However, changes in vapor density, high dust content, foam, electrical and ambient noise, and surface turbulence can affect the speed of the transmission pulse. For instance, a heavy vapor cloud emitted above the liquid surface and creation of stratification layers will cause errors in the reading or produce a loss of signal transmission back to the transducer face. Ultrasonic cannot be used at temperatures exceeding 300°F.
Radar. There are different techniques of propagating radar, such as pulsed and Frequency Modulated Continuous Wave (FMCW). Pulsed wave radar is similar to ultrasonic non-contact technology, but with the signal traveling at the speed of light instead of sound. In pulse radar, microwave signals are evaluated by a method of sequential sampling of discrete echoes over time (Figure 4).
The FMCW technique sends hundreds of thousands signals at different frequencies in a short time and compares those frequencies from transmit to reception, using Fast Fourier mathematics to determine level.
Measuring solids generally is the most difficult application for level; radar technology is becoming the norm for reliable measurement and dealing with the angles of repose of the material, dust in the airspace, and long-range readings.
The technology is non-contacting. Moreover, radar is unaffected by airspace conditions like humidity, heavy vapors, and vapor-layer stratification due to temperature fluctuations. It can perform measurements out to 230 ft. in conditions of heavy dust, which other technologies find very difficult. There are now two-wire, loop powered designs that offer low cost of wiring and ownership. Plus, prices have considerably lowered. For solids’ measurement, most radar transmitters will offer different antenna options. The horn (4 inches in diameter) is most widely used, but parabolic or dish type antennas also are available.
The accuracy of radar technology depends upon the dielectric constant of the material, which affects the reflectivity of the radar wave. A lower dielectric value (such as 1.5 compared to air’s 1.0) will lessen the amplitude or strength of the microwave signal reflecting back to the antenna mounted at the top of the vessel, making the measurement difficult. Radar cannot measure the interface of two immiscible liquids like oil and water. There also are pressure limitations for the antenna seal as well as temperature limitations at the flange area based on the material of the emitter, propagation tip or rod-style antenna.
Guided wave radar. Sometimes referred to as Time Domain Reflectometry (TDR), this technology is similar to non-contact radar, except that it is invasive. Guided wave radar utilizes a flexible cable or various types of rigid probes and generates microwave pulses down the probe or guide (Figure 5).
Figure 5. This technology offers a measurement span that almost equals the vessel height.
When the signal meets the liquid or solid, it is reflected back, with the time from launch to reception a measure of the distance. Like non-contact radar, the amplitude of the signal depends upon the conductivity and dielectric value of the material. This technology has become very successful for measuring solids such as powders in hoppers and bins where the measurement range is 30 ft. or less.
Guided wave radar is unaffected by temperature, pressure or vacuum, and changes in the airspace with vapor. Additionally, the accuracy is independent of the material’s dielectric value, conductivity, density, and moisture content. Another distinct advantage is that calibration can be done without material being present. The technology tends to work well for short-range measurements. Because it has little top-end and bottom-end blanking, this technology can measure from near the top to almost the bottom of the container.
As a contact technology, it poses concerns about material compatibility, which sometimes necessitates expensive, exotic metals. Installation can be quite cumbersome with the rigid probe styles for liquid applications if head space above the vessel is limited. Additionally, the technology can be difficult, if not impossible, to apply to agitated vessels. In solids’ applications, there is a risk of the flexible cable being broken from too much loading of material weight. The loss of a guided wave radar cable into a powder-like material can cause heavy damage to a screw conveyor at the bottom of a silo. Also, abrasiveness of the material as well as excessive coating can cause damage and wear or false readings.
Now that we’ve examined how these various technologies work and their basic advantages and disadvantages, we are ready to see how they compare for a given application. To do this properly, the application must be understood in full. Then, use the following criteria to use to target the most appropriate choices:
- range or height of measurement;
- accuracy and repeatability;
- material to be measured;
- type of measurement (continuous level, interface or point level);
- physical or chemical changes to material in container;
- vapors in the airspace with temperature changes or heavy dust;
- material of container;
- pressure and temperature (minimum and maximum);
- dielectric constant;
- mounting constraints or available container openings (size, type and rating);
- desired output or information from field device (control or simple readout);
- solids or liquids to be measured; and
- potential material build-up.
After reviewing these criteria, you should have narrowed the list of potential technologies. Then you may want to delve into some other factors, such as cost of ownership and direct costs. Typically, a number of technologies can be successfully applied to an application. The important thing to remember is that all technologies work when properly applied. Don’t force fit a technology into an application or you’ll be setting yourself up for problems. Always give special consideration to the principle of operation and where the limitations of each technology lie.
The Table relates key criteria for a continuous level measurement to the various technologies and so will help you select the appropriate options. The color coding within the cells gives an indication of how well the technology fits the criterion, with green meaning okay; yellow, caution (also look at an another technology); and red, no good (use an alternative technology).
Table 1. These criteria provide a basis for winnowing measurement technologies. (Click to enlarge.)
*Note: This pertains to the measurement span.
Once you’ve narrowed the choices using the general criteria in the chart, consider your particular needs. Here, application data sheets offered by companies that provide a variety of level technologies can serve as a useful tool. They raise questions that can lead you to the proper technology for the measurement at hand
Do your level best
Choosing the right technology for continuous level measurement requires an appreciation of the applicability and pros and cons of the various options. It also demands an understanding of the attributes of the material to the monitored and the environment in which measurements will take place. Armed with this information, it becomes easy to determine the appropriate measurement technique for the application.
Jerry Boisvert is a product marketing manager for Siemens Energy & Automation, Grand Prairie, Texas. E-mail him at firstname.lastname@example.org.