Positive displacement flowmeters repeatedly entrap fluid to measure its flow. It can be thought of as repeatedly filling a bucket with fluid before dumping the contents downstream. The number of times that the bucket is filled represents the flow. Many positive displacement flowmeter geometries are available. Entrapment is usually accomplished using rotating parts that form moving seals between each other and/or the flowmeter body. In most designs, the rotating parts have tight tolerances so these seals can prevent fluid from going through the flowmeter without being measured (slippage). In some positive displacement flowmeter designs, bearings are used to support the rotating parts. Rotation can be sensed mechanically or by detecting the movement of a rotating part. When more fluid is flowing, the rotating parts turn proportionally faster. The electronic transmitter processes the signal generated by the rotation to determine the flow of the fluid. Some positive displacement flowmeters have mechanical registers that show the total flow on a local display. |
