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VARIABLE AREA METERS (ROTAMETER)
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- Measuring Gas Flow
| VARIABLE AREA METERS (ROTAMETER) Variable-area instrument for flow measurement is the rotameter. It consists of a tapered vertical tube with afloat inside. The tube is usually made of borosilicate glass. The diameter of the tube is smallest at the bottom and increases toward the top. The fluid to be measured enters at the narrow bottom end and flows upward. 66 Field Instrumentation GEN Rev) The float inside the tube is made so that its largest diameter almost equals the diameter of the tube at the bottom. The density of the float is greater than the density of the fluid. This means that the float sinks to the bottom of the tube if the fluid is not flowing. As fluid flows upward through the tube, the float rises. The area available for the flow of fluid through the rotameter is the ring-shaped area between the tube and the float. If the float rests on the bottom point in the tube, the area between the float and the tube is nearly zero. If the float is higher in the tube, the area between the float and the tube is larger. As the float rises or falls in the tube, the area available for the passage of fluid increases or decreases. This variation is the reason the rotameter is called a variable-area instrument. Measuring Gas Flow Gas flow rates are measured in standard cubic feet per minute (usually abbreviated scfm). One standard cubic foot of gas is a specific quantity of gas one ft 3 measured at standard conditions, which usually are 14.7 psia and F. This quantity of gas will occupy a different volume if either the pressure or the temperature changes. But it is still called one standard cubic feet, because it is still the same amount of gas and will still occupy one ft 3 at standard conditions. It weighs the same amount and contains the same number of gas molecules as it does under standard conditions. 67 Field Instrumentation GEN Rev) A rotameter measures the amount of gas flowing per unit of time. If the flow rate is 100 scfm, the amount of gas flowing out of the rotameter in one minute occupies 100 ft if the pressure is 14.7 psia and the temperature is FA rotameter measures a gas flow rate as though the calibration gas were flowing at standard pressure and temperature. The actual flow rate is different from the indicated flow rate if the actual gas is different from the calibration gas, or if the pressure or temperature varies from standard conditions Download 3.37 Mb. Share with your friends:
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