A Dozen Ways to Measure
Fluid Level
and How They Work
The more you know about fluid level sensors—from sight glasses to guided-wave radar to lasers—the happier you will be with the technology you choose for your own application.
Kevin Hambrice and Henry Hopper
K-TEK Corp.
The demands of sophisticated automated processing systems, the need for ever-tighter process control, and an increasingly stringent regulatory environment drive process engineers to seek more precise and reliable level measurement systems. Improved level measurement accuracy makes it possible to reduce chemical-process variability, resulting in higher product quality, reduced cost, and less waste. Regulations, especially those governing electronic records, set stringent requirements for accuracy, reliability, and electronic reporting. The newer level measurement technologies help meet these requirements.
Level Measurement Technology in Transition
The simplest and oldest industrial level measuring device is, of course, the sight glass. A manual approach to measurement, sight glasses have always had a number of limitations. The material used for its transparency can suffer catastrophic failure, with ensuing environmental insult, hazardous conditions for personnel, and/or fire and explosion. Seals are prone to leak, and buildup, if present, obscures the visible level. It can be stated without reservation that conventional sight glasses are the weakest link of any installation. They are therefore being rapidly replaced by more advanced technologies.
Other level-detection devices include those based on specific gravity, the physical property most commonly used to sense the level surface. A simple float having a specific gravity between those of the process fluid and the headspace vapor will float at the surface, accurately following its rises and falls. Hydrostatic head measurements have also been widely used to infer level.
When more complex physical principles are involved, emerging technologies often use computers to perform the calculations. This requires sending data in a machine-readable format from the sensor to the control or monitoring system. Useful transducer output signal formats for computer automation are
current loops, analog voltages, and digital signals. Analog voltages are simple to set up and deal with, but may have serious noise and
interference issues. 4-20 mA current loops (where the loop current varies with the level measurement) are the most common output mechanism today. Current loops can carry signals over longer distances with less degradation. Digital signals coded in any of a number of protocols (e.g., Foundation Fieldbus, Hart, Honeywell DE, Profibus, and RS-232) are the most robust, but the older technologies such as RS-232 can handle only limited distances. New wireless capabilities can be found in the latest transmitters' signals, allowing them to be sent over tremendous distances with virtually no degradation.
As for the more advanced measurement technologies (e.g., ultrasonic, radar, and laser), the more sophisticated digital encoding formats require digital computer intelligence to format the codes. Combining this requirement with the need for advanced communication capabilities and digital calibration schemes explains the trend toward embedding microprocessor-based computers in virtually all level measurement products (see Figure 1).
Figure 1. Level measurement determines the position of the level relative to the top or bottom of the process
fluid storage vessel. A variety of technologies can be used, determined by the characteristics of the fluid and its
process conditions.
Established Level-Sensing Technologies
Throughout this article we will assume the density of the vapor in the headspace (typically air) to be negligible compared with that of the process fluid. We will assume also that there is only one, uniform process fluid in the tank. Some of these technologies can be used for multilevel applications where two or more immiscible fluids share a vessel.
Floats. Floats work on the simple principle of placing 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 read
out its position. The float sinks to the bottom of the headspace vapor and floats on top of the process fluid. While the float itself is a basic solution to the problem of locating a liquid's surface, reading a float's position (i.e., making an actual level measurement) is still problematic. Early float systems used
mechanical components such as cables, tapes, pulleys, and gears to communicate level. Magnet-equipped floats are popular today.
Early float level transmitters provided a simulated analog or discrete level measurement using a network of resistors and multiple reed switches, meaning that the transmitter's output changes in discrete steps. Unlike continuous level-measuring devices, they cannot discriminate level values between steps.
Hydrostatic Devices. Displacers,
bubblers, and differential-pressure transmitters are all hydrostatic measurement devices. Any change in
temperature will therefore cause a shift in the liquid's specific gravity, as will changes
in pressure that affect the specific gravity of the vapor over the liquid. Both result in reduced measurement accuracy.
Displacers work on Archimedes' principle.
