Level instrument accessories

Disturbances in the liquid tend to complicate liquid level measurement. These disturbances may result from liquid introduced into a vessel above the liquid level (splashing into the liquid’s surface), the rotation of agitator paddles, and/or turbulent flows from mixing pumps. Any source of turbulence for the liquid surface (or liquid-liquid interface) is especially problematic for echo-type level sensors, which only sense interfaces between vapors and liquids, or liquids and liquids.

If it is not possible to eliminate disturbances inside the process vessel, a relatively simple accessory one may add to the process vessel is a vertical length of pipe called a stilling well. To understand the principle of a stilling well, first consider the application of a hydraulic oil reservoir where we wish to continuously measure the oil levels. The oil flow in and out of this reservoir will cause problems for the displacer element:

A section of vertical pipe installed in the reservoir around the displacer will serve as a shield to all the turbulence in the rest of the reservoir. The displacer element will no longer be subject to a horizontal blast of oil entering the reservoir, nor any wave action to make it bob up and down. This section of pipe quiets, or stills, the oil surrounding the displacer, making it easier to measure oil level:

Stilling wells may be used in conjunction with many types of level instruments: floats, displacers, ultrasonic, radar, and laser to name a few. If the process application necessitates liquid-liquid interface measurement, however, the stilling well must be properly installed to ensure the interface level inside the well matches the interface levels in the rest of the vessel. Consider this example of using a stilling well in conjunction with a tape-and-float system for interface measurement:

In the left-hand installation where the stilling well is completely submerged, the interface levels will always match. In the right-hand installation where the top of the stilling well extends above the total liquid level, however, the two levels may not always match.

This potential problem for the non-submerged stilling well is graphically illustrated here:

The problem here is analogous to what we see with sightglass-style level gauges: interfaces may be reliably indicated if and only if both ends of the sightglass are submerged (see section 20.1.2 beginning on page 1163 for an illustrated explanation of the problem).

If it is not possible or practical to ensure complete submersion of the stilling well, an alternative technique is to drill holes or cut slots in the well to allow interface levels to equalize inside and outside of the well tube:

Such equalization ports are commonly found as a standard design feature on coaxial probes for guided-wave radar level transmitters, where the outer tube of the coaxial transmission line acts as a sort of stilling well for the fluid. Coaxial probes are typically chosen for liquid-liquid interface radar measurement applications because they do the best job of preventing dispersion of the electromagnetic wave energy39, but the “stilling well” property of a coaxial probe practically necessitates these equalization ports to ensure the interface level within the probe always matches the interface level in the rest of the vessel.

List of Prominent Manufacturers: ABB, Afrisco, Airmar, Ametek, Anderson-Negele, Aplisens, Aquolabo, ATMI, Autosen, Balluff, Barkssdale, BCM Sensor, BD Sensors, Berthold, Branom Instrument, Broyce Control, Burkert, CAE, Captron, Chemitec, Clarksol, CS Instruments, Danfoss, DHE, di-soric, Divatec, Drexelbrook, Dwyer, EBE, EGE, EIT, Elettrotec, Elobau, Emerson, Endress+Hauser, Engler, Envea, Fae, Fafnir, Feejoy, Fiama, FineTek, Focusens, Fluidwell, Fuji Electric, Gavin, GEA, Gems, Hanla IMS, Hengesbach, Henry, Holykell, Honeywell, Hughes, Hyquest Solutions, Ifm, Introtek, Jacob, Jumo, KoBold, Krohne, LEEG, Littelfuse, Maddalena, Magnetrol, MBA Instruments, MC Techgroup, Meggitt, Metso Outotec, Microsonic, Migraton Corp, Moduloc, Monitortech, Montrans, Moog, Nordson, Ohm Group, Omega, Omni Comm, Peeperl+Fuchs, Primayer, Process Control Devices, Proxitron, Pulsar, Resensys, Reventec, Rittmeyer, Sapco Instruments, Senix, Sensotec, Sera, SMB Group, Solidat, Sommer, Sonotec, Soway, Supmea, TDK, TEK BOX, Technotron, Teledyne Taptone, Temposonics, Toscano, Trafag, Trimod Besta, Tsurumi Pump, UWT, Val.Co, Vega, WAM Group, Wika, Woerner, WoMaster, Xylem, Yuanben, ZHYQ

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Radiation Type Level Measurement

Certain types of nuclear radiation easily penetrate the walls of industrial vessels but are attenuated by traveling through the bulk of material stored within those vessels. By placing a radioactive source on one side of the vessel and measuring the radiation making it through to the other side of the vessel, an approximate indication of level within that vessel may be obtained. Other types of radiation are scattered by process material in vessels, which means the level of process material may be sensed by sending radiation into the vessel through one wall and measuring back-scattered radiation returning through the same wall.

