In what scenario is the uncompensated Thorpe Tube not accurate?

The uncompensated Thorpe Tube is primarily designed to measure gas flow accurately under specific conditions. It operates based on the principle that the flow of gas creates a pressure drop, allowing the float within the tube to rise to a level that corresponds to the flow rate. When gas flows freely, the tube can reflect the actual flow accurately, as it relies on consistent pressure for its readings.

However, when the gas delivery is obstructed, the conditions that the Thorpe Tube relies upon for accuracy are disrupted. The obstruction can create a back pressure that alters the flow characteristics, resulting in an inaccurate representation of the flow rate. In an unobstructed flow scenario, the gas pressure is constant, allowing the Thorpe Tube’s float to correspond accurately to the gas flow rate; but with obstruction, the pressure may vary significantly, leading to a misleading reading.

Therefore, the compromise in flow continuity and pressure variation caused by an obstruction can prevent the Thorpe Tube from providing a correct measurement of flow.