Flow and Level Measurement PDF

TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL Flow and Level Measurement LEARNING OBJECTIVES After completing this lesson, you will be able to: ❑ Define level and flow ❑ Identify the basic level and flow measurement...

TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL Flow and Level Measurement LEARNING OBJECTIVES After completing this lesson, you will be able to: ❑ Define level and flow ❑ Identify the basic level and flow measurements ❑ Identify the Mechanical and Pressure Elements What is Flow and Level? Flow [verb]  (of a fluid, gas, or electricity) move along or out steadily and continuously in a current or stream:  go from one place to another in a steady stream, typically in large numbers: [noun]  the action or fact of moving along in a steady, continuous stream:  a steady, continuous stream of something: Oxford Languages  to issue or move in a stream  to move with a continual change of place among the constituent particles Merriam Dictionary Level [noun]  a horizontal plane or line with respect to the distance above or below a given point:  a position on a real or imaginary scale of amount, quantity, extent, or quality: Oxford Languages  a device for establishing a horizontal line or plane by means of a bubble in a liquid that shows adjustment to the horizontal by movement to the center of a slightly bowed glass tube  a measurement of the difference of altitude of two points by means of a level  the magnitude of a quantity considered in relation to an arbitrary reference value Merriam Dictionary Measurements of Water Level Water level is one of the most commonly measured parameters, as accurate level data are essential for many applications. While climate change, pollution monitoring, and industrial water usage are broad reasons for monitoring water levels, more specific applications are discussed throughout this page. Level is perceived as one of the most straightforward water parameters. In general, it is the level of water in a body of water, in groundwater, in a tank, etc. However, there’s a lot to unpack with this parameter. Not only are there very different water level applications and technologies used to measure it, but there are also a variety of terms used when describing water level, some of which have only subtle differences. These include: 36 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL Water level: The height or elevation of water above (more common) or below (less common) a user-specified point. This term is used in many applications. Depth: When measuring in a groundwater well, this is the distance from the land surface to water in the well. In surface water, depth is the distance from the water’s surface to a specific point, typically the bottom of the water body or the location of a sensor. Gage height: Used to describe the water level of a river or stream. Level measurements in these applications are often collected at stream gage stations. Tide Gage: Describes a water level sensor used to monitor changes in sea level. Elevation: Used when describing the height of water above sea level. Hydraulic head: The height to which a column of water is above a reference elevation (e.g., sea level). Like elevation, this term is often used in groundwater applications. Common applications for water level monitoring As well as time-saving benefits, water level monitoring can have a wide variety of environmental benefits, such as improved visibility on potential flood situations. By utilizing water level monitoring systems, it is much easier to rapidly deploy countermeasures in the event that levels become too high or low. Water level loggers feature highly sensitive pressure sensors which can detect the smallest of changes to water level and can send alerts to preferred contacts alerting them to the situation. This benefit means that water level monitoring is commonly used in some of the following applications:  Flood monitoring  River level monitoring  Wetland studies  Tidal studies  Groundwater monitoring  Surface water monitoring What is a water level sensor? The water level sensor is a device that measures the liquid level in a fixed container that is too high or too low. According to the method of measuring the liquid level, it can be divided into two types: contact type and noncontact type. The input type water level transmitter we call is a contact measurement, which converts the height of the liquid level into an electrical signal for output. It is currently a widely used water level transmitter. Where to use water level sensors? The uses of water level sensors include the following applications:  Water level measurement of pools and water tanks  Water level measurement of rivers and lakes  Marine level measurement  Level measurement of acid-base liquids  Oil level measurement of oil trucks and mailboxes  Swimming pool water level control  Tsunami warning and sea-level monitoring  Cooling tower water level control  Sewage pump level control  Remote monitoring of the liquid level 37 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL What are the benefits of water sensors? 1. Simple structure: There are no movable or elastic elements, so the reliability is extremely high, and there is no need for regular maintenance during use. The operation is simple and convenient. 2. Convenient installation: When using, first connect one end of the wire correctly, and then put the other end of the water level probe into the solution to be measured. 3. Ranges are optional: you can measure the water level in the range of 1-200 meters, and other measurement ranges can also be customized. 4. Wide range of applications: suitable for liquid level measurement of high temperature and high pressure, strong corrosion, high pollution, and other media. Building an electronic water level gauge on the riverbank can be used for tide monitoring. 5. Wide range of measuring medium: High-precision measurement can be carried out from the water, oil to paste with high viscosity, and wide-range temperature compensation is not affected by the foaming, deposition, and electrical characteristics of the measured medium. 