From heavy industrial plants to precision laboratories, Thermal Dispersion Flow Meters have become the ideal solution for gas measurement. Based on the same thermal dispersion principle, Thermal Dispersion flow switches and flow controllers have been developed. These devices measure mass flow directly without moving parts, and can even control flow, solving the maintenance problems associated with mechanical flow meters in demanding applications.
Thermal Dispersion Flow Measurement Technology
Thermal diffusion flow measurement technology is based on the concept of convective heat transfer. When gas/liquid molecules flow over a heated surface, they carry away heat. The faster the flow rate, the more heat is carried away. This relationship can be described by King’s Law, which establishes the correlation between heat dissipation and mass flow rate.
The design of thermal diffusion flow meters, thermal diffusion flow switches, and flow controllers is similar. Two precisely matched resistance temperature detectors (RTDs) are used. These sensors are key components for achieving gas mass flow rate measurement.
- One RTD acts as a reference sensor: This RTD measures the actual process temperature of the fluid. It serves as a benchmark.
- The other RTD acts as an active sensor: This RTD is heated to a specific temperature higher than the reference sensor.
Two methods are typically used to calculate the flow rate:
Constant Temperature Difference: We maintain a constant temperature difference (ΔT) between the two sensors. As the flow rate increases and the heated sensor cools, the electronics increase power to maintain this ΔT. The required power is proportional to the mass flow rate.
Constant Power: We supply a constant amount of power to the heated sensor. As flow rate increases, the temperature difference between the heating sensor and the reference sensor decreases. This change, ΔT, is inversely proportional to the flow rate.
Thermal Dispersion Flow Meters vs Flow Switches vs Flow Controllers
Thermal dispersion flow meters, also known as thermal gas mass flow meters, are primarily used for measuring dry gases. They have a wide measuring range and are available in inline or insertion type configurations, customizable for pipe sizes from DN10 to DN3000.
Thermal dispersion flow switches are mainly used for online or insertion-type installation in pipelines carrying water, gas, oil, and other media to monitor the flow rate. When the media flow rate is above or below a certain set point, an alarm signal is triggered and transmitted to the control unit. The system then receives the signal and performs the corresponding actions.
The main advantages of thermal mass flow controllers lie in flow control and measurement of low flow rates. Using the thermal principle, they offer fast response and are unaffected by temperature and pressure changes. The flow signal measured by the sensor is amplified and then compared with a set voltage. The resulting difference is used to drive the control valve, providing closed-loop control of the flow through the channel to match the set flow rate.
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