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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.