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Laser Level Measurement (Part 1 of 2)by David W Spitzer and Walt Boyes
Laser level measurement sensors emit a laser beam towards the material and measure the remnants of the beam that are reflected from the material. These systems determine the location of the material using the time-of-flight that the laser beam takes to travel to and return from the material. The distance between the sensor and the material can be calculated as one-half of the measured time-of-flight times the speed of the laser beam. Mechanical dimensions can then be used to determine the level in the vessel.
Noting that the laser beam travels to and returns from the material, degradation of the beam strength between the sensors and the material can cause laser level measurement systems to fail. Degradation can occur at the sensor, in transit to/from the material, and at the surface of the material.
Dirt or other coatings on laser transmitter/receiver can cause the received laser signal to be weak. When accumulations over time are normal for the process, routine maintenance may be required to keep the transmitter/receiver operating. In many applications, the sensor may be shielded in a tube and/or continuously purged with gas to keep it in operation. Similar concerns exist when the laser beam travels through a sight glass that can become dirty and cause attenuation of the beam.
Excerpted from The Consumer Guide to Non-Contact Level Gauges.
Calibration Variables: Positioning Diaphragm Seals on DP Level Transmittersby David W Spitzer
In the last two months, we determined that the
capillary tubing for differential pressure measurements should generally be the
same length --- even though the added physical length may seem wasteful,
expensive and cumbersome. In particular,
we discussed a flow measurement system where the capillary tubes were designed
and installed such that their temperatures were often different. Level measurement using a transmitter with
diaphragm seals and capillary tubing can exhibit different problems.
In particular, differential pressure level transmitters
with diaphragm seals are often calibrated in the instrument shop --- especially
prior to initial installation. To simulate the actual installation in the
instrument shop, the diaphragms should be mounted at different elevations that
correspond to the actual installation to calibrate the transmitter. This procedure becomes impractical in many
applications due to tank heights that limit this simulation to but a few
In place of this simulation, differential pressure level
transmitters are often calibrated with their diaphragm seals mounted at the
same elevation. The calibration values
will be different than those in the actual vessel due to the different
elevations of the diaphragm seals in the shop as compared with the field. Sometimes transmitters are calibrated with
their diaphragm seals just "laying around".
Aside from these potential shop calibration issues,
installing the diaphragm seals on the actual vessel may result in minor
calibration shifts due to the physical nature of positioning the diaphragm seal
and tightening its bolts. As a minimum,
a zero calibration should be performed after the transmitter and its diaphragm
seals are permanently installed.
Performing a shop calibration has its virtues because
it can find transmitter and diaphragm seal problems before installation in a
field environment. However, appropriate
valves and calibration ports should be installed to allow field calibrations
that will improve measurement accuracy.
One such device is a "filler flange" that is a thin wafer spool piece
that contains drain and bleed ports that can be used for calibration. Better yet, it would be more convenient to
purchase the diaphragm seals with these features.
This article originally appeared in Flow Control magazine.
The Weight of Waterby David W Spitzer
What is the approximate weight of the water in a
full vertical cylindrical tank that is approximately 6 feet (2 meters) in
diameter and 20 feet (6 meters) high?
Use pi = 3 for estimating purposes.
18 metric tons
72 metric tons
None of the above
The solutions to this problem are
relatively straightforward. The volume
of a full tank of water is pi times the square of the radius times the tank
height (3 x 32 x 20) or approximately 540 cubic feet. The density of water is approximately 62.4
pounds per cubic foot so the water in the tank weighs approximately 33,700
The solution to the example in
metric units is (3 x 12 x 6) or approximately 18 tons (18,000
kilograms) because the density of water is approximately 1 metric ton per cubic
meter. Therefore, Answer A and Answer C
As mentioned, this problem is
relatively straightforward from a technical perspective. The point of performing the calculation in
both the English and metric systems is that the English system is more prone to
error. The metric system is more elegant
--- even though both will yield the same answer for the same size tanks. The metric system is less prone to
calculation error because many of its conversion factors are 1 or multiples of
10. (If you have already attended one of
my seminars you know that I am good at multiplying and dividing by 1.)
This exercise may not seem important, but small errors
can make a big difference in results. I
seem to recall that a space probe crashed into a planet (instead of orbiting)
due to a misunderstanding regarding the measurement units used during design.
Additional Complicating Factors
The above calculations in the English system can
be made even more complicated by using the weight of water per gallon and the
number of gallons per cubic foot. In
addition, the liquid could be different from water and have a density that is
different from that of water so the tank contents could be heavier or lighter
than calculated in either system.
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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