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Factors affecting differential flow
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- Effect of Pressure
- Effect of Temperature
| 3.5 Factors affecting differential flow measurement: Effect of Relative density If we replace the calibration gas usually air with a different gas having twice the density of air. The relative density of the gas is therefore twice the relative density of the calibration gas. This gas has a greater effect on the rotameter. At a specified flow rate, the denser gas causes the float to rise higher in the tube. As a result, the actual volume of gas that flows from the meter in one minute is equal to the indicated flow rate multiplied by the square root of 1/2. This relationship can be expressed as Qa = Qi √ (Drc/Dra) Where, Qa = actual flow rate Qi = indicated flow rate Drc = relative density of the calibrated gas Dra = relative density of the actual gas. Effect of Pressure If the calibration gas is at the calibration temperature, but the pressure of the gas is twice the standard pressure. The actual flow rate is then equal to the indicated flow rate multiplied by the square root of 2. That is Qa = Qi √ (Pa/Pc) Where, Pa = actual pressure Pc = calibration pressure. Effect of Temperature If we use the calibration gas at the calibration pressure, but at twice the calibration temperature. The actual flow rate of gas is then equal to the indicated flow rate multiplied by the square root of 1/2. That is Qa = Qi √ (Tc/Ta) Where, Tc = calibration temperature (absolute) 68 Field Instrumentation GEN Rev) Ta = actual temperature (absolute. We can combine these three equations into a single equation for gas-to-gas conversions. This equation enables us to convert any reading from a flow meter into the correct reading for the specific gas and the specific conditions under which it is measured. The master equation is Qa = Qi √ (Drc)(Pa)(Tc) / (Dra)(Pc)(Ta) It is assumed that the gas being measured is an ideal or perfect gas. A perfect gas is one in which the molecules are infinitely small and exert no forces of attraction or repulsion on each other. Areal that is non-perfect gas has molecules of a finite size that exert forces on each other. To adjust these equations for non perfect gas effects, apply a compressibility factor. The compressibility factor fora perfect gas is 1.00. Fora non perfect gas, the correction factor might be 0.85. Download 3.37 Mb. Share with your friends:
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