There are numerous ways in which a quantity can be measured.
The method is selected according to the accuracy required for the measurement.
The following are the different methods for measuring a physical variable.
In the direct method measurement, the value of a quantity is obtained directly by comparing the unknown with the standard.
It involves, no mathematical calculations to arrive at the results.
Examples:
When direct measurement is not possible, the indirect measurement method is used.
Direct Measurement is measuring exactly the same quantity you want to measure, while Indirect measurement is measuring other quantity and required value is determined by mathematical relationship.
Examples:
(Volume=Mass/Density)
(Speed=Distance/Time)
Output of ‘RTD’ and ‘Thermocouple’ is in ‘Ohm’ and ‘millivolt’ respectively. This is converted into temperature by respective standard conversion tables.
(Ohms Law V=IR i.e. I=V/R) I is current, V is Voltage, R is Resistance
In the comparative measurement method, the quantity to be measured is compared with the known value of the same quantity.
Examples:
The substitution measurement method involves the replacement of the value of the quantity to be measured with the known value of the same quantity.
In this method, the quantity to be measured is measured by direct comparison on an indicating device by replacing the measuring quantity with some other known quantity that produces the same effect on the indicating device.
Example:
In the null measurement method, the quantity to be measured is compared with a known quantity, and the difference between these two is made zero i.e. Indicator Indicates Zero reading.
Null measurement techniques achieve greater accuracy.
Example:
In the fundamental measurement method, the measurement is based on the measurement of the base quantities used to define the quantity.
The quantity to be measured is directly measured and linked with the definition of that quantity.
Examples:
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