By definition, consistency is expressed as a percentage by dividing the mass of solid material by the total mass of a wet sample, resulting in units of mass per unit mass. Mechanically, consistency is the resistance to deformation or shear by fibrous materials, and thus is related to apparent viscosity. Such materials include wood pulp, dough, tomato paste, paint, gelatin, or drilling mud. This section will focus on those methods used for the measurement of consistency in the paper industry, involving pulp–water mixtures. In thelaboratory, consistency is measured using a gravimetric method described in TAPPI Test Method T 240 om-88. 1 General industrial methods used for consistency measurement are described in a number of sources. 2–4 In order to have good control over the basis weight of the paper product, it is necessary to maintain the consistency of the feed constant. An increase in temperature or an increasing inorganic materials content will reduce the viscosity, and thus apparent consistency, while increasing freeness (ability of the suspension to release water), increasing fiber length, or increasing pH will increase the apparent consistency. Pipelinevelocity also can influence the consistency reading for mechanical sensors and, therefore, it is advisable to measure consistency in turbulent locations at constant flow rate. Consistency is also related to turbidity and sludge or suspended solids detectors.Forty years ago, consistency was considered to be a mature measurement, almost totally relying on mechanical devices for shear force measurements in the 2 to 5% range. Today, we still rely on mechanical shear force measurements, but in addition, we have devices utilizing light scattering, light transmission, nuclear radiation, radio waves, and microwaves.