Aconcentric cylinder viscometer may be formed by rotating the inner member of a pair of closely fitting cylinders (see fig). the annular gap is small so that a linear velocity profile will exist in the liquid sample. consider a viscometer with an inner cylinder of 4 in. diameter and 8 in. height, and a clearance gap width of 0.001 in., filled with castor oil at 90°f. determine the torque required to turn the inner cylinder at 400 rpm.