The four most common forms of nuclear radiation are alpha particles (α), beta particles (β), gamma rays (γ), and neutrons (n). Alpha particles are helium nuclei (2 protons bound together with 2 neutrons) ejected at high velocity from the nuclei of certain decaying atoms. They are easy to detect but have very little penetrating power and so are not used for industrial level measurement. Beta particles are electrons37 ejected at high velocity from the nuclei of certain decaying atoms. Like alpha particles, though, they have little penetrating power and so are not used for industrial-level measurement. Gamma rays are electromagnetic in nature (like X-rays and light waves) and have a great penetrating power. Neutron radiation also penetrates metal very effectively but is strongly attenuated and scattered by any substance containing hydrogen (e.g., water, hydrocarbons, and many other industrial fluids), which makes it almost ideal for detecting the presence of a great many process fluids. These latter two forms of radiation (gamma rays and neutrons) are the most common in industrial measurement, with gamma rays used in through-vessel applications and neutrons typically used in backscatter applications.

Nuclear radiation sources consist of radioactive samples contained in a shielded box. The sample itself is a small piece of radioactive substance encased in a double-wall stainless steel cladding, typically resembling a medicinal pill in size and shape. The specific type and quantity of radioactive source material depend on the nature and intensity of radiation required for the application. The basic rule here is that less is better: the smallest source capable of performing the measurement task is the best one for the application.

Common source types for gamma-ray applications are Cesium-137 and Cobalt-60. The numbers represent the atomic mass of each isotope: the sum of protons and neutrons in the nucleus of each atom. These isotopes’ nuclei are unstable, decaying over time to become different elements (Barium-137 and Nickel-60, respectively). Cobalt-60 has a relatively short half-life38 of 5.3 years, whereas Cesium-137 has a much longer half-life of 30 years. This means radiation-based sensors using Cesium will be more stable over time (i.e., less calibration drift) than sensors using Cobalt. The trade-off is that Cobalt emits more powerful gamma rays than Cesium, which makes it better suited to applications where the radiation must penetrate thick process vessels or travel long distances (across wide process vessels).

One of the most effective methods of shielding against gamma-ray radiation is with very dense substances such as lead or concrete. Therefore the source boxes holding gamma-emitting radioactive pellets are lined with lead, so the radiation escapes only in the direction intended:

Radioactive sources naturally emit radiation, requiring no source of energy such as electricity to do their job. As such, they are “always-on” devices and may be locked out for testing and maintenance only by dropping a lead shutter over the “window” of the box. The lever actuating the shutter typically has provisions for lock-out/tag-out so a maintenance person may place a padlock on the lever and prevent anyone else from “turning on” the source during maintenance. For point-level (level switch) applications, the source shutter acts as a simple simulator for either a full vessel (in the case of a through-vessel installation) or an empty vessel (in the case of a backscatter installation). A full vessel may be simulated for neutron backscatter instruments by placing a sheet of plastic (or other hydrogen-rich substance) between the source box and the process vessel wall.

The detector for a radiation-based instrument is by far the most complex and expensive component of the system. Many different detector designs exist, the most common at the time of this writing being ionization tubes such as the Geiger-Muller (G-M) tube. In such devices, a thin metal wire centered in a metal cylinder sealed and filled with inert gas is energized with high voltage DC. Any ionizing radiation such as alpha, beta, or gamma radiation entering the tube causes gas molecules to ionize, allowing a pulse of electric current to travel between the wire and tube wall. A sensitive electronic circuit detects and counts these pulses, with a greater pulse rate corresponding to a greater intensity of detected radiation.

Neutron radiation is notoriously difficult to electronically detect since neutrons are non-ionizing. Ionization tubes specifically made for neutron radiation detection do exist and are filled with special substances known to react with neutron radiation. One example of such a detector is the so-called fission chamber, which is an ionization chamber lined with a fissile material such as uranium-235 (235U). When a neutron enters the chamber and is captured by a fissile nucleus, that nucleus undergoes fission (splits into separate pieces) with subsequent emission of gamma rays and charged particles, which are then detected by ionization in the chamber. Another variation on this theme is to fill an ionization tube with boron trifluoride gas. When a boron-10 (10B) nucleus captures a neutron, it trans mutates into lithium-7 (7Li) and ejects an alpha particle and several beta particles, both of which cause detectable ionization in the chamber.

