Some radar level measurement systems determine the location of the material level using the time-of-flight that the radar signal takes to travel to and return from the material. The distance between the sensor (antenna) and the surface of the material can be calculated as one-half of the measured time-of-flight of the pulse times the speed of the radar signal. Mechanical dimensions can then be used to determine the level in the vessel.

Other radar level measurement systems modulate the frequency of the radar signal. In doing so, the received signal can be compared to the transmitted signal to determine the frequency difference between them. The frequency difference and mechanical dimensions can then be used to determine the level in the vessel. These frequency modulated continuous wave (FMCW) techniques are often better able to discriminate level than pulsed radar level measurement systems. This is because the frequency spectrum that is produced is more amenable to signal processing analysis than is the echo signal returned when using pulse radar techniques. In some applications, the peaks in the frequency spectrum of an FMCW radar unit can be correlated with various levels, such as the actual level, some liquid-liquid interfaces, and stratification due to vapor in the vessel.

Excerpted from The Consumer Guide to Non-Contact Level Gauges.

For example, about 30 years ago I was charged with measuring the flow of a liquid flowing at 120-200 degrees Celsius in steam-jacketed pipe that is blown clear with steam after use.  I seem to recall that the liquid had a specific gravity of approximately 1.10 and a viscosity of approximately 10 centipoise at flowing temperature.  In addition, should the fluid changes to a hard concrete-like substance if it freezes. 

Finding a flowmeter for this application was not easy 20 years ago and it took a considerable amount of analysis to find a flowmeter that would work in this application.  The process and steam flow conditions were somewhat harsh and tended to eliminate most flowmeter technologies.  In the final analysis, finding a flowmeter for this application amounted to finding a flowmeter that would not plug and that could be easily removed and opened to chisel away the concrete-like material without damaging the flowmeter. 

Many new and better flowmeters have been developed in the 20 years since the original analysis was performed.  However, given the same application today, I might quickly look at some of the newer flowmeters in the context of the removal/chiseling requirements, but I would probably select the same flowmeter again --- because the analysis took a long time to perform.

This article originally appeared in Flow Control magazine. 

All of these statements are true in a general sense.  However, investigation of variable speed drive operation in the context of the process may yield contradictory results. 

It is generally presumed that variable speed drives operate equipment in a manner whereby the hydraulic output is matched to the actual process load.  This evokes a sense of efficiency and implies a reduction in electrical energy consumption.  In one application, a variable speed drive was applied to an air compressor to enable the substitution of compressed air for a more expensive gas.  The electrical energy consumption of the compressor increased because more compressed air needed to be produced.  However, the cost of the expensive gas was reduced by a greater amount than the increased electrical cost associated with the compressor.  The simple payback for this project was only 8 months --- despite the increase in electrical energy consumption.  Answer A is wrong in this application. 

It might seem intuitive that reducing the operating speed of equipment will reduce equipment vibration.  This may sound reasonable, but reducing the speed of cooling tower fans can cause the cooling tower fan to operate at its resonant frequency.  Extended operation in resonance might shake the tower to the point of failure.  Many drives have adjustments that allow the drive skip over these frequencies to reduce the time that resonance can occur.  Answer C is wrong in this application. 

Additional Complicating Factors

There are many complicating factors that can come into play when applying variable speed drives to processes.  While there may be some relatively standard ways in which variable speed drives are applied to certain process configurations, all processes are different and deserve custom consideration.  The benefits of applying variable speed drives can be substantial and beneficial to the operation, but you should be careful.

This article originally appeared in Flow Control magazine.