1. Use a ruler of appropriate length depending on the detection surface.
2. When testing, turn the horizontal bubble on the ruler upward, at the surface to be tested, and find the lowest end of the slope, and then slowly lift the end.
3. At the same time, while checking whether the horizontal bubble is centered, insert the wedge-shaped feeler gauge until the bubble reaches the center, and the insertion depth reflected on the feeler gauge scale is the level or slope of the detection surface.
