ehandbook - control global
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eHANDBOOK
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TABLE OF CONTENTSMake sense of sensitive level 5
Radar offers increasing opportunities to improve control at a reasonable price.
Differential pressure level in a purged tank 10
Hydrostatic and head pressures can lead to confusion.
Wireless sensor sweet spots 15
How battery life interacts with update rate to define the most practical applications.
Measuring oceans and ice 17
Accuracy and precision support predictions and balance the sea level budget.
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Krohne, Inc. • optiwave.krohne.com 9
Vega Americas • www.vega.com 2
eHANDBOOK: Level Measurement, Part II 3
www.ControlGlobal.com
Jon Eide Process Consultant for Terminal Management
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Greg: It’s not well recognized that
a sensitive level measurement
can be extremely valuable. For
inventory, a fraction of an inch change in
level in a large-diameter tank is a lot of
material. For closure of material balances,
these seemingly small changes in level
can make a huge difference in account-
ing and in process control of some key
unit operations. For crystallizers, evapo-
rators and reactors, tight residence time
control depends on an accurate level
measurement. Many multi-effect evapo-
rators measure and control the product
density in the last stage by manipulating
a product or feed flow. These multi-effect
evaporators depend upon tight level con-
trol to make sure changes in flow in and
out of each effect are equal. For distilla-
tion columns where the distillate receiver
level controller manipulates reflux flow,
extremely tight level control enables much
better temperature control, particularly
through inherent internal reflux control.
The most predominant level measurement
uses a differential pressure (DP) transmit-
ter. The actual level, of course, depends
on density. A second DP whose high and
low connections are always submersed or
a Coriolis meter in a recirculation line can
be used to measure density to correct the
reading, but the DP sensitivity is not great.
The accuracy seriously deteriorates due to
problems and limitations of impulse lines,
purges and capillary systems.
We have a situation for level similar to
last month’s for mass flow, where Cori-
olis meters had an order of magnitude
Make sense of sensitive levelRadar offers increasing opportunities to improve control at a reasonable price
By Greg McMillan
eHANDBOOK: Level Measurement, Part II 5
www.ControlGlobal.com
better accuracy, much greater reliability,
and much less vulnerability to installa-
tion problems. The level measurement is
radar. Like Coriolis, radar doesn’t depend
on density (and thus associated changes
in composition), and while the hardware
cost is higher, the installation cost and
especially the maintenance cost is less. If
you include the improvement in process
monitoring and control, the return on
investment is a no-brainer.
To help us get sensitive to sensitive level
measurement, we gain the insight and
knowledge offered by Jeff Blair, offer
manager for level, Schneider Electric.
Jeff: Thank you for inviting me to Con-
trol Talk, Greg. It’s an honor to be here.
I enjoyed reading your material along
with items from Hunter Vegas and many
others.
Most of the free space radar (FSR) and
guided wave radar (GWR) applications we
see work flawlessly and get rave reviews.
However, a small number of radars are
misapplied or not installed properly, so
we’ve been on an educational tour to help
teach the operators, technicians and engi-
neers about proper installation procedures
and how to get the best signal return. It’s
for reasons like these that there’s been a
trend for manufacturers to offer additional
field services (including startup services)
for radar. FSR and GWR are actually easier
to commission and use than most instru-
ments, as long as the end user follows
proper vendor instructions and/or has a
rep or vendor either onsite or on speed
dial/Facetime/Skype to help.
Greg: What equipment and process
application details do you need to know
to correctly specify and install FSR
and GWR?
Jeff: When selecting a radar measure-
ment, the best thing to do is to gather the
criteria of process temperature, process
pressure, expected dielectric constant
(Dk), material requirements, available tank
connections and economics. Next, work
with your favorite representative or man-
ufacturer to help narrow the choice of
applicable radar models. They can guide
you on available models and also help
select special models that can perform
interface measurement (GWR) or tolerate
high temperatures and pressures for appli-
cations such as steam drum measurement.
All application parameters are important
to make sure the correct radar is selected
and that it performs for the lifecycle of
the process. Besides ensuring a Dk of 1.1
or higher (1.5 preferred), the other most
important thing to consider when select-
ing radar is to ensure proper selection of
the cable, rod or coaxial probe (GWR) or
antenna (FSR). The reason is that those
items and their design and dimensions
www.ControlGlobal.com
eHANDBOOK: Level Measurement, Part II 6
have a large part in determining the over-
all maximum measurement length, beam
angle (FSR only), and temperature and
pressure limitations. A close runner-up to
the antenna selection is the proper choice
of the physical installation location (i.e.,
what distance from the tank wall, ensuring
location is away from center of tank and
any filling nozzles or streams) on top of
the tank, vessel, sump pit or stilling well.
