Experiment 10
AIM:
5th Semester
Experiment 10
Metrology Lab
To find the flatness error in surface plate.
Apparatus Required: Surface plate, spirit level.
Theory:
The spirit of level consists of a sealed glass tube mounted on a base. The inside
surface of the tube is ground to a convex barrel shape having large radius. The precision of
the level depends on the accuracy of this radius of the tube. A scale is engraved on the top
of the glass tube. The tube is nearly filled with either ether or alcohol, except a small air or
vapor in the form of a bubble.
The bubble always tries to remain at the highest point of the tube. If the base of
the spirit level is horizontal, the centre point is the highest point of the tube. So, that when
the level is placed on a horizontal surface, the bubble rests at the centre of the scale. If the
base of the level is fitted through a small angle, the bubble will more relative to the tube a
distance along its radius corresponding to the angle.
Figure: Surface Plate
Soft copy available @: www.RamaKantRana.com/Metro
Figure: Spirit Level
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Experiment 10
5th Semester
Metrology Lab
The figure shows two positions of the base of the level (OA1 and OA2) and
corresponding positions of the bubble (B1, B2). When the base OA1 is horizontal, the
bubble occupies position B1. Let ‘Ɵ’ be the small angle through which the base is fitted.
The bubble now occupies the position B2.Let L be the distance travelled by bubble along
the tube and ‘h’ the difference in heights between the ends of the base. Then L= R Ɵ and
h =. L Ɵ
Where R = radius of curvature of the tube
L = length of base
Finally h =L/R
Procedure:
1
2
3
4
5
Place the spirit level on the surface plate for which we have to find out the
flatness
Find the base length of the spirit level
Note the radius of curvature of the spirit level tube
Find the tilt in the bubble
Finally find out the difference in heights between the ends of the base.
Soft copy available @: www.RamaKantRana.com/Metro
49
Experiment 10
5th Semester
Metrology Lab
OBSERVATIONS:
PRECAUTIONS:
1 .Clean the surface plate and ensure there is no dust particles
2. Take the bubble reading without any parallax error.
RESULT:
Flatness of the surface plate can be found by using spirit level.
Soft copy available @: www.RamaKantRana.com/Metro
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