Classifications of Methods of Measurements

Classifications of Methods of Measurements

In precision measurements various methods of measurement are followed depending upon the accuracy required and the amount of permissible error.

There are numerous ways in which a quantity can be measured. Any method of measurements should be defined in such a detail and followed by such a standard ​practice that there is little scope for uncertainty. The nature of the procedure in some of the most common measurements is described below. Actual measurements may employ one or more combinations of the following.

(i) Direct method of measurement: In this method the value of a quantity of obtained directly by comparing the unknown with the standard. It involves no mathematical calculations to arrive at the results, for example, measurement of length by a graduated scale. The method is not very accurate because it depends on human insensitiveness in making judgement.

(ii) Indirect method of measurement: In this method several parameters (to which the quantity to be measured is linked with) are measured directly and then the value is determined by mathematical relationship. For example, measurement of density by measuring mass and geometrical dimensions.

(iii) Fundamental method of measurement: Also known as the absolute method of measurement, it is based on the measurement of the base quantities used to define the quantity. For example, measuring a quantity directly in accordance with the definition of that quantity, or measuring a quantity indirectly by direct measurement of the quantities linked with the definition of the quantity to be measured.

(iv) Comparison method of measurement: This method involves comparison with either a known value of the same quantity or another quantity which is function of the quantity to be measured.

(v) Substitution method of measurement: 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 which produce same effect on the indicating device. For example, determination of mass by Borda method.

(vi) Transposition method of measurement: This is a method of measurement by direct comparison in which the value of the quantity to be measured is first balanced by a initial known value A of the same quantity; next the value of the quantity to be measured is put in the place of that known value and is balanced again by a second known value B. When the balance indicating device gives the same indication in both cases, the value of the quantity to be measured is AB . For example, determination of a mass by means of a balance and known weights, using the Gauss double weighing method.

(vii) Differential or comparison method of measurement: This method involves measuring the difference between the given quantity and a known master of near about the same value. For example, determination of diameter with master cylinder on a comparator.

(viii) Coincidence method of measurement: In this differential method of measurement the very small difference between the given quantity and the reference is determined is determined by the observation of the coincidence of scale marks. For example, measurement on vernier caliper.

(ix) Null method of measurement: In this method the quantity to be measured is compared with a known source and the difference between these two is made zero.

​(x) Deflection method of measurement: In this method, the value of the quantity is directly indicated by deflection of a pointer on a calibrated scale.

(xi) Interpolation method of measurement: In this method, the given quantity is compared with two or more known value of near about same value ensuring at least one smaller and one bigger than the quantity to be measured and the readings interpolated.

(xii) Extrapolation method of measurement: In this method, the given quantity is compared with two or more known smaller values and extrapolating the reading.

(xiii) Complimentary method of measurement: This is the method of measurement by comparison in which the value of the quantity to be measured is combined with a known value of the same quantity so adjusted that the sum of these two values is equal to predetermined comparison value. For example, determination of the volume of a solid by liquid displacement.

(xiv) Composite method of measurement: In involves the comparison of the actual contour of a component to be checked with its contours in maximum and minimum tolerable limits. This method provides for the checking of the cumulative errors of the interconnected elements of the component which are controlled through a combined tolerance. This method is most reliable to ensure inter-changeability and is usually effected through the use of composite ―Go‖ gauges, for example, checking of the thread of a nut with a screw plug ―GO‖ gauge.

(xv) Element method: In this method, the several related dimensions are gauged individually, i.e., each component element is checked separately. For example, in the case of thread, the pitch diameter, pitch, and flank angle are checked separately and then the virtual pitch diameter is calculated. It may be noted that value of virtual pitch diameter depends on the deviations of the above thread elements. The functioning of thread depends on virtual pitch diameter lying within the specified tolerable limits. In case of composite method, all the three elements need not be checked separately and is thus useful for checking the product parts. Element method is used for checking tools and for detecting the causes of rejects in the product.

(xvi) Contact and contact less methods of measurements: In contact methods of measurements, the measuring tip of the instrument actually touches the surface to be measured. In such cases, arrangements for constant contact pressure should be provided in order to prevent errors due to excess contact pressure. In contactless method of measurements, no contact is required. Such instruments include tool – maker‘s microscope and projection comparator, etc.

For every method of measurement a detailed definition of the equipment to be used, a sequential list of operations to be performed, the surrounding environmental conditions and descriptions of all factors influencing accuracy of measurement at the required level must be prepared and followed.

​Metrological characteristics of Measuring Instruments:-

Measuring instruments are usually specified by their metrological properties, such as range of measurement, scale graduation value, scale spacing, sensitivity and reading accuracy.

Range of Measurement: It indicates the size values between which measurements may be made on the given instrument.

Scale range: It is the difference between the values of the measured quantities corresponding to the terminal scale marks.

Instrument range: It is the capacity or total range of values which an instrument is capable of measuring. For example, a micrometer screw gauge with capacity of 25 to 50mm has instrument range of 25 to 50mm but scale range is 25mm.

Scale Spacing: It is the distance between the axes of two adjacent graduations on the scale. Most instruments have a constant value of scale spacing throughout the scale. Such scales are said to be linear.

In case of non – linear scales, the scale spacing value is variable within the limits of the scale.

Scale Division Value: It is the measured value of the measured quantity corresponding to one division of the instrument, e.g. for ordinary scale, the scale division value is 1mm. As a rule, the scale division should not be smaller in value than the permissible indication error of an instrument.

Sensitivity (Amplication or gearing ratio): It is the ratio of the scale spacing to the division value. It could also be expressed as the ratio of the product of all the larger lever arms and the product of all the smaller lever arms. It is the property of a measuring instrument to respond to changes in the measurement quantity.

Sensitivity Threshold: It is defined as the minimum measured value which may cause any movement whatsoever of the indicating hand. It is also called the discrimination or resolving power of an instrument and is the minimum change in the quantity being measured which produces a perceptible movement of the index.

Reading Accuracy: It is the accuracy that may be attained in using a measuring instrument.

Reading Error: It is defined as the difference between the reading of the instrument and the actual value of the dimension being measured.

Accuracy of observation: It is accuracy attainable in reading the scale of an instrument. It depends on the quality of the scale marks, the width or the pointer / index, the space between the pointer and the scale, the illumination of the scale, and the skill of the inspector. The width of scale mark is usually kept one – tenth of the scale spacing for accurate reading of indications.

​Parallax: It is apparent change in the position of the index relative to the scale marks, when the scale is observed in a direction other than perpendicular to its plane.

Repeatability: It is the variation of indications in repeated measurements of the same dimension. The variations may be due to clearances, friction and distortions in the instrument‘s mechanism. Repeatability represents the reproducibility of the readings of an instrument when a series of measurements in carried out under fixed conditions of use.

Measuring force: It is the force produced by an instrument and acting upon the measured surface in the direction of measurement. It is usually developed by springs whose deformation and pressure change with the displacement of the instrument‘s measuring spindle.