 I welcome you all for the second lecture in the series. In the first lecture, we started discussion on basic concepts of metrology. Now, we will continue with that. Today, we will see about metrology terminologies. The first one is accuracy of a measurement system. It is the degree of closeness of measurements of a quantity to that quantities actual or true value. So, we can understand the term by referring to this figure. We have x axis, x axis, on the x axis, we have values measurement measured values and along y axis, we have probability density and all the measurement points are scattered like this. Now, we can always take calculate the mean of these values. So, this line represents mean value of all the measured points and this indicates the reference value or the desired value. Now, we can see that there is a gap between reference value and the mean value. If the mean value coincides with the reference value, then we say the measurement system is very, very accurate. Now, we have another set of pictures here. Now, the center point represents the target value or the desired value and measurement points are scattered around the target value. Now, when we calculate the average of these measure points, it will be very close to the target value. So, we say the process is very accurate, but the process is not so precise because of scattering of the measure points. Now, in the second picture, we can see that all the measure points are very close to each other and when we calculate the average, it will be somewhere here. Now, there is a gap between the target value and the average value. So, we say this process, this measurement system is precise, but not so accurate. Now, let us try to understand the precision of the measurement system. It is related to the reproducibility and repeatability of the process. It is a degree to which repeated measurements under unchanged conditions show the same results that we have understood when we studied these two pictures. This process is very precise and here the measurement system is not so precise. Now, the second term is sensitivity of an instrument. This refers to a minimum change in value of quantity being measured that an instrument can respond. If you take an example of a dial indicator, when we apply a displacement of 0.1 millimeter and if the dial indicator gives response to that signal, then we say the sensitivity of the dial indicator is 0.1 millimeter and we have another term resolution. It is also known as discrimination. It is the fineness to which an instrument can be read. In the figure, we can see a digital stop watch and we have two digits beyond the seconds. So, it subdivides time into hundreds of a second. Hence, 1 upon 100 of a second is the resolution of this digital watch. Now, calibration is the quantitative determination of errors of measuring instruments and adjusting them to a minimum. In the previous discussion, we have understood that all the instruments undergo wear due to continuous usage and due to wear, there will be an error in the instrument. So, at regular intervals, we have to compare the accuracy of these instruments with the equipment available at standards room of the plant and then whatever error is there, they should be recorded on the instrument. This process is known as calibration. Now, interchangeability. So, this is the components selected randomly should assemble correctly with any other mating component. This is possible when certain standards are strictly followed. Due to the mass production, all the mating parts, they are made at different places by different operators. When we try to mate them by selecting randomly, they will mate easily without any individual fitting operation. So, this process is known as interchangeability. This will be possible only when we use standard instruments and standard measuring methods. Now, what are the advantages of interchangeability? Now, since the mating parts fit easily, the assembly time will be reduced and assembly cost will be reduced. Now, since the parts produced in mass, they are available in the market readily, the repair work will be cheap and it will be very quick. We need not have to order for the mating part for a new fabrication process. Now, distortion is also known as a base law. The scale of a linear measuring system should be collinear with the displacement to be measured. If you take the example of this Vernier caliper, we can understand that we keep the workpiece between anvil and spindle. So, the axis of the workpiece will be here. It is collinear with the axis of the instrument. So, in that cases, the distortion will be minimum. When we take the example of a Vernier caliper, we can see that the workpiece is kept between the two jaws and when we apply pressure here, there are chances that the jaw may bend like this, introducing an error component shown here. So, the distortion can be eliminated by designing the instrument such that the axis of the workpiece coincides with the axis of the instrument. Now, we have another term known as 0 error. The instrument does not read 0 when the input is 0. So, this happens due to the change in the dimensions of the internal components due to variation in the temperature. This can be easily adjusted by adjusting a kind of 0 offset adjustment provided. Now, in this picture, we can see that when there is no signal applied, the pointer is showing some deflection. So, this is known as 0 error or 0 drift. Parallax error occurs when the pointer on a scale is not observed along a normal, a line normal to the scale. Now, this we can understand with the help of this diagram. We have the scale here and we have the pointer