ERRORS IN MEASUREMENTS
It is never possible to measure the true
value of a dimension there is always some error. The error in measurement is
the difference between the measured value and the true value of the measured
dimension.
Error
in measurement = Measured value - True value
The error in
measurement may be expressed or evaluated either as an absolute error or as a relative
error.
Absolute Error
True
absolute error:
It is the algebraic difference between
the result of measurement and the conventional true value of the quantity
measured.
Apparent
absolute error:
If the series of measurement are made
then the algebraic difference between one of the results of measurement and the
arithmetical mean is known as apparent absolute error.
Relative
Error:
It is the quotient of the absolute error
and the value of comparison use or calculation of that absolute error. This
value of comparison may be the true value, the conventional true value or the
arithmetic mean for series of measurement. The accuracy of measurement, and
hence the error depends upon so many factors, such as:
-calibration standard -Work piece -Instrument
-Person -Environment etc
Types of Errors
1. Systematic
Error
These errors include
calibration errors, error due to variation in the atmospheric condition
Variation in contact pressure etc. If properly analyzed, these errors can be
determined and reduced or even eliminated hence also called controllable
errors. All other systematic errors can be controlled in magnitude and sense
except personal error.
These errors results
from irregular procedure that is consistent in action. These errors are repetitive
in nature and are of constant and similar form.
2. Random Error
These errors are caused
due to variation in position of setting standard and work-piece errors. Due to
displacement of level joints of instruments, due to backlash and friction,
these error are induced. Specific cause, magnitude and sense of these errors
cannot be determined from the knowledge of measuring system or condition of
measurement. These errors are non-consistent and hence the name random errors.
3. Environmental Error
These errors are caused
due to effect of surrounding temperature, pressure and humidity on the
measuring instrument. External factors like nuclear radiation, vibrations and
magnetic field also leads to error. Temperature plays an important role where
high precision is required. e.g. while using slip gauges, due to handling the
slip gauges may acquire human body temperature, whereas the work is at 20°C. A
300 mm length will go in error by 5 microns which is quite a considerable
error. To avoid errors of this kind, all metrology laboratories and standard
rooms worldwide are maintained at 20°C.
Calibration
It is very much
essential to calibrate the instrument so as to maintain its accuracy. In case
when the measuring and the sensing system are different it is very difficult to
calibrate the system as an whole, so in that case we have to take into account
the error producing properties of each component. Calibration is usually
carried out by making adjustment such that when the instrument is having zero measured
input then it should read out zero and when the instrument is measuring some
dimension it should read it to its closest accurate value. It is very much
important that calibration of any measuring system should be performed under
the environmental conditions that are much closer to that under which the
actual measurements are usually to be taken.
Calibration is the process of checking
the dimension and tolerances of a gauge, or the accuracy of a measurement
instrument by comparing it to the instrument/gauge that has been certified as a
standard of known accuracy. Calibration of an instrument is done over a period
of time, which is decided depending upon the usage of the instrument or on the
materials of the parts from which it is made. The dimensions and the tolerances
of the instrument/gauge are checked so that we can come to whether the
instrument can be used again by calibrating it or is it wear out or
deteriorated above the limit value. If it is so then it is thrown out or it is
scrapped. If the gauge or the instrument is frequently used, then it will
require more maintenance and frequent calibration. Calibration of instrument is
done prior to its use and afterwards to verify that it is within the tolerance
limit or not. Certification is given by making comparison between the
instrument/gauge with the reference standard whose calibration is traceable to
accepted National standard.
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