What is SPIRIT LEVEL
What is Spirit level
1- Levelling of the machine is a very important operation before proceeding to conduct geometrical tests.
2- A precision spirit level is used to level the machine tools accurately.
3- Sprit level consists of a curved glass tube called 'VIAL 'containing industrial alcohol 'spirit' and a bubble of 'AIR' trapped in the tube.
4- The spirit and the bubble are both acted upon equally by the force of gravity.
5- Since the spirit has a higher density, it is pulled down to the bottom of the tube and the bubble always floats to the top.
6- The vial is set in a cast iron base and adjusted such that the bubble rests at the center of a scale when the base is horizontal.
How to measurement Precision spirit level
1- Spirit levels used for high precision measurements should have a sensitivity of about 0.02 to 0.05 millimeter per millimeters for each division.
2- If the movement of the bubble by one division corresponding to a change in slope of 6 to 12 seconds of a level of 0.04 mm per 1000 mm is chosen, then
i- 1 division = 0.04 mm/1000 mm
ii- 3/4 division = 0.03 mm/1000 mm
iii- 1/2 division = 0.02 mm/1000 mm
iv- 1/4 division = 0.01 mm/1000 mm
3- It is quite easy to estimate within a quarter of a division.
Hints on spirit level
1- Spirit levels which are too sensitive are difficult to bring to rest in a workshop in which machines are running.
2- Levels with low sensitivity result in insufficient reading accuracy, as very small fractions of a division have to be estimated.
3- The bearing surfaces of spirit levels should be long as possible.
4- For testing medium size machines the level should not be less than 200 mm long.
5- It is often advisable to use a bridge piece the feet of which are about 300 mm apart.
6- The spirit level can then be placed on the scraped surface of the bridge.
7- This method avoids errors which could be caused by irregular scraping of the surface to be measured.
Sensitivity of spirit level
1- The sensitivity E of the spirit level is the movement of the bubble in millimeters which corresponds to a change in slope of 1 millimeter per 1000 millimeters.
E = Movement of bubble in mm/1Millimetre per meter.
2- The inside of the glass tube of a spirit level has a shape of a circular arc of radius R which moves during a change of slope around the center M of its curvature.
3- If the slope is measured as a ratio of h/L, and the movement of the bubble is t then
t/h = h/L and
since E= t÷(h/L).
Radius and sensitivity
1- The sensitivity of the spirit level is equal to the radius of curvature of the barrel shaped bubble tube.
2- Therefore the sensitivity of the level depends only on the radius of the curvature of the bubble tube and not on the length of its bearing surface.
Causes for errors in spirit level reading
1- Wrong position of the vial in the housing.
2- Faulty graduation
3- The surface finish of the piece to be tested .
4- The influence of temperature.
5- Personal errors of the inspector.
6- Reading spirit levels depends on:
i- The quality and length of the bearing surface of the work piece.
ii- Dimensional stability of the metal housing.