Crack tell-tales monitors are used to accurately measure the movement of cracks in walls, ceilings, or foundations. They are composed of two pieces of acrylic and PVC sheet which overlap. One has an opaque marker and the other has an overlay marker on clear acrylic. Each half is fixed to the structure on opposite sides of the crack. When the crack moves the sides displace in relation to one another. The movement is then read by observing where the sight marks cross the displacement grid. Internal or external use with an accuracy of 1mm. Corner, floor, and displacement tell-tales are also available. Call for information and pricing.