Two other types of mechanical flow meters which can be used are the area flow and displacement meters. In addition, there exists much more sophisticated techniques for measurement of flow rate than use of differential pressure devices, such as anemometry, magnetic, and ultrasonic.

EO 1.3DESCRIBE the following types of mechanical flow detectors, including the basic construction and theory of operation.

a. Rotameter

b. Nutating Disk

Area Flow Meter

The head causing the flow through an area meter is relatively constant such that the rate of flow is directly proportional to the metering area. The variation in area is produced by the rise and fall of a floating element. This type of flow meter must be mounted so that the floating element moves vertically and friction is minimal.

Rotameter

The rotameter, illustrated in Figure 6, is an area flow meter so named because a rotating float is the indicating element.

The rotameter consists of a metal float and a conical glass tube, constructed such that the diameter increases with height. When there is no fluid passing through the rotameter, the float rests at the bottom of the tube. As fluid enters the tube, the higher density of the float will cause the float to remain on the bottom. The space between the float and the tube allows for flow past the float. As flow increases in the tube, the pressure drop increases. When the pressure drop is sufficient, the float will rise to indicate the amount of flow. The higher the flow rate the greater the pressure drop. The higher the pressure drop the farther up the tube the float rises.

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Figure 6 Rotameter

The float should stay at a constant position at a constant flow rate. With a smooth float, fluctuations appear even when flow is constant. By using a float with slanted slots cut in the head, the float maintains a constant position with respect to flow rate. This type of flow meter is usually used to measure low flow rates.