This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Anaesthesia
True about Rotameter – 
Select and explain. 
A. Rotameter reading may not get affected by dirt inside the tube, B. Rotameter reading may not get affected by static electricity, C. The height to which bobbin rises indicates the flow rate, D. A rotameter is a variable pressure flowmeter
C: The height to which bobbin rises indicates the flow rate
Answer: C: The height to which bobbin rises indicates the flow rate 
Explanation: As the gas flows through the rotameter, bobbin rises and the upper end of bobbin determines the flow rate.
Rotameter works on variable area (variable orifice) and constant pressure principle (not variable pressure).
Causes of inaccurate reading in rotameter are :- Static electricity, Dirt inside the tube, Non-vertical tube, back pressure by ventilator, Defect in top sealing washer.