The goal of proper physical location of
the radar is to eliminate potential obsta-
cles, and give the radar the best possible
chance at return signal. There are fewer
constraints and restrictions when choos-
ing an installation location for GWR as the
microwave energy always stays within a
radius of the probe.
Greg: What are the installation require-
ments you need to carefully address for
FSR and GWR?
Jeff: The beauty of both types of radar
is their simple installation techniques
compared with other types of level instru-
mentation. There are no requirements for
process tubing runs that are often required
for DP level applications. Regarding GWR,
pay attention to ensure you have enough
overhead clearance to install a rod or
coax unit. It’s no different than ensur-
ing overhead coverage when installing a
capacitance probe or a magnetostrictive
instrument. And some of the probes come
in segments, so several short pieces may
be assembled together rather than a long
10- or 20-ft rod. Cable antennas for GWR
may need to be secured to the bottom of
the tank. If that’s the case, there may be
some potential for confined space permits.
FSR is a bit simpler because it is non-con-
tact; there’s no probe or cable that extends
down into the liquid.
Once those items are accounted for,
both FSR and GWR are simply screwed
into their respective process connections
with the face oriented in accordance with
instructions.
The goal of proper physical location of the radar is to eliminate potential obstacles and give the radar the pest possible chance at a return signal. There are fewer constraints
when selecting GWR as the microwave energy always stays within a radius of the probe.
www.ControlGlobal.com
eHANDBOOK: Level Measurement, Part II 7
Installing radar when the tank is empty is
advised. This allows the radar software
to take a snapshot of the tank and iden-
tify any obstacles (e.g. baffles, ladders)
that may be interpreted as false level
reflections. The snapshot or tank mapping
procedure allows the user to blank out
potential false echoes. It’s worth noting
that high-frequency (80 GHz) types of
FSR have a narrow radar beam that’s
often able to avoid seeing obstacles and
potential disturbances, eliminating the
need to map them out.
Greg: What do you need to know to cor-
rectly install FSR and GWR?
Jeff: In radar, the measurement is inferred
from distance, like how level is inferred
from head pressure when using DP level.
The data needed are three pieces of
information:
1. The tank height—from the process con-
nection of the radar to the tank (where
the radar signal generates) to the
tank bottom.
2. The 4 mA or 0% level—this should nor-
mally be located at an offset from the
very bottom of the tank.
3. The 20 mA or 100% level—this should
be located below the flange and allow
room for the radar dead space. Each
type of radar requires a small amount of
buffer space either below the process
connection or the horn (FSR only).
Greg: When would you use ultrasonic
instead of radar level measurements?
Jeff: Cost-conscious consumers often
chose ultrasonic. Ultrasonic transmitters
using sound waves were about half the
cost of radar, and like radar, they perform
flawlessly if they’re applied and installed
properly. Now, in general, the average
price of process FSR and GWR have come
down, so that the price delta between
radar and ultrasonic closed. Price being
nearly equal, most users opt for radar as
it can handle higher temperatures and
pressures. Now, ultrasonic measurements’
primary advantage is that its unaffected
by Dk. One of the drawbacks to ultrasonic
used to be the buildup of condensation
on the transducer face. Recently, there’s
been some advancement in that field
where some ultrasonic measurements are
better able to shed condensation droplets
to make a reliable measurement.
www.ControlGlobal.com
eHANDBOOK: Level Measurement, Part II 8
products solutions services
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Q When using a differential-pressure
(DP) level transmitter, are the high
and the low pressure ports reversed if
the tank is purged with nitrogen? Purg-
ing is needed because the tank contains a
hydrate inhibitor.
R. REVISH
A In all wet leg-type level installations,
the top connection on the tank is
connected to the high-pressure side of
the DP cell, and the bottom one to the
low-pressure side, because the wet leg is
always full of liquid (a constant head pres-
sure plus the pressure of the head space
or nitrogen purge), while the hydrostatic
head at the bottom connection varies and
is always less than the one on the wet
leg side (the level in the tank is changing,
while the level in the wet leg is constant
and always higher). Naturally, in such
installations, the DP transmitter has to be
reverse-acting.
This reminds me of the Fukushima acci-
dent, with which I was involved, and later
wrote a book about how to fix its controls
(www.amazon.com/Automation-Can-Pre-
vent-Next-Fukushima/dp/087664017X).
