pressure & temperature measurement · 2021. 1. 11. · rules of thumb for this most ubiquitous of...

SPECIAL REPORT

Pressure & Temperature Measurement

PART I

Pressure points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

Prevent steam reformer tube rupture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

Test options for pressure-relief valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Measuring the oceans’ heat content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

AD INDEXEndress+Hauser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

Watlow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

TABLE OF CONTENTS

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Pressure & Temperature Measurement, Part I 2

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Though I’m sure the percentage of field measurements based on pressure is decreas-ing from what I heard at one time was close to 80% of all signals, I’m confident the majority of control inputs continue to be pressure-based, including differential pres-sure (d/p) for flow and level inference . While many of us take pressure measurement for

granted, we must keep in mind several basic rules of thumb, such as placing sensors at the

top or side of a line, and using manifolds for isolation and maintenance .

The main reason for placing the sensing line in the correct location is to minimize the

chances of something (corrosion, unwanted fluids, etc) in the process affecting the reading .

One way in which I have had success in minimizing tap plugging is to use a diaphragm seal

instead of a 1/2- or 3/4-in . pipe nipple which, because of its narrow diameter, is more easily

“bridged” and hence plugged .

When using a diaphragm, I’ve learned the hard way to remember the following lessons . First,

specify the diaphragm face to be 1/2-in . to a maximum of 1-in . from the pipe face, especially

in slurry or abrasive service . This will prevent the face of the sensing diaphragm from being

scoured and damaged by the process fluids . Second, be sure to specify that the inside of the

nozzle is ground flush before attempting to insert the diaphragm, which will have a tight tol-

erance with the pipe wall . You don’t want to damage your meter, and then wait until the next

outage to complete the project because you “smashed the face” on a piece of slag .



Pressure pointsRules of thumb for this most ubiquitous of process measurements

by Ian Verhappen



As pressure meter diaphragms are generally

quite thin, be very careful when selecting

the face materials . In some cases, it may be

preferable to select a brittle ceramic rather

than one of the metals .

Another source of d/p transmitter problems

is the capillary—especially if it extends more

than about 10 feet . At this length, response

grows sluggish, and accuracy is susceptible

to temperature changes if the fill fluid isn’t

carefully selected . Heat tracing helps, but

makes this option quite expensive .

For these reasons and others, several

manufacturers now offer “electronic” im-

pulse lines, where two close-coupled (di-

rectly connected to the process with a

manifold for calibration and maintenance

when needed) transmitters are connected

electronically rather than via capillary . The

electronic-impulse-line approach requires a

minimum distance (hence, pressure drop)

between the taps to ensure an adequate

signal-to-process-noise ratio . Again, de-

pending on the process fluid this distance is

coincidentally about 10 feet .

WIRELESS REMOTE SEALS?Electronic impulse lines have been in use for

about a decade, and I believe it will only be

a matter of time before someone replaces

the electronic impulse line with a wireless

option . To do so, however, will required

continued improvement in wireless power

supplies .

Wireless could also be used to connect an

at-grade transmitter with local indication

to the sensing head in the pipe rack—again,

with a potentially proprietary connection to

keep down costs, arguably for security, and

of course to keep out competitors .

Fortunately, if you have a reasonable rela-

tionship with the supplier of your transmit-

ters, they’ll consider many of the above

items for you, or at least be asking the

questions .

Lastly, though it may be intuitive when you

look at the way pressure transmitters are

made, practically all pressure transmitters

are based on differential pressure sensing,

with the one leg open to atmosphere and,

hence, measuring gauge pressure .

Pressure measurement may be ubiquitous

but that doesn’t mean that we can take it

for granted . In fact, the argument can be

made that because it’s so widely used, it

should be better understood . Hopefully

some of the points made this month will

spur that conversation .

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Q: The tubes in our methanol plant steam reformer ruptured because of overheating, and

the investigation team recommended installation of UV flame detectors. I had bad expe-

rience with UV detectors in furnaces because they led to false alarms, trips and process

disturbances. Can you please guide me on this issue?