here and the observer is observing the pointer from three different positions. This is position number 1, position number 2 and we have position number 3. When the observer observes the scale and pointer from the location 2 is observing the scale normally, then we get the correct reading. When he observes from the location 1, now he may get the reading at this place which is incorrect reading. Similarly, when the observer observes from location 3, again there will be an error. So, this parallax error can be eliminated by reducing the distance between scale and pointer. Now, range of the instrument indicates the maximum and minimum capacity of measurement. What is the maximum physical quantity we can measure and what is the minimum capacity we can measure? The difference gives the range. We can observe this dial indicator. Now, for one complete revolution of this pointer, the displacement will be 1 millimeter. So, now we have in this small dial, we have totally 10 graduations are there. So, the range of this instrument is 10 millimeter. Now, we have the term traceability. It means that a measured result can be related to stated references. Usually, national and international standards throw an unbroken chain of comparison that is step by step comparison with better standards. So, this step by step comparison with better standards, we can understand by studying this flow chart. Now, this is the manufacturing area wherein the production is going on. Components are produced, assembled and then finally, we get the product. During the process of production, the quality control department will inspect all the components produced. Now, due to continuous usage of these instruments, the instruments are subjected to wear and the measurement error creeps in. So, to eliminate the measurement error at regular intervals, we need to inspect or we need to compare all the instruments used in the workshop with the plant standards laboratory equipments which are of higher accuracy. By comparing the workshop instruments with the plant standards laboratory equipment, we can always get the amount of error in the measuring instrument and that can be noted on the instrument. So, while preparing the inspection report, we should give consideration. We should note down what is the error that is indicated on the instrument. Then, the plant standards laboratory equipment should be compared with the equipment available at NABL accredited professional labs which are of superior quality. So, in turn the accredited professional labs instruments are compared with national bureau of standards. Like this, all the measuring instruments are compared with the higher accuracy instruments in step by step level. Now, the term readability, it refers to the ease with which a reading of measuring instruments can be taken. That means, what is the amount of easiness of reading is readability. Now, if you observe this diagram of dial indicator, we can see that we have a bigger dial and we have a smaller dial. We have a bigger pointer and we have a smaller pointer. We can see that this is the reference point 0 and then we have a graduations. So, this is 10th graduation. In between, we have 10 lines. So, each small graduation indicates 1 micrometer. So, now very easily we can read this dial indicator. It indicates that readability of this indicator is very good. In some cases, if the graduations are very fine, then we may have to use magnifying lenses or we may have to use microscope for reading the instruments. Now, nominal value or true value is the required value of the quantity or required size of the quantity. For example, if you take a shaft, the required quantity is 10 millimeter. So, 10 millimeter is suggested by the design engineer and that is the required value or nominal value of the component. Now, let us understand what is the error in the measurement. Error in the measurement of a physical quantity is its deviation from actual value or nominal value. Now, if we see this expression error is equal to indicated value that is the measurement given by the instrument and this is the true value of the component. The difference gives the amount of error. For example, say the indicated value. So, we are measuring the diameter of a spindle and the micrometer gives a value of 9 millimeter, whereas the true value is 10 millimeter. So, the difference is minus 1 millimeter. So, this is the error of measurement. Correction is the value added algebraically to the uncorrected result of measurement to get the true value. This has a sign opposite to the error sign. Again, we can take the example of measurement of diameter of the spindle. Now, uncorrected result of measurement that is indicated value is 9 millimeter. We need to add the correction factor. So, the error is minus 1 millimeter. So, we need to add plus 1 millimeter to get the true value that is 10 millimeter. Now, we can observe that error is minus 1 millimeter, whereas the correction factor is plus 1 millimeter. So, this is sign is opposite to the sign of the error. Now, the term measurement. It is a particular quantity subjected to measurement. For example, length, flatness, roundness, surface finish, etcetera, these are measured quantities or measurements. Now, the next topic is methods of measurement. Different kinds of measurement methods are used and we will discuss a few methods which are relevant to the metrology subject. So, first one is direct method. In this method, the physical quantity is obtained directly without any calculations. For example, measurement of length by 1 year caliper. You can see that we have the component whose dimension, whose length is to be measured. We