GMT METROLOGY DIVISION - SYSTEMS OF MEASUREMENT

SYSTEMS OF MEASUREMENT

Errors in Measurements There is a true fact that, no measurement is exact. All measurements are subject to some error. It is therefore necessary to state not only the measured dimension, but also the accuracy of determination to which the measurement is made. As for as possible the errors inherent in the method of measurement used should be kept to a minimum, and having minimized the error, its probable magnitude, or accuracy of determination should be stated. Along with the actual gauge block size details, there should be details regarding the measured error in the block and the accuracy of determination with it enclosed. The accuracy of determination can be improved by repeating the measurement a number of times and stating the mean value. Types of errors: There are two types of errors, •  Those which should not occur and can be eliminated by careful work and attention. •  Those which are inherent in the measuring process. Misreading an instrument, arithmetic errors, alignment errors, parallax error, errors due to temperature were some of the errors that we can eliminate on proper procedural handling of the system. When we can truly believe a measurement?   We can never have 100% confidence in a measurement. No measurement is ever correct. There is always an unknown, finite, non-zero difference between a measured value and the corresponding true value. Most instruments have specified or implied tolerance limits within which the true value of the measurement should lie if the instrument is functioning correctly. One can never be 100% sure that an instrument is operating within its specified tolerance limits. There are steps we can take to minimize the probability of a measurement falling outside specified tolerance or uncertainty bands. Regular traceable calibration is a method for gaining quantifiable confidence in a measurement system. For example, if we consider about the pressure transducer, there are a number of modes in which electronic circuitry and the digital display can fail or malfunction. Most of the faults and malfunctions would not be visible to an operator therefore it is impossible to verify the absence of faults and electronic drift by simple inspection. We cannot tell by inspection if the instrument has recently been dropped, subjected to an over-range pressure or otherwise mistreated. When we make a measurement in the field we are forced to trust the instrument. The only way we can gain confidence in the electronic manometer is by regularly comparing its response with another similar or preferably superior instrument in which we have a high level of confidence. A quantitative comparison or verification of the performance of an instrument is called a calibration.