Indirect measurements are made by comparing the dimensions of a target to a reference object, such as gauge blocks or ring gauges, using measuring tools like dial gauges.
Due to the fact that a comparison is made using an object having standard dimensions, these measurements are sometimes referred to as comparative measures.
The measurement becomes simpler the more rigidly defined a reference device's shape and dimensions are.
The narrow measurement range of this approach is another drawback, though.
Direct method of measurement
Direct measurements involve taking measurements of the target's dimensions using tools like Vernier calipers, micrometers, and coordinate measuring machines.
They are also referred to as absolute measurements.
The measuring instrument's scale allows measurements to be made across a wide range, however, there is a danger that the measurement will be off due to inaccurate scale readings.
Fundamental method of measurement.
The basic measurement method bases its measurement on the measurement of the base quantities that serve as the quantity's definition.
The definition of the quantity being measured and the quantity itself are both directly measured.
Examples include using a fixed point instrument to measure temperature.
Substitution method of measurement.
In the substitution method, the value of the quantity being measured is not determined indirectly from a reading of the measuring instrument, but rather from the magnitude of the standard, which is chosen or regulated in such a way that the measurement instrument's reading is maintained when the quantity being measured is replaced by the standard.
In order to weigh an object on a beam balance, for instance, the object is removed from the pan and replaced with weights of equal total weight; the balance will display the prior reading (the Borda method).
Measurements of electrical properties including resistance, capacitance, and inductance are frequently conducted using the replacement approach.
Comparison method of measurement.
The value of the quantity to be measured is compared with another quantity that is practically connected to it or with a known value of the same quantity in this method.
Therefore, only deviations from a master gauge, such as dial indications or other comparators, are determined using this method.