keep the workpiece between the moving jaw and measuring jaw and then we slowly apply small pressure and then we get the reading in the display. So, in this case, no calculation is needed. The measuring instrument will directly display the measured quantity. So, it will be fast. Direct method measurement will be fast, but there are some disadvantage in this particular case. If you apply more pressure, then the moving jaw may compress the workpiece and we may get wrong result. So, sense of feel is very important in this case. Now, second method is indirect method in which the value of the quantity is obtained by measuring other quantities which are related with the required quantity. For example, measurement of angle using a sine bar. Now, this picture shows the setup for measurement of angle using sine bar. Now, the component whose taper is to be measured or angle is to be measured should be kept on the datum surface. So, this is the tapered component and we need to measure the taper of this component and now we have to select a sine bar of appropriate length and then it is placed on the tapered component. Now, you can see that there is a gap between this roller and the datum. We have to fill it using slip gauges. So, this h is the height of slip gauge and then L is the length of sine bar. So, this L is distance between center of this roller and this roller. The center distance is L. Now, we can calculate the taper angle of the workpiece with using this relationship sine theta is equal to h divided by L. So, this is how we calculate the taper angle. So, some calculation is involved in indirect method. Now, the third method is comparison method in which the value of the quantity to be measured is compared with a known value of the same quantity or other related quantity. In this method only deviations from master gauges are recorded. For example, use of dial indicator as a comparator. We can observe this diagram wherein we have mounted the dial indicator on the stand on this column and then we have inserted slip gauge between the datum surface and the spindle to set the desired value. And then we have to remove the slip gauge and we have to insert the workpiece whose height is to be measured. Now, when we insert the workpiece between datum and spindle, the pointer moves and it indicates the deviation of the height with reference to the slip gauge height. Now, the fourth method is deflection method in which the value of the quantity to be measured is directly indicated by the deflection of a pointer on a calibrated scale. Again, we can take the example of dial indicator when we insert the workpiece between the datum and the plunger, the pointer deflects and it gives the displacement. Now, the next method is complementary method in which the value of the quantity to be measured is combined with a known value of the same quantity. Example, measuring the volume of a solid by liquid displacement. Now, let me explain this complementary method. So, initially we have to take a beaker and then we have to fill it with some liquid say water. Now, say the water level is 20 units, maybe 20 millimeter cube or 20 centimeter cube. Now, we have a solid whose volume is to be measured that we have to insert into this beaker. Now, you can see there is a displacement of liquid from 20 to 30 millis 30 centimeter cube. So, this displacement gives the volume of the solid that is immersed in the liquid. Now, transposition method in this method quantity to be measured is first balanced by a known value and then balanced by another known value. So, by this method we will come to know about the value of the physical quantity that is to be measured. So, one example is determination of mass by balancing method. First four methods are very important from the metallurgical point of view. Now, let us move to another topic that is selection of instruments. So, how to select the instruments? In order to select the instrument we should know what is that we are measuring, what is the environment of measurement and what is the material to be inspected, what are the physical quantities to be measured. So, all those things we should study before we select the instrument. Some of the essential factors for proper selection of measuring instrument are listed below. The range of the instrument this indicates what is the minimum value we can measure and what is the maximum value that we can measure. If we take the example of a dial indicator the range of movement of the plunger is say 10 millimeter. Then in that case range of the instrument is 0 to 10 millimeter. Then resolution or discrimination smallest dimensional input that the instrument can detect. This depends upon accuracy level what we expect for the measurement process. If we try to measure the physical quantity to a very accurate level for example, length of a spindle we want to measure up to say 0.001 millimeter. Then we should select instrument accordingly. Then accuracy expected demand and accuracy of measurement higher than really needed higher the degree of accuracy higher the cost of measuring instrument. Now, what are the installation requirements sometimes measuring instruments requires certain amount of installation requirements like mounting requirement how the instrument is to be mounted whether any magnetic stand is required or a column is required or vibration isolation system is required or there is any requirement of compressed air for example, air gauges they require compressed air for their operation any wireless system is required. What is the distance between the place of measurement and the control room what are the ambient conditions like whether the measurement