Last month, I laid the groundwork for a discussion about the value and limitations of the venerated Marsh funnel, particularly for high solids mixes or slurries with potential for progressive gelling. Here, we talk about why shear gives Fann viscometers an advantage with high-solids mixes. I wrap up with tips for getting the best readings out of your Marsh funnel.
As we talk about the two tests the logical question is, what is the important difference between a Marsh funnel and Fann rheometer? The answer is surprisingly simple: The Fann rheometer spins constantly during the test. That means the entire volume between the rotor and stator (shaft and bob) remains in motion and under shear. Shear speed can change from 3 to 600 rpm, and the fluid can certainly thin or thicken depending on its profile, but the fluid between the bob and shaft remains in motion except for when we conduct a gel-strength measurement.