During the accident, the water level
reading showed the level rising when it
was dropping because the temperature
around the overheated reactor caused
the condesate in the wet leg to boil off.
The Fukushima installation is shown in
Figure 1.
In my correction of this level control loop,
I added a second DP cell (∆PT in Figure 2)
Differential pressure level in a purged tankHydrostatic and head pressures can lead to confusion
eHANDBOOK: Level Measurement, Part II 10
www.ControlGlobal.com
that is continuously detecting the height
of the level in the wet leg (A in Figure 2).
If, for any reason, the height of this refer-
ence leg is dropping, the amount of drop
is added to the the output signal of the
level transmitter (LT in Figure 2).
Naturally, in your application—if you’re
sure that the wet leg will always be there
and will always be of constant height—you
don’t need the correction I made in Figure
2 for the Fukushima reactor. In your case,
you can just reverse the pressure taps or
use a reverse-acting transmitter.
Béla Lipták
A It’s very difficult to help you with the
minimal detail you provided. However,
if we review some general details of level
measurement, maybe you can understand
some of the difficulties in using differential
pressure to measure liquid level.
In Figure 3, the pressure at the top of the
tank must be presented to the DP level
transmitter (LT) through an impulse line.
The impulse line is not empty. It may be
filled with a non-corrosive fluid designed
for this purpose, or it may be filled with
condensate of the fluid in the tank being
measured. For that reason, I labeled this
tubing as “wet leg” in the drawing. Like-
wise, the pressure at the bottom of the
tank is also presented to the LT via its
impulse line (not labeled in the figure). We
know the pressure due to the liquid level
in the tank is higher at the point of mea-
surement (D) than the pressure at the top
of the tank (P1) in the drawing. However,
the LT sees the pressure at the top as the
CORRECTED INSTALLATIONFigure 2: A second DP cell (∆PT) continuous-ly detects the height of the level in the wet leg (A). If the height of this reference leg is dropping, the amount of drop is added to the output signal of the level transmitter (LT).
A
Steam pressure
Wet leg actual level
Reactor∆PT ∆
LT
Σ
+ +
+
-
-
Wet leg referenceA
∆L = lost wet leg
Correct level
HP
LP
FUKUSHIMA INSTALLATIONFigure 1: The arrangement of the level trans-mitter (LT) in the reactor vessel at Fukushima, according to Dr. Ritsuo Yoshioka, president, Japanese Functional Safety Laboratory.
LT
www.ControlGlobal.com
eHANDBOOK: Level Measurement, Part II 11
weight of the fluid in the wet leg plus P1,
while the pressure at the bottom is the
sum of the weight of the tank fluid (head)
plus the weight of the fill fluid in the
bottom pressure sample line plus P1.
From this analysis, you can see that the
pressure at LT at the wet leg (top of tank)
will actually be higher than the pressure
at LT from the bottom of the tank. This is
due to the pressure head from the wet leg.
Many instrument mechanics are confused
by this, and connect the top and bottom
pressure taps in the reverse positions on
the LT, which is what I would guess has
happened in your case.
Since the only thing you’re interested in
is the level in the tank, even though the
connections are reversed, the difference
between the HP and LP taps will still be
a correct measurement of liquid level.
Sometimes we’re just lucky.
For others, note that using a HART or
Foundation fieldbus smart level trans-
mitter will allow you to do a wet leg
compensation when the DP appears to be
negative because the LP is connected to
the wet leg and the HP is connected to
the bottom of the tank.
Dick Caro
A To measure level in a closed tank
that’s purged with nitrogen, you
need to follow these steps.
1. Determine the span of the measurement:
high point minus low point (in meters,
inches, feet, cm, etc.).
2. Since you’ll be using a purge that adds pres-
sure onto the measurement element, you’ll
need to know the following: pressure (in bar,
psig, etc.) and if the pressure will be con-
stant or variable.
3. Next, you’ll need to define the type of con-
nection on the tank—to know how the DP
cell is to be connected (tubing, seals, wet
leg, etc).
4. Then, identify the location of the transmitter
versus the datum or low level point of mea-
surement of the tank (in meters, inches, feet,
cm, etc.).
PURGED VAPOR SPACEFigure 3: Here, A is the measurement span; B is the distance of the minimum level above the connection; C is the distance from the high side connection to the top of the instrument connection and may be filled with fluid; and D is the actual liquid level above the transmit-ter (LT) low side. In wet leg applications, C is connected to the high side and D is connected to the low side of LT.