The accuracy of radiation-based level instruments varies with the stability of process fluid density, vessel wall coating, source decay rates, and detector drift. Given these error variables and the additional need for NRC (Nuclear Regulatory Commission) licensing to operate such instruments at an industrial facility, radiation instruments are typically used where no other instrument is practical. Examples include the level measurement of highly corrosive or toxic process fluids where penetrations into the vessel must be minimized and where piping requirements make weight-based measurement impractical (e.g. hydrocarbon/acid separators in alkylation processes in the oil refining industry), as well as processes where the internal conditions of the vessel are too physically violent for any instrument to survive (e.g. delayed coking vessels in the oil refining industry, where the coke is “drilled” out of the vessel by a high-pressure water jet).

List of Prominent Manufacturers: Berthold, Endress+Hauser, Vega

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Capacitive Type Level Measurement

Capacitive level instruments measure the electrical capacitance of a conductive rod inserted vertically into a process vessel. When the process level increases, capacitance increases between the rod and the vessel walls, causing the instrument to output a greater signal.

The basic principle behind capacitive level instruments is the capacitance equation:

The amount of capacitance exhibited between a metal rod inserted into the vessel and the metal walls of that vessel will vary only with changes in permittivity (ǫ), area (A), or distance (d). Since A is constant (the interior surface area of the vessel is fixed, as is the area of the rod once installed), only changes in ǫ or d can affect the probe’s capacitance.

Capacitive level probes come in two basic varieties: one for conductive liquids and one for nonconductive liquids. If the liquid in the vessel is conductive, it cannot be used as the dielectric (insulating) medium of a capacitor. Consequently, capacitive level probes designed for conductive liquids are coated with plastic or some other dielectric substance, so the metal probe forms one plate of the capacitor, and the conductive liquid forms the other:

In this style of the capacitive level probe, the variables are permittivity (ǫ) and distance (d), since a rising liquid level displaces low-permittivity gas and essentially acts to bring the vessel wall electrically closer to the probe. This means total capacitance will be greatest when the vessel is full (ǫ is greatest and effective distance d is at a minimum), and least when the vessel is empty (ǫ of the gas is in effect, and over a much greater distance).

If the liquid is non-conductive, it may be used as the dielectric itself, with the metal wall of the storage vessel forming the second capacitor plate. The probe is just a bare metal cable or rod:

In this style of the capacitive level probe, the variable is permittivity (ǫ), provided the liquid has a substantially greater permittivity than the vapor space above the liquid. This means total capacitance will be greatest when the vessel is full (average permittivity ǫ is at a maximum), and least when the vessel is empty.

Permittivity of the process substance is a critical variable in the non-conductive style of capacitance level probe, and so good accuracy may be obtained with this kind of instrument only if the process permittivity is accurately known. A clever way to ensure good level measurement accuracy when the process permittivity is not stable over time is to equip the instrument with a special compensating probe (sometimes called a composition probe) below the LRV point in the vessel that will always be submerged in liquid. Since this compensating probe is always immersed, and always experiences the same A and d dimensions, its capacitance is purely a function of the liquid’s permittivity (ǫ). This gives the instrument a way to continuously measure process permittivity, which it then uses to calculate level based on the capacitance of the main probe. The inclusion of a compensating probe to measure and compensate for changes in liquid permittivity is analogous to the inclusion of a third pressure transmitter in a hydrostatic tank expert system to continuously measure and compensate for liquid density. It is a way to correct for changes in the one remaining system variable that is not related to changes in the liquid level.

Capacitive level instruments may be used to measure the level of solids (powders and granules) in addition to liquids. In these applications, and solid substance is almost always non-conductive, and therefore the permittivity of the substance becomes a factor in measurement accuracy. This can be problematic, as moisture content variations in the solid may greatly affect permittivity, as can variations in granule size.

Capacitive level instruments are generally found in applications where precision is not important. These instruments tend to suffer from errors arising from changes in process substance permittivity, changes in process vapor-space permittivity, and errors caused by stray capacitance in probe cables.

List of Prominent Manufacturers: Autosen, Balluff, Captron, di-soric, Ege, Endress+Hauser, Feejoy, Fiama, Gavin, GEMS, Holykell, Ifm, Meggitt, Omega, Process Control Devices, Proxitron, Reventec, Soway, UWT, Vega, Yuanben

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Weight Based Level Measurement

Weight-based level instruments sense process levels in a vessel by directly measuring the weight of the vessel. If the vessel’s empty weight (tare weight) is known, process weight becomes a simple calculation of total weight minus tare weight. Obviously, weight-based level sensors can measure both liquid and solid materials, and they have the benefit of providing inherently linear mass storage measurement. Load cells (strain gauges bonded to a steel element of the precisely known modulus) are typically the primary sensing element of choice for detecting vessel weight. As the vessel’s weight changes, the load cells compress or relax on a microscopic scale, causing the strain gauges inside to change resistance. These small changes in electrical resistance become a direct indication of vessel weight.

