Classification of measurements precautions for use of instruments Sources of errors
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Classification of measurements:
In the precision measurements, various methods of measurement are followed depending upon the accuracy required and the amount of permissible error. The various methods of measurement are classified as follow :
1. Direct method of measurement
2. Indirect method of measurement
3. Absolute method of measurement
4. Comparative method of measurement
5. Contact method of measurement
6. Contact less method of measurement
Direct method of measurement:
The direct method of measurement is one in which the measurement value in determined directly where as in the indirect method of measurement the dimension in determined by measuring the values functionally related to the required value. The direct method of measurement is simple and most widely employed in production.Indirect measurement:
In many cases, for example, as when checking the pitch diameter of treads, the direct method may lead to large errors in measurement. In this case, it is more expedient to make indirect measurement.Absolute method of measurement:
An absolute method of measurement in one in which the zero division of the measuring tool or instrument corresponding zero value of the measured dimension. eg. Steel rule, vernier Caliper, micrometer, Screw gauge). By absolute method the full value of the dimension is determined.Comparative method:
In the comparative method, only the deviation of the measured dimension from a master gauge are determined (eg. Dial comparator).Contact method:
In contact methods of measurement, the measuring tip of the instrument actually touches the surface to be measured, eg. By dial comparator, screw gauges etc. In such cases arrangements for constant contact pressure should be provided in order to prevent errors due to excess contact pressure.Contact less method:
In Contact less method of measurement, no contact is required. Such instruments include tool maker's micrometer and projection comparator.According to the functions, the measuring instruments classified as.
1. Length measuring instruments
2. Angle measuring instruments
3. Instrument for checking deviation from geometrical forms
4. Instrument for determining the quality of surface finish.
According to the accuracy of measurement, the measuring instrument are classified as follows.
Most accurate instrument
eg : light – interference instruments.
Second group
consists of less accurate instruments. Such as tool room Microscopes, comparator optimeter etc.
Third group
consists of , still less accurate instruments eg: dial indicators, vernier caliper and rules with vernier skills.
Measuring instrument are also classified in accordance with then metrological proper ties:
such as range of instrument, scale graduation value, scale spacing, sensitivity and reading accuracy.
• The measurement must be made at right angles to the surfaces of the component.
• The component must be supported so that it does not collapse under the measuring pressure or under its own weight.
• The work piece must be cleaned before being measured, and coated with oil or a corruption inhibitor after inspection.
• Measuring instrument must be handled with care so that they are not damaged or strained.
• They must be kept in their cases when not in use and kept clean and lightly oiled on the bright surfaces.
• They should be regularly checked to ensure that they have not lost their mutual accuracy.
• It must be emphasized that it is not good practice to rely on the accuracy of the instruments and on the readings taken – readings should be double checked and the instruments should be periodically checked against the appropriate standards.
• Measuring instruments are produced to a high degree of accuracy, form the engineer's common rule to the most complex optical instrument, and they should be treated accordingly.
• Instruments are easily damaged, and very often the damage is not noticeable. Always handle instrument with great care, and report immediately any accidental damage.
• Protect highly polished surfaces from corrosion by handling them as little as possible and by covering them with petroleum jelly when not in use.
1. Alignment Principle
2. Location of the measured part
3. Temperature
4. Parallax Effect.
An alignment error of 2° overin introduces an error of approximately 0.6mm.
Error in introduced to dial indicator readings if the plunger axis does not coincide with the axis or line of measurement.
To ensure correct displacement readings on the dial indicator the plunger must, of course be normal to the surface in both mutually perpendicular planes.
A second source of error will illustrated by the vernier Caliper and similar instruments or circumstance is associated with measuring pressure or "feel". The measuring pressure in applied by the adjusting screw which is adjacent and parallel to the scale. A bending moment in introduced equal to the product of the force applied by the adjusting screw and the perpendicular distance between the screw centre line and the line of measurement.
Variation of force applied at the screw are augmented at the line of measurement and a hot unusual form of damage to Vernier Caliper is permanent distortion to the measuring jaws presumably from this source.
