How do you read a rotameter scale?

• A. Top of the scale
• B. Center of the ball
• C. Bottom of the scale
• D. On the side of the tube

Answer: (B)

Reading a rotameter is done at the center of the float inside the tapered tube. The float rises until the upward flow force balances the weight and drag, reaching an equilibrium position that corresponds to a specific flow rate. The scale is calibrated so that the value shown matches the centerline of the float; that center aligns with the numbers on the scale. Reading at the top, bottom, or from the side would give an incorrect value because those positions aren’t how the scale is calibrated and because parallax and the tube’s curvature affect those readings. For accuracy, look straight at the center of the float and read the number where its center line intersects the scale.