Thermal Mass Flowmeters (TMF for short), originated in the United States,
are generally used in gas and air measurement in environments with high
accuracy requirements, trade settlement, or difficult-to-measure environments.
Compared with traditional air measuring instruments, thermal mass flow meters are widely used in various industries such as thermoelectricity, chemical
industry, oil and gas, etc. due to their many advantages in gas and air measurement
accuracy and maintenance.
Working principle of thermal mass air flow meter
How
does a mass air flow meter work ?
Thermal mass air flow meter consists of a
transmitter and a sensor, where the sensor is a temperature probe made of two
platinum thermal resistors, two probes are placed in the air pipeline, one
platinum thermal resistance is heated, and the other is sensed process temperature.
When the air density remains unchanged, the air flow rate increases, the heated
platinum thermal resistor cools faster, and the temperature difference between
the two platinum thermal resistors decreases rapidly. When the air in the
pipeline has no flow, the temperature difference reaches the maximum.
The temperature difference in the sensor can be converted into a DC
voltage signal through the control circuit, and finally the 4-20 mA current is
output at the output terminal, and the measured mass flow can be obtained from
the on-site setting calculation circuit of the transmitter. Thermal Dispersion
Technology provides a measuring instrument that can easily configure the signal
output, while the display signal can be adjusted on-site using the built-in
keyboard or a communicator.
Main features of thermal mass flow meters
Advantages of thermal mass flow meter for air
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(1) It is easy to install and has various structures. There are two types
of installation: insertion thermal mass air flow meter and pipeline type. It is
easy to disassemble and easy to use. It can be used in high temperature air measurement
occasions and corrosive environments.
-
(2) There are no moving parts, which reduces the measurement error caused
by the dirty air or the blockage of the medium particles. The integrated design
is adopted, and there is no need to add additional instruments, which reduces
the difficulty of maintenance, and will not be affected by the leakage of the
air.
-
(3) A platinum RTD sensor is used, and two RTDs are used in the sensor to
balance the mass, so that it can accurately measure the mass flow of the air when
the temperature, pressure, density, etc. change.
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(4) High accuracy, high repeatability, no need for temperature and
pressure compensation, the air mass flow is directly measured without
temperature and pressure compensation, the accuracy can reach ±1% FS
repeatability is generally about 0.05%, it is high accuracy air mass flow
meter.
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(5) It is more sensitive to low air flow measurement, with strong overload
capacity and large range ratio. When the actual flow rate is much higher than
the full range, a corresponding alarm will be made and the instrument will not
be damaged, it can be used at low pressure air flow measurement.
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(6) Thermal mass air flow meter cost is not very high ,especially for
large pipelines air lines or ducts, such as 8 inch, 10”, 20”.,etc ,insertion
thermal mass air flow meter is an economical solution for compressed air or air
measurement.
Disadvantages of thermal mass flow meters
(1) If there is liquid in the pipeline, or the medium contains other
impurities, it will cause air measurement errors.
(2) If the mixed air is measured and the gas composition changes, the
internal parameters need to be recalculated and set according to the dynamic
curve. If the gas composition is unstable, there will be errors in the
measurement.
Measurement of acid gas by insertion thermal mass flow meter
The wear-resistant and anti-pollution characteristics of thermal mass flow
meters make it useful in highly corrosive environments, improving the accuracy
and feasibility of measuring acid mixtures. Insertion flow meters combine
unique mass thermal dispersion sensing element, precise electronics, and
rigorous fluid calibration in a rugged industrial package, making it an ideal
air measurement instrument. The insertion air flow sensor can be installed and
maintained online, but the sensor must be installed on the valve and inserted
into the center of the pipe, otherwise it will cause measurement errors. The
measurement principle of the flow meter is to convert the heat taken away by
the measured medium into flow. When the measured air is not transported in the
pipeline, the air will still take away part of the heat, resulting in a small
amount of false flow when no medium passes through the pipeline. At this time,
a small signal can be used to cut off about 5%.
Epilogue
In the past ten years, thermal mass air flowmeters have been continuously
innovating in combination with modern microelectronics new technologies and new
processes. With their unique characteristics, SILVER thermal mass air flow
meters can measure acid gas, fuel gas, purified air, compressed air, flue air, blower
air flow measurement, hydrogen and other single gases or Mixed gas, quickly in
industrial production, successfully applied in measurement conditions and
environments that other flow meters are incapable of.
