Flow Sensors

                For the majority of this term, this class has focused on intelligent buildings and new technology. However, as engineers we deal with lots of basic measurements and calculations we encounter in an everyday work environment. An example of these measurements, is the measurement of flow done through the use of flow meters and sensors.  Flow sensors are detecting elements within a flow meter that record the flow of fluids or gases. In the figure below, there’s a variety of flow sensors that measure liquid flow, but  vary in the form of which they measure the flow.

             As can be seen on the left hand side, the rotor in turbine flow meters measures the flow because the rate of the flow causes a proportional movement in the rotary wheel. The rate at which the wheel is spinning, is also the rate of the flow. Magnetic flow meters as can be seen on the bottom right side of the image above, operate on Faraday’s law of electromagnetic induction. This means that the flow meters are triggered by conductive liquids because the flow is measured as a counter reaction to the conductivity. This counter reaction is a voltage that is produced by  a current applied to coils mounted on or outside the flow pipe. The voltage produced is a magnetic field that is proportional to flow rate, an and its measured by electrodes in the system. Thermal flow meters as pictured above (second one down, left hand side) measure mass flow directly. The thermal flow meters measure flow by heating the liquid within, and take the rate at which it takes to dissolve. Other thermal sensors just input heat into a system, and measure the amount of energy used for the system to stay at that temperature. This type of thermal system is more often used for gases, along with multivariable differential pressure transmitters. These type of meters are based on temperature sensors, which measure the heat within the moving medium, along with velocity to calculate the rate.

                 I found it neat that the multivariable differential pressure transmitters, can act as temperature sensors as well. They can measure pressure and temperature , to calculate mass flow. This was really interesting because it shows an overlap within sensors, since the flow meters use resistive temperature detectors (RTDs), which EldaCifligu describes as temperature sensors. Like Matthew Tedesco stated, “aside from mechanical flow meters, fluid velocity and flow can be measured using optic sensors.” None of the meters above show this type of sensor because this “laser-based interferometry is often used for air flow measurement but not for liquid flow.

               

Sources:

http://en.wikipedia.org/wiki/Flow_sensor

http://www.pc-control.co.uk/flow_sensors.htm