Definition
A flow-measuring instrument that indicates the rate of fluid or gas flow by means of a tapered, vertical tube containing a float. The flow enters at the bottom and lifts the float to a height proportional to the flow rate, which is read against a calibrated scale on the tube.
Plain English
A device that measures how fast a liquid or gas is flowing by letting the flow push a small float up a tapered glass tube. The faster the flow, the higher the float rises, and the height shows the flow rate on a printed scale.
Context Anchor
Seen in aircraft maintenance, system testing, and equipment checks where fuel, air, oxygen, or other fluid flow must be measured.
Derivation
From 'rotate' plus 'meter.' The float in many designs has small slots that cause it to spin (rotate) as fluid passes, keeping it centered in the tube. The name reflects that spinning action combined with its job as a measuring device.
Why Pilots Care
On aircraft fitted with built-in oxygen systems, a rotometer-style flow indicator confirms that oxygen is actually reaching the mask at the correct rate. Seeing the float at the right mark is direct visual proof the system is working.
Analogy
Think of a clear vertical tube with a small marker inside it. As more fluid flows upward, the marker rises higher, and the scale beside it shows the flow rate.
Intuition Check
A rotometer is not a tachometer. It measures flow through a line, not how fast something is spinning.
Example Sentence 1
The technician used a rotometer to verify that the oxygen system was delivering the correct flow rate to each crew station.
Example Sentence 2
During the engine run-up check, the mechanic used a rotometer to confirm steady fuel delivery before flight.