6. Long service life: Generally, the liquid level sensor can be used for 4-5 years in a normal environment, and it can also be used for 2-3 years in a harsh environment. 7. Strong function: It can be directly connected to the digital display meter to display the value in real-time, or it can be connected to a variety of controllers and set the upper and lower limits to control the water volume in the container. 8. Accurate measurement: The built-in high-quality sensor has high sensitivity, fast response, and accurately reflects the subtle changes of the flowing or static liquid level, and the measurement accuracy is high. 9. Variety of types: liquid level sensors have various structural designs such as input type, straight rod type, flange type, thread type, inductive type, screw-in type, and float type. It can meet the measurement needs of all different places. What are the types of water sensors? Types of liquid level sensors for your reference: OPTICAL WATER LEVEL SENSOR The optical sensor is solid-state. They use infrared LEDs and phototransistors, and when the sensor is in the air, they are optically coupled. When the sensor head is immersed in the liquid, the infrared light will escape, causing the output to change. These sensors can detect the presence or absence of almost any liquid. They are not sensitive to ambient light, are not affected by foam when in air, and are not affected by small bubbles when in liquid. This makes them useful in situations where state changes must be recorded quickly and reliably, and in situations where they can operate reliably for long periods without maintenance. Advantages: non-contact measurement, high accuracy, and fast response. Disadvantages: Do not use under direct sunlight, water vapor will affect the measurement accuracy. CAPACITANCE LIQUID LEVEL SENSOR Capacitance level switches used two (2) conductive electrodes (usually made of metal) in the circuit, and the distance between them is very short. When the electrode is immersed in the liquid, it completes the circuit. 38 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL Advantages: can be used to determine the rise or fall of the liquid in the container. By making the electrode and the container the same height, the capacitance between the electrodes can be measured. No capacitance means no liquid. A full capacitance represents a complete container. The measured values of “empty” and “full” must be recorded, and then 0% and 100% calibrated meters are used to display the liquid level. Disadvantages: The corrosion of the electrode will change the capacitance of the electrode, and it needs to be cleaned or recalibrated. TUNING FORK LEVEL SENSOR The tuning fork level gauge is a liquid point level switch tool designed by the tuning fork principle. The working principle of the switch is to cause its vibration through the resonance of the piezoelectric crystal. Every object has its resonant frequency. The resonant frequency of the object is related to the size, mass, shape, force of the object. A typical example of the resonant frequency of the object is: the same glass cup in a row Filling with water of different heights, you can perform instrumental music performance by tapping. Advantages: It can be truly unaffected by flow, bubbles, liquid types, etc., and no calibration is required. Disadvantages: Cannot be used in viscous media. DIAPHRAGM LIQUID LEVEL SENSOR The diaphragm or pneumatic level switch relies on air pressure to push the diaphragm, which engages with a micro switch inside the main body of the device. As the liquid level increases, the internal pressure in the detection tube will increase until the microswitch is activated. As the liquid level drops, the air pressure also drops, and the switch opens. Advantages: There is no need for power in the tank, it can be used with many types of liquids, and the switch will not come into contact with liquids. Disadvantages: Since it is a mechanical device, it will need maintenance over time. FLOAT WATER LEVEL SENSOR The float switch is the original level sensor. They are mechanical equipment. The hollow float is connected to the arm. As the float rises and falls in the liquid, the arm will be pushed up and down. The arm can be connected to a magnetic or mechanical switch to determine on/off, or it can be connected to a level gauge that changes from full to empty when the liquid level drops. The use of float switches for pumps is an economical and effective method to measure the water level in the pumping pit of the basement. Advantages: The float switch can measure any type of liquid and can be designed to operate without any power supply. Disadvantages: They are larger than other types of switches, and because they are mechanical, they must be used more frequently than other level switches. 39 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL ULTRASONIC LIQUID LEVEL SENSOR The ultrasonic level gauge is a digital level gauge controlled by a microprocessor. In the measurement, the ultrasonic pulse is emitted by the sensor (transducer). The sound wave is reflected by the liquid surface and received by the same sensor. It is converted into an electrical signal by a piezoelectric crystal. The time between the transmission and reception of the sound wave is used to calculate the Measure of the distance to the surface of the liquid. The working principle of the ultrasonic water level sensor is that the ultrasonic transducer (probe) sends out a high-frequency pulse sound wave when it encounters the surface of the measured level (material), is reflected, and the reflected echo is received by the transducer and converted into an electrical signal. The propagation time of the sound wave. It is proportional to the distance from the sound wave to the surface of the object. The relationship between the sound wave transmission distance S and the sound speed C and the sound transmission time T can be expressed by the formula: S=C×T/2. Advantages: non-contact