Sensitiveness or the sensitivity of a level tube is its capability of exhibiting small deviations of the tube from the horizontal. This quality depends mainly upon the radius of curvature of the level tube which may vary from 10 to 300 metres —the larger the radius, the greater the sensitiveness. Sensitiveness is also increased by increase in the length of bubble and by decrease of viscosity and surface tension of the liquid in the level tube.
Sensitiveness is sometimes designated in terms of the radius of curvature of the level tube, but the better way is to state the angle through which the axis must be tilted to cause the bubble to travel through one division of the scale (i.e., the angular value of one division of the level tube.)
The angular value of one ‘2 mm’ division may vary from 8 to 45 seconds depending upon the type of the instrument. When the bubble is not much sensitive, the error in reading over long sights is very marked and the great accuracy is not possibly.
And if the sensitiveness is greater than is really necessary, time is wasted in levelling the instrument and the accuracy in readings is no way increased. Therefore for better efficiency, only such instruments as have the requisite sensitiveness should be used for different purposes.
ADVERTISEMENTS:
Measurement of Sensitiveness:
To find either the radius of curvature of the level tube or the angular value of one division of the level tube, proceed as follows: (Fig 7.30)
(i) Select a base line of length 50 to 100 m on a fairly level ground and measure it accurately by a steel tape.
ADVERTISEMENTS:
(ii) Set up and level the instrument at one end of the base line and hold the staff at other end ‘P’.
(iii) By using the foot-screw beneath the telescope, bring the bubble near one end of its run (extreme left-hand position) and read both ends of the bubble.
(iv) Observe the staff reading in this position of the bubble (say it is AP)
(v) As before bring the bubble to the other end of the tube (extreme right-hand position) and note the two end readings of the bubble.
ADVERTISEMENTS:
(vi) Again read the staff (say it is BP).
Note:
(i) The two end readings of the bubble are necessary since the length of the bubble changes with varying temperature.
(ii) The observations should be repeated several times and the results averaged.
ADVERTISEMENTS:
(iii) Calculation work may be simplified by taking the first reading on the staff with the bubble exactly in the centre of its run and then moving it to one side and taking the second staff reading.
Let D = the horizontal distance between the instrument and the staff
AB= S = the average length of the staff intercepted between the upper and lower lines of sight (i.e., the difference of the two staff readings AB and BP).
= the angle between the lines of sight
ADVERTISEMENTS:
n = the average number of divisions through which the centre of the bubble is moved. In (iii) and (v) above, in each case, the position of both ends of the bubble is recorded and that of its centre can be deduced in terms of the tube graduations. These are say ‘E’ and ‘F’ in (iii) and (v) respectively.
OE = OF = R = the radius of curvature of the tube.
l = the length of one division on the tube, expressed in the same unit as D and S.
The length through which the centre of the bubble travels = the length of the are EF = nl.
Note:
1 radian = 206, 265 Seconds.