Safdar Toor

Brega, Libya

safdarrashid@hotmail .com

A1: Your investigation team is wrong, and your question deserves a more

detailed explanation.

In steam reforming, a mixture of gases at high pressure and temperature are sent through

catalyst-filled pipes, where they break down into their constituents and, depending on the

catalyst used, recombine (reform) into product gases . If desulfurized hydrocarbons are

mixed with steam (water), this process can be used to produce hydrogen-rich synthesis

gas, methane, methanol, ammonia, etc .

This is a heat-balance process, where the heat sources are burner heads on the outside of

catalyst-filled pipes, located inside refractory-walled furnaces . This heat passes through

the pipe walls and heats the gases in the radiant section of the reformer furnace, reform-

ing the gas mixture as it passes through the pipes . Under steady state conditions, the



Prevent steam reformer tube ruptureInfrared and thermocouple solutions are best for preventing overheating

by Béla Lipták



temperature of the reformed gas product

leaving the furnace (TI-5 in Figure 1) is

controlled constant by modulating the fuel

flow to the furnace burners . Plant produc-

tivity rises with temperature, but the pipe

temperature must stay below the “creep

limit” of the pipe . Tube wall temperatures

range from 700 °C to 1,200 °C (1,292 °F to

2,192 °F) and industry rule of thumb is that

a 20 °C increase in tube wall temperature

above its maximum allowable temperature

will halve the tube life . So, plants maximize

procuction by operating as close as pos-

sible to the temperature limits of the pipes

without exceeding them .

The startup of these processes is slow, and

speeding it up can cause accidents because,

if the heat input of the pipes increases with-

out a commensurate cooling by increased

gas flow inside the pipes, overheating can

occur . Catalyst maldistribution may cause

local overheating or channeling and, hence,

shorter life of the reformer tubes . The cata-

lyst should be mechanically stable under

all process conditions, as well as condi-

tions during start-up and shutdown of the

reformer . In particular, resistance to condi-

tions during upsets may become critical .

Breakdown of catalyst pellets may cause

partial or total blockage of some tubes .

Similarly, during the production phase,

build-up of catalyst powder or coke

formation on its surface can increase

pressure drop through the pipe, reduce

gas flow, and cause overheating . The

lifetime of pipes is often estimated us-

ing the Larson-Miller parameter (LMP),

which predicts the lifetime of material vs .

time and temperature . This parameter

is usually expressed as LMP = T(C + log

t), where C is a material-specific con-

stant (often approximated as 20), t is the

time in hours, and T is the temperature in

Kelvins . Because reformer tubes expand

when heated and because this expansion

is related to the tube’s temperature, over-

heating can theoretically be detected by

measuring “tube growth .” (In this answer

to your question, I will not cover such—in

my view—unreliable techniques .)

CATALYST PROTECTION, EXCESS STEAM CONTROLSSteam must be provided in excess of the

reaction requirements to prevent the side

reaction of coke formation on the cata-

lyst . The coking of the catalyst deactivates

it, and can even plug the pipe, requiring

expensive replacement . To minimize cok-

ing, steam is usually supplied in a ratio of

3 .5:1 by weight, relative to feed gas . Figure 1

shows the contol system that maintains the

excess steam at all times, since even a few

seconds of interruption in the steam flow,

while feed gas continues, can completely

ruin the catalyst charge .



As Figure 1 also shows, feed gas flow is

maintained by using a pressure-compen-

sated flow controller (FIC-1) . Pressure com-

pensation of flow corrects the measure-

ment for fluctuations in feed gas pressure .

The steam rate is maintained by means of

FRC-2, and the ratio of steam to feed gas

flow is manipulated to keep the steam-

hydrocarbon ratio constant .

