Hydrostatic? RF capacitance? Conductivity? Radar? Ultrasonic? The level measurement methods in control systems range from manual to those that can be integrated with automated control systems. According to a study from analysts at Frost & Sullivan, “The drive towards automation is resulting in an increase in the deployment of advanced technologies in most process industries. However, a major difficulty encountered by end users while switching or upgrading technologies is the installation of new level-sensing systems. The intrusive nature of traditional sensing technologies makes the installation process cumbersome and adds downtime. As a result, there is a notable shift towards non-intrusive sensing technologies, such as free propagating radar/microwave and ultrasonic level sensing systems, which offer greater value and accuracy. Further, since these technologies have simplified circuitry and offer network connectivity, demand for such systems is bound to see an increase in the near future.”
So, how do you choose the right technology for your application? Suppliers often provide recommendations if you specify your needs. Here is a check list from Gabor Vass of Princo Instruments to help you get the right technology for your application:
- Give the generic name of the process material, such as a 5% sodium hydroxide, for your specific application.
- Specify whether you need to measure a liquid, slurry, solid, granular or powder.
- Give the value of the material’s dielectric constant, K, conductivity in microsiemens per centimeter, viscosity in centipoise and density in pounds per cubic foot. Also describe consistency, such as watery, oily, batter, molasses or send the supplier a sample of the material.
- Give values of the normal temperature and pressure, as well as the minimum and maximum. If turbulence is present, indicate its degree as light, medium or heavy.
- Describe vessel material such as metallic, nonmetallic or lined. Give materials of construction of wetted materials, for example 316 stainless, Kynar, Teflon or other.
- Describe area classifications such as non-hazardous or corrosive.
- Describe the main function of the vessel such as sump, reactor, storage, water separation at bottom, etc.
- Provide a schematic diagram showing the vessel size and shape, the probe mounting and location, 0% and 100% of level and the presence of an agitator or other internal obstruction.
- Specify power requirements.
SANITARY LEVEL MEASUREMENT
Echotel Model 960 AS-Interface ultrasonic level switch uses the AS-Interface low-cost cabling solution that is a digital replacement for traditional hard wiring of field devices. It provides a digital serial interface with a single unshielded two-wire cable for power and data transfer. Available in industrial and sanitary versions, the Model 960 uses a tip-sensitive transducer that measures level within ¼ in. of the vessel bottom. Magnetrol International; 630/969-4000; www.magnetrol.com
INSENSITIVE TO MATERIAL BUILDUP
CLS2 capacitive level switch uses impulse RF admittance combined with an active guard to provide level measurement that is insensitive to material build up. It auto-calibrates using an external magnet to initiate without having to open the enclosure. Dwyer Instruments; 219/879-8000; www.dwyer-inst.com
NO TROUBLE WITH DUST OR FOAM
The LevelFlex M top-mounted smart transmitter is designed for continuous level measurement in liquids and bulk solids. It is not influenced by fluctuations in density, temperature or build-up of dust. It offers a menu-guided, on-site operation with four-line plain text display or remote display and operation conducted via a PC. Endress+Hauser; 317/535-7138; www.us.endress.com
WORKS WITH DIFFICULT SOLIDS
SITRANS LR 460 radar level transmitter uses 24 GHz FMCW technology and Process Intelligence echo processing for measurement on difficult solids. With a range of 328 ft., it can measure cement powder, fly-ash, coal, gypsum, flour, grain, aggregates and plastics. Siemens Energy & Automation; 215/646-7400; www.sea.siemens.com
SIMPLE FLUID LEVEL CONTROL
LL100 level controller simplifies the monitoring of tank and barrel fluid levels without costly controllers. Attach a magnetic-relay switch or other low-level logic source. Set up the controller for AC/DC or NO/NC operation, depending upon the application. Internal relay is rated for up to 7 amp. APT Instruments; 217/498-0342; www.aptinstruments.com
IMMUNE TO BUILDUP
Resonator line of vibrating liquid level switches provides immunity to buildup even in high-viscosity applications to 20,000 cP. Designed for a variety of chemical and petrochemical applications, they are based on a robust tuning fork design. A self-test option that automatically checks for fault conditions, such as excessive product buildup on the sensor is available. K-TEK; 225/673-6100; www.ktekcorp.com
RADAR LEVEL SENSORS
Radar level transmitters for continuous level measurement are designed for applications requiring non-contact liquid level measurement. They adjust to materials to be measured, vessel configuration and system interface. The transmitters are unaffected by environmental conditions, such as temperature, vacuums and gas vapors and are suited for reflective liquids, foaming surfaces, condensation or dusty conditions. Omega Engineering; 203/359-1660; www.omega.com
SUBMERSIBLE LEVEL TRANSDUCER
Series 705 submersible level transducer, with static accuracy of +/-0.25% FSO, is suited for highly viscous or slurry environments. It is available in custom level ranges from 6 ft to 115 ft and has analog outputs of 4-20 mA or 0-5 VDC. Standard housing construction is 316 stainless steel with titanium optional. Pressure Systems; 800/328-3665; www.pressuresystems.com
NO SCUM BUILDUP
ORCA sonar series interface level transmitter features industrial scum-cleaning mechanisms that require virtually no maintenance. The transducers are continuously cleaned. Providing measurements up to 100 ft, the ORCA is suited for a variety of applications, such as sewage and wastewater treatment, water treatment, mining and processing and is compatible with MODBUS, Profibus and HART. Hawk Measurement America; 978/304-3000; www.hawkmeasure.com