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The idea behind level measurement is nothing more than being able to determine the location of the interface between a liquid and a gas or between a solid and a gas. The units of measure are typically a linear percentage range from 0% to 100%, where 0% implies empty (or minimum level) and 100% means full (or maximum level). In some cases, the measurement is converted into units of volume or weight.
The big picture
The first thing to do when you need a level-measuring device is to assess the environment in which the hardware will operate and determine the range you need to measure (e.g., 0 ft. to 100 ft.). Not all level measuring devices provide the same capabilities. Evaluate sensor accuracy, response time and durability for the process. Select appropriate material of construction for wetted parts, which are the components in contact with process materials.
Next, decide on the output signal, choosing between analog (4 - 20 mADC, 3 - 15 psig) and discrete (ON or OFF at 120 VAC or 24 VDC). Fieldbus and similar network protocols allow accurate and reliable digital data transfer for both signal and diagnostics between devices and may be used as the “output signal.” Also, ensure that the equipment will withstand any fluctuations in the output from the available power supply.
Select an enclosure that will withstand the hazardous areas, dusty locations and wet surroundings in the environment. If you foresee low ambient temperatures, winterize the enclosure with steam or electric tracing while assessing the effects of heating failure on device performance. In high-temperature applications, protect the electronics from the process using remotely mounted electronics to keep things cool.
For ease of maintenance, both the sensing element and the transducer should be accessible from grade or a platform. Determine if the device can be removed for service while the process is in operation. Consider availability of service and support that maintains equipment functionality. Assess carefully the vendor support, difficulty and frequency of calibration (on site or at vendor's facility), capabilities of in-house maintenance staff and the need for training.
Each type of level sensor has advantages and disadvantages. Your selection depends on a good understanding of the unit’s suitability for the anticipated process conditions.
The easiest way
A gage, also known as “sight glass,” operates on the U-tube principle (Figure 1). It’s used commonly as a local indicator for open or pressurized vessels. The three gage types are glass, reflex and magnetic. The glass type, used only on safe applications, consists of the fluid level to be measured held in a vertical housing between two glass strips. This type shouldn’t be used with hazardous liquids.
The reflex type, used for low- and medium-pressure applications, has a single glass with cut prisms. In the space above the liquid level, vapor refracts light and appears as a light color. Liquid absorbs light and appears as a dark color.
The magnetic gages, used for high-pressure applications or toxic fluids, have a float inside a nonmagnetic chamber and a vertical series of metal wafers on the outside. The wafers have different colors on opposite sides. The float supports a magnet that rotates the wafers in response to level changes.
Isolating valves facilitate gages maintenance. This type of level indicator is low-cost, accurate and easy to install. Except for the magnetic type, it’s not suitable for remote indication or for dark, dirty liquids.
The DP type
The differential pressure type, also known as “hydrostatic” or “pressure head,” is based on the liquid height and the pressure the liquid produces (Figure 2). To facilitate maintenance, differential pressure instruments are typically isolated from the process by a shutoff valve. These devices are easy to install, simple, accurate and have a wide range of measurement. Their calibration is simple and requires no special tools. However, differential pressure devices are sensitive to changes in liquid density, which affect the liquid head. Parts of the instrument, as well as the connecting process tubing, are exposed to the process fluid. If this is intolerable, use diaphragm seals filled with a fluid that’s compatible with the process fluid.
A third type of level device is the bubbler level sensor (Figure 3). It consists of four components. A rotameter provides constant airflow, a pressure regulator fosters smooth operation, a dip tube (or pipe), and a pressure gauge or a differential-pressure transmitter mounted at the top of the tube to measure air pressure, which is converted to liquid head. The tube extends to about 3 in. from the bottom and is typically notched to produce small air bubbles. Often, a tee connection at the top of the dip tube facilitates rodding because fluid residue or dirt can plug the tube. Bubblers are easy to maintain, have a low cost and operate without electrical power. However, density variations affect the bubbler’s readout.
The sound of level
Sonic and ultrasonic units use echoes to locate the interface (Figure 4). The main components are a transmitter and a receiver. A sonic transmitter emits 10 kHz pulses and ultrasonic units operate at 20 kHz or more. After each pulse, the receiver detects the pulse reflected off the liquid surface. The pulse’s transit time is converted into a distance -- the liquid level. These devices are noncontacting, reliable, cost-effective and accurate. They have no moving parts and are unaffected by changes in liquid density. However, strong mechanical vibration close to the unit’s operating frequency and airborne dust degrade signal accuracy. In addition, process material that deposits on the sensor may attenuate the signal and affect performance. Vapor concentration, humidity and foreign gases and vapors alter the speed of sound and thus affect the instrument’s accuracy.
