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Quality assurance errors

This document discusses types of errors that can occur in measurement. It describes absolute and relative errors, and how errors can be expressed. There are various sources of error, including the instrument, workpiece, person, and environment. Errors are classified as systematic/controllable or random. Systematic errors include calibration errors, environmental errors, stylus pressure errors, and avoidable errors. Random errors cause fluctuations that are positive or negative.

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Metrology & Quality Assurance Department of Mechanical Engineering University of Engineering & Technology Lahore
ERRORS  Measure dimension difference between the measured value and the true value.  Never possible to measure a dimension without error.  Error in measurement may be expressed or evaluated as:  Absolute error  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 used for 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: I. Calibration standard II. Work piece III. Instrument IV. Person V. Environment etc.
Types of Errors  During measurements several types of error may arise, these may be 1. Static Error 2. Reading errors 3. Characteristics error 4. Environmental error 5. Instrument loading error 6. Dynamic error
Static Errors Occurs due to physical nature of the various components of measuring system. Static Errors include  Reading Errors  Characteristics Errors  Environmental errors
Reading Errors  Reading errors apply exclusively to the read-out device.  Don't have any direct relationship with other types of errors within the measuring system.  Parallax error and Interpolation error.  Attempts have been made to reduce or eliminate reading errors by relatively simple techniques
Characteristics Error  Deviation of the output of the measuring system from the theoretical predicted performance or from nominal performance specifications.  Linearity errors, repeatability, hysteresis and resolution errors are part of characteristic errors if the theoretical output is a straight line.  Calibration error is also included in characteristic error.
Environmental Error  Occur the effect of surrounding such as temperature, pressure, humidity etc. on measuring system.  External influences like magnetic or electric fields, nuclear radiations, vibrations or shocks etc. also lead to environmental errors.  Environmental errors of each component of the measuring system make a separate contribution to the static error.  Reduced by controlling the atmosphere according to the specific requirements.
Instrumental Loading Error  Difference between the value of the measurand before and after the measuring system is connected/contacted for measurement. For example, the soft or delicate components are subjected to deformation during measurement due to the contact pressure of the instrument and cause a loading error
Dynamic Error  Error caused by time variations in the measurand.  Occur due to inability of the system to respond faithfully to a time varying measurement.  It is caused by inertia, damping, friction or other physical constraints in the sensing or readout or display system.  For statistical study and the study of accumulation of errors, these errors classified into two categories:  1. Systematic or controllable errors  2. Random errors.
Systematic Error  Systematic errors are regularly repetitive in nature. They are of constant and similar form.  Occur when improper conditions or procedures that are consistent in action.  Out of the systematic errors all except the personal error varies from individual to individual depending on the personality of observer.  Other systematic errors can be controlled in magnitude as well as in sense.  If properly analyzed they can be determined and reduced. Hence, these are also called as controllable errors.
Types of Systematic Error  Calibration Errors  Ambient or Atmospheric conditions (Environmental Errors)  Stylus Pressure  Avoidable Errors  Random Errors
1. Calibration Errors  These are caused due to the variation in the calibrated scale from its normal value.  The actual length of standards such as slip gauge and engraved scales will vary from the nominal value by a small amount. This will cause an error in measurement of constant magnitude.  Sometimes the instrument inertia and hysteresis effect do not allow the instrument to transit the measurement accurately
2. Ambient or Atmospheric Conditions (Environmental Errors)  Variation in atmospheric condition at the place of measurement from that of internationally agreed standard values (20° temp. and 760 mm of Hg pressure).  Give rise to error in the measured size of the component.  Instruments are calibrated at these standard conditions; therefore error may creep into the given result if the atmosphere conditions are different at the place of measurement.
3. Stylus Pressure  Common source of error is the pressure with which the work piece is pressed while measuring.  Ideally, the stylus should have simply touched the work piece. Besides the deformation effect the stylus pressure can bring deflection in the work piece also.  Variations in force applied by the anvils of micrometer on the work to be measured results in the difference in its readings.  Error is caused by the distortion of both micrometer frame and work-piece.
Random Error  Random errors are positive and negative fluctuations that cause about one-half of the measurements to be too high and one-half to be too low. Sources of random error cannot always be identified. Possible sources of random errors are as follows:  Observational  Environmental