Drive process engineers require more reliable and precise level measurement systems because of the needs of sophisticated automated processing systems, an increasingly stringent regulatory environment and the need for ever-tighter process control.
Better level measurement accuracy makes it achievable to reduce chemical-process variability, resulting in reduced cost, less waste, and higher product quality. Regulations, particularly those concerning electronic records, dictate strict requirements for electronic reporting, accuracy, and reliability. More modern level measurement technologies assist in meeting these requirements.
Level Measurement Technology in Transition
The sight glass is the oldest and simplest industrial level measurement device. Sight glasses have always had restrictions, because they are a manual solution to measurement. The material employed for its transparency can suffer catastrophic failure, with hazardous conditions for users, fire and explosion and harm to the environment.
Seals often leak, and if buildup is present, this obscures the visible level. Conventional sight glasses are the weakest link of any installation. They are consequently being quickly replaced by modern technologies.
Some other level-detection devices are based on specific gravity, the physical attribute most often used to sense the level surface. A simple float having a specific gravity between those of the headspace vapor and the process fluid floats at the surface, precisely following its ups and downs. Hydrostatic head measurements are also widely used to understand level.
The oldest and simplest industrial signal communication is 4-20 mA current loops (where the loop current deviates with the level measurement) the most widely used output mechanism of recent times.
Current loops can carry signals further and with less degradation. Digital signals coded in any of a number of protocols (e.g. Hart, Honeywell DE, Foundation Fieldbus, RS-232, and Profibus) are the most robust, but the older technologies like RS-232 can only handle short distances. New wireless proficiency can be established in the latest transmitters’ signals, which allows them to be sent over immense distances with hardly any degradation.
Emerging technologies generally use computers to execute the calculations when more complex physical principles are involved. This requires sending data from the sensor to the control or monitoring system in a machine-readable format.
Useful transducer output signal formats for computer automation are digital signals, current loops, and analog voltages. Analog voltages are easy to setup and deal with, but may have serious interference and noise issues.
As for the more advanced measurement technologies (e.g., laser, ultrasonic, and radar), digital computer intelligence to is needed to format the codes for the more sophisticated digital encoding formats.
Combining this requirement with the need for digital calibration schemes and advanced communication capabilities explains the trend for embedding microprocessor-based computers in almost all level measurement products (see Figure 1).
Established Level-Sensing Technologies
We assume the density of the vapor in the headspace (typically air) to be negligible compared with that of the process fluid throughout this article. We will also assume the tank contains only one, uniform, process fluid. Some of these technologies can be used for multilevel applications where multiple immiscible fluids share a vessel.
1. Floats. Floats operate on the simple principle of positioning a buoyant object with a specific gravity intermediate between those of the process fluid and the headspace vapor into the tank, then attaching a mechanical device to record its position.
The float sits on top of the process fluid and sinks to the bottom of the headspace vapor. The float itself is a basic solution to the problem of locating a liquid’s surface, but calculating a floats position (i.e., making an actual level measurement) is still difficult. Magnet-equipped floats are popular today.
Early float systems used mechanical components such as gears, cables, tapes, and pulleys to show level. Early float level transmitters gave a simulated discrete or analog level measurement utilizing multiple reed switches and a network of resistors, causing the transmitter’s output to change in discrete steps. They cannot discriminate level values between steps, unlike continuous level-measuring devices.
2. Glass Level Gauge. Glass gauges have been used for over 200 years as a simple method to measure liquid level. Available in a variety of designs, both unprotected and armored. The down side is the risk of glass breakage resulting in spills or harm to personnel. The benefit of this design is the ability to observe the true level through the clear glass.
3. Differential Pressure Transmitters, 4. Bubblers and 5. Displacers are all hydrostatic measurement devices. Any shift in temperature will consequently cause a change in the liquid’s specific gravity, as will pressure changes that affect the specific gravity of the vapor over the liquid. Both result in less accurate measurements.
A differential pressure (DP) level sensor is shown in Figure 2. The essential measurement is the difference between the static or head pressure in the vessel and the total pressure at the bottom of the tank (hydrostatic head pressure of the fluid plus static pressure in the vessel).
