Measuring Instruments in Experimental Research

There are very few experiments in science and technology, even when only an object or a phenomenon is to be observed, in which no measuring instruments are involved. And when the change in a measurable quantity constitutes the effect (sometimes referred to as outcome or quality characteristic) part of the hypoth­esis, the need for measuring instruments is all too obvious. The extent and kind of instruments needed, besides the elementary ones for the measurement of lengths, weights, volumes, time, temperature, and so forth, which are taken for granted as avail­able to all researchers, depend on the field of study and the accu-racy required, as dictated by the hypothesis. A few terms and concepts, which apply to most instruments, follow.

Instruments may be broadly divided in a somewhat overlap­ping fashion into two classes: gages and meters.

Gages are usually replicas of the corresponding shape and dimensions to be measured, used on a comparative basis as refer­ences, for example, the “Go and Not Go” gage for checking the tolerance of a hole. But we also speak of “pressure gages,” where pressure is directly read on the instrument; in strict terminology, it ought to be a “pressure meter.” With similar freedom, we also refer to the “depth gage,” “strain gage,” “radius gage,” and so on. The word “gage” has traditionally been used for other purposes also as indicating thickness of sheet materials, the diameters of wires, the width between railroad rails, the size of gun barrels, to name a few.

Meters, in conservative terminology, are instruments that read in real time on a cumulative basis the quantity involved (e.g., water meter) or the intensity of a property over a wide range (e.g., thermometer). But the word “gage” has been widely used, often subsuming the meaning of “meter.”

Resolution is the smallest difference in dimension that an instrument can present. For instance, a ruler with a millimeter division has a resolution of 1 mm. On the other hand, a micrometer (without a vernier) has a resolution of 0.1 mm. Sensi­tivity is an alternate term used.

Precision refers to the reproducibility or repeatability of the same reading at different times and in different environments; for example, a steel ruler is likely to be more precise for measuring lengths than a wooden ruler, considering that wood expands with high humidity and contracts with high temperature. Often, wrongly, the term accuracy is used in place of precision.

Accuracy refers to the extent of closeness of a measurement to its “true” value, as judged by an appropriate standard. For instance, a ruler measurement of, say, 2.31 cm may be compared with 2.31 cm obtained by stacking gage blocks on a surface plate. To the extent that the ruler reading coincides with that obtained with gage blocks, the ruler is accurate.

All the above references were made, for convenience, to length measurements, other length-related measures being thickness, diameter, taper, flatness, tolerances, and so forth. All of these are dealt with in an area of study known as metrology. The terms described above also apply to measurements of volume, weight, and time and, further, even to measuring instruments such as pressure gages, ammeters, and voltmeters.

In the course of their application, hence, at the point of selec­tion, the measuring instruments are to be judged relative to a few more considerations:

• Speed of response indicates how rapidly the instrument indi­cates the measurement. This is of particular importance when many readings are to be taken at short intervals.
• Stability is the virtue by which an instrument holds its reading over an extended duration (without fluctuations).
• Linearity is referred to as being “high” when an instrument presents accurate readings over its full working range.

Source: Srinagesh K (2005), The Principles of Experimental Research, Butterworth-Heinemann; 1st edition.