The ratio relay (RY-2) divides the gas flow

signal (FT-1) with the steam flow (FT-2) . If

this ratio falls below approximately 3:1, a

low flow ratio alarm (LRA-2) is sounded,

and if the ratio continues to fall below ap-

proximately 2 .7:1, the feed gas is shut off by

closing the valve (MV-1) .

If there’s no separate shutoff valve (HV-1),

the feed valve (FV-1) must be a quick-clos-

ing valve (4 to 5 seconds for full closure), so

the gas flow can be stopped almost instant-

ly, thus protecting the reformer catalyst .

An effluent analyzer (not shown in figure)

can be used to determine reaction comple-

tion by measuring the product concentra-

tion in the effluent stream . This analyzer can

be either an infrared or a chromatographic

type . Because of the excess steam present,

water removal devices are required in the

analyzer sampling system . The degree of

conversion can be controlled by the furnace

temperature profile, and the fuel flow to

the burners can be throttled by the effluent

temperature (TI-5) . During plant transients,

PT1

FT1

FY1

LFA1

FRC1

FV1

MV1

LRA2

RY2

LFA2

FY2

LLRS

FRC2

FT2

T14

T15

TJA3

FLUE GAS

HYDROCARBON GAS FEED

STEAM TOCARBON RATIO

LOW RATIO ALARM

SUPERHEATEDSTEAM

LOW-LOW RATIO INTERLOCK

PREHEAT COIL

AIR & FUEL

PEEPHOLE

PRODUCT GAS

CATALYST FILLED TUBES

FURNACE

S

M

FO

÷

Figure 1: A key reformer control challenge is to ensure that radiant heating of the tubes is balanced by sufficient flow of gas through the catalyst-packed tubes.



when flow rates are well below design val-

ues, reliance on standard plant instrumenta-

tion, such as in Figure 1, (controlling header

or pigtail exit temperature) alone is danger-

ous because under low-flow conditioons,

the reformer gas exit temperature doesn’t

indicate the temperature of the tubes .

Now, turning to your question concerming

tube overheat detection in furnaces, the

two most commonly used principles are

based on either thermocouple (TC) or infra-

red (IR) technologies . These topics are too

large to be discussed in this column . (They

take up about 1,000 pages in the 5th edition

of my handbook .) Therefore, I’ll make only

a few brief comments about them . I might

also mention that production departments

tend to use spot infrared pyrometers, while

inspection departments often use portable

thermal imagers .

INFRARED PYROMETERS THERMAL IMAGERS Every object radiates IR energy, in propor-

tion to the temperature and the emissivity

of the object . Noncontact IR thermometers

measure temperature by detecting the

intensity of this radiated energy . The re-

former imager allows measurement of the

tube wall temperature profile along the full

length of the reformer tube, and will allow

measurement of the maximum tube wall

temperature . A further benefit with the

reformer imager is it provides a high-resolu-

tion image rather than a single-point mea-

surement . It would be impractical to take

the same number of measurements as the

reformer imager does using point sensors .

When measuring the surface temperature

of reformer tubes, the main limitation of IR

sensors is that the emittance of both the

target and its background varies because of

intervening dust, smoke, moisture and dirt .

These all affect the amount of radiated IR

energy received by the detector .

GOLD CUP IR PYROMETERSGold cup pyrometers are the most ac-

curate contacting tube wall temperature

sensors . These spot temperature detecting

pyrometers measure the surface tempera-

ture of objects by simulating a blackbody

condition in order to eliminate errors

caused by reflected temperature sources .

By using a hemispherical reflector, a mea-

surement area is produced that is indepen-

dent of emissivity .

Such a radiometric infrared borescope

imaging camera is ideal for measuring the

temperature of tubes of unknown emissiv-

ity . It measures temperatures from 600 to

800 °C (1,122 to 3,272 °F), and is ideal for

applications with high differential tem-

perature between the target and its sur-

roundings . These sensors are often used

as reference sensors to calibrate ther-

mocouples or non-contact thermal imag-



ers . They’re relatively expensive, hard to

manipulate, and their operation requires a

fair amount of training .

