PRINCIPLES OF MEASUREMENTS
Definition:-
Measurement is the process of determining the amount/degree/capacity by comparison with accepted standards of system units being used.
Listed below are some popular measuring instruments
# Calculator
# Ammeter
# Clock
# Multimeter
# Speedometer
AND……………
Here are some which you perhaps don’t know
# Anemometer: - (for wind velocity)
# Spectrophotometer: - (for intensity)
# Tachymeter: - (for distances)
Measurement is the process of determining the amount/degree/capacity by comparison with accepted standards of system units being used.
Listed below are some popular measuring instruments
# Calculator
# Ammeter
# Clock
# Multimeter
# Speedometer
AND……………
Here are some which you perhaps don’t know
# Anemometer: - (for wind velocity)
# Spectrophotometer: - (for intensity)
# Tachymeter: - (for distances)
What are the basic performance characteristics of a system?
1} STATIC CHARACTERISTICS
The static characteristics of an instrument are, in general, considered for instruments which are used to measure an unvarying process condition. The various static Characteristics are as follows:-
Accuracy: The degree of exactness (closeness) of a measurement compared to the expected desired value.
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).
Repeatability: It describes the closeness of output reading when the same input is applied over a short period of time with same environmental location, measurement condition, observer, and instrument.
Reproducibility: It describes the closeness of output reading for same input when there are changes in method of measurement environmental location, observer, and instrument.
Linearity: It is defined as the ability to reproduce input characteristics, symmetric and linear
Resolution: The smallest change in a measured variable to which an instrument will respond
Sensitivity: The ratio of the change in output (response) of the instrument to a change of input or measured variable.
2} DYNAMIC CHARACTERISTICS
Instruments rarely respond instantaneously to changes in the measured variables. Instead, they exhibit slowness or sluggishness. In addition to this, pure delay in time is often encountered where the instrument waits for some reaction to take place. Therefore; the dynamic and transient behaviour of the instrument is as important as the static behaviour.
The dynamic characteristics of an instrument are
(1) Speed of Response: It is the rapidity with which an instrument responds to changes in the measured quantity.
(2) Fidelity: It is the degree to which an instrument indicates the changes in the measured variable without dynamic error.
(3) Lag: It is the retardation or delay in the response of an instrument to changes in the measured variable.
Let’s take a peek at the various errors that we encounter while taking measurements
TYPES OF ERRORS
Three types of errors are generally studied in detail.
1} Gross Errors
· This error is mainly due to human mistakes in reading or in using instruments or errors in recording observations.
· Errors may also occur due to incorrect adjustments of instruments and computational mistakes.
· These errors cannot be treated mathematically. The complete elimination of gross errors is not possible, but one can minimize them.
· Some errors are easily detected while others may be elusive.
2} Systematic Errors
These errors occur due to shortcomings of, the instrument, such as defective or worn parts, or ageing or effects of the environment on the instrument. There are basically three types of systematic errors.
i) Instrumental Errors
· Instrumental errors are inherent in measuring instruments, because of their mechanical structure.
· For example, irregular spring tensions, stretching of the spring or reduction in tension due to improper handling or over loading of the instrument.
· Instrumental errors can be avoided by
(a) Selecting a suitable instrument for the particular measurement applications.
(b) Applying correction factors after determining the amount of instrumental error.
(c) Calibrating the instrument against a standard.
ii) Environmental Errors
· Environmental errors are due to conditions external to the measuring device, including conditions in the area surrounding the instrument, such as the effects of change in temperature, humidity, barometric pressure or of magnetic or electrostatic fields.
· These errors can also be avoided by (i) air conditioning, (ii) hermetically sealing certain components in the instruments, and (iii) using magnetic shields.
iii) Observational Errors
· Observational errors are errors introduced by the observer.
· The most common error is the parallax error introduced in reading a meter scale, and the error of estimation when obtaining a reading from a meter scale.
· These errors are also can be caused by the habits of individual observers.
3} Random errors
· They are seen once gross & systematic errors are observed & removed.
· They are generally an accumulation of large number of small errors.
· These kinds of error are to be determined only where high degree of accuracy is required.
· These errors can be determined statistically, the causes of these errors are unknown, and cannot be determined in ordinary process of measurement.
1} STATIC CHARACTERISTICS
The static characteristics of an instrument are, in general, considered for instruments which are used to measure an unvarying process condition. The various static Characteristics are as follows:-
Accuracy: The degree of exactness (closeness) of a measurement compared to the expected desired value.
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).
Repeatability: It describes the closeness of output reading when the same input is applied over a short period of time with same environmental location, measurement condition, observer, and instrument.
Reproducibility: It describes the closeness of output reading for same input when there are changes in method of measurement environmental location, observer, and instrument.
Linearity: It is defined as the ability to reproduce input characteristics, symmetric and linear
Resolution: The smallest change in a measured variable to which an instrument will respond
Sensitivity: The ratio of the change in output (response) of the instrument to a change of input or measured variable.
2} DYNAMIC CHARACTERISTICS
Instruments rarely respond instantaneously to changes in the measured variables. Instead, they exhibit slowness or sluggishness. In addition to this, pure delay in time is often encountered where the instrument waits for some reaction to take place. Therefore; the dynamic and transient behaviour of the instrument is as important as the static behaviour.
The dynamic characteristics of an instrument are
(1) Speed of Response: It is the rapidity with which an instrument responds to changes in the measured quantity.
(2) Fidelity: It is the degree to which an instrument indicates the changes in the measured variable without dynamic error.
(3) Lag: It is the retardation or delay in the response of an instrument to changes in the measured variable.
Let’s take a peek at the various errors that we encounter while taking measurements
TYPES OF ERRORS
Three types of errors are generally studied in detail.
1} Gross Errors
· This error is mainly due to human mistakes in reading or in using instruments or errors in recording observations.
· Errors may also occur due to incorrect adjustments of instruments and computational mistakes.
· These errors cannot be treated mathematically. The complete elimination of gross errors is not possible, but one can minimize them.
· Some errors are easily detected while others may be elusive.
2} Systematic Errors
These errors occur due to shortcomings of, the instrument, such as defective or worn parts, or ageing or effects of the environment on the instrument. There are basically three types of systematic errors.
i) Instrumental Errors
· Instrumental errors are inherent in measuring instruments, because of their mechanical structure.
· For example, irregular spring tensions, stretching of the spring or reduction in tension due to improper handling or over loading of the instrument.
· Instrumental errors can be avoided by
(a) Selecting a suitable instrument for the particular measurement applications.
(b) Applying correction factors after determining the amount of instrumental error.
(c) Calibrating the instrument against a standard.
ii) Environmental Errors
· Environmental errors are due to conditions external to the measuring device, including conditions in the area surrounding the instrument, such as the effects of change in temperature, humidity, barometric pressure or of magnetic or electrostatic fields.
· These errors can also be avoided by (i) air conditioning, (ii) hermetically sealing certain components in the instruments, and (iii) using magnetic shields.
iii) Observational Errors
· Observational errors are errors introduced by the observer.
· The most common error is the parallax error introduced in reading a meter scale, and the error of estimation when obtaining a reading from a meter scale.
· These errors are also can be caused by the habits of individual observers.
3} Random errors
· They are seen once gross & systematic errors are observed & removed.
· They are generally an accumulation of large number of small errors.
· These kinds of error are to be determined only where high degree of accuracy is required.
· These errors can be determined statistically, the causes of these errors are unknown, and cannot be determined in ordinary process of measurement.