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Pressure Regulators are found in many common home and industrial applications. For example, pressure regulators are used in gas grills to regulate propane, in home heating furnaces to regulate natural gases, in medical and dental equipment to regulate oxygen and anesthesia gases, in pneumatic automation systems to regulate compressed air, in engines to regulate fuel and in fuel cells to regulate hydrogen. As this partial list demonstrates there are numerous applications for regulators yet, in each of them, the pressure regulator provides the same function. Pressure regulators reduce a supply (or inlet) pressure to a lower outlet pressure and work to maintain this outlet pressure despite fluctuations in the inlet pressure. The reduction of the inlet pressure to a lower outlet pressure is the key characteristic of pressure regulators.
When choosing a pressure regulator many factors must be considered. Important considerations include: operating pressure ranges for the inlet and outlet, flow requirements, the fluid (Is it a gas, a liquid, toxic, or flammable?), expected operating temperature range, material selection for the regulator components including seals, as well as size and weight constraints.
A wide range of materials are available to handle various fluids and operating environments. Common regulator component materials include brass, plastic, and aluminum. Various grades of stainless steel (such as 303, 304, and 316) are available too. Springs used inside the regulator are typically made of music wire (carbon steel) or stainless steel.
Brass is suited to most common applications and is usually economical. Aluminum is often specified when weight is a consideration. Plastic is considered when low cost is of primarily concern or a throw away item is required. Stainless Steels are often chosen for use with corrosive fluids, use in corrosive environments, when cleanliness of the fluid is a consideration or when the operating temperatures will be high.
Equally important is the compatibility of the seal material with the fluid and with the operating temperature range. Buna-n is a typical seal material. Optional seals are offered by some manufacturers and these include: Fluorocarbon, EPDM, Silicone, and Perfluoroelastomer.
The chemical properties of the fluid should be considered before determining the best materials for your application. Each fluid will have its own unique characteristics so care must be taken to select the appropriate body and seal materials that will come in contact with fluid. The parts of the regulator in contact with the fluid are known as the &#;wetted&#; components.
It is also important to determine if the fluid is flammable, toxic, explosive, or hazardous in nature. A non-relieving regulator is preferred for use with hazardous, explosive, or expensive gases because the design does not vent excessive downstream pressure into the atmosphere. In contrast to a non-relieving regulator, a relieving (also known as self-relieving) regulator is designed to vent excess downstream pressure to atmosphere. Typically there is a vent hole in the side of the regulator body for this purpose. In some special designs, the vent port can be threaded and any excess pressure can be vented from the regulator body through tubing and exhausted in a safe area. If this type of design is selected the excess fluid should be vented appropriately and in accordance to all safety regulations.
The materials selected for the pressure regulator not only need to be compatible with the fluid but also must be able to function properly at the expected operating temperature. The primary concern is whether or not the elastomer chosen will function properly throughout the expected temperature range. Additionally, the operating temperature may affect flow capacity and/or the spring rate in extreme applications.
The inlet and outlet pressures are important factors to consider before choosing the best regulator. Important questions to answer are: What is the range of fluctuation in the inlet pressure? What is the required outlet pressure? What is the allowable variation in outlet pressure?
What is the maximum flow rate that the application requires? How much does the flow rate vary? Porting requirements are also an important consideration.
In many high technology applications space is limited and weight is a factor. Some manufactures specialize in miniature components and should be consulted if overall size and weight are critical. Material selection, particularly the regulator body components, will impact weight. Also carefully consider the port (thread) sizes, adjustment styles, and mounting options as these will influence size and weight.
A pressure regulator is comprised of three functional elements
In operation, the reference force generated by the spring opens the valve. The opening of the valve applies pressure to the sensing element which in turn closes the valve until it is open just enough to maintain the set pressure. The simplified schematic &#;Pressure Regulator Schematic&#; illustrates this force balance arrangement. (see below)
Most commonly, regulators employ a spring loaded &#;poppet&#; valve as a restrictive element. The poppet includes an elastomeric seal or, in some high pressure designs a thermoplastic seal, which is configured to make a seal on a valve seat. When the spring force moves the seal away from the valve seat, fluid is allowed to flow from the inlet of the regulator to the outlet. As the outlet pressure rises, the force generated by the sensing element resists the force of the spring and the valve is closed. These two forces reach a balance point at the set point of the pressure regulator. When the downstream pressure drops below the set-point, the spring pushes the poppet away from the valve seat and additional fluid is allowed to flow from the inlet to the outlet until the force balance is restored.