THERMOCOUPLESThermocouples are usually welded onto

the tube’s surface or embedded in the tube

wall, and connected to a reader or recorder .

They’re inexpensive point sensors . Their

limitations include that, due to weld mass,

they don’t detect the actual tube tem-

perature . They also drift and are difficult to

shield against furnace radiation or recali-

brate . Also, they don’t last long in the harsh

furnace environment . Daily Thermetrics’

CatTracker system claims to use special

thermocouple technology that can handle

the harsh environment, when directly con-

nected to the reformer tubes, overcoming

these limitations .

Béla Lipták

liptakbela@aol .com

Headlinedeckbyline

Q: Are there testing systems commercially available for verifying the discharge capacity

and blowdown of pressure-relief valves (PRVs)? I want to perform all tests required by NB-

18 and ASME for their PRV certifications. Also, can you introduce me to some suppliers? Fi-

nally, is it possible to create a simulation unit (pilot unit) to examine the discharge capacity

in addition to performing cold differential test pressure (CDTP) and seat tightness testing?

Medhi Mehdipoor

mehdipoor .mehdi@gmail .com

A1: Allow me to start by defining some of the pressure terms that are used in connection

with PRV operation and testing:

• Maximum allowable working pressure (MAWP) is the design pressure rating of the equip-

ment that the PRV protects .

• Set pressure is the pressure at which the PRV is set to start to open . Set pressure can

equal, but not exceed MAWP .

• Reseat pressure (seal-off pressure) is the pressure at the valve inlet after closing . At that

point, process fluid is no longer detected downstream of the seat .

• Overpressure is the pressure increase (accumulation) over the set pressure caused by the

pressure drop through the downstream piping of the PRV . It’s 6% for steam systems gov-

erned by ASME, Section I, Power Boiler Code . If the set pressure of the valve is less than

the MAWP, the overpressure can be 10% or more .



Test options for pressure-relief valvesSome parameters can be verfied in house, others will require outside assis-tance

by Béla Lipták



• Blowdown is the difference between the

set and the reseat pressures caused by

the pressure drop through the inlet piping

to the PRV . It’s recommended to be 4%

to 6% per ASME I and 7% per ASME VIII .

If more, the valve closes slowly, if less, it

might chatter .

• Chatter (simmer, warn) is the conse-

quence of too small blowdown or of the

PRV being oversized . This is the condition

just prior to opening, at which point zero

or negative forces are holding the valve

closed . Under these conditions, as soon as

the valve disc attempts to rise, the spring

develops enough force to close it again .

In general, in-place testing of PRVs isn’t

performed because of the high cost of the

necessary valves and bypasses . PRVs for

boiler overpressure protection require code

stamps to certify that their capacity and de-

sign meets ASME’s boiler and pressure ves-

sel code, Section 1 . These PRVs are certified

to reach full lift at 3% over setpoint . Section

II of API-520 also endorses this pressure

loss limit for the inlet piping .

Normally, pressure relief valves are tested

with air or nitrogen, and often there’s not

enough gas capacity to verify discharge

capacity . PRVs must be serviced after

each time they pop, however, and servic-

ing should include the replacement of the

valve seat gasket . Also, it’s a good idea to

detect their popping by regular or wireless

limit switches .

All PRVs should be tested before their instal-

lation, after each fire event, and at a frequen-

cy set by the plant . Certified technicians are

required, and the testing itself is done either

in a test workshop or, if field connections

are provided, in situ . After testing, inspec-

tors should check for foreign objects, weld-

ing beads, rust, signs of tampering, leakage,

erosion, wear and corrosion, and verify that

the flanges are clean . In addition, they should

check if the tag is correct, securely tied, and

that the test results are properly reported .

