What device measures true flow of oxygen?

The Thorpe tube is the correct choice for measuring the true flow of oxygen. It is designed to provide a precise reading of flow rate due to its construction and design characteristics. In a Thorpe tube, the gas flow enters at the bottom of a tapered tube and rises within it, moving a float that indicates the flow rate on a calibrated scale. The variable diameter of the tube allows for accurate measurement of flow, as the float will rise higher with increased flow due to the lifting force of the gas, accurately reflecting the actual flow rate in liters per minute.

This device is particularly advantageous because it remains accurate under varying pressures as long as it is calibrated for the specific gas being used, making it especially suitable for medical oxygen delivery systems where precise flow rates are critical for patient safety and effective treatment. The clear visibility of the float also allows for easy monitoring by healthcare personnel.