The second type of eye level can be used in the same manner as the first for determining level, the only difference being that the line inside the instrument will be lined up with the zero mark on the degree wheel in the level instead of a bubble.
Typically these types of levels have two different calibration markings on the wheel inside. One side will be in degrees the other will be percentage. You should refer to your documentation to determine which is which, as these are two very different types of measure. Unlike the bubble level these types of instruments are meant to be used at different angles. The housing or outer part of the level rotates around the wheel inside. The wheel always remains in the same position relative to the ground. When looking through the level with both eyes open an optical illusion occurs that produces a line that can be seen outside of the level.
Determining the percentage of slope on a driveway could be done in the following way. A benchmark should be set at exactly the same height that your eye is off the driveway. You could for instance pick a spot on your machine that is at the same height as your eye, this will serve as your benchmark when you sight back to it. Walk down or up the driveway and then look back to your mark and put the line in the level on your benchmark. Read the numbered markings on the wheel where the line intersects it, this is the percentage or degree of slope depending on what scale you use.
Eye levels are a very handle tool for an operator to have. It can be very difficult at times to determine if an excavation is level. Using an eye level to confirm that you are digging level will help educate your eye to recognize level. As your experience increases you will find that what appears to be level from the seat of the machine, will be, when you check it with your level. part 3