measurement, the measured medium is almost unlimited, and it can be widely used for measuring the height of various liquids and solid materials. Disadvantages: The measurement accuracy is greatly affected by the temperature and dust of the current environment. RADAR LEVEL GAUGE A radar liquid level is a liquid level measuring instrument based on the principle of time travel. The radar wave runs at the speed of light, and the running time can be converted into a level signal by electronic components. The probe sends out high frequency pulses that travel at the speed of light in space, and when the pulses meet the surface of the material, they are reflected and received by the receiver in the meter, and the distance signal is converted into a level signal. Advantages: wide application range, not affected by temperature, dust, steam, etc. Disadvantages: It is easy to produce interference echo, which affects the measurement accuracy. Measurement of water flow Flow is the measurement of the fluid velocity per unit area in a pipe or a conduit. By using a flow sensor, we could measure the amount of gas, liquid, or vapor. A flow meter consists of a transducer and also a transmitter. According to the method of measuring the liquid flow, it can be divided into two: direct method and indirect method. DIRECT METHOD In this type of flow measurement, the quantity which is to be measured would be compared to a standard value. So, we are exactly measuring the quantity, which is to be measured, mostly in this measurement we are measuring the fluid mass. Types of direct flow measurement  Dipsticks  Lead line  Level glass  Float gauge  Turbine flow sensor  Coriolis sensor 40 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL INDIRECT METHOD This is the most common method of flow measurement because in most cases direct measurement of the flow is not possible. So indirect method is mostly used in this method the sensor would measure the flow of the fluid indirectly like transmitting sound waves through the fluid flow. Types of indirect fluid flow measurement  Differential pressure  Magneto inductive  Vortices frequency  Ultrasonic  Venturi meter  Orifice plate  Pitot tube  Annubar  Flow nozzle  Thermal mass flow sensor  Positive displacement flow sensor WHAT ARE THE TYPES OF FLOW SENSORS?  Inferential flow sensor  Differential pressure flow sensor  Velocity flow sensor  Volumetric flow sensor  Mass flow sensor INFERENTIAL FLOW SENSOR In this type of flow sensor, the measurement of flow is done indirectly, like temperature and pressure of the flow would be measured to determine the flow rate. Types of inferential flow sensors are magnetic sensors, turbine sensors, target sensors, etc. MASS FLOW SENSOR Fluid mass would be directly measured in this type of flow measurement. It would measure the mass or amount of fluid that flows through a tube to determine the flow. Coriolis and thermal mass flow sensors are examples of the mass flow sensor. 41 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL DIFFERENTIAL PRESSURE FLOW SENSOR In this type of flow sensor, the pressure difference of the flow due to the pipe construction would be utilized to determine the flow rate. VELOCITY FLOW SENSOR This type of flow sensor would measure the flow velocity through a pipe. So the velocity of the fluid will be multiplied by the area that the fluid flows through. VOLUMETRIC FLOW SENSOR Flow measurement would be done by measuring the volume of the fluid which flows through a pipe. TYPES OF THE INFERENTIAL FLOW SENSOR TURBINE FLOW SENSOR In this type of flow measurement, a rotor will be placed in the pipe and this rotor is supported by two bearings. A magnetic pick-up would be placed at the top of the pipe’s section where the rotor is installed. When the fluid flows the rotor would rotate and this would create a frequency. The rotor pulse would be calculated to determine the flow rate. Advantages of the turbine flow sensor Disadvantages of the turbine flow sensor  Easy installation  It can’t be used with high viscous fluids  It is not affected by the variation in the fluid density  Due to the moving parts damages could happen if  Compact the speed increases  Less head loss Applications of the turbine flow sensor  Military applications  Good temperature and pressure ratings  Petroleum industry  Good repeatability and range  Energy fuel and cryogenic flow measurement TARGET FLOW SENSOR In a target flow sensor, an obstruction would be placed in the flow path so when the fluid comes in contact with the object a force would be exerted on the object. This force can be measured and this measured force will be proportional to the flow rate. The target size would vary according to the measured liquid. Advantages of the target flow sensor Applications of the target flow sensor  These flowmeters can be used with liquid slurries  It can be used to measure dirty fluids  It can be used for low turbulence flow metering  Liquid, vapor, and gas measurements can be done 42 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL TYPES OF DIFFERENTIAL PRESSURE FLOW SENSORS VENTURI METER This type of sensor is a tube that has a cone structure. The operation of the Differential pressure flow meters is based on Bernoulli’s principle which states that if the fluid velocity increases then the pressure would decrease and vice versa. The tube structure is in a way that there is a convergent part and also a divergent part. So, when the fluid flows through the converging part of the tube then it would be accelerated and during this process, the fluid pressure will be dropped. The end part of the tube section is expanded and in this part, the fluid flow would almost gain its actual pressure. So, the velocity and pressure relation are checked and according to this, we could determine the flow rate. Advantages of venturi meter Applications of venturi meter  Good accuracy  Measurement of compressible and non-  High velocity and pressure recovery compressible fluids  It can be used with fluids that have small solid  Gases and liquid flow