Piston style designs are often used when higher outlet pressures are required, when ruggedness is a concern or when the outlet pressure does not have to be held to a tight tolerance. Piston designs tend to be sluggish, as compared to diaphragm designs, because of the friction between the piston seal and the regulator body.
In low pressure applications, or when high accuracy is required, the diaphragm style is preferred. Diaphragm regulators employ a thin disc shaped element which is used to sense pressure changes. They are usually made of an elastomer, however, thin convoluted metal is used in special applications. Diaphragms essentially eliminate the friction inherent with piston style designs. Additionally, for a particular regulator size, it is often possible to provide a greater sensing area with a diaphragm design than would be feasible if a piston style design was employed.
The reference force element is usually a mechanical spring. This spring exerts a force on the sensing element and acts to open the valve. Most regulators are designed with an adjustment which allows the user to adjust the outlet pressure set-point by changing the force exerted by the reference spring.
The accuracy of a pressure regulator is determined by charting outlet pressure versus flow rate. The resulting graph shows the drop in outlet pressure as the flow rate increases. This phenomenon is known as droop. Pressure regulator accuracy is defined as how much droop the device exhibits over a range of flows; less droop equals greater accuracy. The pressure versus flow curves provided in the graph &#;Direct Acting Pressure Regulator Operating Map&#;, indicates the useful regulating capacity of the regulator. When selecting a regulator, engineers should examine pressure versus flow curves to ensure the regulator can meet the performance requirements necessary for the proposed application.
The term &#;droop&#; is used to describe the drop in the outlet pressure, below the original set-point, as flow increases. Droop can also be caused by significant changes in the inlet pressure (from the value when the regulator output was set). As the inlet pressure rises from the initial setting, the outlet pressure falls. Conversely, as the inlet pressure falls, the outlet pressure rises. As seen in the graph &#;Direct Acting Pressure Regulator Operating Map&#;, this effect is important to a user because it shows the useful regulating capacity of a regulator.
Increasing the valve orifice can increase the flow capacity of the regulator. This may be beneficial if your design can accommodate a bigger regulator however be careful not to over specify. A regulator with an oversized valve, for the conditions of the intended application, will result in a greater sensitivity to fluctuating inlet pressures, and may cause excessive droop.
&#;Lockup pressure&#; is the pressure above the set-point that is required to completely shut the regulator valve off and insure that there is no flow.
Hysteresis can occur in mechanical systems, such as pressure regulators, due to friction forces caused by springs and seals. Take a look at the graph and you will notice, for a given flow rate, that the outlet pressure will be higher with decreasing flow than it will be with increasing flow.
Single-stage regulators are an excellent choice for relatively small reductions in pressure. For example, the air compressors used in most factories generate maximum pressures in the 100 to 150 psi range. This pressure is piped through the factory but is often reduced with a single-stage regulator to lower pressures (10 psi, 50 psi, 80 psi etc.) to operate automated machinery, test stands, machine tools, leak test equipment, linear actuators, and other devices. Single stage pressure regulators typically do not perform well with large swings in inlet pressure and/or flow rates.
A two-stage pressure regulator is ideal for applications with large variations in the flow rate, significant fluctuations in the inlet pressure, or decreasing inlet pressure such as occurs with gas supplied from a small storage tank or gas cylinder.
With most single-stage regulator regulators, except those that use a pressure compensated design, a large drop in inlet pressure will cause a slight increase in outlet pressure. This happens because the forces acting on the valve change, due to the large drop in pressure, from when the outlet pressure was initially set. In a two-stage design the second stage will not be subjected to these large changes in inlet pressure, only the slight change from the outlet of the first stage. This arrangement results in a stable outlet pressure from the second stage despite the significant changes in pressure supplied to the first stage.
A three-stage regulator provides a stable outlet pressure similar to a two-stage regulator but with the added ability to handle a significantly higher maximum inlet pressure. For example, the Beswick PRD4HP series three-stage regulator is rated to handle an inlet pressure as high as 3,000 psi and it will provide a stable outlet pressure (in the 0 to 30 psi range) despite changes to the supply pressure. A small and lightweight pressure regulator that can maintain a stable low output pressure despite an inlet pressure that will decrease over time from a high pressure is a critical component in many designs. Examples include portable analytical instruments, hydrogen fuel cells, UAVs, and medical devices powered by high pressure gas supplied from a gas cartridge or storage cylinder.
