in terms of inaccuracy or limits of errors and can be expressed in the following ways:
a) Point accuracy:
This is the accuracy of the instrument only at one particular point on its scale.The specification of this accuracy does not give any information about the accuracy at other points on the scale or in the words,does not give any information about the general accuracy of the instrument.
b) Accuracy as percentage of scale span:
When an instrument as uniform scale, its accuracy may be expressed in terms of scale range.
c)Accuracy as percentage of true value:
The best way to conceive the idea of accuracy is to specify it in terms of the true value of the quantity being measured within +0.5% or 0.5% of true value.
[Note: Simply can says Accuracy is a measure of how close the measured value is to the true value.]
a) Point accuracy:
This is the accuracy of the instrument only at one particular point on its scale.The specification of this accuracy does not give any information about the accuracy at other points on the scale or in the words,does not give any information about the general accuracy of the instrument.
b) Accuracy as percentage of scale span:
When an instrument as uniform scale, its accuracy may be expressed in terms of scale range.
c)Accuracy as percentage of true value:
The best way to conceive the idea of accuracy is to specify it in terms of the true value of the quantity being measured within +0.5% or 0.5% of true value.
[Note: Simply can says Accuracy is a measure of how close the measured value is to the true value.]
2. Precision:
This is a measure of the deviation from a mean value computed from a set of readings obtained for a single given input. In other words It is the measure of reproducibility i.e., given a fixed value of a quantity, precision is a measure of the degree of agreement within a group of measurements.
The precision is composed of two characteristics:
a) Conformity:
Consider a resistor having true value as 2385692 , which is being measured by an ohmmeter. But the reader can read consistently, a value as 2.4 M due to the nonavailability of proper scale. The error created due to the limitation of the scale reading is a precision error.
b) Number of significant figures:
The precision of the measurement is obtained from the number of significant figures, in which the reading is expressed. The significant figures convey the actual information about the magnitude & the measurement precision of the quantity.
Where, P = precision
Xn = Value of nth measurement
Xn = Average value the set of measurement values
3. Sensitivity :
The sensitivity denotes the smallest change in the measured variable to which the instrument responds. It is defined as the ratio of the changes in the output of an instrument to a change in the value of the quantity to be measured.
Thus, if the calibration curve is liner, as shown, the sensitivity of the instrument is the slope of the calibration curve. If the calibration curve is not linear as shown, then the sensitivity varies with the input. Inverse sensitivity or deflection factor is defined as the reciprocal of sensitivity. Inverse sensitivity or deflection factor = 1/ sensitivity.
For example, a temperature measuring system that uses a platinum resistance temperature device (RTD) produces a change in resistance as the temperature changes. The input is temperature and the output is resistance. The output over the input is therefore,
Sensitivity = DR/DT Units = W/°C
4. Linearity:
It defines the proportionality between input quantity and output signal. If the sensitivity is constant for all values from zero to full scale value of the measuring system,then the calibration characteristic is linear and is a straight line passing through origin. If it is an indicating or recording instrument the scale may be made linear. In case there is a zero error the characteristic assumes the form of equation given by y=mx+c where y is output,x is input,m is slope and c is intercept.
Linearity is the closeness of the calibration curve of a measuring system to a straight line.If an instruments calibration curve for desired input is not a straight line, the instrument may still be highly accurate. In many applications, however, linear response is most desirable.
Linearity is defined as,
linearity=Maximum deviation of o/p from idealized straight line/Actual readings
Linear curve Non-linear curve
5. Reproducibility:
Reproducibility is defined as the degree of closeness by which a given value can be repeatedly measured.The reproducibility is specified for a period of time.Perfect reproducibility signifies that the given readings that are taken for an input, do not vary with time..
6. Repeatability:
It is the characteristic of precision instruments. It describes the closeness of output readings when the same input is applied repetitively over a short period of time, with the same measurement conditions, same instrument and observer, same location and same conditions of use maintained throughout. It is affected by internal noise and drift. It is expressed in percentage of the true value. Measuring transducers are in continuous use in process control operations and the repeatability of performance of the transducer is more important than the accuracy of the transducer, from considerations of consistency in product quality.
7. Resolution :
The smallest change of the magnitude of the measurand that produces a minimum observable output of the instrument.
If the input to an instrument is increases slowly from some arbitrary nonzero value, it will be observed that the output of the instrument does not change at all until there is a certain minimum increment in the input. This minimum increment in what is input is called resolution of the instrument.Thus, the resolution is defined as the smallest incremental of the input quantity to which the measuring system responds..
8. Threshold:
If the instrument input is increased very gradually from zero there will be some minimum value below which no output change can be detected. This minimum value defines the threshold of the instrument. In specifying threshold, the first detectable output change is often described as being any noticeable measurable change.
9. Drift:
The change in the transducer output for a zero input or its sensitivity over a period of time, change in temperature, humidity or some other factor.Drift is classified into three categories:
1)Zero drift
2)Span drift or sensitivity drift
3) Zonal drift
10. Stability:
The ability of a measuring system to maintain standard of performance over prolonged periods of time. Zero stability defines the ability of an instrument restore to zero reading after the input quantity has been brought to zero,while other conditions remain the same.
(Or simply)
The ability of an instrument to retain its performance throughout its specified storage life and operating life is called as Stability.
11. Tolerance:
The maximum allowable error in the measurement is specified in terms of some value which is called tolerance.
For example: A 1000W resistor with a tolerance of ±5% has an actual resistance between 950 and 1050W.
12. Range or span:
The minimum & maximum values of a quantity for which an instrument is designed to measure is called its range or span.
For exapmple: For a standard thermometer given the range 0° C to 100°C then span is 100°C . If the thermometers range is 30 to 220°C, then the span is equal to 250°C.
13. Hysteresis:
This refers to the situation where different readings (outputs) are sometimes observed for the same input because the input was approached from different directions. For example a thermometer exposed to an increasing temperature input (i.e. going from 0 to 100°C) may show a slightly different profile to that for the decreasing input (i.e. decreasing from 100 to 0°C).
(Or simply)
The noncoincidence between the loading (increasing) and the unloading (decreasing) measurement curves.
Hysteresis curve
14. Bias:
The constant error which exists over the full range of measurement of an instrument is called bias. Such a bais can be completely eliminated by calibration. The zero error is an example of bais which can be removed by calibration.
15. Dead zone:
It is the largest change of input quantity for which there is no output of the instrument. For instance, the input applied to the instrument may not be sufficient to overcome the friction and will, in that case not move at all.
It is due to either static friction(stiction), backlash or hysteresis. Dead zone is also known as dead band dead Space. All elastic mechanical elements used as primary transducers exhibit effects of hysteresis, creep and elastic after effect to some extent.
16. Static error:
It is the deviation from the true value of the measured variable. It involves the comparison of an unknown quantity with an accepted standard quantity. The degree to which an instrument approaches to its excepted value is expressed terms of error of measurement.
17. Backlash:
The maximum distance or angle through which any part of mechanical system may be moved in one direction without applying appreciable force or motion to the next part in a mechanical sequence.
18. Conformance:
For a nonlinear transducer, the tightness of fit to a specified curve is known as conformance of conformity.
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