Before using the level, firstly we must check if there are cracks, pores, etc.

On the surface of the level, this vial if clean and transparent. 

After determining that the level is in good condition, we will calibrate it. 

Put the level on the wall, drawing a line on the wall along the edge of the ruler.

Take a reading of the bubble’s position.

Rotate the level 180° end-to-end and align the level with place it on the original line. 

Take another read, If the level coincides with the line, the bubble will be in the same position for both, the level is accurate

To test the vertical vial, follow the same procedure against a flat vertical surface.