A
B
Vapor spacein tank
P1Maximumlevel
Minimumlevel
LT
CWetleg
D
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eHANDBOOK: Level Measurement, Part II 12
Depending on the location of the trans-
mitter to the datum line, type of tank
connection, etc., and with the formulas of
the links below, it will be very clear why
the low-pressure side may have a higher
pressure causing a negative value.
It would appear that you have a differen-
tial pressure transmitter with a wet leg.
That means that the low-pressure side
is filled with liquid to the very top of the
tank connection point. This wet leg will
always have a pressure of the wet leg
height multiplied by the density of the
fill liquid plus the purge pressure on the
top of the tank. This means the pressure
will always be higher than the one being
sensed on the lower connection, which
will vary.
The easiest method to handle this is to
interchange the connection points—then
you’ll be subtracting the level head from
the wet leg head, giving a positive value.
But be careful, since as the level drops,
the value of the signal will increase.
Reference links:
• www.controlglobal.com/assets/
wp_downloads/pdf/mma_070921_
endress_liquidlevelpart1.pdf
• https://instrumentationtools.com/
closed-tank-level-measurement-us-
ing-dp-transmitters
• www.emerson.com/documents/
automation/technical-data-sheet-lev-
el-measurement-pressure-rose-
mount-en-74346.pdf
• www.ibiblio.org/kuphaldt/socratic/sinst/
output/level.pdf
• https://automationforum.co/
open-closed-level-measurement
• https://automationforum.in/t/
what-is-lrv-and-urv-how-to-
obtain-lrv-urv-for-level-measurment-us-
ing-differential-
pressure-transmitter/2396
• https://instrumentationtools.com/
open-and-closed-tank-level-calculations
Alex (Alejandro) Varga
www.ControlGlobal.com
eHANDBOOK: Level Measurement, Part II 13
— LWT series guided wave radar With LevelExpert™: the expert inside
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Untitled-1 1 9/19/2019 9:44:55 AM
Though the majority of instrument
and controls engineers have an elec-
trical background (confirmed by the
surprise people still have when I tell them I
was trained as a chemical engineer and, like
them, “fell” into this profession), we all need
to remember that the reason we are install-
ing all our sensors, control elements and
control systems is to control and manage
the process. The point of this message is
that process dynamics need to be part of
the design process.
With wired devices that are not power-con-
strained, the update rate is decided by the
I/O card and controller. Battery-powered
wireless devices, however, do need to
manage their energy consumption, and the
most common way of doing so is by config-
uration settings of the update rate.
Though update rates for wireless sensor
networks (WSN), WirelessHART and
ISA100.11a can be as short as 0.5 sec., as
the update frequency is increased, there is
an associated exponential decrease in bat-
tery life. As expected, the largest impact
is at the faster update rates that might be
required for closed-loop control. Longer
update periods (beyond 60 sec.) are con-
strained by basic battery life physics more
than the update rate, thus setting the limit
on the slower-update-rate side of the
equations.
So how do we balance the update rate
and battery life? Basic control theory
recommends that the update rate of the
measurement shall be a minimum of three
times faster than the process time constant.
I personally prefer an update rate of six
Wireless sensor sweet spotsHow battery life interacts with update rate to define the most practical applications
By Ian Verhappen
eHANDBOOK: Level Measurement, Part II 15
www.ControlGlobal.com
times the process time constant, if possible,
because then I am sure to observe all stages
of an oscillatory process. However, using
the three-times-faster basis for a tempera-
ture loop (where measured temperature
changes with a sensor inside a thermowell
can be 16 sec. or longer, given how much
time is required for heat to penetrate the
thermowell and its mass), the required
wireless update rate would be roughly 5
sec. Since WSN cycles increase by doubling
each time, the closest approximation for
this loop is a 4-sec. update rate.
Industry practice and experience also rec-
ommend the update rate should be four
to 10 times faster than the time constant
of the process for regulatory closed-loop
control, so though it’s at the low end, the
4 sec. update rate would also work in
this example.
Another, non-process-related consideration
in addition to battery life is the impact traf-
fic may have on the network itself, and in
particular, the access point or gateway. One
WSN manufacturer recommends keeping
update rates no faster than 4 sec. since
doing so can impact the total number of
wireless devices that can be put on a gate-
way. Therefore, the 4 sec. update rate for
this example works well by meeting all
three minimum criteria.
