Where do you read the flow on a Thorpe tube flowmeter?

In a Thorpe tube flowmeter, the correct way to read the flow is at the center of the metal ball. The design of the Thorpe tube features a float, typically a ball, that moves up and down within a tapered glass tube according to the flow of gas. As the flow increases, the ball is lifted higher in the tube. The center of the ball accurately indicates the flow rate on the graduated markings along the side of the tube. This method provides a precise measurement since the center of the ball corresponds with the specific flow rate where the ball is stabilized and floating freely in the gas stream. Reading at this point ensures that variations in ball shape or contact with the tube walls do not affect the measurement accuracy.

In contrast, reading at the bottom or top of the ball can lead to inaccuracies due to the influence of the ball's position and shape on the flow readings. Additionally, checking the side of the tube would not give a correct indication of the flow rate, as the flowmeter is calibrated based on the ball’s position in relation to the mid-point of the ball itself.