Measuring viscosity

The Hagen-Poiseuille expression is used as the basis for measuring the viscosity of liquids and gases. If a straight pipe of uniform diameter can be set up horizontally, and a liquid or gas made to flow through it, the pressure drop and the rate of flow can be measured and, from this, the value of the coefficient of viscosity can be deduced. Any attempt to do this will show that the coefficient of viscosity varies with temperature and a further constraint must be imposed on the method of measuring the coefficient of viscosity; the measurement must be made at uniform temperature. As laminar flow necessarily  involves internal heating, the flow can only be at constant temperature if it is slow with a small pressure drop and undertaken in an environment that is temperature controlled. Then, for liquids and gases for which m is independent of the velocity gradient it is possible to measure m. We, as engineers, must accept that physicists are quite capable of doing this and of producing accurate values of viscosity of all the common liquids and gases. We should note that laminar flow at constant temperature is a very special case and normally, especially in oil hydraulics, the heating is significant and, if we choose to use the Poiseuille expression to deal with such a flow, we should expect it to become inaccurate in such cases and be prepared to create a computer model that allows for the heating.

 

The method outlined above is regarded as a primary method of measuring viscosity and, without doubt, it is a method for use in a laboratory.  One must contemplate the position of engineers in respect of viscosity.