AbstractA steady‐flow capillary viscosimeter, using dry air as the test gas, has been demonstrated at 150, 230, 330 and 430° Centigrade and over a pressure range of 35–150 atmospheres. Measured viscosities showed a maximum isothermal increase of about 4%. An extrapolation procedure was used to correct for flow and temperature discrepancies and a modified Hagen‐Poiiseuille equation, using variables evaluated at the mean capillary temperature, was applied to those data taken under nonisothermal conditions. The overall assembly and technique are capable of relative viscosity measurements having a standard deviation within 11/2% in the cited