Magnetic flow meters operate on Faraday’s law of electromagnetic induction that states that a voltage is induced when a conductor moves through a magnetic field. The liquid serves as the conductor and the magnetic field is created by energized coils outside the flow tube. Electromagnetic flow meters can only be used for electrically conductive fluids, such as water. They cannot measure De-ionized water or most hydrocarbon based liquids. The output voltage produced by the electromagnetic induction is directly proportional to the flow rate. Electrodes mounted in the pipe wall detect the voltage which is measured by a secondary element.
Electromagnetic flow meters have no moving parts. They are selected for their abilities to measure difficult and corrosive liquids and slurries with a very low pressure drop parts and are ideal for wastewater applications or any dirty liquid which is electrically conductive. Most meters offered have accuracies between 0.3 to 1% of reading. Flow ranges of up to 100:1 are available. Insertion style meters are typically in the range of $400 to $1000. The majority of In-Line style version cost $1,200 to $5,000.
The effect of motion to a sound source, relative to the frequency shift of the sound, was first observed and described by Christian Johann Doppler. A practical application for flow measurements is made by sending bursts of sound waves through a fluid filled pipe. The measurement of flow is based on the principle that sound waves travelling in the direction of the fluid flow travel faster than sound waves travelling against the fluid flow. At zero velocity, the transit time differential or delta T is zero. Since ultrasonic signals can penetrate solid materials, the transducers can be mounted onto the outside of the pipe. Ultrasonic flow meters use one of these two principles; The Doppler Effect, or Time of Flight measurement.
Ultrasonic flow meters use very high frequency sound waves to measure the velocity of particles in the process fluid or the time it takes for the sound waves to pass through the fluid.
The Doppler Effect Flow method requires that there are reflecting particles in the fluid. This method is not suitable for clear liquids. Since the Doppler flow meters performance is so highly dependent on the physical properties of the fluid, such as the sonic conductivity, particle density, and flow profile, this method is only suitable for applications that do not require a high accuracy.
Time of Flight Ultrasonic Flow meters measure the time for the sound to travel between a transmitter and a receiver. This method is not dependent on the particles in the fluid. Two transceivers are located on each side of the pipe. The transmitters send pulsating ultrasonic waves in a predefined frequency from one side to the other. The difference in frequency is proportional to the average fluid velocity.
Ultrasonic flowmeters are chosen due to their clear flow path, low pressure drop, corrosion resistance, and relatively low power consumption (compared to Magnetic flow meters). Typical accuracy is about 1% of flow rate, although multi-beam ultrasonic meters can offer higher levels of accuracy. Clamp-on or bolt-on ultrasonic flow meters are available at very economical prices. Inline styles typically range from one thousand to five thousand dollars.
Reliable operation of ultrasonic flowmeters requires high frequency sound transmitted across the pipe. Liquid slurries with excess solids or with entrained gases may block the ultrasonic signals. Ultrasonic flow meters are not recommended for primary sludge, mixed liquor, aerobically digested sludge, dissolved air flotation thickened sludge and its liquid phase, septic sludge and activated carbon sludge.
Turbine flow meters are a propeller blade that spins within a pipe as fluid moves through it.
There are many different manufacturing designs of turbine flow meters, but in general they are all based on this same simple principle. The turbine blade motion is sensed by a magnetic pick-up and an electrical pulse is generated. The number of electrical pulses counted for a given period of time is directly proportional to flow volume. To achieve a repeatable result, the liquid flow must be fully developed and stable across the cross section of the pipe. To achieve this stable profile, a few diameters of straight pipe on the inlet and the outlet are required to achieve accurate measurements. They can be used for clean liquids with viscosities up to 50 centistokes. The turndown ratio can be as high as 100:1 if the turbine meter is calibrated for a single fluid and used at constant conditions. The majority of turbine meters have an accuracy of 0.5% to 1.5% of reading.
Paddle wheel flow meters are cost effective devices typically used for water or water like fluids. These meters, like the turbine meter, require a few pipe diameters of straight pipe on the inlet and on the outlet. Chemical compatibility should be verified when not using water. The rotor of the paddlewheel sensor is perpendicular to the flow and contacts only a limited cross section of the flow. Prices typically are in the range of $100 to $600. Accuracy is typically 1 to 3% of full scale. An accuracy measured against the full range of the device will double at 50% of full scale and triple at 33% of full scale. Caution should be used when measuring with a device that states a “full scale accuracy” as opposed to an “of reading accuracy”.