is required at certain temperature or pressure or humidity levels or whether we require some measurement to be taken under water or in space like that. And what about the need of telemetry so all these things we should understand and accordingly we should select the instruments for example, if it is necessary to measure underwater measurements then we need to have instruments which are leak proof. Then final data requirement whether the data required is immediate or later use whenever we require data to be read immediately then we should go for instruments having indicators may be mechanical indicators or dial indicators or digital indicators. So, that we can record the measurement result if it is required if the measurement data is required at a later for a later use then we can always store the data and we can use it at a later stage in that case we need to have data equation systems. Now, what is the data format that is required indication or recording accordingly we have to select if indication is required we should select the instruments with indications like the dial indicator with digital display and if recording is required recording of data is required then we have to select data equation system having necessary data equation speed and data equation capacity. And if the measurement has to be taken place at a very difficult to assess environment may be we have to select the miniature probes. So, that we can comfortably measure the difficult to assess places dimensions. Then whether single or multiple quantity how many instruments at a time we need to use if we want two three or four or more than two instruments are there which are taking measurements then a data equation system we should select accordingly which are having multiple channels of acquisition. Sometimes we may have to dump the data measurement data to the softwares data management softwares for getting the statistical characteristics. So, in such cases we need to select the measurement which are having the data transmission capability may be in the form of RS 232 or USB ports and then necessary cabling also we should select. Then cost factor is very very important supposing we need to have some advanced technologies like coordinate measuring machines or some laser based systems or some automatic measurement systems non-contract measurement systems. Then normally they are costly and we should see the financial health of the manufacturing plant whether it can support such advanced technologies and whether recovery the economic aspects we have to study. And then nature of measuring whether the quantity measure quantity is static or dynamic whether the signal or the physical quantity is slowly changing or rapidly changing. And what is the response of the instrument all those things we need to understand before we select the instrument. Then what is the parameter to be measured whether length is to be measured or diameter is to be measured, surface finish is to be measured, some form of the physical component is to be measured like taper angle or the shape of the component like the drum shape or barrel shape. So, accordingly we should select the dimensional measurement instrument or form measurement instrument or surface finish measurement instrument. Then what is the skill required? So, depending upon that also whether measurement skill is needed or any unskilled operator can use that is also another important point before we select the instrument. And what is the life expectancy or stability of the instrument if you want to use for a longer period then we should always go for instruments which have better life expectancy. And then what are the environmental effects whether readings are affected by the changes in pressure, temperature etcetera. So, environmental compatibility also we need to understand then any need for compensation for example, there is deviation from the temperature, then in that case we should know what is the amount of change in the dimension due to the temperature change. And we need to compensate for that where temperature errors due to changes or we need to calibrate the instruments at that particular temperature where the measurements are taken. Then let us take an example of selection of a pressure gauge to measure 6 bar. Now, if we select a pressure gauge having 6 range of 6 bar then it becomes exact range and it will be most sensitive less errors, but prone to overload. If the pressure increases beyond 6 bar then there the pressure gauge may be subjected to some damage. Then if we select 60 bar the if we select the pressure gauge having 0 to 60 bar range then it becomes excessive range low sensitivity at lower end of the range errors will be too high. So, if we select a pressure gauge between 0 to 25 bar then it will be alright. Sometimes we need to measure both pressure as well as vacuum in that case in that case we need to go for combined gauges. Now, there is another important term rule of 10 the measuring instrument should be 10 times more precise than the tolerance to be measured that means we the tolerance levels of the component are say in terms of 0.1, 0.2 millimeter. Then we in order to measure such a component we need to select the instrument which are capable of giving 0.01, 0.02 millimeter readings. So, magnification needed so magnification is the process of enlarging something only in appearance not in the physical size. So, we need to understand whether any magnification is needed. If the measuring instrument is very simple like steel rule we need not have to use any magnifying systems, but if we need to measure very minute dimensions like the lens in terms of 0.01 millimeter or 0.02 