The unit in Figure 2 uses atmospheric pressure as a reference. A vent at the top keeps headspace pressure equal to the atmospheric pressure. The hydrostatic pressure difference equals the height of fluid in the vessel multiplied by the process fluid density.
DP sensors can be utilized in unvented (pressurized) vessels. All that is required is to connect the reference port (the low-pressure side) to a port in the vessel above the maximum fill level. Liquid purges or bubblers might still be needed, depending on the transmitter’s location relative to the process connections and/or the process’s physical conditions.
Figure 3 shows a bubbler-type level sensor. This technology is employed in vessels that operate under atmospheric pressure.
A dip tube with its open end near the vessel open, carries a purge gas (typically air, although an inert gas such as dry nitrogen can be used when there is risk of an oxidative reaction with the process fluid or contamination) into the tank.
The pressure in the tube rises until it overcomes the hydrostatic pressure generated by the liquid level at the outlet, as gas flows down to the dip’s tube outlet. The pressure equals the process fluid’s density multiplied by its depth from the end of the dip tube to the surface and is monitored by a pressure transducer connected to the tube.
A column of solid material (the displacer) is suspended in the vessel, as shown in Figure 4. Displacers work on the Archmedes’ principle. The displacer must extend from the lowest level required to at least the highest level to be measured and the displacers density is always greater than that of the process fluid (it will sink in the process fluid).
The column displaces a volume of fluid equal to the column’s cross-sectional area multiplied by the process fluid level in the displacer as the process fluid level gets higher. A buoyant force equivalent to this displaced volume multiplied by the process fluid density pushes upward on the displacer, reducing the force needed to support it against gravitys pull. The transducer, which is linked to the transmitter, observes and relates this change in force to level.
6. Magnetic Level Gauges. These are similar to float devices, but they communicate the liquid surface location magnetically. They (see Figure 5) are the preferred replacement for the sight glasses.
Carrying a set of strong permanent magnets, the float rides in an auxiliary column (float chamber) attached to the vessel by means of two process connections. This column confines the float laterally so that it is consistently close to the chamber’s side wall.
A magnetized shuttle or bar graph indication moves with the float as it rides up and down the fluid level, showing the position of the float and thereby providing the level indication. The chamber walls and auxiliary column are constructed of material that is non-magnetic for the system to work.
Many manufacturers produce float designs optimized for the specific gravity of the fluid being measured, whether acid, water, propane, oil, butane, or interfaces between two fluids, made from a choice of float materials.
This means the gauges can handle corrosives fluids, high temperatures and high pressures. High-buoyancy floats and oversized float chambers and are available for applications where buildup is expected.
Flanges, process connections and chambers and can be made from exotic alloys such as Hastelloy C-276 or engineered plastics such as Kynar. Special chamber configurations can handle extreme conditions such as:
- temperature designs for liquid nitrogen and refrigerants
- steam jacketing for liquid asphalt
- oversized chambers for flashing applications
Many alloys and metals such as Monel, titanium and Incoloy can be used for varying combinations of corrosive-fluid applications, high-temperature, low-specific-gravity and high-pressure.
Modern magnetic level gauges can also be equipped with guided-wave and magnetostrictive radar transmitters so that the gauge’s local indication can be converted into digital communication and 4-20 mA outputs that can be sent to a control system or controller.
7. Capacitance Transmitters. These devices (see Figure 6) operate on the fact that process fluids usually have dielectric constants, ᶓ, significantly different from that of air, which is very close to 1.0. Oils have dielectric constants from 1.8 to 5. Pure glycol is 37; aqueous solutions are between 50 and 80.
This technology requires an alteration in capacitance that changes with the liquid level, created by either an un-insulated rod attached to the transmitter or an insulated rod attached to the transmitter and the process fluid, and either a reference probe or the vessel wall.
The overall capacitance rises proportionately as the fluid level rises and fills more of the space between the plates. An electronic circuit called a capacitance bridge measures the overall capacitance and gives a continuous level measurement.
8. Load Cells. A load cell or strain gauge device is basically a bracket or mechanical support equipped with one or more sensors that identify small changes in the support member. As the force on the load cell alters, the bracket flexes slightly, causing output signal changes. Calibrated load cells have been made with force capacities ranging from fractional ounces to tons.
