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Principle of Mass Flow Instruments
- 2. Principle of mass flow sensor
- The flow sensor used in mass flow is called a thermal flow sensor in general.
The principle of detection is as follows.
Structure of sensor section
A resistive element with a large temperature coeffi cient of resistanceis wound on the upstream side (Rus) and the downstream side (Rds), respectively, around the capillary tube that is a sensor as shown in Figure 3.
When electric current flows through these sections, the two resistive elements generate heat. When no fluid flows in
the capillary tube at that time, the temperature of the upstream side is the same as that of the downstream side, matching each other.
(The solid line in Figure 3: Zero fl ow = Position of the zero point used
for mass fl ow instruments.)
When the fluid begins to flow in this state, the temperature distribution changes as shown by the broken line in Figure 3.
The heat of the upstream side is drawn at that time, and the heat is transferred by the fl ow to the downstream side conversely. In other words, a temperature difference (∆T) arises between the upstream and downstream sides.
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When electric current flows through these sections, the two resistive elements generate heat. When no fluid flows in
the capillary tube at that time, the temperature of the upstream side is the same as that of the downstream side, matching each other.
(The solid line in Figure 3: Zero fl ow = Position of the zero point used
for mass fl ow instruments.)
When the fluid begins to flow in this state, the temperature distribution changes as shown by the broken line in Figure 3.
The heat of the upstream side is drawn at that time, and the heat is transferred by the fl ow to the downstream side conversely. In other words, a temperature difference (∆T) arises between the upstream and downstream sides.
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Structural chart of mass flow meter
As the temperature difference (∆T) has a functional relation to the mass flow of fluid, mass flow instruments pick up electric signals that represent the change in respective resistance values and amplify and correct the signal to permit the mass flow to be measured under a
certain condition.
This is the function of the mass flow meter shown in Figure 4.
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certain condition.
This is the function of the mass flow meter shown in Figure 4.
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Structural chart of mass flow controller
In the mass flow controller shown inFigure 5, the opening of the flow control valve is controlled by a high-velocity, high-resolution piezo or solenoid actuator based on the comparison between the external flow setting signal and the flow signal output from the sensor.
This system permits stable mass flow control, which will hardly be affected by changes of various conditions such as temperature and pressure.
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This system permits stable mass flow control, which will hardly be affected by changes of various conditions such as temperature and pressure.
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