During measurements of crack dimensions, the temperature of the specimens must be equal to that of the
laboratory. Deviation of their temperature from the ambient temperature in the laboratory must be no greater
than
±
3
°
C in order to be considered acceptable. When measuring crack width, the measuring line must be
perpendicular to the crack contour; the margins within which the measuring line may deviate from the normal
direction must not exceed
±
5
°
. The operator who takes these measurements must have no pathological
changes in eyesight, and the visual acuity of each eye must be no less than 100% without correction.