The mechanical device which is used to measure flow velocity, where the number of revolutions of the wheel per unit time are proportional to the velocity of the flowing water is the
- Apitot tube
- Bcurrent meterCorrect
- Cfloat
- Drotameter
Solution
Concept
A current meter has a wheel or cups that rotate in flowing water. The revolutions per second are proportional to the flow velocity.
Formula
v = a Ns + b (Ns = revolutions per second, a and b = meter constants)
Step-by-step solution
1. The device has a rotating wheel, and its revolutions per unit time relate to velocity. 2. A current meter works on this principle: v = a Ns + b. 3. A pitot tube uses pressure difference, a float uses travel time, and a rotameter uses a float in a tapered tube. 4. So the answer is the current meter (B).
Explanation
A current meter is put in a stream. The water turns its wheel or cups, and the speed of rotation is proportional to the water velocity. Using the calibration equation, the velocity is found from the revolutions per second.
Why the other options are wrong
A: Pitot tube measures velocity from the difference between stagnation and static pressure, with no rotating wheel. C: Float gives surface velocity from distance travelled divided by time. D: Rotameter measures discharge in pipes using a float in a tapered tube, not a rotating wheel.
Shortcut
Rotating wheel or cups in a river means current meter.
Common mistake
Choosing rotameter because the name sounds like 'rotation'. It has no rotating wheel.
Correct answer: B) current meter