# Engineering Metrology Questions and Answers – Calibration of Polygons

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This set of Engineering Metrology Multiple Choice Questions & Answers (MCQs) focuses on “Calibration of Polygons”.

1. How many auto-collimators are required when the polygons are calibrated by using only auto-collimators?
a) 1
b) 2
c) 3
d) 4

Explanation: Any precision polygon is usually calibrated by two autocollimators before using, in order to know the errors. For this method, precision polygon is mounted on a table and this table is a rotating table.

2. What is the accuracy of comparison method used for calibration of polygons?
a) ±0.1 seconds
b) ±0.2 seconds
c) ±0.3 seconds
d) ±0.4 seconds

Explanation: If all the exterior angles of the upper polygon are compared with the angle between any two faces of the lower polygon then by comparison method of calibration of polygons, the accuracy of ± 3 seconds can be obtained.

3. What is the relation between angle of two autocollimators (R) and angle between polygon faces (P) when polygon is calibrated? (R1, R2 are the readings.)
a) P = R + (R1+R2)
b) P = R + (R2-R1)
c) P = R – (R1+R2)
d) P = R – (R1-R2)

Explanation: The angle between two polygon face is P and between two autocollimators is R. Readings are taken by using autocollimators let readings are R1 and R2. Then the angle between polygon face is equal to autocollimator angle plus difference in readings.

4. How many auto-collimators are required for comparison method of calibration of polygons?
a) 1
b) 2
c) 3
d) 4

Explanation: In comparison method of calibration of polygons, two polygons which are identical to each other and one table which can rotate is used. For viewing purpose a single autocollimator is used.

5. Which of the following is the correct option for given statements about calibration of polygons?
Statement 1: Autocollimators angle is constant during calibration.
Statement 2: Wild rotary table can be used as the base of precision polygons.
a) F, F
b) T, T
c) F, T
d) T, F

Explanation: Angle between two autocollimators (R) is constant that is ∑R/n is equal to R where n is the number of polygon faces. Precision polygons are placed on a rotary table for calibration and wild type is used in many places.

6. What is formula for the angle between autocollimator (R) when polygon is calibrated by using two autocollimators?
a) (3600/n)-∑(R2-R1)/n
b) (1800/n)-∑(R2-R1)/n
c) (3600/n)-∑(R2-R1)*n
d) (1800/n)-∑(R2-R1)*n

Explanation: Summation of angle between polygon faces is equal to 360 degree. If there be n number of sides in a polygon then dividing the equation throughout by n that is (3600/n)-∑(R2-R1)/n.

7. Which of the following option is true for calibration of polygons by using auto-collimators?
Statement 1: Imperfect centering has a great effect on calibration.
Statement 2: This method gives a high degree of accuracy without reference to any angular standard.
a) T, T
b) T, F
c) F, T
d) F, F

Explanation: It is not necessary for calibration of polygons that the autocollimators be aimed at the centre of polygons as there is no effect of imperfect centering in this method of calibration or autocollimators.

8. By computed least square method, what is the accuracy obtained for polygons up to 12 faces?
a) ±1.0 second
b) ±2.0 second
c) ±3.0 second
d) ±4.0 second

Explanation: If each faces pair of the upper polygon is compared with all the faces of the lower polygon and the angles are computed by the least square method then the better accuracy will achieve. By this, the accuracy of ± 1.0 second may be obtained for up to 12 faces polygons.

9. Which of the following is not true for comparison method used for calibration of polygons?
a) Identical polygons are used
b) Polygons are mounted one above the other
c) Polygons are mounted on the fixed table
d) Reflecting faces of both the polygons are viewed simultaneously

Explanation: Two identical polygons are mounted one above the other on a table which can rotate. The autocollimator is placed in a position that it views simultaneously the reflecting faces of both the polygon.

10. What is correct for the angle of autocollimators used for calibration of polygons?
a) Angle between autocollimators is equal to the nominal angle between adjacent faces
b) Angle between autocollimators is more than the nominal angle between adjacent faces
c) Angle between autocollimators is less than the nominal angle between adjacent faces
d) Angle between autocollimators is more or less to the nominal angle between adjacent faces 