As shown in Figure 2, a column of solid
material (the displacer) is suspended in the vessel. The displacer's density is always greater than that of the process fluid (it will sink in the process fluid), and it must extend from the lowest level required to at least the highest level to be measured. As the process fluid level rises, the column displaces a volume of fluid equal to the column's cross-sectional area multiplied by the process fluid level on the displacer. A buoyant
force equal to
Figure 2. Displacement level gauges operate on Archimedes’ principle. The force needed to support a column of material (displacer) decreases
the weight of the process fluid displaced. A force transducer measures
e support force and reports it as an analog signal.
by th  this displaced volume multiplied by the process fluid density pushes upward on the displacer, reducing the force
needed to support it against the pull of gravity. The transducer, which is
linked to the transmitter, monitors and relates this change in force to level.
A bubbler-type level sensor is shown in Figure 3. This technology is used in
vessels that operate under atmospheric pressure. A dip tube having its open end near the vessel bottom carries a purge gas (typically air, although an inert gas such as dry nitrogen may be used when there is danger Fi pr  gure 3. Bubblers sense process fluid depth by measuring the hydrostatic essure near the bottom of the storage vessel.
pulleysof contamination of or an oxidative reaction with the process fluid) into the tank. As gas flows down to the dip tube's outlet, the pressure in the tube rises until it overcomes the hydrostatic pressure produced by the liquid level at the outlet. That 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 differential pressure (DP) level
sensor is shown in Figure 4. The essential measurement is the difference between total pressure at
the bottom of the tank (hydrostatic head pressure of the fluid plus static
pressure in the vessel) and the static or head pressure in the vessel. As with the bubbler, the hydrostatic pressure
difference equals the process fluid
density multiplied by the height of fluid in the vessel. The unit in Figure 4 uses atmospheric pressure as a reference. A vent at the top keeps the headspace pressure equal to atmospheric pressure.
Figure 4. Differential pressure sensors monitor the process fluid level by measuring e total pressure difference between the fluid at the bottom of the tank and the ssel pressure.  th ve
In contrast to bubblers, DP sensors can be used 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 may still be required, depending on the process's physical conditions and/or the transmitter's location relative to the process connections.
Load Cells. A load cell or strain gauge device is essentially a mechanical support member or bracket equipped with one or more sensors that detect small distortions in the support member. As the force on the load cell changes, 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. Knowing the vessel's geometry (specifically, its cross-sectional area) and the fluid's specific gravity, it is a simple matter to convert the load cell's known output into the fluid level.
While load cells are advantageous in many applications because of their noncontact nature, they are e
xpensive and the vessel support structure and connecting piping must be designed around the load cell's requirements of a floating substructure. The total weight of the vessel, piping, and connecting structure supported by the vessel will be weighed by the load cell system in addition to the desired net or product weight. This total weight often creates 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 reflected as level, as can side load, wind load, rigid piping, and binding from overturn-prevention hardware
(for
bottom-mounted load cells). In short,
load cell weighing system requirements must be a paramount
consideration throughout initial vessel support and piping design, or performance is quickly degraded.
Magnetic Level Gauges. These
gauges (see Figure 5) are the preferred replacement for sight glasses. They are similar to float devices, but they communicate the liquid surface location magnetically. The float, carrying a set of strong
permanent magnets, 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 always close to the chamber's side wall. As the float rides up and down with the fluid level, a magnetized shuttle or bar graph indication moves with it, showing the position of the float and thereby providing the level indication. The system can work only if the auxiliary column and chamber walls are made of nonmagnetic material.
Fi floa  gure 5. Magnetic level gauges use a magnetically coupled shuttle to locate a
t’s position inside the chamber.  Many manufacturers provide float designs optimized for the specific gravity of the fluid being measured, whether butane, propane, oil, acid, water, or interfaces between two fluids, as well as a large selection of float materials. This means the gauges can handle high temperatures, high pressures, and corrosive fluids. Oversized float chambers and high-buoyancy floats are available for applications where buildup is anticipated.
Chambers, flanges, and process connections can be made from engineered plastics such as Kynar or exotic alloys such as Hastelloy C-276. Special chamber configurations can handle extreme conditions such as steam
jacketing for liquid asphalt, oversized chambers for flashing applications, and cryogenic temperature designs for liquid nitrogen and refrigerants. Numerous metals and alloys such as titanium, Incoloy, and Monel are available for varying combinations of high-temperature, high-pressure, low-specific-gravity, and corrosive-fluid applications. Today's magnetic level gauges can also be outfitted with magnetostrictive and guided-wave radar transmitters to allow the gauge's local indication to be converted into 4-20 mA outputs that can be sent to a controller or control system.

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