Now that you have chosen the regulator that best suits your application it is important that the regulator is installed and adjusted properly to insure that it functions as intended.
Most manufacturers recommend the installation of a filter upstream of the regulator (some regulators have a built-in filter) to prevent dirt and particulates from contaminating the valve seat. Operation of a regulator without a filter could result in a leaking to the outlet port if the valve seat is contaminated with dirt or foreign material. Regulated gases should be free from oils, greases, and other contaminants which could foul or damage the valve components or attack the regulator seals. Many users are unaware that gases supplied in cylinders and small gas cartridges can contain traces of oils from the manufacturing process. The presence of oil in the gas is often not apparent to the user and therefore this topic should be discussed with your gas supplier before you select the seal materials for your regulator. Additionally, gasses should be free of excessive moisture. In high flow rate applications, icing of the regulator can occur if moisture is present.
If the pressure regulator will be used with oxygen, be aware that that oxygen requires specialized knowledge for safe system design. Oxygen compatible lubricants must be specified and extra cleaning, to remove traces of petroleum based cutting oils, is typically specified. Make certain that you inform your regulator supplier that you plan to use the regulator in an oxygen application.
Do not connect regulators to a supply source with a maximum pressure greater than the rated inlet pressure of the regulator. Pressure regulators are not intended to be used as shutoff devices. When the regulator is not in use, the supply pressure should be turned off.
STEP 1
Begin by connecting the pressure source to the inlet port and the regulated pressure line to the outlet port. If the ports are not marked, check with the manufacturer to avoid incorrect connections. In some designs, damage can occur to the internal components if the supply pressure is mistakenly supplied to the outlet port.
STEP 2
Before turning on the supply pressure to the regulator, back off the adjustment control knob to restrict flow through the regulator. Gradually turn on the supply pressure so as not to &#;shock&#; the regulator with a sudden rush of pressurized fluid. NOTE: Avoid turning the adjustment screw all the way into the regulator because, in some regulator designs, the full supply pressure will be delivered to the outlet port.
STEP 3
Set the pressure regulator to the desired outlet pressure. If the regulator is non-relieving, it will be easier to adjust the outlet pressure if fluid is flowing rather than &#;dead ended&#; (no flow). If the measured outlet pressure exceeds the desired outlet pressure, vent the fluid from the downstream side of the regulator and lower the outlet pressure by turning the adjustment knob. Never vent fluid by loosening fittings, as injury may result.
With a relieving style regulator, excess pressure will be automatically vented to atmosphere from the downstream side of the regulator when the knob is rotated to lower the output setting. For this reason, do not use relieving style regulators with flammable or hazardous fluids. Be sure the excess fluid is vented safely and in accordance with all local, state and federal regulations.
STEP 4
To obtain the desired outlet pressure, make the final adjustments by slowly increasing the pressure from below the desired set point. Setting the pressure from below the desired setting is preferred to setting it from above the desired setting. If you overshoot the set point while setting the pressure regulator, back off the set pressure to a point below the set point. Then, again, gradually increase the pressure to the desired set point.
STEP 5
Cycle the supply pressure on and off several times while monitoring the outlet pressure to confirm the regulator is consistently returning to the set point. Additionally, the outlet pressure should also be cycled on and off to ensure the pressure regulator returns to the desired set point. Repeat the pressure setting sequence if the outlet pressure does not return to the desired setting.
Beswick Engineering specializes in miniature liquid and pneumatic fittings, quick disconnects, valves and regulators. We have a team of degreed Application Engineers ready to assist you with your questions. Custom designs are available upon request. Submit your inquiry on our Contact Us page or click the Live Chat icon in the bottom right of your screen.
A flow meter (or a flow sensor) is an flow instrument that is used to indicate the amount of liquid, gas, or vapor moving through a pipe or conduit by measuring linear, non-linear, mass, or volumetric flow rates. Since flow control is often essential, measuring the flow of liquids and gasses is a critical need for many industrial applications &#; and there are many different types of flow meters that can be utilized depending on the nature of the application.
When choosing a flow meter to buy, one should consider such intangible factors as familiarity of plant personnel, their experience with c and maintenance, spare parts availability, and meant time between failure history, etc., at the particular plant site. It is also recommended that the cost of the installation be computed only after taking these steps. One of the most common flow measurement mistakes is the reversal of this sequence: instead of selecting a sensor which will perform properly, an attempt is made to justify the use of a device because it is less expensive. Those &#;inexpensive&#; purchases can be the costliest installations.