measurement particles  Chemical industries  High repeatability  Oil and gas  Less maintenance  Power industries Disadvantages of venturi meter  Installation cost is high  Abrasive or sticky fluid would affect the measurement ORIFICE PLATE This type of flow sensor has a disc-type structure and it is installed in the straight run of the pipe. The orifice plate would be installed perpendicular to the fluid flow. This plate would have a hole in the center of it so when the fluid comes in contact with the plate then it would flow through the hole and the flow velocity would be increased and the pressure would be decreased. So, when the fluid flows beyond the orifice plate the flow velocity and pressure would change. The relation between the pressure drop and the velocity can be observed to determine the flow rate. Advantages of the orifice plate Disadvantages of orifice plates  Economical  Permanent pressure loss is high  Maintenance is easy  It can’t be used with dirty fluids  Generates high differential pressure  Frequent calibration is needed  It can be easily replaced  Measurement is affected due to the variation in  No moving parts density, viscosity, etc.  High accuracy Applications of the orifice plate  Neutral gas transfer  Gas, oil and fluid measurement  Refining 37 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL PITOT TUBE A pitot tube can be considered as a probe that has two parts and is capable to sense static and dynamic pressure. This type of flow sensor is used for the non-uniform flow measurement, so due to the non-uniform flow, there will be variations in the pressure and velocity of the flow. The probe would be inserted into the fluid, the impact unit of this flow meter consists of a tube with one end bent at right angles towards the direction of the flow. The static section of the tube end will be closed but it would have a small slot that can be seen in the side of the unit. The static pressure would be measured at the wall of the flow conduit and the dynamic or impact pressure is calculated from the flowing pressure. Velocity head will be converted to impact pressure. The flow measurement is done by the measuring difference between the static and dynamic pressure. Mostly these tubes are installed by welding a coupling on the pipe and after that, the probe would be inserted through the coupling. Advantages of Pitot tube Applications of Pitot tube  Installation is easy  It can be used to measure the non-uniform flow  No pressure loss  Measurement of velocity and direction in Disadvantages of Pitot tube combustion studies and in heat transfer work  It can’t be used with dirty fluids  It is used to determine the magnitude and direction  Not suitable to measure the sticky fluids of the gas velocity  It is sensitive to the upstream disturbances  Measurement of the aircraft’s airspeed  Low accuracy ANNUBAR The operation of the Annubar is similar to the Pitot tube. The only difference is that the Annubar has more than one hole in the pressure measuring chambers. So this device has many velocity sensing openings which are subjected to the fluid flow. So from this, the static and dynamic pressure is calculated and the difference between these two can be calculated to determine the fluid flow. Advantages of Annubar Disadvantages of Annubar  Easy installation  It can’t be used with high viscous fluids  Low-pressure drop  Measurement is single point  Low maintenance Applications of Annubar  High accuracy  Used for liquid and gas flow  It can be used for low and high- pressure applications 37 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL FLOW NOZZLE This is another type of differential pressure flow sensor and it can handle a higher flow rate than the orifice plate. This type of flow meter is similar to the venturi but there is no recovery cone. Basically, flow nozzle is a restriction to fluid flow and it has a cylindrical throat section. Pressure taps are required to measure the difference in static pressure which is created by the flow nozzle, and it will be located, upstream and downstream in the pipe. This type of flow meter can be used to determine the flow of liquids with suspended solids. The cost and size of this meter are very less, the pressure loss of this meter is really high. This type of flow meter is used when high velocity and pressure recovery are needed. Advantages of the flow nozzle Applications of the flow nozzle  It can be used to detect high flow  High-pressure steam flow  Measurement of liquids with solid particles is  High-temperature steam flow possible  Mostly it is used for gas measurement than  Low-pressure drop than the orifice plate fluids Disadvantages of the flow nozzle  It is used with a pressure transmitter  It can’t be used with viscous fluids  It is used with flow control valves  It must not be used with fluids that have sticky particles  More maintenance  It can’t be used with large size pipes TYPES OF VELOCITY FLOW SENSORS MAGNETIC FLOW SENSOR In this type of flow sensor, a magnetic field would be applied to the pipe or conduit in which the flow is meant to be measured. This operating principle of this flow sensor is based on Faraday’s law, which states that when a conductive fluid passes through a magnetic flux an EMF will be created. So, when the conductive fluid passes the magnetic field then a voltage will be induced. The voltage which is formed will be proportional to the flow velocity. Advantages of the magnetic flow sensor Disadvantages of the magnetic flow sensor  It doesn’t have any moving parts and also there is  Only conductive fluid can be measured, we won’t no fluid obstruction be able to measure gases and hydrocarbons with  Very less pressure drop this  Non-contact measurement  Expensive  Good electrical insulation and corrosion resistant Applications of the magnetic flow sensor  It can be used with extremely low flow  Measurement of slurries and dirty fluids can be  It can be used for bidirectional flow measurement done  Measurement won’t be affected by viscosity  It can be used with acid, base, water, and