Bubble Tube Level Transmitters. These devices use a constant volumetric rate of gas flow, usually air, fed through a bubble tube submerged in a liquid. The pressure required to force the bubble gas out of the bottom of the bubble tube is equal to the hydrostatic pressure at that point. These transmitters have no moving parts, making them suitable for measuring the level of sewage, drainage water, sewage sludge, night soil, or water with large quantities of suspended solids. The only part of the sensor that contacts the liquid is a bubble tube which is chemically compatible with the material whose level is to be measured. The device generates an electrical signal that can be used to control a readout or other device. These sensors are designed to provide continuous monitoring.
Things to Consider: Range in feet or meters of water, Output current, Relay option, Power supply, Wiring, Inaccuracy as % full scale, Characteristics of liquid, Dimensions, Weight, Warranty.
Capacitive Level Transmitters. These devices, also called RF level transmitters, use a radio frequency technique based on differing dielectric constants. The impedance detected by the radio frequency changes whenever material approaches the sensing region. These transmitters are used with wide variety of solids, aqueous and organic liquids, and slurries. They contain no moving parts, are rugged, simple to use, easy to clean, and can be designed for high temperature and high pressure applications. The device generates an electrical signal that can be used to control a readout or other device. These transmitters can provide both point level detection and continuous monitoring.
Things to Consider: Input power supply, Supply voltage effect, Dimensions, Ambient temperature range, Humidity range, Current output, Accuracy, Repeatability, Linearity, Resolution, Response time, Temperature effect, Operator interface for data entry, Operator LCD display, Analog output, Error indication, Range, Auto ranging, Damping, Maximum remote distance, Microprocessor lock-up prevention, Fail-safe operation features, CE approval, Warranty.
DP Cell Level Transmitter. A DP, or differential pressure, level transmitter measures the head pressure of a liquid in a tank, using a detector mounted at the bottom of the tank being monitored. The higher pressure, caused by the fluid in the tank, is compared to a lower reference pressure, usually atmospheric, and the result is converted into a height measurement. The device generates an electrical signal that can be used to control a readout or other device. DP level transmitters provide output as a continuous reading of the level.
Things to Consider: Pressure range, Span and range limits, Signal current, Linearity, Hysteresis, Repeatability, Static line shift, Ambient temperature effect, LCD Indicator, Transmitter dust tight per IEC IP66, Environmental protection of NEMA type 4X, Overpressure protection, Push button adjust, Weight, Dimensions, Warranty.
Magnetorestrictive Level Transmitter. These transmitters operate using magnetorestrictive sensing material, usually in the form of a coiled wire. The sensing wire is pulsed with a current at fixed intervals to create a magnetic field which then interacts with the float’s magnetic field, which is in direct relation to the liquid level. This creates a stress in the sensing wire. This stress wave propagates along the sensing wire at sonic speed until the pulse is detected at the transmitter. The measured lapse time is then converted into an analog output proportional to level. These devices have high sensitivity and dynamic range, no measurable mechanical hysteresis, a linear response, and physical robustness. They are ideal for continuous level monitoring of storage and shipment containers for oil, beer and other beverages, pharmaceuticals, flammable fluids, solvents, and hydrocarbons.
Things to Consider: Resolution, Linearity, Repeatability, Maximum operating pressure, Operating temperature range, Output current/voltage, Output connector type, Power supply, Power consumption, Enclosure (e.g. IP66), Float dimensions, Length of stem required, Terminal type, Cable type, Warranty.
Sight Glass Level Indicator. Generally, a transparent tube through which an operator can observe the level of liquid contained in a tank. Simple sight glasses consist of a plastic or glass tube connected to the bottom of the tank at one end and to the top of the tank at the other. The level of liquid in the sight glass will be the same as that in the tank. Sight glasses intended for use in more demanding environments may have wire reinforced glass tubes, or metal tubes with a narrow reinforced glass window.
Things to Consider: Direct or indirect reading, Measurement units, Working pressures, Working temperatures, Materials, Chemical compatibility with liquid, Shutoff valves, Dimensions, Warranty.
Level Switch. A device that detects powder or liquid surfaces or interfaces, and generates an electrical signal that can be used either to trigger an alarm or to control a device. Level switches are designed for several different applications. They can detect liquid and powder levels, liquid to liquid interfaces, liquid to solid interfaces, and can differentiate foam from air or liquid. They can operate at process temperatures to 900ºF and pressures to 10,000 PSIG.
Things to Consider: Accuracy, Repeatability, NEMA 6 (IP68) rating for liquid level switches, Switch rating (VA), Switch poles and throw, Switch type (NO or NC), Mounting orientation, Mounting type (e.g. 1/2″ NPT) Operating temperature range, Maximum pressure rating, Switch actuation, Lead wires (gauge and length), Stem and float materials, Float specific gravity, Dimensions, Warranty.
Ultrasonic Level Transmitter. This device measures the level of liquids, bulk solids, and slurries by measuring the length of time it takes for a reflected sound wave to travel from a sensor to the surface of the material and back. Transmission time is proportional to level. The device generates an electrical signal that can be used to control a readout or other device. Ultrasonic sensors can be designed to provide point level control or continuous monitoring.
Things to Consider: Typical accuracy, Calibratable accuracy, Repeatability, Rangeability/turndown, Gas temperature, Flow range, Flow sensitivity, Data refresh rate, Minimum pressure, Pipe size range, Display, RTD temperature sensor, Outputs, Analog inputs, Programmability, Optional outputs, Status/Alarm/IO features, Data logger memory size and programmable functions, NEMA rating, Weight, Dimensions, Power, Warranty.