The use of differential pressure as an inferred measurement of a liquid&#;s rate of flow is well known. Differential pressure flow meters are, by far, the most common units in use today. These meters, which boast high accuracy, calculate fluid flow by reading pressure loss across a pipe restriction. Estimates are that over 50 percent of all liquid flow measurement applications use this type of unit.
The basic operating principle of differential pressure flow meters is based on the premise that the pressure drop across the meter is proportional to the square of the flow rate. The flow rate is obtained by measuring the pressure differential and extracting the square root.
Differential pressure flow meters, like most flow meters, have a primary and secondary element. The primary element causes a change in kinetic energy, which creates the differential pressure in the pipe. The unit must be properly matched to the pipe size, flow conditions, and the liquid&#;s properties. And, the measurement accuracy of the element must be good over a reasonable range. The secondary element measures the differential pressure and provides the signal or read-out that is converted to the actual flow value.
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Orifice flow meters are the most popular liquid flow meters in use today. An orifice is simply a flat piece of metal with a specific-sized hole bored in it. Most orifices in use are of the concentric type, but eccentric, conical (quadrant), and segmental designs are also available.
In practice, the orifice plat is installed in the pipe between two flanges. Acting as the primary device, the orifice constricts the flow of liquid to produce a differential pressure across the plate. Pressure taps on either side of the plate are used to detect the difference. Major advantages of orifices are that they have no moving parts, and their cost does not increase significantly with pipe size.
Conical and quadrant orifices are relatively new. The units were developed primarily to measure liquids with low Reynolds numbers. Essentially constant flow coefficients can be maintained at R values below . Conical orifice plates have an upstream bevel, the depth and angle of which must be calculated and machined for each application.
The segmental wedge is a variation of the segmental orifice. It is a restriction orifice primarily designed to measure the flow of liquids containing solids. The unit has the ability to measure flows at low Reynolds numbers and still maintain the desired square-root relationship. Its design is simple, and there is only one critical dimension the wedge gap. Pressure drop through the unit is only about half that of conventional orifices.
Integral wedge assemblies combine the wedge element and pressure taps into a one-piece pipe coupling bolted to a conventional pressure transmitter. No special piping or fittings are needed to install the device in a pipeline.
Metering accuracy of all orifice flowmeters depends on the installation conditions, the orifice area ratio, and the physical properties of the liquid being measured.
Venturi tubes have the advantage of being able to handle large flow volumes at low pressure drops. A venturi tube is essentially a section of pipe with a tapered entrance and a straight throat. As liquid passes through the throat, its velocity increases, causing a pressure differential between the inlet and outlet regions.
The flowmeters have no moving parts. They can be installed in large diameter pipes using flanged, welded or threaded-end fittings. Four or more pressure taps are usually installed with the unit to average the measured pressure. Venturi tubes can be used with most liquids, including those having a high solids content.
Pitot tubes are generally installed by welding a coupling on a pipe and inserting the probe through the coupling. Use of most pitot tubes is limited to single point measurements. The units are susceptible to plugging by foreign material in the liquid. Advantages of pitot tubes are low cost, absence of moving parts, easy installation, and minimum pressure drop.
Operation of these units consists of separating liquids into accurately measured increments and moving them on. Each segment is counted by a connecting register. Because every increment represents a discrete volume, positive-displacement units are popular for automatic batching and accounting applications. Positive-displacement meters are good candidates for measuring the flows of viscous liquids or for use where a simple mechanical meter system is needed.
Reciprocating piston meters are of the single and multiple-piston types. The specific choice depends on the range of flow rates required in the particular application. Piston meters can be used to handle a wide variety of liquids. A magnetically driven, oscillating piston meter is shown in Fig. 1. Liquid never comes in contact with gears or other parts that might clog or corrode.
Figure 1: Oscillating-piston meter operates on magnetic drive principle so that liquid will not come in contact with parts. A partition plate between inlet and outlet ports forces incoming liquid to flow around a cylindrical measuring chamber and through the outlet port. The motion of the oscillating piston in the unit is transferred to a magnetic assembly in the measuring chamber, which is coupled to a follower magnet on the other side of the chamber wall.
Oval-gear meters have two rotating, oval-shaped gears with synchronized, close fitting teeth. A fixed quantity of liquid passes through the meter for each revolution. Shaft rotation can be monitored to obtain specific flow rates.