also for aqueous solution 37 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL VORTEX FLOW SENSOR In this type of flow measurement, an obstruction would be placed perpendicular to the flow. When the fluid passes through the obstruction then the flow would be disrupted and vortices would be formed. This vortex shedding frequency can be used to determine the velocity of the fluid. Mostly pressure transducers are utilized to measure the vortices. Advantages of the vortex flow sensor Applications of the vortex flow sensor  High accuracy  Cryogenic fluids  There are no moving parts so less maintenance  Hot and chilled water  It can be used with liquid, gas, and steam  Condensate measurement  Pressure drop is really low  Acids  It can be used with fluids with different density  Solvents Disadvantages of the vortex flow sensor  Measurement of steam  It can’t be used with abrasive or dirty fluids  Measurement would be affected if the flow velocity is low  In order to create proper vortices, a specified straight pipe run is required for the upstream and downstream of the sensor TYPES OF VOLUMETRIC FLOW SENSORS POSITIVE DISPLACEMENT FLOW SENSOR This type of flow sensor is used for the measurement of gases and liquids. There are several types of positive displacement sensors such as rotating paddle sensors, oscillating piston sensors, oval gear sensors, sliding vane sensors, and bi rotor sensors. Mostly a positive displacement sensor has a moving part, the mechanical components of the PD sensors are driven due to the energy from the flow. The energy that drives the sensor would create a pressure drop in the inlet and outlet of the meter. Advantages of positive displacement flow sensors Disadvantages of positive displacement sensors  It can be used with viscous liquids  It can’t be used with liquids that have suspended  Certain type of PD meter doesn’t need particles electrical power for their operations ▪  Chances of mechanical wear are high because of Good rangeability the moving parts  Over-speeding could damage the sensor Applications of the positive displacement flow sensors  Measurement of very low and very high viscous fluids 37 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL ULTRASONIC FLOW SENSOR In this type of flow sensor, sound waves would be used to determine the flow rate. There are three methods of measurement in this type of sensor the Doppler shift method, deflecting beam method, and the transit time method. In the above image, we can see two transducers are placed in opposite directions. Both these transducers would emit sound waves to each other and the time taken to receive the two signals is measured to determine the fluid velocity. Doppler shift method In this method, the sound waves would be transmitted through the fluid and these waves would be reflected from the fluid to the receiver in the flow sensor. The received sound wave frequency can be measured to determine the flow rate. Deflecting beam method In this method, sound waves are transmitted at the right angle to the flow. The liquid would deflect the sound wave. This deflection can be measured to determine the flow rate. Transit time flow In this method, a diagonal beam would be transmitted across the flow path and this beam would be transmitted against the flow. The sound travel against the flow would be slow and this can be measured to determine the flow rate. Advantages of ultrasonic flow sensors Disadvantages of the ultrasonic flow sensor  Non-intrusive  It can’t be used with dirty liquids  There is no pressure drop  A straight pipeline would be required for the flow  Good accuracy measurement  Good range  The transmission path will be affected by  High reliability attenuation TYPES OF MASS FLOW SENSORS CORIOLIS MASS FLOW SENSOR The working principle of this flow sensor is based on newton’s second law which states that force is equal to the value of the mass multiplied by the acceleration. In this meter, the flow will be divided into two parallel tubes and these tubes would be vibrated by the electromagnetic drive coil. So when the fluid flows through these tubes then there will be an upward and downward force and due to this there will be a tube deflection and this deflection is known as the Coriolis effect. The deflection of the tube corresponds to the mass flow in the tube. Advantages of Coriolis flow sensor Applications of Coriolis flow sensor  High accuracy  Metering of natural gas consumption  Very low-pressure drop  Custody transfer  It can be used for liquid and gas flow  Syrups and oil monitoring Disadvantages of Coriolis flow sensor  Gas & liquid flow measurement  Costly  Solid content concentration can be determined  Mounting is difficult 37 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL THERMAL MASS FLOW SENSOR In this type of sensor, the flow measurement is done by utilizing the thermal properties of the fluid. In this sensor, a specified amount of heat will be provided to the heater which is situated inside the sensor. So when the fluid flows, a portion of the heat will be lost and if the flow is increased then the heat loss will be increased too. This loss of heat would be measured by the sensor’s temperature measuring instrument. Advantages of thermal mass flow meter Disadvantages of thermal mass flow meter  Measurement is not affected by variation in  Measurement would be affected by moistures pressure  Sensitivity is less for high flow  Pressure drop is really low  If there is any variation in gas composition then it  Less maintenance because there are no moving would require recalibration parts  Accuracy would be reduced due to the flow sensor  Installation is really easy build-up  Economical Applications of the thermal mass flow sensor  Measurement of gases like nitrogen, hydrogen, helium  Measurement of pure gases HOW SHOULD WE SELECT A FLOW SENSOR FOR OUR APPLICATION?  It must be selected according to the type of liquid, like clean or dirty  Properties of the liquid should be considered like pressure, density, viscosity, conductivity, temperature, pressure drop, etc.  