The comparator reading in thus an indication of the displacement of the upper surface of the measured part from the datum.
Faults at the location surface of the part damage, geometrical variations from part to part or the presence of foreign matter are also transmitted to the indicator.
This provides false information regarding the true length of the part by introducing both sine and cosine error.
Where location conditions may not be ideal, ex:inter stage measurement during production, sensors, operating on each side of the component can be used which eliminate the more serious sine type error.
A two probe system measures length rather than surface displacement and highly sensitive electronic comparators of this type are used for slip gauge measurement.
When this is not possible and the length at reference temp. must be known, a correction is made to allow for the difference between ambient and reference temp.
The correction value required to – 0.001375mm, when steel object exactly 25mm long at 20° C and Co-efficient of linear expansion 11Mm c/m in measured at 25° C, Which is rather larger than the increment step the M88/2 stip gauge set.
However, for less stringent measurement requirements it is not essential that correction to reference temperature is made provided that the following precautions and conditions are observed.
a) The temp. at which measurement is made is not changing significantly.
b) The gauge and work being compared are at the same temp and the temp is the same as ambient temp.
c) The gauge and work have the same Co-efficient of linear expansion.
Conditions a) and b) can be met if gauge and work allowed sufficient time to reach equal temp with surrounding after being arranged in the measuring positions.
If the measurement can be carried out on the surface of a large mass, eg: Surface plate, then temp. equalization will be family vapid as heat will be conducted away form the work and gauge but will not contribute any significant temp. change to the plate.
A component having a co-efficient of linear expansion significantly different from the gauge may be said to correct to size only at a given temp.
It is then essential to observe the pointer along a line normal to the scale otherwise a reading error will occur.
This effect is shown in fig. Where a dial is shown observed from three positions where the pointer is set at zero on the scale, observed from position 1) ie, from the left, the pointer appears to indicate some value, to the right off zero, and from position 2) Some value slightly to the left of zero, while only at position. 3) With the pointer Coincide with zero on the scale.
Rules and micrometer thimbles are beveled to reduce this effect and on dials the indicates may be arranged to lie in the same plane as the scale, thus completely eliminating parallax, or a silvered reflector may be incorporated on the scale so that the line between the of eye and pointer is normal to the scale only when the pointer obscures in own image in the reflector.
Range of Measurement :
It indicates the size values between which measurements may be made on the given instrument.Scale Spacing :
It is the distance between the axis of two adjacent graduations on the scale.Scale division Value :
It the measured value corresponding to one division of the instrument scale, eg. For Vernier Caliper the scale division value 0.1mm.Sensitivity (amplification or gearing ratio ):
It is the ratio of the scale spacing to the space division value. It would also be expressed as the ratio of the product of all the larger lever arms and the product of all the smaller lever arms.Sensitivity Threshold :
It is defined as the minimum measured value which may cause any movement whatsoever of the indicating hand..Reading Accuracy :
It is the accuracy that may be attained in using a measuring instrument.Reading Error :
It is defined as the difference between the reading of the instrument and the actual value of the dimension being measured.Important precautions for use of instruments:
few important precautions for use of instruments towards achieving accuracy in measurement are as follows :• The measurement must be made at right angles to the surfaces of the component.
• The component must be supported so that it does not collapse under the measuring pressure or under its own weight.
• The work piece must be cleaned before being measured, and coated with oil or a corruption inhibitor after inspection.
• Measuring instrument must be handled with care so that they are not damaged or strained.
• They must be kept in their cases when not in use and kept clean and lightly oiled on the bright surfaces.
• They should be regularly checked to ensure that they have not lost their mutual accuracy.
• It must be emphasized that it is not good practice to rely on the accuracy of the instruments and on the readings taken – readings should be double checked and the instruments should be periodically checked against the appropriate standards.
• Measuring instruments are produced to a high degree of accuracy, form the engineer's common rule to the most complex optical instrument, and they should be treated accordingly.
