How to convert the reading displayed by a rotameter into standard cubic units
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This post was last edited by ch4_hcho on 2012-2-1 22:08. I bought a rotameter, and I’m encountering some issues with its use that I can’t figure out; please help me understand. The meter’s dial indicates: hydrogen, 298K, 0.4MPa, 0.084 kg/Nm3. I understand that this meter was calibrated using pure H2, at a pressure of 0.4 MPa (absolute) and a temperature of 298K, at which point the density of hydrogen is 0.084 kg/Nm3. If the meter is used under the same conditions of 0.4 MPa (absolute) and 298K, with pure H2 as the gas, then if the meter displays a value of 10, it means that the flow rate of H2 under those temperature and pressure conditions is 10 m3/h, not 10 Nm3/h. I’m not sure if my understanding is correct My actual operating conditions are as follows: the gas is a mixture of H2 and N2, with H2 accounting for 80% and N2 accounting for 20%; the pressure is 2.5 MPa (absolute pressure), the temperature is 398 K, and the flow meter reads 6. So what is the actual flow rate of this gas mixture in Nm3/h? How is this calculated?
The gauge on the flow meter indicates: hydrogen, 298K, 0.4MPa, 0.084 kg/Nm3; the flow meter is calibrated using air as a reference for conversion purposes. As for whether the flow rate you mentioned refers to the condition under standard conditions or under operating conditions, it’s impossible to tell since you haven’t specified the units on the gauge. As for the fact that the parameters you are using now do not match those used at the time of ordering, if a conversion is necessary, they can all be converted to air values before making a comparison; however, if the range differences are too large, such conversion becomes meaningless.
The scale unit on the dial is m3/h, not Nm3/h
A rotameter is inherently a rough measuring device, especially those with on-site indication; it should not be used with precision. It merely provides an approximate indication of the flow rate of a medium. The composition, density, pressure, and temperature of the medium in use can all cause variations in the rotor’s height, which in turn results in different flow rates. Converting these values each time is quite troublesome. If the medium in use differs little from the one used for calibration, it is reasonable to assume that the flow rate indicated by the rotor corresponds to the actual flow rate of the medium.
Hydrogen, 298 K, 0.4 MPa, 0.084 kg/Nm3 – these are the operating conditions provided by the process! Flow meter manufacturers will not use h2 as a designation! As for the unit, look at the dial! The gas being tested has changed now; please contact the manufacturer so that they can calculate it for you, and they will then tell you how to convert the scale
Hydrogen: 298 K, 0.4 MPa, 0.084 kg/Nm3. The flow rate range is 10 m3/h; by making conversions, this value is first transformed into the flow rate of air under standard conditions: 10.528, 21.057, 31.585, 42.113, 52.641, 63.170, 73.698, 84.226, 94.754, 105.283. In practice, a mixed gas is used, with a pressure of 2.5 MPa (absolute pressure) and a temperature of 398 K; the flow meter indicates a value of 6. Based on standard conversion values, this corresponds to a flow rate of 143.235, 286.469, 429.704, 5612.938, 7016.173, 8419.407, 9822.642, 11225.876, 12629.111, 14032.345. In other words, when the pointer shows 6, the actual flow rate is 14 Nm3/h
I think some of the issues you’re considering are too complicated. Your flow meter has been calibrated, so the value it displays should be the accurate one, as calibration is done using a medium with a density similar to that of the actual fluid being measured. I’m not sure if my understanding is correct. The accuracy of that type of flow meter isn’t high; it’s only for rough estimates.
Gas flow meters are greatly affected by changes in temperature and pressure on the flow rate; therefore, temperature and pressure corrections are taken into account here. It is also possible to make temperature and pressure corrections without considering changes in density
The flow rate under standard conditions is roughly calculated using the gas equation