millimeter. In that case we need to magnify the quantity physical quantity that is measured. So, we can magnify the readings by using mechanical means or optical means. So, this picture shows how we can magnify the physical quantity using a mechanical means. We can use a rack and pinion and then the gear trains. So, that the physical quantity is amplified and we can comfortably read the readings. We can also amplify the signals using optical means in mechanical means the magnification of 100, 200, 300 are possible whereas, if we use optical means more than 300 magnification is easily achievable. Now, let us move to the next topic measurement errors. Now, measurement error is the difference between the actual size of the workpiece and the measured value. If the same workpiece is successively measured with an instrument each successive measurement will show differences in dimension. Now, what are the reasons for this? Reasons for inaccuracy in measurement are listed below. The inaccuracy may be due to instrument error that is calibration error or hysteresis, backlash, misalignment of the instrument. So, these terminologies we will discuss subsequently. Then the inaccuracy may be due to environmental effects like vibration or temperature changes. Inaccuracy may be also due to human error that means, sense of feel or it may be due to reading error like parallax error or it may be due to fatigue of the operator. The error may be due to bad assembly of the instrument that means, the measuring instrument is not assembled in a proper way. There are some slackness in the assembly of various components. So, due to that also the error will creep in and then imperfect datum used it is very essential that the datum we use should be very accurate. So, that we get correct readings sometimes the datum may be like this it has some errors. Now, we have the workpiece like this we have kept the workpiece on the datum and now we use an instrument to measure the height of this workpiece. Now, this is the spindle. Now, this is the depth measuring instrument. Now, if the datum is like this perfect flat then this will be the reading of the instrument. Now, there is some error in the datum itself. So, we will get an error which will be equal to this size. So, it is very essential that we always use good datum surfaces. Now, the measurement errors can be classified into controllable errors and non controllable errors. Under controllable, controllable error means we can control the amount of these errors by adjusting the working environment. That means under controllable errors errors are due to environmental conditions, errors are due to elastic deformation, deflection errors, alignment errors, parallax errors and error due to improper instrument. Now, under non controllable errors these are these cannot be controlled, but we can always measure what is the amount of error and then we can while preparing the inspection reports we have to account for these non control what is the amount of non controllable error we should know and accordingly we should present the inspection reports. Now, under controllable errors the first one is environmental errors. These are due to the effect of surrounding temperature variation in the surrounding temperature pressure and humidity or it may be due to vibration and shock electric fields. Now, we need to understand that when we conduct the measurement process we have to maintain a standard room temperature of 20 degree Celsius which is internationally accepted. When we keep the work pieces for measurement, when we take the work pieces for measurement we have to keep the work pieces at this temperature and we have to allow for temperature stabilization then only we should go for measurement. Then, if the temperature is deviated from this standard temperature say 22 degree or 23 degree then we need to compensate for that variation in the temperature. So, that compensation we can calculate using this relationship error is equal to L times alpha times T minus T 2 where L is the nominal value and then alpha is the coefficient of thermal expansion of the material work piece material and T minus T 2 is the deviation from standard temperature. If T is 20 degree and T 2 is 22 degree then deviation is 2 degree. Using this relationship we can always calculate the error due to environmental factors and we have to account for this when we give the measurement results. Now, if we have two different kinds of materials then the error can be calculated using this relationship L times alpha minus alpha 2 times T minus T 2 where alpha 1 is coefficient of thermal expansion of the first material and alpha 2 is coefficient of expansion for second material. So, measurement error may be also due to improper datum surface which we discussed already and may be there is presence of dirt and presence of chips due to this also the measurement error may creep in. That means, we need to clean the datum surface properly so that there is no dirt or chip. So, this we can understand by the simple diagram say there is a dust particle or dirt particle and then if we use place the instrument like this then this much error will be introduced. So, we should clean the datum surface and then we should go for measurement process. Now, we are coming to the end of second lecture. So, in this lecture we discussed about various meteorological terminologies and what are the different methods of measurement. We also discussed about selection of instruments based upon the parameters to be checked and then we started discussion on measurement errors. In the next lecture we will continue this discussion on measurement errors. Thank you.