Float devices. Perhaps the oldest method and the easiest to understand, float technology is available in many versions that also can be connected to micro switches, mercury switches or even pressure switches. Continuous measurement float devices are excellent choices for continuous measurement of thin fluids, but are of limited use for interfaces and thick fluids. They ignore foam, but can be used to measure it. Point level switches perform only fair with thick fluids. Their moving parts limit most designs to clean service, and only density-adjusted floats can detect interfaces.
Laser devices (non-contact). With laser devices, a laser beam is reflected from the material's surface, and the time lag to read the reflection bouncing off the material is proportional to the space above the level. Laser devices are excellent choices for continuous measurement of thick fluids, chunks and sticky solids. However, they are of limited use with thin fluids, powders, locating interfaces and foam.
Magnetostrictive devices. These devices incorporate Ferro-magnetic wire inside a non-magnetic sensing tube with a movable magnetic float outside the sensing tube. A current pulse sent down the wire interacts with the magnetic field of the float, producing a torsional strain pulse on the wire. The pulse travels back up the wire from that point at a known speed, interacting with a device that produces electrical pulse. The time interval between the initial pulse and the return of strain pulse is converted into 4-20-mA output proportional to level.
Magnetostrictive devices work extremely well for continuous measurement of thin fluids, but are of limited use for thick fluids. They ignore foams and can be used with a density-adjusted float to locate interfaces.
Continuous Ultrasonic Device
Microwave switches (non-contact). With microwave switch technology, a pulsed microwave signal is radiated by a directional antenna. A directional antenna receiver compares the two signal strengths to determine the level above or below a set threshold-to-trip switch.
Microwave switches are excellent choices for conducting fluids and detecting interfaces, but are of limited use with insulating fluids and powders and chunks. They ignore organic foams and can measure aqueous foams. Fluids with a low dielectric constant and thick coatings are a problem; microwave signals are absorbed almost entirely by water, causing problems with products that are water-based or contain water.
Radar devices (non-contact). With radar technology, a strong frequency modulated continuous wave (FMCW) signal allows accurate reading of the echo bouncing off the material. The time for return is proportional to level. The strength of the signal also requires plants to meet specific installation criteria to comply with Federal Communications Commission regulations.
Radar devices do an excellent job in the continuous measurement of conducting fluids, aqueous slurries and powders. They are of limited use with insulating fluids, aqueous foam and chunks or sticky solids. Low dielectric materials limit their range, and condensation or crystallization on antenna can cause errors.
Radiation devices (nuclear) (can be non-contact). Radiation level technology requires a radioactive source(s) to be mounted on one side of the vessel and radiation detector(s) to be mounted on the opposite side. The high-energy signal penetrates the vessel walls and shoots through the material. The amount received by the detector is dependent on level and density. This technology's big advantage is that the vessel is not penetrated.
Continuous measurement radiation devices are excellent choices for continuous measurement of most fluids and solids; however, they are of limited use in aqueous foams and slurries. As switches, radiation devices are of limited use in detecting interfaces and with aqueous slurries. They can ignore foams, but can measure them. To use the devices, plants must obtain a Nuclear Regulatory Commission license. Source disposal can be a problem, and heavy coatings can limit reliability.
Resistance tapes. With this technology, resistor wire is helically wound around steel tape mounted vertically in a vessel. The pressure of the fluid causes the tape to be short-circuited, changing the total resistance in direct relation to level.
Resistance tapes do an excellent job in the continuous measurement of thin fluids, but have only limited use with thick fluids. They perform only fair in aqueous slurries. They will ignore foams, but have limited temperature and pressure ranges and are adversely affected by large specific gravity changes.
Rotating paddle switches. This technology has a low-power synchronous motor that keeps the paddles moving at a low speed and stops when the material stalls the paddles. A change in motor torque triggers the switch.
Rotating paddle switches are excellent choices for powders, but perform only fair with chunks of solids. They are limited to the detection of dry, noncorrosive low-pressure solids.
Sight gauges. Glass gauges are for nonaggressive process applications at moderate temperatures and pressures. If a process is clear, a reflex-type gauge can be used to create a visual indication. In more aggressive conditions in which glass is not suitable or acceptable, a magnetic float type can be used and linked to an outside tracking component.
Sight gauges boast excellent performance with thin fluids, but are of only limited use for interfaces, thick fluids and foams. They must have the same temperature as the tank. Foam and boiling are potential problems; opaque coatings cause incorrect readings.