It should be selected by considering the plant conditions, as the size of the pipeline, straight pipe runs, enclosures, etc.  Flow sensors should be selected according to the required performance like accuracy, repeatability, stability, and pressure loss  Range requirements  Installation space  It must be selected by considering the flow stream like if it has corrosive or erosive materials What could be the major cause of the errors in flow meters?  It could be due to the variations in the fluid properties such as density, viscosity, pressure, and temperature  Meter orientation  Flow disturbance Level and Fluid Flow Measurement Flow rate is the volume of fluid that passes through a given cross-sectional area per unit time. Accurate flow rate measurement using an appropriate flowmeter is paramount to ensuring fluid control processes run smoothly, safely and cost-effectively. The majority of flow rate measurement devices provide an inferential flow rate calculation based on the fluid’s velocity. Velocity is determined by the pressure differential that forces the fluid through the pipe. The pipe’s cross- sectional area is constant and this is what makes average velocity a reliable indicator of the flow rate. Flowmeters are the foremost flow measurement and control devices for these types of applications. Here are three of the main advantages of accurate flow rate measurement: 37 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL Quality control Flow rate measurement is an essential parameter in many applications. Whether you are measuring gas or liquid, it is vitally important to ascertain that the right fluid is going to the right place at the right time. Flow rate measurement is especially important in applications requiring stringent quality control, such as batch control or time-pressure dosing processes in the pharmaceutical industry. Health and safety The health and safety of personnel is at the forefront of all industrial processes and none more so than those involving liquids and gases. Taking regular, reliable and accurate flow rate measurements is one of the best ways to protect the safety of personnel. A safer working environment is likely to be a more productive environment, with machinery operating efficiently and downtime kept to a minimum. Cost-efficiencies There are inherent cost-efficiencies that arise as a direct consequence of adhering to high standards of quality control and plant safety. In pharmaceutical and F&B applications, there is an obvious cost benefit of using flowmeters to detect dead legs and minimize wastage of expensive media. Furthermore, if pipelines and machinery are kept operating optimally, the subsequent reduction in maintenance delivers tangible savings. AIR FLOW RATE MEASUREMENT INSTRUMENTS FOR FPM OR CFM: This section defines air flow rate or cubic feet per minute (CFM) as the term is used to describe building air conditioners, heating systems, or building air movement rates. This section described the types of devices or instruments used to measure air flow, comparing the features, operation, and accuracy of each approach. This section includes a list of air flow rate measurement instrument or tool suppliers – where to buy CFM measurement equipment. LEARNING OBJECTIVES After completing this lesson, you will be able to:  Definitions, Procedures & Tools for Measuring Air Flow Rates (CFM)  Discussed here: how to measure air movement or flow rates in buildings; how to measure HVAC duct supply or return air flow rates in CFM or by other standards.  What tools to use to measure air flow rate, accuracy, procedures, & where to buy.  Definitions, Procedures & Tools for Measurements of Air Flow Rates (CFM) in Buildings. Also, after completing this section, you will be able to learn the following:  Air Flow Measurement CFM  Air Flow Rate Measurement Definition  Air Flow Rate Measurement Accuracy  Air Velocity Measurement & Standards  Air Flow Rate Cfm Measurement Devices & Methods  ANEMOMETERS: Cup Type anemometers used to measure wind speed  ANEMOMETERS: Vane / Fan Blade Anemometers used for air flow rate measurements  ANEMOMETERS: Swing Vane Anemometers used for air flow measurements  HVAC Duct Air Flow Monitors  Pitot Tubes probes for air flow measurements  Pressure Transducers for air flow measurements  "HOT WIRE" CFM measurements using a hot wire anemometer  Capture Hoods for air flow measurements  Liquid Column Gauges - liquid column manometers used for air pressure or air flow measurements  Toilet Paper or Tissue Test Confirmation of Air Flow 37 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL DEFINITION OF AIR FLOW RATE & FLOW RATE MEASUREMENT: AIR FLOW QUANTITY & AIR VELOCITY The Air flow rate for an HVAC system is defined as the volume of air being delivered at some speed or "rate". Typically flow rate, or quantity of air being moved, is measured in m3/sec (cubic meters per second) or CFM (cubic feet per minute). Air velocity or air speed without necessarily knowing the total quantity of air being moved. Air velocity or air speed is measured in m/s (meters per second) or feet/minute. Some instruments can measure both air velocity (air speed) and air flow rate (air quantity). DEFINITION OF AIR FLOW MEASUREMENT Air flow rate is measured by calculating an average velocity for the conduit of interest, and then, multiplying this velocity by the cross-sectional area of the duct at the measurement location. The air velocity value may estimate using a single reading, or a survey across the duct at a station. We make the actual measurement of air flow in a cooling or heating system by using an instrument that is sensitive to the passage of air. This article describes the different types of air flow measurement instruments and how each works. The accuracy of air flow rates measured in cooling or heating system air handler or duct systems is discussed at accuracy of CFM or other air flow measurements on HVAC systems. SIMPLE AIR FLOW CALCULATION If I held up a one-foot square sensor in front of an air source (say an air supply register) and