For more information on National Board- and

ASME-accepted testing laboratories, visit:

https://www .nationalboard .org/index .

aspx?pageID=142&ID=63 and

https://www .asme .org/certification-accreditation/pres-

sure-relief-device-testing-laboratories

Béla Lipták

Liptakbela@aol .com

A2: The National Board (NB) has a VR

Stamp program that incorporates all areas

of relief valve repair and testing, and part of

that effort includes the test stand itself . This

PowerPoint presentation provides a lot of

detail on the accuracy of setting the valves,

including details on the test bench:

http://www .asmeconferences .org/nrcasme10/

pdf/3b/3b_prv .pdf

There are several manufacturers who make

test benches . If you search “relief valve test

equipment,” you’ll find them . Each manu-

facturer offers a variety of options but the

most important features are:



• Volume: The more volume the test stand

has, the more accurate the PRV setting

will be .

• Minimal piping: The volume chamber must

be as close as possible to the valve . Even

a few inches can make a large difference

when the PRV lifts .

• NIST traceability of the pressure measure-

ment .

Before buying or building a test bench, you

must decide what size range of relief valves

you want to test and what is the maximum

test pressure you’ll test . (Obviously, a bench

that can handle larger valve sizes and/or

higher pressures will cost a great deal more .)

One last thing: the NB test centers have

tremendous steam/air/water capacity at

their test centers . (Some are tied to working

power plants!) Your test bench will never

have that kind of capacity, so you won’t be

able to set the blowdowns very accurately .

P . Hunter Vegas

phvegas50@gmail .com

A3: To the best of my knowledge, the relief

valves you install aren’t designed for tight

shutoff once they’ve “popped .” The ratings on

relief valves and their testing is a “type test”

PRESSURE RELATIONSHIPS FOR PRV TESTING Figure 1: This schematic illustrates the relationships among key press relief valve design

and operating parameters.

Pressure% of Set Pressure

110

100

95

90

Where PRVs normally sized(except fire case)

Set pressure tolerance

Reset pressure of PRV Typical operation pressure

MAWPTypical set pressure

Allowableoverpressure

Blowdown % of set pressure

Leak test pressure



performed by the manufacturer . In a typi-

cal application, the relief valve is designed to

relieve overpressure in a tank/vessel at some

relatively high pressure to avoid rupturing

the tank/vessel . Since the relief fluid is almost

always a gas or vapor, and the blowdown col-

lection system is at atmospheric pressure, the

flow through an open relief valve will be at

sonic velocity; sonic flow is highly predictable

and isn’t dependent on the degree of opening

of the relief valve . The rating of a relief valve

as supplied by the manufacturer is the result

of tests performed on air pressure and gives

the capacity of all that manufacturer’s valves

of that size .

You don’t need to test relief valves in service,

and you shouldn’t test them unless you intend

to service the valve and replace the valve

seat gasket after your test . In fact, a relief

valve must always be serviced each time it

pops . Unfortunately, most installations have

no measurement to detect the popping of

a relief valve, resulting in leakage of process

gas/vapor to the blowdown system or at-

mosphere . This isn’t acceptable for process

safety or in the case of toxic gases/vapors

for preservation of human life . In either event,

atmospheric contamination will likely occur .

My advice is to install a simple limit switch to

detect the popping of a relief valve . If wiring

this limit switch is too costly, I suggest using

any of several wireless transmitters built for

that purpose, or a wireless limit switch .

Dick Caro

RCaro@Caro .us

A4: There are methods of measuring the

discharge capacity during relief-valve test-

ing, but you must be aware that said mea-

surement is not standard, so many compa-

nies will not offer the services .

I’d suggest you contact the relief valve

manufacturer to see if they offer that ad-

ditional test .

Normally, the relief valves are tested with

air or nitrogen to see when they open, so

there’s not enough volume to perform a

discharge capacity test . For that, additional

air or nitrogen needs to be supplied and a

flowmetering station needs to be added—all

at an additional cost .