Temperature is one example of a slow
process. Level measurement, especially
in large tanks, is another. These sorts of
measurements are well suited to wireless
sensing because they can operate with
slower update rates, and when you consider
that large tanks and tank farms are widely
distributed, not having to install cable infra-
structure makes a lot of sense.
With today’s computing power, the smart
people working in our industry have
developed a number of fancy tricks, such
as custom P&ID algorithms for wireless
networks that consider lag, other con-
trol algorithms (such as Smith Predictors,
developed in 1957), or other math to
compensate for the effects of delayed
measurements. (Some would say mask
rather than compensate, especially if
they are used improperly by a person not
understanding and applying first princi-
ples correctly.)
Despite all the advances we have made
and continue to make with our control
systems, it is always good to remember
why we are implementing the applica-
tion, as well as the underlying associated
engineering principles, and basic laws
of physics and chemistry that need to
be followed.
Not everyone makes a good controls engi-
neer, technician or practitioner, however, if
you remember the basic laws of physics and
chemistry, the chances of succeeding going
up significantly.
www.ControlGlobal.com
eHANDBOOK: Level Measurement, Part II 16
I’m optimistic about climate change, and
not only because the 2018 United Nations
Climate Change Conference has shown
that mankind is waking up, or because in
March, a 17-year-old girl was able to mobi-
lize an estimated 1.6 million high school
students in 125 countries around the world
to protest the present inaction about
global warming. I’m particularly optimistic
because of the contributions our tech-
nology—the monitoring capability of the
process control profession—are making.
Consider the fact that our scientific model-
ing was able to convincingly and accurately
predict the future and thereby give us
time to fix things. Consider that, based on
measuring only a few millimeters of ocean
level rise and less than a degree of global
temperature rise, our models were capable
of predicting that this process, if left uncon-
trolled, spells disaster for the future. This is
a fantastic achievement!
Here I’ll focus on the accuracy of two
additional measurements: the level of the
oceans and the changes in the mass of the
ice as polar and glacier ice formations melt.
Accurate measurements are important not
only to resolve the debate between deni-
alists and alarmists, but also to establish
the dynamics of the global warming pro-
cess, so we can more accurately predict its
speed, time constants, rate of rise and tip-
ping points.
Accurate measurement of ocean levels
must take into account that both sea and
land experience vertical motion. Land can
move vertically due to glacial and tectonic
Measuring oceans and iceAccuracy and precision support predictions and balance the sea level budget
By Béla Lipták
eHANDBOOK: Level Measurement, Part II 17
www.ControlGlobal.com
processes. Sea level is affected by tides
caused by the gravitational forces of the
moon and sun, and by “weak tides” (also
called meteorological tides) generated by
winds. Because these measurements are
on the scale of millimeters, not only accu-
rate detectors but sophisticated models are
needed to correct observed sea levels for the
effects of regular and meteorological tides.
The level models depend on two mea-
surements. One is the relative sea level
(the height of the water relative to the
land), which is corrected for any earth
movements. The second measurement
is provided by satellite altimeters, which
measure the distance between the ocean’s
surface and the center of the Earth. (Some
of the following paragraphs are taken
from a NASA document at https://sea-
level.nasa.gov/understanding-sea-level/
global-sea-level/ice-melt.)
ACCURATE ALTIMETERS
Laser altimetry: By the 1990s, laser altim-
etry from aircraft revealed thinning of the
ice sheets on Greenland’s coastal margins
[Abdalati, et al., 2001] and later surveys,
using NASA’s Airborne Topographic
Mapper, showed a further increase in that
thinning [Thomas, et al., 2009]. NASA’s
P-3B is a four-engine turboprop equipped
with radar and laser altimeters (Figure 1).
Researchers use highly sophisticated air-
borne instruments on these retrofitted
aircraft to measure annual changes in the
thicknesses and movements of the ice.
Satellite altimetry: A new NASA satellite,
Ice, Cloud and land Elevation Satellite
(ICESat2) was launched in September 2018
(Figure 2). It’s expected to gauge future
changes of the Greenland and Antarctic
ice sheets at an accuracy of the width
AIRBORNE TOPOGRAPHIC MAPPERFigure 1: NASA's P-3B four-engine turboprop is equipped with radar and laser altimeters to measure annual changes in thicknesses and movements of ice. Source: www.nasa.gov/mis-sion_pages/icebridge/instruments/p3b.html
SATELLITE MEASURES SHEET ICEFigure 2: NASA's ICESat2 (cloud and land ele-vation satellite) accurately measures ice sheet thickness at an accuracy of the width of a pencil. Source: www.click2houston.com/news/national/nasa-to-launch-laser-device-into-space-to-mea-sure-earths-polar-ice
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eHANDBOOK: Level Measurement, Part II 18
of a pencil. In the Arctic,
NASA’s Icebridge opera-
tion studies the effect of
the polar ice on the Earth’s
climate. ICESat2 measures
the time it takes for laser
beams to travel from the
satellite to Earth and back.