To measure level, the load cell must be incorporated into the vessel’s support structure. As process fluid fills the vessel, the force on the load cell increases. Converting the load cells known output into the fluid level is simple when you know the fluid’s specific gravity and the vessel’s geometry (specifically, it’s cross-sectional area).
Due to their noncontact nature, load cells are advantageous in many applications, but they are expensive and the vessel support structure and connecting piping must be designed to fit the load cell’s requirements of a floating substructure.
The total weight of the vessel, piping, connecting structure and supported by the vessel will be weighed by the load system in addition to the desired net or product weight. This total weight regularly gives a very poor turndown to the net weight, meaning that the net weight is a very small percentage of the total weight.
Finally, the supporting structure’s growth, caused by uneven heating (e.g., morning to evening sunshine) may be shown as level, as can wind load, side load, rigid piping and binding from overturn-prevention hardware (for bottom-mounted load cells). So, load cell weighing system requirements must be the primary consideration throughout piping design and initial vessel support, or performance is lost.
Probably the biggest difference between earlier continuous liquid-level measuring technologies and those now gaining favor is the use of time-of-flight (TOF) measurements to transducer the liquid level into a conventional output.
These devices generally operate by measuring the distance between a reference point at a sensor or transmitter near the top of the vessel and the liquid level. The system normally produces a pulse wave at the reference point, which travels through either the conductor or vapor space, reflects off the liquid surface, and returns to a pickup at the reference point. An electronic timing circuit calculates the total travel time.
The distance to the surface of the fluid is calculated by dividing the travel time by twice the wave’s speed gives. The technologies differ mainly in the kind of pulse used to make the measurement. Light, ultrasound, and microwaves (radar), are all useful.
9. Magnetostrictive Level Transmitters. The advantages of using a magnet containing a float to determine liquid level have already been established, and magnetostriction is a proven technology for reading the float’s location very precisely. Magnetostrictive transmitters use the speed of a torsional wave along a wire, instead of mechanical links, to find the float and report its position.
The float carries a series of permanent magnets in a magnetostrictive system (see Figure 7). A tension fixture is attached to the opposite end of the sensor tube and a sensor wire is connected to a piezoceramic sensor at the transmitter. The tube is either adjacent to the float outside of a nonmagnetic float chamber or runs through a hole in the center of the float.
To find the float, the transmitter dispatches a short current pulse down the sensor wire, setting up a magnetic field along its entire length. A timing circuit is triggered ON at the same time. The field interacts immediately with the field generated by the magnets in the float.
The overall effect is that a torsional force is produced in the wire, much like an ultrasonic vibration or wave, throughout the time the current flows. This force travels back to the piezoceramic sensor at a characteristic speed.
The sensor produces an electrical signal that notifies the timing circuit that the wave has arrived and stops the timing circuit when the tensional wave is detected.
The timing circuit measures the time interval (TOF) between the wave’s arrival and the start of the current pulse. From this information, the float’s location is determined very precisely and shown as a level signal by the transmitter.
Advantages of this technology are that the signal is not affected by foam, beam divergence, or false echoes, and the signal speed is known and constant with process variables such as pressure and temperature. The only moving part is the float that rides up and down with the fluid’s surface which is another benefit is that.
10. Ultrasonic Level Transmitters. Ultrasonic level sensors (see Figure 8) measure the distance between the transducer and the surface, using the time required for an ultrasound pulse to travel from a transducer to the fluid surface and back (TOF).
These sensors utilize frequencies in the tens of kilohertz range; transit times are ~6 ms/m. The speed of sound (340 m/s in air at 15 degrees C, 1115 fps at 60 degrees F) depends on the blend of gases in the headspace and their temperature. While the sensor temperature is compensated for (if the sensor is at the same temperature as the air in the headspace), this technology is limited to atmospheric pressure measurements in air or nitrogen.
11. Laser Level Transmitters. Designed for slurries, bulk solids, and opaque liquids such as liquid styrene, dirty sumps, and milk, the lasers run on a principle like that of ultrasonic level sensors. Instead of using the speed of sound to detect the level, however, they use the speed of light (see Figure 9).
