**Core Concept**
A rotameter is a variable area flowmeter that measures fluid flow rate by detecting the position of a float (bobbin) in a tapered tube. The flow rate is directly related to the height of the bobbin, which rises as flow increases due to the balance between fluid pressure and the float’s weight.

**Why the Correct Answer is Right**
In a rotameter, the bobbin rises in the tapered tube as flow increases. The height to which it rises is directly proportional to the flow rate because the cross-sectional area of the tube decreases with height, and the pressure drop balances the float’s weight. This principle ensures that the bobbin position provides a real-time, visual indication of flow rate without requiring external pressure measurements.

**Why Each Wrong Option is Incorrect**
Option A: Dirt inside the tube can obstruct flow and alter the actual flow rate, leading to inaccurate readings. Thus, it does affect the reading.
Option B: Static electricity can cause the bobbin to stick or move erratically, especially in low-flow conditions, leading to measurement errors.
Option D: A rotameter is not a variable pressure flowmeter; it is a variable area flowmeter. Variable pressure flowmeters (like differential pressure meters) measure pressure differences, whereas rotameters rely on changing cross-sectional area.

**Clinical Pearl / High-Yield Fact**
Rotameters are commonly used in anaesthesia for monitoring oxygen flow in breathing circuits, but they require regular cleaning and are sensitive to air entrainment or contamination. Always ensure the tube is clean and the float is free of obstruction.

✓ Correct Answer: C. The height to which bobbin rises indicates the flow rate