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Principle of Mass Flow Instruments
- 1.Volume flow and mass flow
- Gas flow meters can be roughly divided into volume flow meters and mass flow meters. Volume flow meters include area flow meters, positive displacement flow meters, and differential pressure flow meters, while mass flow meters include coriolis flow meters, vortex flow meters, and thermal flow meters.
The float type flow meters produced by KOFLOC are classified as area flow meters in the category of
volume flow meters, while the mass flow instruments produced by KOFLOC are classified as thermal flow meters in the category of mass flow meters.
n terms of classifi cation, the terms, "thermal mass flow controllers and thermal mass flow meters," are used according to the basic principle.
In this text, however, the commonly used mass flow instruments mean mass flow controllers and mass flow meters in general.
Difference between volume flow meters and mass flow meters
The difference between volume flow meters and mass flow meters is explained below using some simple examples.
Most of the volume flow meters are used when each section of a flow meter is exposed to the atmosphere as shown in Figure 1, namely, when no pressure is applied to the inside of the flow meter.
When pressure is applied, the reading of the volume flow meter calibrated in the atmosphere will not be correct,
and a calculation for correcting the reading is necessary.
Soap film flow meters and dry/wet gas meters are especially susceptible to even a small resistance, and they are used in the atmosphere in principle. The same applies to float type flow meters; their reading cannot be correct when the gas density changes because of a substantial change in pressure or gas temperature.
Therefore, the pressure and temperature conditions must be determined in advance, or calculation for correcting respective factors is necessary for the reading.
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Most of the volume flow meters are used when each section of a flow meter is exposed to the atmosphere as shown in Figure 1, namely, when no pressure is applied to the inside of the flow meter.
When pressure is applied, the reading of the volume flow meter calibrated in the atmosphere will not be correct,
and a calculation for correcting the reading is necessary.
Soap film flow meters and dry/wet gas meters are especially susceptible to even a small resistance, and they are used in the atmosphere in principle. The same applies to float type flow meters; their reading cannot be correct when the gas density changes because of a substantial change in pressure or gas temperature.
Therefore, the pressure and temperature conditions must be determined in advance, or calculation for correcting respective factors is necessary for the reading.
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Detecting by means of weight
Meanwhile, as the name suggests, mass flow meters detect flow by means of weight, permitting the flow to be defined in the same state even if the density changes due to compression of fluid.
When gas is detected by means of mass, the reading of the flow mentioned above will be the same even in a ressurized state as shown in Figure2.
Therefore, flow meters can be placed at any location on the
flow chart, permitting a system to be confi gured without signifi cant flow reading errors.
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When gas is detected by means of mass, the reading of the flow mentioned above will be the same even in a ressurized state as shown in Figure2.
Therefore, flow meters can be placed at any location on the
flow chart, permitting a system to be confi gured without signifi cant flow reading errors.
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