People have been using various devices to measure level since about 3000 BC. The first historical references to a measuring device were dipsticks or staff gauges used to indicate the amount of water stored in reservoirs for farming. Next came crude float-type devices to indicate the height of drinking water reserves in underground cisterns or storage caverns.
Modern instrumentation uses more than 20 different technologies to satisfy the ever-growing requirements for level measurement in a wide variety of applications. Some instruments measure level directly, while others measure some other variable that is related to level. Direct-measurement instruments include sight glasses, dipsticks and floats. Indirect-measurement devices include differential pressure, sonic, nuclear and radio frequency (RF).
Point vs. continuous
Level measurement instrumentation can be divided into two major types, point level and continuous level devices.
Point level (on/off) measurement devices indicate the absence or presence of a material at a certain point within a vessel. Point level switches are used as high-level and spill prevention alarms and low-level and pump protection alarms, as well as to turn pumps on and off.
Continuous level (proportional) measurement devices indicate the level in a vessel over the full span of measurement. These devices typically are used for process control, inventory control and management.
Technologies used to measure both continuous or point level are affected differently by varying process conditions. Factors such as pressure, temperature, agitation, corrosion, foaming and explosion hazards need to be considered before device selection. Other conditions that affect level measurement are a material's density, chemical composition, buildup and electrical properties.
RF Capacitance Device
Level measurement technologies
Many technologies currently are in use.
Bubblers (continuous). Perhaps the simplest devices currently available, bubblers measure backpressure by forcing gas (air) down into an immersed vertical pipe until gas bubbles emerge. That pressure is related to the change in the height of the liquid in the vessel, as well as density.
Bubblers ignore foam and provide excellent performance in continuous level measurement of thin fluids, both conductive and insulating. However, they require a high degree of maintenance and need a clean gas supply.
Capacitance/RF devices (point level or continuous). RF admittance is by far the most versatile level measurement technology, good for a wide range of conditions, from cryogenics to high-temperature and from vacuum to 10,000-pounds-per-square-inch (psi) pressure. It works with virtually all types of materials.
RF current from constant voltage applied to one electrode measures the impedance change between two capacitor electrodes (often a single electrode and the vessel wall, or the material itself and the insulating coating on the sensor), based on rise and fall of material in vessel. A second sensor to monitor dielectric can provide compensation.
RF devices feature excellent performance with thin fluids and for measuring interfaces, and perform fairly well to excellent in thick fluids and slurries. They can ignore organic foam, but can measure both aqueous and organic foams.
Switches generally are very good for powdery, chunky or sticky solids, but continuous measurement versions are more limited in this area. Continuous measurement versions also cannot measure the interface between conductive layers or between liquids and solids. With point level switches, conductive coatings can produce a false high without a guard-type probe, and short insertions can be a problem.
Conductivity switches. This technology recognizes that the resistance between two probes, or a probe and vessel wall, is significantly higher in air than in conductive liquid. When conductive liquid is brought into contact with the probe, the drop in resistance being measured between the two will change the output status.
Conductivity switches work very well with conductive thin fluids and perform fairly well in detecting interface. However, they are of limited use for thick fluids and slurries or solids. They can detect conductive process materials, but thin insulating coatings produce false lows while conductive thick fluids can show false highs. Conductivity switches ignore organic foams but can measure aqueous foams.
Diaphragm devices. The use of immersed sensor measurement in nonpressurized vessels is quite straightforward, using a single measurement of head pressure. When corrected for density, the measurement is equal to liquid level.
Diaphragm continuous measurement instruments are of limited use in thin fluids. They perform fairly well in thick fluids and will ignore foam. Submerged sensors need a reference to atmospheric pressure. Switches are used infrequently and are suitable for granular solids only.
Differential Pressure Device
Differential pressure devices. Perhaps the most frequently used instruments, differential pressure devices measure the head pressure at the sensor location resulting from the height of the process material multiplied by density. Continuous measurement versions are excellent choices for the continuous measurement of both thin and thick liquids and ignore foam. However, they require diaphragm seals, repeaters, purging or sealing legs to prevent slurry-related plugging. Point level switches are limited to clean liquids with constant specific gravity.
Displacer devices. Displacer devices are based on Archimede's principle: "When a body is immersed in fluid, its weight loss is equal to the weight of the fluid displaced." A heavy body (displacer) is suspended in fluid, often in a side chamber, and is buoyed up in proportion to the fluid level multiplied by density.
Continuous measurement versions are excellent choices for the continuous measurement of thin fluids, but perform only fair in locating interfaces and are of limited use for thick fluids. They ignore foam. Point level switches perform excellent in thin fluids, but only fair in thick fluids and in locating interfaces. Neither is recommended for sludge or slurries, and vacuum and/or a high viscosity can cause dynamic instability.