The divergence of a typical transmitter beam is 8° to 15° (compared to 0.3° for a laser transmitter). This produces an increasingly large sonic footprint as the distance increases. Therefore, avoid placing braces, stiffeners or other cross-members in the path of the beam.
The concept behind radar-based level measuring devices is similar to that of sonic and ultrasonic sensors (Figure 5). Instead of high-frequency sound, radar units use a 24-GHz electromagnetic pulse. Because radar pulses won’t penetrate metal, the transducer can sometimes be mounted outside the vessel if it’s provided with a plastic or glass window. This “see through” capability provides reliable noncontact measurement. Radar sensors are expensive and spurious reflections from metal objects produce interference, but the 24-GHz signal provides a relatively narrow bandwidth compared to that of sonic and ultrasonic devices.
The two types of laser-based level sensors are continuous wave and pulsed-type (Figure 6). The transmitter in a continuous wave laser sensor directs a laser beam at a target constantly. The phase of the reflected beam is shifted and the degree of phase shift corresponds to the distance to the target and, therefore, the level.
The transmitter in a pulsed-type unit directs a series of pulses at a target. The transit time to the liquid surface and back is converted into distance, i.e., level. The laser beam’s large range and ability to penetrate vapors and dust make the pulsed-type sensor common in industrial applications.
Laser sensors can measure through a sight glass, so they can be mounted outside a metal vessel and accessed without having to interrupt the process. They provide accuracy and are used in noncontact measurement for difficult applications. However, while they’re relatively expensive, they’re more economical than a radioactive level sensor.
Radioactive level measurement requires an isotope and a detector (Figure 7). The detector senses the gamma radiation the source sent through the vessel walls. When the vessel is empty, the count rate is high. As the vessel fills, the count rate diminishes. Being external to the vessel, this type of device can be added or removed without disturbing the process. It’s highly reliable, has no moving parts and is unaffected by temperature, pressure or corrosion. However, it requires special licensing for the application and extreme care when locating and installing the radioactive source. It’s expensive to install and operate and is difficult to calibrate. Radioactive level measurement is typically applied when other types of measurement can’t be used.
Capacitance level sensors measure the change in that electrical property as the level in a tank varies (Figure 8). If the material to be measured is nonconductive, the probe and the vessel wall constitute two conductive plates separated by an insulator (the material being measured). A dry, nonconductive, solid material can have moisture content high enough for it to become conductive. Measuring any conductive material by means of a capacitance level sensor requires an insulated probe. This insulation serves as the dielectric and the process material as one of the plates. When installing insulated probes, take special care not to damage its coating. Capacitance level sensors are simple in design, have no moving parts, are easy to install, can be used for pressurized or unpressurized vessels, and can be used for conductive or nonconductive fluids. If the fluid’s dielectric constant can change, select a capacitance unit that compensates for the variation. Calibrating these sensors may be time consuming.
Other less-common types of level measuring devices exist. A displacement unit has a displacer that is restricted from moving freely in the liquid and transmits its change in buoyancy (mechanical force) to a transducer through a torque-tube unit. This type of level device should be used only for liquids having a fixed specific gravity. It is relatively difficult to calibrate and has numerous mechanical components. Beware if the process material can crystallize or coat the displacer.
A float-type unit features a hollow ball bobbing on the liquid surface. The float is connected to an arm that operates a microswitch (or a pointer and scale). The float’s position is a direct indication of level. Its construction typically limits the float’s effective travel to 30° from the horizontal. Floats are used for clean services only and the liquids must have a constant specific gravity. Accumulated buildup on the float affects performance.
Thermal sensors have a heating element mounted next to a temperature switch. Liquid rising above the switch dissipates the heat and the temperature switch changes state. This sensor can’t be used if heating affects product quality. It’s sensitive to coating or caking and provides point measurement only. However, it’s reasonably priced, has no moving parts and has a simple and reliable design.
Another point measurement device uses a vibrating tuning fork. Contact with a material alters the frequency and switches a relay. When the level drops below the fork, the vibrating frequency increases and the relay reverses. The switch shouldn’t be used in vibrating bins, especially if the two frequencies are close and one frequency can’t be changed readily.
Finally, a diaphragm point measurement device consists of a diaphragm connected to a switch. Hydrostatic pressure pushing on the diaphragm flips a switch. Diaphragm switches are affected by coatings. Changes in specific gravity affect the unit’s accuracy.
When selecting a level measuring device, first understand the application well, then assess your options and select the unit that will do the job at the lowest installed cost.
N. (Bill) Battikha, P.Eng., is principal at BergoTech Inc. Contact him at [email protected] and (416) 456-7533.