About Thermal and Electromagnetic Flow Measurement

Thermal Mass Flow Meter

Thermal flow meters are primarily used in air and gas flow measurement applications. The meters consist of a transmitter and probe with temperature sensors (RTD's) located in the pins at the bottom of the probe. One sensor measures the process temperature and the other sensor is heated to a specific temperature above this.

As the flow rate increases heat gets taken away from the heated sensor. Some manufacturers use a variable power operation to keep the temperature difference constant, while others keep the power constant and measure the temperature difference. The Magnetrol Model TA2 measures the power it takes to maintain a constant temperature difference between the sensors. This relationship between power and mass flow rate is established during calibration.

Thermal Dispersion Flow/Level/Interface Switch

Similar to thermal flow meters the switches are based on heat transfer. One sensor is at the process temperature and the other is being heated by a constant power. As the flow rate increases, the temperature difference between the sensors decreases. A set point is established so when that specific temperature difference is reached the relay changes state. This can be on either increasing or decreasing flow or flow/no flow. When used in a level or interface application it is primarily the thermal conductivity of the fluid that will provide the difference in heat transfer.

Electromagnetic Flow Meter

The electromagnetic flow meter consists of a sensor through which the measured liquid flows and an electronic unit where the low-level signal from the sensor is modified to a standardized form suitable for further processing in various industrial electronic devices.

The output signal is proportional to the volumetric flow rate of the measured liquid. The only factor limiting the application of electromagnetic flow meters is the requirement that the measured liquid shall be conductive and non-magnetic.

The electromagnetic flow meter can be designed either as an integral device or with the sensor separated from the associated electronic unit. In the former case, the electronic unit is fitted directly onto the sensor, in the latter case it is connected to the sensor by a remote cable.

The sensor design shall take into consideration the type of the measured liquid and its operational parameters. To facilitate fitting into the liquid piping, the sensor can be provided with end flanges or as a wafer style design.

The function of an electromagnetic flow meter is based on Faraday's law of induction. The sensor consists of a non-magnetic and non-conductive tube with two embedded measuring electrodes. To create an alternating magnetic field, two coils are fitted onto the tube in parallel with the plane defined by the active parts of the measuring electrodes. If a conductive liquid flows across the magnetic field, a voltage will appear on the measuring electrodes proportional to the flow velocity and the conductor length.

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The TA2 offers excellent performance at an exceptional value. This advanced mass flow meter features powerful, yet easy to use, electronics that are contained in an explosion proof enclosure. The TA2 is also available with insertion probes and flow body design for smaller pipe sizes.

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