The following photograph shows three bins used to store powdered milk, each one supported by pillars equipped with load cells near their bases:

A close-up photograph shows one of the load cell units in detail, near the base of a pillar:

When multiple load cells are used to measure the weight of a storage vessel, the signals from all load cell units must be added together (“summed”) to produce a signal representative of the vessel’s total weight. Simply measuring the weight at one suspension point is insufficient, because one can never be sure the vessel’s weight is distributed equally amongst all the supports.

This next photograph shows a smaller-scale load cell installation used to measure the quantity of the material fed into a beer-brewing process:

Weight-based measurements are often employed where the true mass of a quantity must be ascertained, rather than the level. So long as the material’s density is a known constant, the relationship between weight and level for a vessel of the constant cross-sectional area will be linear and predictable. Constant density is not always the case, especially for solid materials, and so weight-based inference of vessel level may be problematic.

In applications where batch mass is more important than height (level), weight-based measurement is often the preferred method for portioning batches. You will find weight-based portion measurements used frequently in the food processing industries (e.g. consistently filling bags and boxes with the product), and also for custody transfer of certain materials (e.g. coal and metal ore).

One very important caveat for weight-based level instruments is to isolate the vessel from any external mechanical stresses generated by pipes or machinery. The following illustration shows a typical installation for a weight-based measurement system, where all pipes attaching to the vessel do so through flexible couplings, and the weight of the pipes themselves is borne by outside structures through pipe hangers:

Stress relief is very important because any forces acting upon the storage vessel will be interpreted by the load cells as more or less material stored in the vessel. The only way to ensure that the load cell’s measurement is a direct indication of material held inside the vessel is to ensure that no other forces act upon the vessel except the gravitational weight of the material.

A similar concern for weight-based batch measurement is vibration produced by machinery surrounding (or on) the vessel. Vibration is nothing more than oscillatory acceleration, and the acceleration of any mass produces a reaction force (F=ma). Any vessel suspended by weight sensing elements such as load cells will induce oscillating forces on those load cells if shaken by vibration. This concern makes it quite difficult to install and operate agitators or other rotating machinery on a weighed vessel.

An interesting problem associated with load cell measurement of vessel weight arises if there are ever electric currents traveling through the load cell(s). This is not a normal situation, but it can happen if maintenance workers incorrectly attach arc welding equipment to the support structure of the vessel, or if certain electrical equipment mounted on the vessel such as lights or motors develop ground faults. The electronic amplifier circuits interpreting a load cell’s resistance will detect voltage drops created by such currents, interpreting them as changes in load cell resistance and therefore as changes in material level. Sufficiently large currents may even cause permanent damage to load cells, as is often the case when the currents in question are generated by arc welding equipment.

A variation on this theme is the so-called hydraulic load cell which is a piston-and-cylinder mechanism designed to translate vessel weight directly into hydraulic (liquid) pressure. A normal pressure transmitter then measures the pressure developed by the load cell and reports it as material weight stored in the vessel. Hydraulic load cells completely bypass the electrical problems associated with resistive load cells but are more difficult to network for the calculation of total weight (using multiple cells to measure the weight of a large vessel).

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Echo (Magnestrictive) Type Level Measurement

A variation on the theme of echo-based level instruments, where the level of some process material in a vessel is measured by timing the travel of a wave between the instrument and the material interface, is one applied to float-type instruments: magnetostriction.

In a magnetostrictive level instrument, the liquid level is sensed by a lightweight, donut-shaped float containing a magnet. This float is centered around a long metal rod called a waveguide, hung vertically in the process vessel (or hung vertically in a protective cage like the type used for displacement-style level instruments) so that the float may rise and fall with process liquid level.

The magnetic field from the float’s magnet at that point, combined with the magnetic field produced by an electric current pulse periodically sent through the rod, generates a torsional stress pulse at the precise location of the float. This torsional (twisting) stress travels at the speed of sound through the rod toward either end. At the bottom end is a dampener device designed to absorb the mechanical wave.

One might argue that a magnetostrictive instrument is not an “echo” technology in the strictest sense of the word. Unlike ultrasonic, radar, and laser instruments, we are not reflecting a wave off a discontinuous interface between materials. Instead, a mechanical wave (pulse) is generated at the location of a magnetic float in response to an electrical pulse. However, the principle of measuring distance by the wave’s travel time is the same. At the top end of the rod (above the process liquid level) is a sensor and electronics package designed to detect the arrival of the mechanical wave.

