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Laser Level Measurement (Part 2 of 2)by David W Spitzer and Walt Boyes
The laser beam travels from the sensor to the material and back. The beam can be attenuated in transit due a number of phenomena, including optical attenuation caused by nature of the vapors in the beam path, and dust/dirt in the beam path to/from the material. Note that the presence of dust/dirt in the path can be temporary, such as when filling occurs and causes a dust cloud to form in the path of the beam.
The surface of the material can cause the intensity of the reflected laser beam to degrade when it exhibits poor reflective qualities, such as when the material itself or contaminants cause the laser beam to reflect poorly.
In addition, accuracy can be degraded based upon the surface on which the laser beam is reflected. For example, the laser beam may measure the top of a layer of foam by reflecting off the top of the foam. If the foam is transparent to the laser energy, the beam may reflect from the foam/liquid interface and measure the liquid level. Translucent foam might cause the level measurement to represent a location within the foam. Further, the foam conditions may vary over time and cause erratic level measurements,
Many laser level measurement sensors utilize Class 1 lasers that do not generally pose a hazard under normal operating conditions. Other sensors use Class 3 lasers that can pose a risk of eye injury when viewed with the naked eye for more than a moment.
Laser level measurement sensors can be used for sanitary applications where the laser beam enters and leaves the vessel via a sight glass.
Excerpted from The Consumer Guide to Non-Contact Level Gauges.
Inspector Clou'fleau': How Process Fluid Temperature Affects Vortex Flow Performanceby David W Spitzer
Engineering can be an exercise in discovery in
which causes and effects are analyzed.
However, in many instances, the use of "first principles" can provide an
appropriate explanation of an observed phenomenon. For example, consider a vortex shedding
flowmeter that worked fine when it was installed in November but stopped
working in April. Investigation revealed
that the flowmeter was fully functional and worked accurately on the flow loop
in the instrument shop. In addition, the
spare flowmeter installed in the process does not work either.
Having analyzed the problem
without solution, one might have a strong bent towards replacing the flowmeter
and using another technology. Before
doing this, let's take a look at the overall operation of the flowmeter.
The flowmeter is sized for a
process flow of 60 gallons per minute in a 2-inch pipe of a fluid that freezes
at 20 degrees Celsius with a specific gravity of 1.00 and a viscosity of 5 centipoise. A quick calculation shows that the Reynolds
number is approximately 19,000 at maximum flow that is acceptable for most
vortex shedding flowmeters.
However, further investigation
reveals that the viscosity is 5 centipoise at 90 degrees Celsius. During the winter, steam tracers maintain the
pipe temperature above 100 degrees Celsius, so the liquid does not freeze. The tracers are turned off during the warmer
months because process heat keeps the liquid from freezing by maintaining the
pipe temperature over 40 degrees Celsius. However,
the liquid viscosity can increase to 20 centipoise at these cooler temperatures so
Reynolds number can fall under 5000 at maximum flow. Operating at (say) 50 percent flow drops the
Reynolds number under 2500 --- where most vortex shedders cease to
In short, turning off the steam tracer reduced the
liquid temperature increasing its viscosity and decreasing Reynolds number to
the extent that the flowmeter became inoperative. The vortex shedding flowmeter could be
replaced with a flowmeter that uses another technology, but it might be more
pragmatic to just leave the tracer on all year.
This article originally appeared in Flow Control magazine.
Gas Flow Pressure & Temperature Compensationby David W Spitzer
When performing pressure and temperature
compensation for differential pressure gas flow measurement should one:
A. pressure and temperature compensate the differential
pressure measurement and then take the square root
B. take the square root and then pressure and temperature
C.pressure and temperature compensate with no square root
D.None of the above
For differential pressure
flowmeters operating in the turbulent flow regime, the volumetric flow is
proportional to the square root of the ratio of the differential pressure divided by the fluid density. The form of this equation
indicates that the differential pressure measurement is divided by the gas
density and then the square root of the result is taken. Answer B and Answer C are not correct.
A closer look reveals that the
density of the gas "corrects" or compensates the differential pressure
measurement. Gas density is a function
of pressure and temperature and can be calculated using various techniques to
include the Gas Laws, density tables, and equations of state. Afterwards, the square root of the result is
taken. Therefore, Answer C is correct.
This question surfaces every so often --- especially
when the implementation of pressure and temperature compensation is not
implemented in a flow computer.
Nonetheless, it is an important question because failure to properly
implement the correct calculation can result in flow errors. Further, operators and engineers may
unknowingly use these flawed measurements.
Investigation as to the accuracy of the flawed measurements will likely
occur only when the flawed measurements yield unreasonable results in important
calculations. An instrumentation
specialist is often required to find the flaw in the calculations.
Additional Complicating Factors
There are many reasons for instrument users to
implement gas flow pressure and temperature compensation in their control
system instead of in a flow computer.
However, flow computers should be used in custody transfer applications
and considered when the computational technique used to calculate density (such
as the Gas Laws or equations of state) introduces significant measurement
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.
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