Nutating-disk meters have a moveable disk mounted on a concentric sphere located in a spherical side-walled chamber. The pressure of the liquid passing through the measuring chamber causes the disk to rock in a circulating path without rotating about its own axis. It is the only moving part in the measuring chamber.
A pin extending perpendicularly from the disk is connected to a mechanical counter that monitors the disk's rocking motions. Each cycle is proportional to a specific quantity of flow. As is true with all positive-displacement meters, viscosity variations below a given threshold will affect measuring accuracies. Many sizes and capacities are available. The units can be made from a wide selection of construction materials.
Rotary-vane meters are available in several designs, but they all operate on the same principle. The basic unit consists of an equally divided, rotating impeller (containing two or more compartments) mounted inside the meter's housing. The impeller is in continuous contact with the casing. A fixed volume of liquid is swept to the meter's outlet from each compartment as the impeller rotates. The revolutions of the impeller are counted and registered in volumetric units.
Helix flow meters consist of two radically pitched helical rotors geared together, with a small clearance between the rotors and the casing. The two rotors displace liquid axially from one end of the chamber to the other.
These instruments operate linearly with respect to the volume flow rate. Because there is no square-root relationship (as with differential pressure devices), their rangeability is greater. Volumetric meters have minimum sensitivity to viscosity changes when used at Reynolds numbers above 10,000. Most velocity-type meter housings are equipped with flanges or fittings to permit them to be connected directly into pipelines.
Turbine meters have found widespread use for accurate liquid measurement applications. The unit consists of a multiple-bladed rotor mounted with a pipe, perpendicular to the liquid flow. The rotor spins as the liquid passes through the blades. The rotational speed is a direct function of flow rate and can be sensed by magnetic pick-up, photoelectric cell, or gears. Electrical pulses can be counted and totalized, Fig. 2.
Figure 2: Turbine flow meter consists of a multiple-bladed, free-spinning, permeable metal rotor housed in a non-magnetic stainless-steel body. In operation, the rotating blades generate a frequency signal proportional to the liquid flow rate, which is sensed by the magnetic pickup and transferred to a read-out indicator.
The number of electrical pulses counted for a given period of time is directly proportional to flow volume. A tachometer can be added to measure the turbine's rotational speed and to determine the liquid flow rate. Turbine meters, when properly specified and installed, have good accuracy, particularly with low-viscosity liquids.
A major concern with turbine meters is bearing wear. A "bearingless" design has been developed to avoid this problem. Liquid entering the meter travels through the spiraling vanes of a stator that imparts rotation to the liquid stream. The stream acts on a sphere, causing it to orbit in the space between the first stator and a similarly spiraled second stator. The orbiting movement of the sphere is detected electronically. The frequency of the resulting pulse output is proportional to flow rate.
Vortex 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 through the meter, Fig. 3.
Figure 3: Vortex meters operate on the principle that when a non-streamlined object is placed in the middle of a flow stream, a series of vortices are shed alternately downstream of the object. The frequency of the vortex shedding is directly proportional to the velocity of the liquid flowing in the pipeline.
The three major components of the flowmeter are a bluff body strut-mounted across the flowmeter bore, a sensor to detect the presence of the vortex and to generate an electrical impulse, and a signal amplification and conditioning transmitter whose output is proportional to the flow rate, Fig. 4. The meter is equally suitable for flow rate or flow totalization measurements. Use for slurries or high viscosity liquids is not recommended.
Figure 4
Electromagnetic meters can handle most liquids and slurries, providing that the material being metered is electrically conductive. The flow tube mounts directly in the pipe. Pressure drop across the meter is the same as it is through an equivalent length of pipe because there are no moving parts or obstructions to the flow. The voltmeter can be attached directly to the flow tube or can be mounted remotely and connected to it by a shielded cable.
Electromagnetic flow meters operate on Faraday's law of electromagnetic induction that states that a voltage will be induced when a conductor moves through a magnetic field. The liquid serves as the conductor; the magnetic field is created by energized coils outside the flow tube, Fig. 5. The amount of voltage produced is directly proportional to the flow rate. Two electrodes mounted in the pipe wall detect the voltage, which is measured by the secondary element.
Figure 5: Major components of obstruction-free electromagnetic flow meter&#;s flow tube include electrodes and coils.
Electromagnetic flow meters have major advantages: They can measure difficult and corrosive liquids and slurries; and they can measure forward as well as reverse flow with equal accuracy. Disadvantages of earlier designs were high power consumption, and the need to obtain a full pipe and no flow to initially set the meter to zero. Recent improvements have eliminated these problems. Pulse-type excitation techniques have reduced power consumption, because excitation occurs only half the time in the unit. Zero settings are no longer required.
