Definition:”Calibration is the comparison of measurement valuesdelivered by a device under test with those of a calibration standard of knownaccuracy. Such a standard could be another measurement device of knownaccuracy, a device generating the quantity to be measured such as a voltage, ora physical artefact, such as a meter ruler.” (Wikipedia)When is it needed?Calibration is needed when we manufacture new instruments orwhen an instrument has been repaired. It is also needed after the instrument usageexceeds certain hours. It may also be needed if the observations from theinstrument are questionable.
Why is it Important?By calibrating an instrument, we can be certain about itsquality and accuracy. Certain quantity like length tend to change withtemperature. So, we need to make sure that we get correct values of samequantity over time and reliable readings are needed for industry standards.
Themain aim of calibration is to assure the accuracy of the instruments.For example:If we need the length of a particular component to be 50mm with tolerance of ±.05mmbut our instrument gives different readings, then we cannot get accurate results.
Thus, calibration is required.(Tempcon Instrumentation, 2012)Calibration of Outside Micrometer:Purpose:The aim of this procedure is to give general instructions forthe calibration of outside micrometers along with digital, Vernier, orcombination scales.Prior Instructions andNotes:We start calibration in a place that adheres to given specifications.We will now refer the outside micrometer as micrometer. The first step is to assure that the micrometer doesn’t haveany dust. Then we check the micrometer for any condition that could createerrors in the process. If there is a need to make changes, we do it and makesure that there is no damage done to the instrument.If there happens to be a defect or malfunction while calibrating,we stop the calibration and make necessary corrective steps.
If corrective stepsaffect a measurement that we took before, the value maybe recalibrated beforethe rest of the procedure is done. Figure 1: Outside Micrometer(Source: http://www.starrett.com/metrology/product-detail/Outside-Micrometers/Micrometers/Precision-Hand-Tools/Precision-Measuring-Tools/v230mxfl)Equipment Required:The following standard equipment is used: Hard Arkansas stone,Lint free clean cloth, cleaning solution, Gage oil and Gage block set.Detailed Procedure:1)The First step is to use a clean leant free cloth along withcleaning solution to clean the outside surfaces of the micrometer.
2) The micrometer is closed by a friction stop3) A source of light is kept nearby to visually inspect themicrometer.4) The spindle is then disassembled.5) The screw and oil spindle is removed carefully.
6) We have to clean the nut which is fixed in the barrel and alsooil it.7) Now we can assemble the micrometer back.8) The measuring screw can be checked for damage by moving themeasuring screw forward and backward. For a perfect movement they should bealigned properly.9) We can compensate for the error if found by pressing the nuton barrel to make it tight. This adjustment should be made to get a perfectfit.
10) Now the micrometer can be closed and check for zero fit.11) The accuracy of the micrometer can be checked by using a gaugeblock. A variety of sizes in gauge block can be used to do this. Readings mightbe taken multiple times for accuracy.12) For using the gauge blocks we first combine the gauge blocksto a required size and press them tightly, then measure them with micrometerand compare the values.