Alejandro Varga

vargaalex@yahoo .com

A5: To determine discharge capacity and

blowdown during test, a huge capacity test

bench is required—likely available only at

ASME laboratories in the U .S . Some PRV

companies in India like Tyco-Sanmar have

large boiler test facilities up to 2,000-psig .

But even there, blowdown and capacity

can be measured only on small-to-medium

sized valves . To cover the full range of a

larger sized PRV, an ASME laboratory is

the right place .

H S Gambhir

Harvindar .S .Gambhir@ril .com

Headlinedeckbyline

More solar energy is radiating down onto our planet than the energy radiated back into space . This difference is called radiative forcing (RF), and according to the Fourth Assessment Report (AR4) of the United Nations Intergovernmental Panel on Climate Change (IPCC), the current level of radiative forcing is 1 .6 watts per square meter

(the range of uncertainty of this value is from 0 .6 to 2 .4) . 93% of this extra energy is absorbed

by the oceans, resulting in a yearly rise in the ocean’s temperature of about 0 .015 °C/yr .

While this temperature rise is small, the amount of energy required to cause it equals about

50 times the total energy use of mankind or the energy released by a couple of Hiroshima

bombs every second . Global warming has heated the oceans for the past 150 years, which

means that while the atmosphere has been spared from the full extent of global warming

for now, this heat already stored in the oceans will eventually be released, causing addition-

al warming of the Earth in the coming decades .

Measuring the heat content of the oceans is a more accrate indication of global warming

than its surface temperature . The heat content of the oceans has increased by about 15 x

1022 Joules in the last 30 years or about 0 .5 x 1022 Joules each year, and is still rising . Natu-

rally, the total heat received by the planet is more than what is absorbed by the oceans,

but the oceans still absorb the bulk of it; oceans reflect less than 10% of radiation received,

while the planet reflects about 34% overall .



Measuring the oceans’ heat contentThe Argo project profiles temperature and salinity at 3,000 points around the globe

by Béla Lipták



We should also remember

that, even if we were able

to cut our emissions to zero

today, the carbon already

accumulated in the atmo-

sphere will still mantain the

elevated greenhouse effect .

Plus, the heat that’s accu-

mulated in the oceans will

continue to heat our planet

for decades or centuries to

come . In other words, a very

small rise in the tempera-

tures of the oceans indicates

the absorption of immense

quantities of heat .

So, how do we measure this

yearly, oceanic temperature

rise of 0 .015 °C/yr, when the

detection error of our most

accurate Class AA RTDs is

on the order of ±0 .01 °C?

The answer is we can’t, at

least not with traditional

methods of temperature

measurement .

TEMPERATURE BASED ON HEAT CONTENTMeasuring the rise in the

surface temperature of the

oceans doesn’t represent

the total heat absorbed

because that tempera-

ture changes with depth .

Therefore, a more accurate

reading is obtained if aver-

age temperature along with

that profile is measured .

Today, the Argo project is

doing just that—measuring

temperature and salinity

profiles from the surface to

2,000 meters deep at 3,000

points around the global .

The project was conceived

and designed by U .S . aca-

demics and scientists at the

National Oceanic and At-

mospheric Admindstration

(NOAA) to obtain a uniform

and systematic method

for measuring the physical

state of the upper layers of

the oceans, and place that

information into climate

computer models in near

DESIGN OF THE ARGO SOLO-II FLOATFigure 1: A hydraulic bladder at the tip of the float and a pneumat-ic bladder on the side serve to vary the float's density. When the density rises the float sinks, and when it decreases the float rises.(Courtesy of Michael McClune at Scripps Institution of Oceanograpy at http://www-argo .ucsd .edu/How_Argo_floats .html)

Temperature probe

Air pump/valve

110 cm

Gear motor

Single stroke pump

Batteries

Pressure sensor

Circuit boards & satellite transmitter

Stability disk

Sataellite antenna

Pneumatic bladder

Hydraulic pump (piston)

Hydraulic oil

Hydraulic bladder

Foam support



real-time . The name ‘Argo’

comes from the name of the

ship Argo in the tale of Ja-

son and the Argonauts told

in Greek mythology .