A precision electronic timing circuit measures the time elapsed between the electric current pulse (called the interrogation pulse) and the received mechanical pulse. So long as the speed of sound through the metal waveguide rod remains fixed, the time delay is strictly a function of the distance between the float and the sensor, which we already know is called ullage.

The following photograph (left) and illustration (right) show a magnetostrictive level transmitter propped up against a classroom wall and the same style of transmitter installed in a liquid-holding vessel, respectively:

The design of this instrument is reminiscent of a guided-wave radar transmitter, where a metal waveguide hangs vertically into the process liquid, guiding a pulse to the sensor head where the sensitive electronic components are located. The major difference here is that the pulse is a sonic vibration traveling through the metal of the waveguide rod, not an electromagnetic pulse as is the case with radar. Like all sound waves, the torsional pulse in a magnetostriction-based level transmitter is much slower traveling than electromagnetic waves.

It is even possible to measure liquid-liquid interfaces with magnetostrictive instruments. If the waveguide is equipped with a float of such density that it floats on the interface between the two liquids (i.e., the float is denser than the light liquid and less dense than the heavy liquid), the sonic pulse generated in the waveguide by that float’s position will represent interface level. Magnetostrictive instruments may even be equipped with two floats: one to sense a liquid-liquid interface, and the other to sense the liquid-vapor interface, so that it may measure both the interface and total levels simultaneously just like a guided-wave radar transmitter:

With such an instrument, each electrical “interrogation” pulse returns two sonic pulses to the sensor head: the first pulse representing the total liquid level (upper, light float) and the second pulse representing the interface level (lower, heavy float). If the instrument has digital communication capability (e.g. HART, FOUNDATION Fieldbus, Profibus, etc.), both levels may be reported to the control system over the same wire pair, making it a “multivariable” instrument.

Perhaps the greatest limitation of magnetostrictive level instruments is mechanical interference between the float and the rod. For the magnetostrictive effect to be strong, the magnet inside the float must be near the rod. This means the inside diameter of the donut-shaped float must fit closely to the outside diameter of the waveguide. Any fouling of the waveguides or float’s surfaces by suspended solids, sludge, or other semi-solid materials may cause the float to bind and therefore not respond to changes in the liquid level.

List of Prominent Manufacturers: ABB, Ametek-Drexelbrook, Balluff, Fafnir, Feejoy, Gems, General Instruments, Holykell, Soway, Temposonics, Val.Co, Wika

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Echo (Radar) Type Level Measurement (Part III)

Continues from the previous post.

Factors influencing the dielectric constant of gases include pressure and temperature, which means the accuracy of a radar level instrument will vary as gas pressure and/or gas temperature vary! This is often referred to as the gas phase effect. Whether or not this variation is substantial enough to consider for any application depends on the desired measurement accuracy and the degree of permittivity change from one pressure/temperature extreme to another. In no case should a radar instrument be considered for any level measurement application unless the dielectric constant value(s) of the upper media are precisely known? This is analogous to the dependence on liquid density that hydrostatic level instruments face. It is futile to attempt level measurement based on hydrostatic pressure if the liquid density is unknown, and it is just as futile to attempt level measurement based on the radar if the dielectric constants are unknown.

One way to compensate for the gas phase effect in radar level instruments is to equip the instrument with a reference probe of fixed length oriented in such a way that its entire length is always above the liquid level (i.e. it only senses gas). If the permittivity of the gas is constant, the echo time along this reference probe will remain the same. If, however, the gas permittivity changes, the reference probe’s echo time will correspondingly change, allowing the instrument’s microprocessor to measure gas permittivity and consequently adjust calculations for liquid level based on this known change. This concept is analogous to the compensating probe sometimes used in capacitive level sensors, designed to measure fluid permittivity so as to compensate for any changes in this critical parameter.

As with ultrasonic level instruments, radar level instruments can measure the level of solid substances in vessels (e.g. powders and granules). The same caveat of repose angle applicable to ultrasonic level measurement, however, is a factor for radar measurement as well. When the particulate solid is not very dense (i.e. much air between particles), the dielectric constant may be rather low, making the material surface more difficult to detect.

Modern radar level instruments provide a wealth of diagnostic information to aid in troubleshooting. One of the most informative is the echo curve, showing each reflected signal received by the instrument along the incident signal’s path of travel. The following image is a screen capture of a computer display, from software used to configure a Rosemount model 3301 guided-wave radar level transmitter with a coaxial probe:

Pulse P1 is the reference or fiducial pulse, resulting from the change in dielectric permittivity between the extended “neck” of the probe (connecting the transmitter to the probe tube) and the coaxial probe itself. This pulse marks the top of the probe, thereby establishing a point of reference for ullage measurement.

