It is inevitable that total station needs to be calibrated in the long-term using. How to verify that the horizontal axis of a circular bubble is parallel to the instrument rotation and how to correct it? Today let’s share it with you.

1. Principle
As shown in the graphics, “V” is the rotation axis of total station, and “l” is the horizontal axis of the circular bubble. It is assumed that there is a crossing angle α when “V” axis and “l” axis are not parallel to each other. When the bubble is centered, the mentioned horizontal axis “l” is vertical, and there is an angle “a”, the deviation between the instrument rotation axis and the vertical position. Rotate the instrument in Figure(a) by 180°to be what shown in Figure(b). The instrument is rotated around rotation axis “V”, so the space position of “V” is fixed. After the instrument is rotated, bubble will remain at its highest point because the liquid in the circular bubble is under the action of gravity. However, the horizontal axis of the circular bubble will be at position “l”. As shown in Figure(b), it can be found that the angle between horizontal axis l and vertical line l′ is 2a. At this time, the bubble in circular bubble is not centered any more but it is tilted to other side. The center angle of arc length offset by the bubble is equal to 2a.

 2. Testing Method
Firstly, center the bubble through foot screws. Secondly, rotate the instrument by 180°. If the bubble is still at the center position, it indicates that it has met the condition that horizontal axis is parallel to the rotation axis of the instrument but failed if not. According to the above testing principle, it can be found that the length of bubble offset can be regarded as to be twice as much as the intersection angle between the rotation axis of the instrument and horizontal axis.

3. Calibration
Calibration is performed by the calibration screws installed below the circular bubble. Generally, there are three calibration screws. During operation, adjust respectively three calibration screws to make the bubble move to the center position, half of the length of circular bubble. If the operation is completely accurate, horizontal axis l will be parallel to the instrument's rotation axis v after calibration. Calibration is recommended to be operated repeatedly for several times. Importantly, circular bubble shall be leveled first for each calibration. Next is to rotate the instrument by 180° and observe the position of the bubble to confirm whether it is needed to do calibration again. If the bubble can always be centered no matter where the instrument is rotated after leveling, the calibration is completed.

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