21/02/2016 · So then plotted the rheogram rpm Vs weights the rpm values converted to actual rate of shear and weight converted into units of shear stress, dy/cm2 by using appropriate constants. Mathematical treatment: For, rotational viscometers, the relationship can be expressed as, η = Kv w/v where v, rpm generated by weight w, in gm Kv is obtained by analyzing material of known viscosity in .