Ultrasonic flow meters can be divided into Doppler meters and Time-of-Travel (or Transit) meters. Doppler meters measure the frequency shifts caused by liquid flow. Two transducers are mounted in a case attached to one side of the pipe. A signal of known frequency is sent into the liquid to be measured. Solids, bubbles, or any discontinuity in the liquid, cause the pulse to be reflected to the receiver element, Fig. 6. Because the liquid causing the reflection is moving, the frequency of the returned pulse is shifted. The frequency shift is proportional to the liquid's velocity.
Figure 6
A portable Doppler meter capable of being operated on AC power or from a rechargeable power pack has recently been developed. The sensing heads are simply clamped to the outside of the pipe, and the instrument is ready to be used. Total weight, including the case, is 22 lb. A set of 4 to 20 millampere output terminals permits the unit to be connected to a strip chart recorder or other remote device.
Time-of-travel meters have transducers mounted on each side of the pipe. The configuration is such that the sound waves traveling between the devices are at a 45 deg. angle to the direction of liquid flow. The speed of the signal traveling between the transducers increases or decreases with the direction of transmission and the velocity of the liquid being measured. A time-differential relationship proportional to the flow can be obtained by transmitting the signal alternately in both directions. A limitation of time-of-travel meters is that the liquids being measured must be relatively free of entrained gas or solids to minimize signal scattering and absorption.
The continuing need for more accurate flow measurements in mass-related processes (chemical reactions, heat transfer, etc.) has resulted in the development of mass flowmeters. Various designs are available, but the one most commonly used for liquid flow applications is the Coriolis mass flow meter. Its operation is based on the natural phenomenon called the Coriolis force, hence the name.
Coriolis flow meters are true mass meters that measure the mass rate of flow directly as opposed to volumetric flow. Because mass does not change, the meter is linear without having to be adjusted for variations in liquid properties. It also eliminates the need to compensate for changing temperature and pressure conditions. The meter is especially useful for measuring liquids whose viscosity varies with velocity at given temperatures and pressures.
Coriolis flow meters are true mass meters that measure the mass rate of flow directly as opposed to volumetric flow. Because mass does not change, the meter is linear without having to be adjusted for variations in liquid properties. It also eliminates the need to compensate for changing temperature and pressure conditions. The meter is especially useful for measuring liquids whose viscosity varies with velocity at given temperatures and pressures.
Coriolis meters are also available in various designs. A popular unit consists of a U-shaped flow tube enclosed in a sensor housing connected to an electronics unit. The sensing unit can be installed directly into any process. The electronics unit can be located up to 500 feet from the sensor.
Inside the sensor housing, the U-shaped flow tube is vibrated at its natural frequency by a magnetic device located at the bend of the tube. The vibration is similar to that of a tuning fork, covering less than 0.1 in. and completing a full cycle about 80 times/sec. As the liquid flows through the tube, it is forced to take on the vertical movement of the tube, Fig. 7. When the tube is moving upward during half of its cycle, the liquid flowing into the meter resists being forced up by pushing down on the tube.
Figure 7
Having been forced upward, the liquid flowing out of the meter resists having its vertical motion decreased by pushing up on the tube. This action causes the tube to twist. When the tube is moving downward during the second half of its vibration cycle, it twists in the opposite direction.
Having been forced upward, the liquid flowing out of the meter resists having its vertical motion decreased by pushing up on the tube. This action causes the tube to twist. When the tube is moving downward during the second half of its vibration cycle, it twists in the opposite direction. The ammount of twist is directly proportional to the mass flow rate of the liquid flowing through the tube. Magnetic sensors located on each side of the flow tube measure the tube velocities, which change as the tube twists. The sensors feed this information to the electronics unit, where it is processed and converted to a voltage proportional to mass flow rate. The meter has a wide range of applications from adhesives and coatings to liquid nitrogen.
Thermal-type mass flowmeters have traditionally been used for gas measurements, but designs for liquid flow measurements are available. These mass meters also operate independent of density, pressure, and viscosity. Thermal meters use a heated sensing element isolated from the fluid flow path. The flow stream conducts heat from the sensing element. The conducted heat is directly proportional to the mass flow rate. The sensor never comes into direct contact with the liquid. The electronics package includes the flow analyzer, temperature compensator, and a signal conditioner that provides a linear output directly proportional to mass flow.
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