Vortex flow meters make use of a natural phenomenon that occurs when a liquid flows around a bluff object. Eddies or vortices are shed alternately downstream of the object. The frequency of the vortex shedding is directly proportional to the velocity of the liquid flowing past the bluff object. The three necessary components of a vortex flow meter are a bluff body mounted in the flow meter bore, a sensor to detect the presence of the vortex and generate an electrical impulse, and a signal amplification circuit. The main advantages of vortex meters are their low sensitivity to variations in process conditions and low wear relative to orifices or turbine meters. Also, initial and maintenance costs are low. Performance with slurries or high viscosity liquids is poor. Used in both gas and liquid applications, Vortex flow meters are used abundantly in steam measurement applications. The majority of Vortex flow meters cost $1,000 to $3,000. Accuracies are typically in the range of 0.5% to 1.0% of reading.
Target flow meters sense and measure forces caused by liquid impacting on a target or disk suspended in the liquid stream. A direct indication of the liquid flow rate is achieved by measuring the force exerted on the target. In its simplest form, the meter consists only of a hinged, swinging plate that moves outward, along with the liquid stream. In such cases, the device serves as a flow indicator. More sophisticated version uses a precision, low-level force transducer sensing element. The force on the target caused by the liquid flow is sensed by a strain gage. Target meters are useful for measuring flows of dirty or corrosive liquids. They are relatively inexpensive and offer 10:1 flow ranges with accuracies of 0.5% to 1.0% of full scale.
Variable area flow meters consist of a vertically oriented glass (or plastic) tapered tube with a larger inside diameter at the top, and a metering float which is free to move within the tube. Fluid flow causes the float to rise in the tube as the upward pressure differential and buoyancy of the fluid overcome the effect of gravity. The float rises to a level where the area between the float and tube reach a state of dynamic equilibrium between the upward differential pressure and buoyancy factors, and downward gravity force. The height of the float is an indication of the flow rate. The tube can be calibrated and graduated in appropriate flow units. Magnetic floats can be used for alarm and signal transmission functions. Similar designs use a spring load instead of gravity to act against the flow. These designs can be mounted horizontally. Widely used because of their low cost, simplicity, and low pressure drop, these meters typically have a turndown ratio up to 10:1. The accuracy ranges from 1% to 5% of full scale. Prices generally fall in the range of $50 to $1,000.
Orifice plate flow meters output is inferred through the difference in pressure from the upstream side to the downstream side of a partially obstructed pipe. The plate obstructing the flow offers a precisely measured obstruction that narrows the pipe and forces the flowing fluid to constrict. Orifice plate meters are simple, cheap, and can be delivered for almost any low viscosity application in any material.
The turndown ratio for orifice plates is less than 5:1. Their accuracy is poor at low flow rates. A high accuracy requires that the orifice plate be in good shape, with a sharp edge to the upstream side. Wear to the plate edges reduces the accuracy.
Flow nozzles are often used as measuring elements for air and gas flow in industrial applications. The flow nozzle is relatively simple and cheap, and available for many applications in many materials. The turndown ratio and accuracy are similar to the orifice plate.
Venturi tube flow meters are often used in applications that require higher turndown rates, or lower pressure drops, than the orifice plate can provide. In the Venturi Tube, the fluid flow rate is measured by reducing the cross sectional flow area in the flow path, generating a pressure difference. After the constricted area, the fluid passes through a pressure recovery exit section, where up to 80% of the differential pressure generated at the constricted area, is recovered. With proper instrumentation and flow calibration, the Venturi Tube can measure over a 10:1 turndown.
The Pitot tube is used to measure the local velocity at a given point in the flow stream and not the average velocity in the pipe or conduit. The Pitot tube is widely used to determine the airspeed of an aircraft and to measure air and gas velocities in industrial applications. The Pitot tube measures the fluid flow velocity by converting the kinetic energy of the flow into potential energy. The use of the Pitot tube is restricted to point measuring. With the “annubar”, or multi-orifice pitot probe, the dynamic pressure can be measured across the velocity profile, and the annubar obtains an averaging effect. The Pitot tube is a very common (and inexpensive) way to measure fluid flow, especially in air applications for ventilation and HVAC systems. They are easy to install and have a very low pressure loss. Typical turndown is 10:1.