A laser transmitter at the top of a vessel fires a short pulse of light down to the process liquid surface, which reflects it back to the detector. A timing circuit calculates the elapsed time (TOF) and measures the distance. The key is that lasers have no false echoes and virtually no beam spread (0.2 degree beam divergence). They are precise, even in vapor and foam and can be directed through space as small as 2 in.2.
Lasers are ideal for use in vessels with obstructions and can measure distances up to 1500 ft. For high-pressure or high-temperature applications, such as in reactor vessels, lasers must be used in conjunction with specialized sight windows to isolate the transmitter from the process. These glass windows pass the laser beam with minimal diffusion and attenuation.
12. Radar Level Transmitters. Through-air radar systems beam microwaves downward from either a horn or a rod antenna at the top of a vessel. The signal reflects off the fluid surface back to the antenna, and a timing circuit calculates the distance to the fluid level by measuring the round-trip time (TOP).
The dielectric contact of liquid is the key variable in radar technology because the amount of reflected energy at microwave (radar) frequencies is dependent on the dielectric constant of the fluid, and if Er is low, most of the radar’s energy enters or passes through. Water (Er=80) produces an excellent reflection at the change or discontinuity in Er.
Guided wave radar (GWR) transmitters (see Figure 10) are also very reliable and accurate. A rigid probe or flexible cable antenna system guides the microwave down from the top of the tank to the liquid level and back to the transmitter. As with through-air radar, a change from a lower to a higher Er causes the reflection.
The guide produces a more focused energy path which means that guided wave radar is 20X more efficient than through-air radar. Different antenna configurations allow measurement down to ER=1.4 and lower.
These systems can be installed either vertically, or in some cases horizontally with the guide being bent up to 90 degree angled, and still provide a concise measurement signal.
Radar’s wave speed is largely unaffected by vapor space gas composition, temperature, or pressure. GWR exhibits most of the advantages and few of the liabilities of ultrasound, laser, and open-air radar systems.
It works in a vacuum with no recalibration needed, and can measure through most foam layers. Confining the wave to follow a probe or cable eliminates beam-spread problems and false echoes from tank walls and structures.
General trends across different measurement technologies reflect market drivers. Refined digital electronics are making level sensors and other measurement devices more user-friendly, easier to set up, less expensive, and more reliable. Improved communication interfaces feed level measurement date into a company’s existing control and/or information system.
Today’s level sensors incorporate an increasing variety of materials and alloys to combat harsh environments such as oils, acids, and extremes of temperature and pressure. Modern materials help process instruments fulfill specialized requirements as well, such as assemblies made of PTFE jacketed material for corrosive applications and electro polished 316 stainless steel for cleanliness requirements. Probes made of these new materials allow contact transmitters to be used in virtually any application.
Figure 1: Level Measurement defines the position of the level relative to the top or bottom of the process fluid storage vessel. A variety of technologies can be utilized, depending on the characteristics of the fluid and its process conditions.
Figure 2: Differential pressure sensors monitor for the process fluid level by measuring the total pressure difference between the fluid at the bottom of the tank and the vessel pressure.
Figure 3: Bubblers sense process fluid depth by measuring the hydrostatic pressure near the bottom of the storage vessel.
Figure 4: Displacement level gauges operate on Archimedes’ principle. The force needed to support a column of material (displacer) decreases by the weight of the process fluid displaced. A force transducer measures the support force and reports it as analog signal.
Figure 5: Magnetic level gauges use a magnetically coupled shuttle to locate a float’s position in the chamber.
Figure 6: Capacitive level sensors measure the change in capacitance between two plates produced by changes in level. Two versions are available, one for fluids with high dielectric constants (A) and another for those with low dielectric constants (B).
Figure 7: Magnetostrictive level transmitters use the speed of a torsional wave in a wire to produce a level measurement
Figure 8: Ultrasonic level transmitters use the speed of sound to calculate level
Figure 9: A laser transmitter uses a short burst of laser energy to measure level.
Figure 10: Guided wave radar uses a waveguide to conduct microwave energy and from the fluid surface.
This information has been sourced, reviewed and adapted from materials provided by ABB Measurement & Analytics.
For more information on this source, please visit ABB Measurement & Analytics.