Argo uses robotic floats

(Figure 1) that spend most

of their life drifting below

the ocean’s surface . They

make temperature and

salinity measurements, and

when they surface, they

transmit collected data to

satellites . After that, they

sink again to drift for anoth-

er 10 days . Currently, these

drifting floats, produce

some 100,000 tempera-

ture/salinity profiles per

year . Compared to satel-

lite observations of surface

temperatures, profiles gath-

ered by these floats provide

much better data for better

understanding and quanti-

fying climate change .

The Argo floats use den-

sity change to drive their

movements . The density of

any object is the ratio of its

weight and its volume . If

the density of an object in

the ocean drops below that

of the water around it, the

object will rise and when its

density is greater than the

surrounding water, it will

sink . The Argo float (Figure

2) uses flexible bladders to

change its volume (just as

fish do) . A hydraulic blad-

der at the tip of the float

and a pneumatic bladder on

the side, serve to vary the

float’s density . When the

density rises, the float sinks

and when it decreases, the

float rises .

Roughly 3,000 Argo floats

are deployed in the ice-free

areas of the world’s oceans,

profiling temperature and

salinity down to a depth of

about 2,000 meters . These

floats dramatically reduce

100,000 PROFILES ANNUALLYFigure 2: At 10-day intervals, each of the 3,000 Argo floats sinks to a depth of 1,000 meters, where it drifts for nine days. Following that, it sinks to a depth of 2,000 meters, and then slowly rises to the surface over a six-hour period, during which it collects the tem-perature and salinity profile along the way. When at the surface, it transmits profile data to a satellite, and the cycle starts again.

20 min on sea surface

9 days drifting

1000 m

Collect T/S profile on ascent

2000 m



the amount of time and cost of gathering

such data by research ships using tradition-

al over-the-side methods of ocean observa-

tions . On top of that, the data gathered is

accurate to about ±0 .002°C .

These floats are battery-powered, autono-

mous units that spend most of their life

drifting at a “parking depth” of about 1,000

meters for about 9 days during each mea-

surement cycle, where they’re stabilized

by automatically matching their density

with that of the water around them (Fig-

ure 2) . At 10-day intervals, they automati-

cally reduce their own volume by lowering

the volume of their bladders by pumping

some of the fluid from them, and increas-

ing the density of the float, so it will sink

to a depth of 2,000 meters . After that, the

float automatically reduces its density by

increasing its volume (expanding its blad-

ders ) and rises to the surface over about

a 6 hour period, during which it measures

the temperature and salinity in profile .

When they surface, the floats transmit

their profile data to satellites . After about

20 minutes on the surface, the bladders

deflate, the float sinks, and the cycle is

repeated . The floats are designed to make

about 150 such cycles yearly . By this

method, a relatively accurate heat content

of the oceans can be calculated, and when

tracked over time gives an accurate mea-

sure of the oceans’ rate of warming .

APPROXIMATE SURFACE TEMPERATURES AND RATES OF INCREASEThe average temperature at the surface of planet Earth is currently 15 .1 °C (59 .0 °F), ris-

ing at the rate of 0 .15 °C (0 .27 °F) per decade . Over land, the average surface tempera-

ture is currently 10 °C (50 °F), rising at the rate of 0 .1 °C (0 .18 °F) per decade . And at the

surface of the ocean, the average temperature is currently more than 17 °C (62 .6 °F),

rising at the rate of 0 .25 °C (0 .5 °F) per decade . Since pre-industrial times, that's up 1 .1

°C (2 .0 °F) on average, inclusing 1 .5 °C (2 .7 °F) over land and 0 .7 °C (1 .26 °F) over the sea .

pressure & temperature measurement · 2021. 1. 11. · rules of thumb for this most ubiquitous of...

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