the sensor measured air velocity at 12 inches per minute, I'd be measuring 1 CFM of airflow. (One cubic foot = 12 x 12 x 12 inches). Or if we measured an air velocity at an air supply register of one foot per minute and we knew that the duct work was a 12-inch square duct, we'd figure we were seeing one cubic foot per minute of air supply at that location. Actually, here are more than one answer to your question about how airflow is measured in an HVAC system because there is a range of air flow measurement instruments on the market. The measuring devices vary in price, accuracy, and in operating principle, and there are also of course multiple sources of CFM data: manufacturers specifications, theoretical numbers, and actual measurements. We are most interested in the last category. AIR VELOCITY STANDARDS & MEASUREMENTS How do we measure air velocity? ❑ The most common air flow measurement device used in HVAC systems is a hand-held vane anemometer or a hand-held hot-wire anemometer. ❑ The vane anemometer is in essence a small fan driven by the movement of air across the fan blades. ❑ The hot wire anemometer uses a heated wire that is cooled by the movement of air across the wire. AIR FLOW RATE CFM & AIR VELOCITY FPM MEASUREMENT DEVICES & APPROACHES CUP-TYPE WIND SPEED ANEMOMETERS & DATA LOGGERS The cup type anemometer is familiar if you've looked around a local weather station or airport. The movement of air across the rotating cups causes the shaft to turn at a rate that is measured by a wind speed indicator or in the case of this instrument from Spectris, a wind speed data recorder. 38 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL VANE / FAN BLADE ANEMOMETERS Vane or Fan Blade Anemometers, for Fan type air flow measurement: these are the most commonly used lower-cost CFM measurement devices used by home inspectors and HVAC technicians. A hand-held portable fan blade anemometer device is held in the air path and moving air rotates a fan blade. The instrument measures fan blade rotation to calculate a flow rate or pressure equivalent that is combined with the known cross-sectional area of the measurement device. An advantage of measuring CFM with an anemometer is that you don't need to correct the measurement for temperature (variation in air density). Another advantage is that some of these instruments are very low in costs. Choose a unit that measures in the range of air speed in CFM specified by the standards against which you are measuring for your application. PITOT TUBE PROBES USED FOR AIR FLOW MEASUREMENTS Pitot tube is a device that measures air (or other flowing gas or liquid) pressure when the tube is inserted or placed in the proper position (pointed into the direction from which air flow emanates) for sensing airflow. The pressure is converted to a flow rate by considering the cross-sectional area of the duct or opening through which air is being delivered. (There are some assumptions behind this including that air flow rate is uniform across the cross section of the opening.) Pitot tubes are familiar to air travelers who have noticed that little tube sticking down and pointing forward from the bottom of many aircraft where the pitot tube is used to measure the air speed of the craft. Indeed, pitot tubes are used for high velocity airflow measurements where a vane anemometer could not possibly be up to the task. Pitot tubes are the most accurate technology for measuring air flow rates and are generally used to provide the accuracy standard for comparison with other CFM measurement devices. HOW PRESSURE TRANSDUCERS ARE USED FOR AIR FLOW MEASUREMENTS? Pressure transducers: also measure pressure from a flowing gas or air and permit conversion to CFM measurements in the same manner as a pitot tube – knowing the cross-sectional area of the duct or opening. Pressure sensors measure the force exerted by a "fluid" including air or liquid by measuring the force that would be necessary to stop that movement. These devices are also called pressure transmitters, pressure senders, pressure indicators, piezometers, and in HVAC equipment and testing, manometers. Actual measurements of airflow in an HVAC system or at air supply registers are expressed in cubic feet per minute and are most often made in the field using a hand-held flow meter through which air moves. The flow meter is calibrated based on the its input area and the resistance offered by its own fan blades. DUCT AIRFLOW MONITORS Airflow Measurement Station utilizes an airflow averaging element generating a velocity pressure signal similar to the orifice, venturi, and other primary elements. Single or multiple airflow elements are factory mounted and pre-piped in a casing designed for flanged connection to the ductwork. Multiple elements are joined together for connection to a differential measurement device (gage, transmitter, etc.) for flow measurement and indication purposes. 39 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL "HOT WIRE" CFM MEASUREMENTS USING A HOT WIRE ANEMOMETER An anemometer type device that uses a heated wire and measures the cooling effect of low velocity air flow can also be used to estimate air flow rates provided that air temperature is also considered to provide a correct estimate of air flow rate. CAPTURE HOODS FOR AIR FLOW MEASUREMENTS Capture Hoods can be used to make accurate measurements of air flow rates at HVAC system air supply registers. Capture hoods cover the entire supply air register and use a differential pressure device or a hot wire device to obtain an air flow CFM number. LIQUID COLUMN GAUGES Liquid column manometers are used for air pressure or air flow measurements. Liquid Column gauges - liquid column manometers are a special form of liquidcolumn manometer used to measure low velocity air flow by comparing air pressure inside and outside of two spaces. At left the U-shaped plastic tube filled with a blue liquid is connected at its left end to the interior of a 6" plastic vertical exhaust duct forming part of a radon mitigation system. The right end of