Spirit Levels price The details of Extra Long Big Aluminium Spirit Level as follow Description Material: Aluminium frame Size: 40cm 50cm 60cm 80cm How the level calibrates the error 1. Zero error calibration of horizontal position After the bubble is stabilized, the reading at one end of the bubble in the horizontal position is a, and then, the level ruler is turned 180 degrees, placed in the original position of the plate, and the reading b at the other end of the bubble is written according to the side of the first reading, and half of the difference between the two readings is a zero error. 2. Zero error calibration of the plumb position Adjust the indexing head so that one end of the horizontal position bubble is aligned with the long engraving line, the indexing head is turned 90 degrees, and the corresponding end of the bubble in the plumb position is read c, and the zero error of c plus the horizontal position is the zero error of the plumb position. 3. Zero error calibration at 45 degrees Adjust the indexing head so that one end of the horizontal position bubble is aligned with the long engraving line, the indexing head is turned 45 degrees, the corresponding end of the bubble at 45 degrees is read d, and the zero error of d plus the horizontal position is the zero error of the 45 degree position.Spirit Levels price website:http://www.dingqimaxtool.com/measuring-tools/spirit-levels/