This next screen capture shows the same level transmitter measuring a water level that is 8 inches higher than before. Note how pulse P2 is further to the left (indicating an echo received sooner in time), indicating a lesser ullage (greater level) measurement:

Several threshold settings determine how the transmitter categorizes each received pulse. Threshold T1 for this radar instrument defines which pulse is the reference (fiducial). Thus, the first echo in time to exceed the value of threshold T1 is interpreted by the instrument to be the reference point. Threshold T2 defines the upper product level, so the first echo in time to exceed this threshold value is interpreted as the vapor/liquid interface point. Threshold T3 for this transmitter is used to define the echo generated by a liquid-liquid interface. However, threshold T3 does not appear in this echo plot because the interface measurement option was disabled during this experiment. The last threshold, T4, defines the end-of-probe detection. Set at a negative value (just like the reference threshold T1), threshold T4 looks for the first pulse in time to exceed that value and interprets that pulse as the one resulting from the signal reaching the probe’s end.

All along the echo curve, you can see weak echo signals showing up as bumps. These echoes may be caused by discontinuities along with the probe (solid deposits, vent holes, centering spacers, etc.), discontinuities in the process liquid (suspended solids, emulsions, etc.), or even discontinuities in the surrounding process vessel (for non-coaxial probes which exhibit varying degrees of sensitivity to surrounding objects). A challenge in configuring a radar level transmitter is to set the threshold values such that “false” echoes are not interpreted as a real liquid or interface levels.

A simple way to eliminate false echoes near the reference point is to set a null zone where any echoes are ignored. The upper null zone (UNZ) setting on the Rosemount 3301 radar level transmitter whose screen capture image was shown previously was set to zero, meaning it would be sensitive to all echoes near the reference point. If a false echo from a tank nozzle or some other discontinuity near the probe’s entry point into the process vessel created a measurement problem, the upper null zone (UNZ) value could be set just beyond that point so the false echo would not be interpreted as a liquid level echo, regardless of the threshold value for T2. A “null zone” is sometimes referred to as a hold-off distance.

Some radar-level instruments allow thresholds to be set as curves themselves rather than straight lines. Thus, thresholds may be set high during certain periods along with the horizontal (time/distance) axis to ignore false echoes and set low during other periods to capture legitimate echo signals.

Regardless of how null zones and thresholds are set for any guided-wave radar level transmitter, the technician must be aware of transition zones near the extremes of the probe length. Measurements of liquid level or interface level within these zones may not be accurate or even linearly responsive. Thus, it is strongly advised to range the instrument in such a way that the lower- and upper-range values (LRV and URV) lie between the transition zones:

The size of these transition zones depends on both the process substances and the probe type. The instrument manufacturer will provide you with appropriate data for determining transition zone dimensions.

List of Prominent Manufacturers: Burkert, Chemitec, Drexelbrook, Emerson, Flowline, Holykell, Ifm, Krohne, Magnetrol, Nivelco, Omega, Prisma, Pulsar, Siemens, Tecfluid

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Echo (Radar) Type Level Measurement (Part II)

In the previous illustration of the last post, the two media were air (ǫr ≈ 1) and water (ǫr ≈ 80) – a nearly ideal scenario for strong signal reflection. Given these relative permittivity values, the power reflection factor has a value of 0.638 (63.8%) or -1.95 dB. This means well over half the incident power reflects off the air/water interface to form a strong echo signal, with the remaining 0.362 (36.2%) of the wave’s power making it through the air-water interface and propagating into water. If the liquid in question is gasoline rather than water (having a rather low relative permittivity value of approximately 2), the power reflection ratio will only be 0.0294 (2.94%) or -15.3 dB, with most of the wave’s power successfully penetrating the air-gasoline interface.

The longer version of the power reflection factor formula suggests liquid-liquid interfaces should be detectable using radar, and indeed they are. All that is needed is a sufficiently large difference in permittivity between the two liquids to create a strong enough echo to reliably detect. Liquid-liquid interface level measurement with radar works best when the upper liquid has a substantially lesser permittivity value than the lower liquid24. A layer of hydrocarbon oil on top of the water (or an aqueous solution such as an acid or a caustic) is a good candidate for guided-wave radar level measurement. An example of a liquid-liquid interface that would be very difficult for a radar instrument to detect is water (ǫr ≈ 80) above glycerin (ǫr ≈ 42).

If the radar instrument uses a digital network protocol to communicate information with a host system (such as HART or any number of “Fieldbus” standards), it may perform as a multi-variable transmitter, transmitting both the interface level measurement and the total liquid level measurement simultaneously. This capability is rather unique to guided-wave radar transmitters and is very useful in some processes because it eliminates the need for multiple instruments measuring multiple levels.