the liquid column gauge is simply open to the atmosphere of the room, in this case a basement. The differential in air pressure between the two ends of the tube is marked on a scale indicating the air flow rate inside of the column. The difference in height between the two ends of the column of blue liquid is always in direct proportion to the difference between the two air pressures (inside & outside of the exhaust duct). If no air were flowing inside of the white exhaust duct, the two ends of the blue liquid would be at the same level. In this application, air flowing past the end of the flexible plastic tube inserted into the column interior causes a reduction of air pressure in the tube that is a function of the speed of air flow past the tube opening. In this application the liquid column gauge reading of differential air pressure does not have to be precise as its function is simply to indicate that there is some difference in air pressure between the room interior and the exhaust duct interior. As long as the room is at higher pressure than the column interior, the exhaust system is working and any radon gas below the floor slab (in this application) tends to exhaust through the duct rather than enter the room. TOILET PAPER OR TISSUE CONFIRMATION OF AIR FLOW A simple test for air movement at the return air inlet is illustrated in our sketch. Just hold a tissue or piece of toilet paper near the inlet grille face. If air is moving into the grille the tissue will be pulled against the opening. This toilet paper or tissue test can confirm air flow as well as the direction of air movement at an HVAC air supply or return air register, and is a useful, if trivial, demonstration that can help confirm air movement when air flow in the system is weak or uncertain. 40 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL Mechanical and Electrical Pressure Elements ELECTRICAL PRESSURE ELEMENTS: STRAIN GAUGE PRESSURE SENSOR Resistance Strain Gauges are a supplementary component that may be used to monitor pressure. Bellows and diaphragms can be used to connect them so that they can measure pressure jointly. The differential pressure measuring system is depicted in the images below. CAPACITANCE TRANSDUCER The principle behind a capacitance pressure transducer is that under pressure, the dielectric constants of liquids, solids, and gases change. A cylindrical capacitor configuration that can tolerate high pressure is shown in the image below. It is only useful for high pressure changes since the change in dielectric constant is relatively tiny (only around a 2% change for a pressure shift of roughly 10 MPa). PIEZOELECTRIC TRANSDUCER A piezoelectric crystal transducer/sensor is an active sensor that generates its own power and does not require external assistance. OPTICAL PRESSURE SENSOR In recent years, optical type pressure measurement, in which the movement of a bellows element, a diaphragm, or other fundamental sensors is sensed by optical methods, has drawn a lot of interest. Although the idea is not novel, there are many different commercialization adaption techniques. 41 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL FIBRE-OPTIC PRESSURE SENSOR Since fiber-optic type pressure monitoring may be used in a variety of fields, it is steadily gaining in popularity. Its adaptability in the bio-medical field has also been shown; thus, pressure in the human circulatory system may be monitored using this technology. SURFACE ACOUSTIC WAVE (SAW) SENSOR Two inter-digital transducers (IDT) make up a surface acoustic wave (SAW) delay line. When the SAW delay line is stretched along the propagation direction or bent as a cantilever beam, the substrate becomes strained and elongates, increasing the center-to-center distance between the two IDTs. High stress also alters the material's elastic constants, which alters the surface acoustic wave's velocity (Vs). The delay line can be used as a sensor for temperature, pressure, force, displacement, and other variables since these changes can also be caused by variations in temperature, pressure, and force. MECHANICAL PRESSURE ELEMENTS: A pressure transducer turns pressure into any other quantifiable type of energy. In general, transducers are devices that change one form of energy into another. The pressure in the system is converted into mechanical displacement using a mechanical pressure transducer. BOURDON TUBE The Bourdon tube functions according to the straightforward tenet that a bent tube will alter its form when subjected to changes in internal and external pressure. The tube straightens when pressure is applied internally; when the pressure is released, the tube assumes its initial shape. HELIX AND SPIRAL TUBES The tubing is formed into the forms designated by the names of the helix and spiral tubes. The pressure applied to the tube forces it to straighten out when one end is shut. The pressure used determines the amount of straightening or uncoiling. SPRING AND BELLOWS A bellows is an expandable element and is made up of a series of folds which allow expansion. One end of the Bellows is fixed and the other moves in response to the applied pressure. A spring is used to oppose the applied force and a linkage connects the end of the bellows to a pointer for indication. MANOMETER The most basic type of manometer is a liquid-filled U-shaped tube. On the tube's open ends, the reference pressure and the measurement pressure are applied. The heights of the liquid on the two sides of the tube will differ if there is a change in pressure. 42 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL DIAPHRAGM Numerous pressure sensors base their measurement on the diaphragm's deflection. The diaphragm is a flexible disc made of sheet metal with highly precise measurements. It can be flat or have concentric corrugations. The diaphragm can be employed for high-pressure applications as well as for separating the process fluids. It may also be used to measure pressure using electrical transducers. 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