A coating is placed into the open area between the plates, forming a wedge shaped film (Fig. 1). By sliding the top plate back and forth, a sharp line of demarcation alternatively appears and disappears (Fig. 2). The point at which the demarcation line appears is read on the engraved scale (Fig. 3). These cale readings are easily converted into thickness in mils, or coverage in square feet per gallon using the table furnished with the instrument. The value of the “wedge constant” of a top plate is the thickness in mils of the wedge of wet paint exactly over the demarcation line when the top plate is centered over this line and the scale reading is 25. Top plates with different wedge constants are included, depending on the opacity of the material to be tested:
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