How flow sensors work?

Flow sensors are used for wide range of fluids in multiple industries for various measurements. There are many types of flow sensors and they all measure “volume or area per unit time”. And depending of the sensor type there are multiple ways this volume per unit time can be measured, but they all use basic concept of fluid flow principles with concept such as the Bernoulli’s principle. Some of the basic flow sensors used today are orifice meter, venture meter, flow nozzle, and pitot tubes which us the principle of difference in pressure from the Bernoulli equation. Then there are sensors which use direct force to measure the flow which include rotameter, turbine meter, propeller flow meter, coriolis mass flow meter. Using pressure differences and direct force are the most common methods used to measure flow rate but there are other complicated methods such as ultrasonic flow meters, magnetic flow meter, calorimetric flow meter, gear flow meter, thermal flow meter, and couple more.

All the flow sensors which use pressure difference use Bernoulli equation which is 

Where the condition on both sides of the sensors are inversely related to each other, therefore the equation can be manipulated into the pressure drop across the flow sensor is equal to velocity of the flow squared. When calculating the flow using the different sensors the area of the opening on both sides of the sensor, density of the liquid, and pressure readings from the sensors are known an can be plugged into the Bernoulli equation to find the velocity because V₁ = V₂ = V. Below are few images portraying how some of this sensors operate and how the pressures differences (dp) can be measured.     

Orifice Plate Flow Sensor

Venturi Tube Flow Sensor

Flow Nozzles Flow Sensor

Sensors using direct force to measure velocity use methods of balancing forces with in systems where the force applied by the fluid flowing through the sensor is measured and manipulated with a proportion factor to get the flow of fluid in the system. Below are some images which portray how the forces applied by the fluid are with few different types of direct force flow sensors.

Rotameter: resistance of gravity force of the bolt is being measured here.

Turbine meter: work is being measured here where work equals force times distance, and the distance id known so the force can be calculated from the work measured by the sensor.

Sources:

http://www.princeton.edu/~asmits/Bicycle_web/Bernoulli.html

https://controls.engin.umich.edu/wiki/index.php/FlowSensors

Flow Sensors (Fluidic-flow measurement sensors)

            Flow sensors are devices that sense the rate of fluid or gas flow. The sensors installed in different field varied depending on the properties of the fluid or gas being measured. Sensors have different meters and principles therefore, the most appropriate should be chosen for the desire application. The most common sensors in our daily life are the ones used to measure water flow and electrical consumption at our home.

During my research through AccessScience I found two types of Fluidic-Flow measurement sensors known as fluidic-oscillator meter and fluidic flow-sensor. The fluidic-oscillator meter works on the principle of Coanda effect. The Coanda effect of a jet fluid attaching to a nearby surface, and it remains attached even when the surface curves away from the initial fluid direction. In the case of the fluidic-oscillator meter the fluid comes into the device and the fluid attaches to one of the side walls (see attach figure). Part of the flow splits off and goes through the feedback passage forcing the incoming flow to attach to the other side of the sidewall. The frequency of the oscillation back and forth is proportional to the volume flow through the meter. The sensor records the oscillations and transmits the signal to record the flow. These types of meters can be used for fluids and flow meters. The fluidic-flow sensor measures the flow of gas. It consists of air or another gas directed from an outer nozzle onto two small openings. The flow of the gas being measure will bend the air or gas and therefore changes the relative pressure on the two ports. This activates the signal and allows recording the gas velocity.

Rita Pauliushchyk on her blog decided to focus on the common types of flow sensors such as flow of water and energy consumption of a building or household.  These are sensors we use/activate every day for our usage. This sensors are the one in charge of saying how much we have consume at  home shown in our monthly bills. Sensors although they are measuring fluid, gas, air they are also helping to control the usage. Sensors are today recording and serving data to make building more efficient and sustainable. They are a powerfull device that is making buildings sustainable. It is important to know which sensors to install accordingly to the application and characteristics.

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

Measuring Flow with Primary Sensors

Flow measurement of fluids, including air and liquids, are measured in a variety of ways. Each type of measurement technique has advantages and disadvantages and therefor implementation varies across the broad field of flow measurement. The applications of different technology vary due to some of the inherent properties of the fluid being measured, resolution of the results, life-span/capability of materials, cost, pipe size, pressure and velocity. Other factors such as cost vary and are sometimes a function of the primary variables, such as reading resolution and operating pressures.

Measurement of flow parameters are often performed by positive displacement methods. An analogy to this method is a bucket and a stopwatch. The bucket is filled with the fluid and the time is recorded that it takes to reach a filled state. The volumetric capacity of the bucket is known and the time duration to fill is known, which constitutes a flow rate. The sensors that can be implemented for this type of monitoring would be a mechanical switch paired with a floating device, which trigger the timer when the cavity is empty and full. The most popular types of positive displacement meters use pistons that operate in a cavity of known volume. Every time the cavity is filled to capacity, the piston is forced to move and subsequently rotate an axle that it is connected to. The signal from the rotating axle can be transmitted to the user by a magnetic drive, needle dial and a counter such as an odometer. A turbine also uses mechanics to produce flow measurements, but instead of positive displacement, the fluid is exerting force on the components and creating work. A turbine is place in the path of the fluid being measured, so that the fluid produces a force on the area of the turbine. The force causes the turbine to rotate, which once established at a steady speed, is proportional to the velocity of the fluid.

A vortex meter uses the phenomenon of Van Karman forces that are created using an object that is located in the flow path of the pipe or channel. The object that is placed in the path is known as a bluff body and results in vortices created in the wake of the body. The Van Karman forces vary between the two sides of the bar at a rate proportional to the fluid velocity. For measurement, a piezoelectric sensor records the number of times the vortices are created by transmitting a voltage pulse.

Similar to the Van Karmen forces, where an object is placed in the path of the pipe, Jalpesh describes in his post the affect that a restriction can cause. The post did an excellent job explaining how the flow was measured using the differential pressures through the restrictions. Each type of restriction was presented and explained correctly. This type of meter was one of the few I saw in the flow sensors postings that relied mostly on heavy theory from fluid dynamics. The other sensors definitely included theory, some of which might have been more complicated than Bernoulli Theorem, but they didn't seem as classic. One thing that wasn't mentioned in the posts I looked was the types of sensors that relied heavily on more calibration and empirical data, such as those that are transcribing through magnetic needles and other sensitive components.

Aside from mechanical flow meters, fluid velocity and flow can be measured using optic sensors. The optic sensors take advantage of lasers of light passing through a tube or pipe containing fluid. Two lasers are contained in a small area of the pipe which track particles suspended in the flow path. The first laser sends a beam of light through the medium and the particles scatter the laser. On the oppisite side of the pipe, a photo detector records the amount of light and sends an electric pulse through a circuit. The same suspended particle then passes another laser beam that completes the same process the first has done with a photo detector sending another pulse. The time between pulses is known as well as the distance between the two lasers, therefor it is possible to calculate the flow rate.

Source:

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