The Basic Components
A mass flow meter measures the flow rate of gas mass based on the convective heat transfer of a heated surface to the flowing gas or fluid. The basic components of a mass flow meter include two temperature sensors with an electric heater between them.
The heater can protrude into the gas or fluid stream or can be external to the pipe. They are basically a meter for measuring the amount of a gas or liquid instead of the volume moving through a closed tube or pipe.
What is being read is the fluctuation of the temperature within the closed circuit. As far as what mass flow meters are and how they work, several different kinds of mass flow meters perform different functions and can handle different amounts of material and still give an accurate measurement.
In the past, mass flow was measured from the outputs of a volumetric flow meter and a densitometer. Density was directly measured using the outputs of process temperature and pressure transmitters. These mass flow measurements were not accurate because the relationship between process pressure or temperature and density are not always known.
Each sensor adds its own level of inaccuracy to the overall measurement, and the speed of response of such calculations is usually not enough to detect changes in flow. This creates a lag in the reporting, and what makes it back to the sensor may no longer be accurate. Thermal mass flow meters operate by utilizing thermal-sensing technology.
As flow enters the device, a small portion is redirected into a small tube containing two coils—one downstream from the other. The first coil introduces a small amount of heat into the gas stream. As the gas passes through the tube, the smart electronics sense the amount of heat transferred from one coil to the other.
Mass flow meters operate either by introducing a known amount of heat into the flowing stream and measuring an associated temperature change or by maintaining a probe at a constant temperature and measuring the energy required to do so.
Chemical Reaction Control and Low Gas-Flow Detection
Mass flow meters provide high range ability if they are operated at a constant temperature difference mode. They are often used in monitoring or controlling mass-related processes like chemical reactions that depend on the relative masses of unreacted ingredients.
In detecting the mass flow of compressible vapors and gases, the measurement is unaffected by changes in pressure and temperature. One of the capabilities of thermal mass flow meters is to accurately measure low gas-flow rates or low gas velocities that are much lower than can be detected with other devices.
If the heat input is constant, the ability to detect exceedingly small temperature differences is limited, and precision will drop off. At normal flow, basic measurement errors are usually in the full-scale range.
The final explanation of what a mass flow meter is and how it works is related to its frequent use in monitoring or controlling mass-related processes like chemical reactions, which depend on the relative masses of unreacted ingredients, like recipe formulations, material balance determinations, and billing and custody-transfer operations in the case of natural gas providers.
As these are the most critical flow measurements in a processing plant, the reliability and accuracy of mass flow measurement is very important.
Image Credit: mass flow meter by envato.com
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