Based on that information,
scientists can accurately
calculate the height of gla-
ciers, sea ice, etc.
This is the largest airborne
operation to survey polar
ice, and it’s designed to
plug the gap between
ICESat that ceased func-
tioning in 2003 and the
launch of ICESat2 in Sep-
tember 2018. Researchers
used data from these
yearly surveys to deter-
mine the rate of ice
melting, and found it
was increasing [Thomas,
et al., 2011]. The study
also relied on data from
NASA’S Airborne Topo-
graphic Mapper, as well as
the University of Kansas’
ice-depth sounder, both of
which have made almost
yearly surveys since 1991
of the Jakobshavn Isbrae
glacier in Greenland, and
since 2002, of the Pine
Island glacier in Antarctica.
The early 1980s saw the
first attempt to measure
ice sheet thicknesses using
satellite radar altimetry,
with observations of only
limited parts of Greenland
and Antarctica [Remy and
Parouty, 2009]. The ice
thickness on Greenland is
about a mile, and about
three miles on Antarc-
tica. Since then, however,
altimetry technology has
advanced a great deal
and became an important
means of determining ice
sheet and glacial mass
balance, measuring the
gains and losses in ice
mass. While the primary
measurement is by laser
altimetry (ICESat2), with
high accuracy and a very
small footprint, radar
altimetry was also used
in the European Space
Agency’s (ESA) Cryo-
Sat-2 mission.
Radar interferometry: Sat-
ellite synthetic aperture
radar interferometry tech-
niques measure the speed
at which ice streams move,
as well as the position of
SATELLITE ALTIMETRY SHOWS ICE THINNINGFigure 3 : These photos from NASA's Ice, Cloud and Land Ele-vation Satellite reveal areas of dynamic thinning (red) in Ant-arctica and Greenland (left). Source: https://sealevel.nasa.gov/understanding-sea-level/global-sea-level/ice-melt
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eHANDBOOK: Level Measurement, Part II 19
the grounding line, which
separates ice grounded
over bedrock from ice
floating on the ocean, as it
breaks off from the same
ice stream. One example of
the power of this technique
was the measurement of
the retreat of Antarctic gla-
ciers [Rignot, et al., 2014].
The researchers used data
from the European Remote
Sensing (ERS) 1 and 2 syn-
thetic aperture radar to
measure the rapid retreat
of the glaciers in West
Antarctica. The study pro-
duced color-coded maps
(Figure 3) of the velocity of
recession for the glaciers,
which showed movement
of the glacial ground-
ing lines.
Satellite gravimetry: The
advent of gravimetric
measurements with the
twin Gravity Recovery
and Climate Experiment
(GRACE) satellites in 2002,
along with more recent
deployment of floating
Argosensors, opened
the way to closure of the
sea level budget—that is,
when the sum of observed
ocean mass and density
changes equals total sea
level change [Leuliette and
Willis, 2011].
GRACE measures changes
in water mass, including
terrestrial storage in the
form of groundwater,
rivers, snow and ice, and
mass changes in the ocean
itself, as well as the move-
ment of water between
land and ocean (Figure 4).
Early attempts didn’t
achieve closure of the sea
level budget for four-year
trend lines [Willis, et al.,
2008, Chang, et al., 2010],
leading to concerns about
possible instrument drift.
However, more recent
efforts led to reports of
closure for more extended
periods, including a NOAA
report, “The Budget of
Recent Global Sea Level
Rise, 2005-2013” by Eric
Leuliette, covering 2005 to
2013.
SEA LEVEL BY SATELLITEFigure 4: The advent of gravimetric measurements with the twin Gravity Recovery and Climate Experiment (GRACE) satellites in 2002, along with more recent deployment of floating Argosen-sors, opened the way to closure of the sea level budget (when the sum of observed ocean mass and density changes equals total sea level change). Source: http://ossfoundation.us/projects/ environment/global-warming/sea-level-rise
50
40
30
20
10
01994 1996 1998 2000 2002 2004 2006
Year
Sea
leve
l (m
m)
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eHANDBOOK: Level Measurement, Part II 20