Exam Question

### **1. Principle of Operation**
The continuous viscosity monitoring device, often called a Viscotherm or Viscosity Controller, operates on the principle of the Hagen-Poiseuille law for laminar flow. This physical law states that for a constant, non-pulsating, laminar flow of a Newtonian fluid through a long cylindrical pipe (or capillary tube) of constant diameter, the pressure drop across the tube is directly proportional to the dynamic viscosity of the fluid.
* **Constant Flow Rate**: A constant displacement pump (typically a gear pump) draws a sample of the fuel and forces it through a capillary tube at a fixed volumetric flow rate (Q).
* **Laminar Flow**: The dimensions of the capillary tube (small diameter 'r' and sufficient length 'L') and the low flow rate ensure that the flow remains laminar (Reynolds Number SP)**: The controller increases its output, causing the steam valve to open further. This admits more steam to the heater, increasing the fuel oil temperature. As temperature rises, viscosity falls.