One reason why a lesser-ǫ fluid above a greater-ǫ fluid is easier to detect than the inverse is due to the necessity of the signal having to travel through a gas-liquid interface above the liquid-liquid interface. With gases and vapors having such small ǫ values, the signal would have to pass through the gas-liquid interface first in order to reach the liquid-liquid interface. This gas-liquid interface, having the greatest difference in ǫ values of any interface within the vessel, will be most reflective to electromagnetic energy in both directions. Thus, only a small portion of the incident wave will ever reach the liquid-liquid interface, and a similarly small portion of the wave reflected off the liquid-liquid interface (which itself is a fraction of the forward wave power that made it through the gas-liquid interface on its way down) will ever make it through the gas-liquid interface on its way back up to the instrument. The situation is much improved if the ǫ values of the two liquid layers are inverted, as shown in this hypothetical comparison (all calculations25 assume no power dissipation along the way, only reflection at the interfaces):

As you can see in the illustration, the difference in power received back at the instrument is nearly two to one, just from the upper liquid having the lesser of two identical ǫ values. Of course, in real life you do not have the luxury of choosing which liquid will go on top of the other (this being determined by fluid density), but you do have the luxury of choosing the appropriate liquid-liquid interface level measurement technology, and as you can see here certain orientations of ǫ values are less detectable with radar than others.

Another factor working against radar as a liquid-liquid interface measurement technology for interfaces where the upper liquid has a greater dielectric constant is that fact that many high-ǫ liquids are aqueous in nature, and water readily dissipates microwave energy. This fact is exploited in microwave ovens, where microwave radiation excites water molecules in the food, dissipating energy in the form of heat. For a radar-based level measurement system consisting of gas/vapor over water over some other (heavier) liquid, the echo signal will be extremely weak because the signal must pass through the “lossy” water layer twice before it returns to the radar instrument.

Electromagnetic energy losses are important to consider in radar level instrumentation, even when the detected interface is simply gas (or vapor) over liquid. The power reflection factor formula only predicts the ratio of reflected power to incident power at an interface of substances. Just because an air-water interface reflects 63.8% of the incident power does not mean 63.8% of the incident power will return to the transceiver antenna! Any dissipative losses between the transceiver and the interface(s) of concern will weaken the signal, to the point where it may become difficult to distinguish from noise.

Another important factor in maximizing reflected power is the degree to which the microwaves spread out on their way to the liquid interface(s) and back to the transceiver. Guided-wave radar instruments receive a far greater percentage of their transmitted power than non-contact radar instruments because the metal probe used to guide the microwave signal pulses help prevent the waves from spreading (and therefore weakening) throughout the liquids as they propagate. In other words, the probe functions as a transmission line to direct and focus the microwave energy, ensuring a straight path from the instrument into the liquid, and a straight echo return path from the liquid back to the instrument. Therefore guided-wave radar is the only practical radar technology for measuring liquid-liquid interfaces.

A critically important factor in accurate level measurement using radar instruments is that the dielectric permittivity of the upper substance(s) (all media between the radar instrument and the interface of interest) be accurately known. The reason for this is rooted in the dependence of electromagnetic wave propagation velocity to relative permittivity. Recalling the wave velocity formula shown earlier:

In the case of a single-liquid application where nothing but gas or vapor exists above the liquid, the permittivity of that gas or vapor must be precisely known. In the case of a two-liquid interface with gas or vapor above, the relative permittivity of both gas and upper liquids must be accurately known to accurately measure the liquid-liquid interface. Changes in the dielectric constant value of the medium or media through which the microwaves must travel, and echo will cause the microwave radiation to propagate at different velocities. Since all radar measurement is based on time-of-flight through the media separating the radar transceiver from the echo interface, changes in wave velocity through this media will affect the amount of time required for the wave to travel from the transceiver to the echo interface and reflect the transceiver. Therefore, changes in the dielectric constant are relevant to the accuracy of any radar level measurement.

Please check the previous and next post to complete the topic.

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Echo (Radar) Type Level Measurement (Part I)

Radar level instruments measure the distance from the transmitter (located at some high point) to the surface of a process material located farther below in much the same way as ultrasonic transmitters – by measuring the time-of-flight of a traveling wave. The fundamental difference between a radar instrument and an ultrasonic instrument is the type of wave used: radio waves instead of sound waves. Radio waves are electromagnetic in nature (comprised of alternating electric and magnetic fields), and very high frequency (in the microwave frequency range – GHz). Sound waves are mechanical vibrations (transmitted from molecule to molecule in a fluid or solid substance) and of much lower frequency (tens or hundreds of kilohertz – still too high for a human being to detect as a tone) than radio waves.