• Instruments are easily damaged, and very often the damage is not noticeable. Always handle instrument with great care, and report immediately any accidental damage.
• Protect highly polished surfaces from corrosion by handling them as little as possible and by covering them with petroleum jelly when not in use.
Sources of errors in precision measurement:
Failure to consider the following factors may introduce errors in measurement :1. Alignment Principle
2. Location of the measured part
3. Temperature
4. Parallax Effect.
Alignment Principle (Abbe's Principle) :
Abbe's principle of alignment states that " the axis or line of measurement of the measured part should consider with the measuring scale or axis or measurement of measuring instrument ".An alignment error of 2° overin introduces an error of approximately 0.6mm.
Error in introduced to dial indicator readings if the plunger axis does not coincide with the axis or line of measurement.
To ensure correct displacement readings on the dial indicator the plunger must, of course be normal to the surface in both mutually perpendicular planes.
A second source of error will illustrated by the vernier Caliper and similar instruments or circumstance is associated with measuring pressure or "feel". The measuring pressure in applied by the adjusting screw which is adjacent and parallel to the scale. A bending moment in introduced equal to the product of the force applied by the adjusting screw and the perpendicular distance between the screw centre line and the line of measurement.
Variation of force applied at the screw are augmented at the line of measurement and a hot unusual form of damage to Vernier Caliper is permanent distortion to the measuring jaws presumably from this source.
Location :
when using a sensitive comparator, the measured part in located on a table which forms the datum for comparison with the standard.The comparator reading in thus an indication of the displacement of the upper surface of the measured part from the datum.
Faults at the location surface of the part damage, geometrical variations from part to part or the presence of foreign matter are also transmitted to the indicator.
This provides false information regarding the true length of the part by introducing both sine and cosine error.
Where location conditions may not be ideal, ex:inter stage measurement during production, sensors, operating on each side of the component can be used which eliminate the more serious sine type error.
A two probe system measures length rather than surface displacement and highly sensitive electronic comparators of this type are used for slip gauge measurement.
Temperature :
The standard reference temp. at which line and end standards are said to be at their true length is 20° and for highest accuracy in measurement this temp Should be maintained.When this is not possible and the length at reference temp. must be known, a correction is made to allow for the difference between ambient and reference temp.
The correction value required to – 0.001375mm, when steel object exactly 25mm long at 20° C and Co-efficient of linear expansion 11Mm c/m in measured at 25° C, Which is rather larger than the increment step the M88/2 stip gauge set.
However, for less stringent measurement requirements it is not essential that correction to reference temperature is made provided that the following precautions and conditions are observed.
a) The temp. at which measurement is made is not changing significantly.
b) The gauge and work being compared are at the same temp and the temp is the same as ambient temp.
c) The gauge and work have the same Co-efficient of linear expansion.
Conditions a) and b) can be met if gauge and work allowed sufficient time to reach equal temp with surrounding after being arranged in the measuring positions.
If the measurement can be carried out on the surface of a large mass, eg: Surface plate, then temp. equalization will be family vapid as heat will be conducted away form the work and gauge but will not contribute any significant temp. change to the plate.
A component having a co-efficient of linear expansion significantly different from the gauge may be said to correct to size only at a given temp.
Parallax Effect :
On most dials the indicating finger or pointer lies in a plane parallel to the scale but displaced a small distance away to allow free movement of the pointer.It is then essential to observe the pointer along a line normal to the scale otherwise a reading error will occur.
This effect is shown in fig. Where a dial is shown observed from three positions where the pointer is set at zero on the scale, observed from position 1) ie, from the left, the pointer appears to indicate some value, to the right off zero, and from position 2) Some value slightly to the left of zero, while only at position. 3) With the pointer Coincide with zero on the scale.
Rules and micrometer thimbles are beveled to reduce this effect and on dials the indicates may be arranged to lie in the same plane as the scale, thus completely eliminating parallax, or a silvered reflector may be incorporated on the scale so that the line between the of eye and pointer is normal to the scale only when the pointer obscures in own image in the reflector.
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