Spirit level. Close-up of the bubble viewing area in a spirit level. This device is used to measure how level a surface is. It consists of a tube filled with a coloured fluid (here yellow) and a single air bubble. When the spirit level is placed on an even, flat surface, the air bubble will remain between the two lines in the centre of the tube (as here). However, if the surface slopes, the air bubble will move to the left or right of the markings.