Some radar level instruments use waveguide “probes” to guide the electromagnetic waves to and from the process liquid while others send electromagnetic waves out through open space to reflect off the process material. The instruments using waveguides are called guided-wave radar instruments, whereas the radar instruments relying on open space for signal propagation are called non-contact radar. The differences between these two varieties of radar instruments are shown in the following illustration:

Non-contact radar transmitters are always mounted on the top side of a storage vessel. Modern radar transmitters are quite compact, as this photograph shows:

Non-contact radar devices suffer much more signal loss than guided-wave radar devices, due to the natural tendency of electromagnetic radiation to spread out over space. Waveguides combat this signal loss by focusing the radio energy along a straight-line path. Probes used in guided-wave radar instruments may be single metal rods, parallel pairs of metal rods, or a coaxial metal rod-and-tube structure. Single-rod probes suffer the greatest energy losses, while coaxial probes excel at guiding the microwave energy to the liquid interface and back. However, single-rod probes are much more tolerant of process fouling than two-rod or (especially) coaxial probes, where sticky masses of viscous liquid and/or solid matter cling to the probe. Such fouling deposits, if severe enough, will cause electromagnetic wave reflections that “look” to the transmitter like the reflection from an actual liquid level or interface.

Non-contact radar instruments rely on antennas to direct microwave energy into the vessel and to receive the echo (return) energy. These antennas must be kept clean and dry, which may be a problem if the liquid being measured emits condensable vapors. For this reason, non-contact radar instruments are often separated from the vessel interior by means of a dielectric window (made of some substance that is relatively “transparent” to electromagnetic waves yet acts as an effective vapor barrier):

Electromagnetic waves travel at the speed of light, 2.9979 × 108 meters per second in a perfect vacuum. The velocity of an electromagnetic wave through space depends on the dielectric permittivity (symbolized by the Greek letter “epsilon,” ǫ) of that space. A formula relating wave velocity to relative permittivity (the ratio of a substance’s permittivity to that of a perfect vacuum, symbolized as ǫr and sometimes called the dielectric constant of the substance) and the speed of light in a perfect vacuum (c) is shown here:

As mentioned previously, the calibration of an echo-based level transmitter depends on knowing the speed of wave propagation through the medium separating the instrument from the process fluid interface. For radar transmitters sensing a single liquid below a gas or vapor, this speed is the speed of light through that gas or vapor space, which we know to be a function of electrical permittivity.

The relative permittivity of air at standard pressure and temperature is very nearly unity (1). This means the speed of light in the air under atmospheric pressure and ambient temperature will very nearly be the same as it is for a perfect vacuum (2.9979 × 108 meters per second). If, however, the vapor space above the liquid is not ambient air, and is subject to large changes in temperature and/or pressure, the permittivity of that vapor may substantially change and consequently skew the speed of light, and therefore the calibration of the level instrument. This calibration shift is sometimes referred to as the gas phase effect.

The permittivity of any gas is related to both pressure and temperature by the following formula:

From this equation, we can see that the permittivity of a gas increases with increasing pressure and decreases with increasing temperature. This means the speed of light through a gas decrease with increasing pressure and increases with increasing temperature. For radar level instruments operating in gas environments subject to significant pressure and temperature variations, the consequent variations in the speed of light through that gas will compromise the instrument’s accuracy.

With ultrasonic level instruments, the necessary condition for an echo to occur is that the sound wave encounters a sudden change in material density. With radar level instruments, the necessary condition for wave reflection is a sudden change in dielectric permittivity (ǫ). When an electromagnetic wave encounters a sudden change in dielectric permittivity, some of that wave’s energy will be reflected in the form of another wave traveling the opposite direction, while the balance of the wave’s energy continues forward to propagate into the new material. The strength of the reflected signal depends on how greatly the two materials’ permittivities differ:

This same principle explains reflected signals in copper transmission lines as well. Any discontinuities (sudden changes in characteristic impedance) along the length of a transmission line will reflect a portion of the electrical signal’s power back to the source. In a transmission line, continuities may be formed by pinches, breaks, or short circuits. In a radar level measurement system, any sudden change in electrical permittivity is a discontinuity that reflects some of the incident wave energy back to the source. Thus, radar level instruments function best when there is a large difference in permittivity between the two substances at the interface. As shown in the previous illustration, air and water meet this criterion, having an 80:1 permittivity ratio.

The ratio of reflected power to incident (transmitted) power at any interface of materials is called the power reflection factor (R). This may be expressed as a unitless ratio, or more often as a decibel figure. The relationship between dielectric permittivity and reflection factor is as follows:

Please check the next post for the rest of this topic.

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