Methods of Measurement
Measurement of any quantity involves two parameters: the magnitude of the value and unit of measurement. For instance, if we have to measure the temperature we can say it is 10 degree C. Here the value “10" is the magnitude and “C" which stands for “Celsius" is the unit of measurement. Similarly, we can say the height of wall is 5 meters, where “5" is the magnitude and “meters" is the unit of measurement.
There are two methods of measurement: 1) direct comparison with the standard, and 2) indirect comparison with the standard. Both the methods are discussed below:
1) Direct Comparison with the Standard
In the direct comparison method of measurement, we compare the quantity directly with the primary or secondary standard. Say for instance, if we have to measure the length of the bar, we will measure it with the help of the measuring tape or scale that acts as the secondary standard. Here we are comparing the quantity to be measured directly with the standard.
Even if you make the comparison directly with the secondary standard, it is not necessary for you to know the primary standard. The primary standards are the original standards made from certain standard values or formulas. The secondary standards are made from the primary standards, but most of the times we use secondary standards for comparison since it is not always feasible to use the primary standards from accuracy, reliability and cost point of view. There is no difference in the measured value of the quantity whether one is using the direct method by comparing with primary or secondary standard.
The direct comparison method of measurement is not always accurate. In above example of measuring the length, there is limited accuracy with which our eye can read the readings, which can be about 0.01 inch. Here the error does not occur because of the error in the standards, but because of the human limitations in noting the readings. Similarly, when we measure the mass of any body by comparing with some standard, it’s very difficult to say that both the bodies are of exactly the same mass, for some difference between the two, no matter how small, is bound to occur. Thus, in direct method of measurement there is always some difference, however small, between the actual value of the quantity and the measured value of the quantity.
2) Indirect Method of Measurement
There are number of quantities that cannot be measured directly by using some instrument. For instance we cannot measure the strain in the bar due to applied force directly. We may have to record the temperature and pressure in the deep depths of the ground or in some far off remote places. In such cases indirect methods of measurements are used.
In the indirect method of measurements some transducing devise, called transducer, is used, which is coupled to a chain of the connecting apparatus that forms the part of the measuring system. In this system the quantity which is to be measured (input) is converted into some other measurable quantity (output) by the transducer. The transducer used is such that the input and the output are proportional to each other. The readings obtained from the transducer are calibrated to as per the relations between the input and the output thus the reading obtained from the transducer is the actual value of the quantity to be measured. Such type of conversion is often necessary to make the desired information intelligible.
The indirect method of measurements comprises of the system that senses, converts, and finally presents an analogues output in the form of a displacement or chart. This analogues output can be in various forms and often it is necessary to amplify it to read it accurately and make the accurate reading of the quantity to be measured. The majority of the transducers convert mechanical input into analogues electrical output for processing, though there are transducers that convert mechanical input into analogues mechanical output that is measured easily.
1) Book: Mechanical Measurements by Thomas G. Beckwith and N. Lewis Buck