TECHNICAL AND MARKETING SERVICES
FOR INSTRUMENTATION SUPPLIERS AND END-USERS
Non-Contact Radar Level Measurement (Part 1 of 3)by David W Spitzer and Walt Boyes
Non-contact radar level measurement sensors emit a radar signal towards the material and measure the remnants of the signal that are reflected from the material.
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.
Flowmeter Selection: When It Makes Sense to Change Your Old Habitsby David W Spitzer
Flowmeter selection can be quirky. Sometimes you have already seen the
application before and have the benefit of having previously evaluated it. In this case, you might select a flowmeter
with reasonable confidence that you have made a good selection. The advantage of previous experience is that
a flowmeter selection for the current application is made quickly and
efficiently. The disadvantage of
previous experience is that it can "color" your thinking. I suggest that the confidence with which one
makes a flowmeter selection based upon previous evaluation is proportional to
the amount of time spent on the original evaluation and inversely proportional
to the amount of time since the original evaluation was performed.
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.
Variable Speed Drivesby David W Spitzer
Which of the following statements
is true about using variable speed drives to operate equipment in a process
plant?Variable speed drives:
A.reduce electrical energy consumption.
B.improve control as compared to a control valve.
D.decrease power surges.
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
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
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.
ABOUT SPITZER AND BOYES, LLC
In addition to over 40 years of experience as an instrument user, consultant and expert witness, David W Spitzer has written over 10 books and 500 articles about flow measurement, level measurement, instrumentation and process control. David teaches his flow measurement seminars in both English and Portuguese.
Spitzer and Boyes, LLC provides engineering, technical writing, training seminars, strategic marketing consulting and expert witness services worldwide.
Copyright 2024 Spitzer and Boyes, LLC
The content of this message is protected by copyright and trademark laws under U.S. and international law. All rights reserved.