Patent ID: 8347693

Claim:
A method of measuring the shear stress of a test liquid at an elevated test pressure, the method comprising the steps of: placing the test liquid in the internal chamber of a container; selecting a second liquid which is immiscible in the test liquid; placing the second liquid in the chamber in contact with the test liquid to form a liquid interface between the test liquid and second liquid; positioning a shaft to extend between the first and second liquids and through the liquid interface, the shaft having a test liquid contacting member on the one end positioned to be in contact with the test liquid; selecting a test pressure different from 1 atmosphere and bringing the test liquid to the test pressure; while the test liquid is at the test pressure, rotating the shaft to move the member in the test fluid while determining the relative viscosity of the test liquid by measuring the drag created by the movement of the member; and additionally comprising replacing at least a portion of the test liquid with a second and different test liquid while maintaining the liquid interface in the reduced cross section area, mixing the test liquids and thereafter determining relative viscosity of the mixture of test liquids at test pressure above atmospheric pressure.