Spirit levels are used for checking and leveling the surfaces during installation and checking.
Figure 1: Spirit Level.
Construction of Spirit Level
A glass tube, whose surfaces are grounded, is fixed in a frame as shown in Fig. 1. A glass tube has got radius ‘R’ which is very large and filled with ether liquid, as well as ether bubble (vapor).
Working Principle of Spirit Level
Figure 1: Spirit Level.
The position of the bubble in the tube will be always at the topmost level which is the principle of working of the spirit level. Fig. 2 shows the principle sketch of spirit level. In this, R-radius of curved glass tube A – is the topmost position of the bubble at horizontal position of spirit level. If bubble moves one division, i.e. from B to B’, the angle of tilt is θ. The level sensitivity per division i.e. the tilt causing the displacement of bubble by one division of the scale is expressed in mm per meter, e.g. 0.02 mm/meter.
For measuring very small angle in terms of seconds, the movement of bubble in terms of divisions is measured and multiplied by least count/level sensitivity. If R is the radius of curvature of the inside surface of the glass tube and α is angle of tilt in radians,
Displacement of bubble, l = R · α
The sensitivity of a sprit level depends on R As R increases, sensitivity increases. If α is expressed in seconds,
\[l=\frac{R{\alpha }”}{206265}\]
[1 radians = 206265 seconds of arc]
If the space between two lines is 2 mm and for a radius of R = 206 m,
\[{\alpha }”=\frac{2\times 206265}{206\times 1000}={2}”\]
This indicates inclination of 2″ of a spirit level will cause a bubble to move by 1 division or 2 mm. Spirit levels are to be handled very carefully and they are sensitive to temperature.
Care of Spirit Level
The following care is to be taken when spirit level is used.
- Spirit level should be used properly for avoiding errors.
- Temperature variation may effect on the accuracy of spirit level.
- Calibration should be carried out time to time.
- Care to be taken to avoid deformation of glass tube to avoid error.
- It should not made vertical during handling.
- As it contains glass tube, should be handled carefully.
