Underway pCO2

Craig Neill CSIRO Oceans and Atmosphere

- History of the G.O. system- Description of the components and how it works- Some of the contributions to uncertainy- Importance of diagnostic sensors- Installation on vessels- Lab practices

NOAA Miami workshop October 2002 Expert panel to produce design criteria for an underway pCO2system for VOS.

• https://www.aoml.noaa.gov/ocd/gcc/uwpco2/workshops/workshop2002/specs.html

• Produced technical criteria for a contract to build an underway pCO2

system for NOAA VOS program, in a nutshell it should be:

• Robust

• Accurate

• Designed for unattended use

• The latter two items actually conflict with the first one

1st generation: 6 systems produced in 2003 by CCN Consulting (me)Equilibrators made by Offshore Analytical (Dave Chipman)

2003 Inter-comparison exercise in Japan

A very useful exercise

Vent equilibrator

Air return from NDIR

Air to NDIR

The inter-comparison excesise showed a small bias toward ambient air concentration which was fixed by added a pre-equilibrator to the vent of the main equilibrator.

2nd generation: 12 systems produced in 2004 at the University of Bergen

Chilled condenser to remove more water vapour

PID control of EQU air pump and cooling fan

Magnetic H2Oflow meter

NOAA workshop on data reduction procedures, October 2005

G.O. system from 2006 - ???

2009 Inter-comparison exercise in Japan

Two G.O systems: NIO India and NOAA PMEL

Good agreement with most other systems

• pCO2 – partial pressure, micro-atmospheres, mechanical pressure

• fCO2 – fugacity, micro-atmospheres, chemical reactive pressure, this is what drives the water-air equilibration

• xCO2 – mole fraction, parts per million (ppm), this is what the NDIR or spectrometer measures.

Dry box

Wet box

PNDIR (built-in)

PNDIR

(added)

PAtmosphere

Pequil

(differential)

Pressure sensors

+/- 1 hPaAtmospheric pressure range

Atmospheric pressure range

Low delta magnitude depends on elevation difference and ship, ventilation system

TNDIR (built-in)

TCondenser

TEquil

TIntake

ca. room temp + 5 to 10 degTintake + 0 to 1 deg

(constant delta)

ca. 4 deg, stable

In-situ

FNDIR air

Fequil-vent-air

Fequil, seawater

70 sccm reference gas100 sccm equil or atm

0-10 sccmreplace often

Determine minimum for each system

Floor of wet box

Condenser overflow (for both EQU and ATM tubes)

LI-7000 NDIR

Diagnostic parameters:

FlowV Reference cell flow rate estimate, voltsCO2 AGC gain being applied to CO2 channelH2O AGC gain being applied H2O channelRH % %RH in the chopper housingDiag Multi-component flag, should be zero

Diagnostics from calculated deltas

• Reference gases• xCO2ref – xCO2meas for individual gases

• Atmospheric pressure deltas (lab, outside, equilibrator)

• Tequil – Tintake• Ideally < 0.5 deg

• Must be < 1.0 deg

• Whatever it is, it should be stable

Pressure measurements

• Equilibrator pressure sensor is differential – it measures the difference in pressure between the equilibrator headspace and the room

• The Licor NDIR pressure sensor measures absolute pressure in the room

• The absolute equilibration pressure is the sum of the two

The licor pressure sensor must be well calibrated, and checked regulary, to get accurate equilibration pressure. An easier but more expensive option is to add an accurate, stable pressure sensor (Druck or other).

• Measure Patm and correct to sea level

Seawater temperature

• Must have accurate temperature in the equilibrator and at the seawater intake.

• Intake sensor must be:• Upstream of the pump

• Ideally right at the intake

• Insulate all pipes if possible

• check with hand-lowered CTD and correct if necessary

C_AGC

A quality flag of “A” OR “B” in SOCAT has several requirements related to calibration

Minimizing uncertainty in equilibration temperatureSeawater jacketed equilibrator for cold waters

Warming of seawater spray on the walls of a non-jacketed equilibrator can lead to a bias of 2 ppm

Equilibrator response time

Response time is dependent on H2O flow rate. If you want to correct for it, measure at a certain flow and keep it costant.

Reference gases

• Should be in natural air*

• WMO traceable*

• Ideally Aluminum cylinders*

• Check steel cylinders for drift at end of life*

• End of life means cylinder pressure of 300 PSI, not empty

* all of these have stricter requirements for high quality atmospheric measurements

H2O correctionsDilution

done automatically by LI-6262NOT done automatically by LI-7000

mistake in Pierrot et al says that both analyzers do this correction

Band broadeningDone automatically by both (as the G.O. system configures them)

Proper H2O corrections require proper H2O measurements

Set the NDIR water channel zero using dry reference gas

Do not touch the water channel span unless you have a dew point generator or another way to produce a reference (I can show you one).

Diagnostics

Dry reference gases should read zero water vapor (assuming no plastic tubing)

If the drying system is working and H2O span is not way off, EQU and ATM xH2O will be about 1.2 – 2.2 mmol per mole

Sequence of analyses

Leak testing: blow where air will be drawn in if there is a leak.

In some parts of the system this on the suspect

fitting, in others it is the equilibrator vent.

Equilibrator vent

Licor OF-CEASLI – 7815 CO2/H2OLI – 7810 CO2/H2O/CH4

Picarro CRDS

Many species combinations available including some stable isotopes.

As far as know the equilibrator design has not been assessed for other gases. Some papers on d13C fractionation.

Los Gatos Oa-ICOS

Other analyzers

Scrubbers

Must be maintained

Setting air flows

Air pump repair

Gas regulators

• Flush regulators, see:

• Allow time for equilibration with new gases

• Leak test high and low pressure side separately

• Two stage, SS diaphragm, low internal volume

https://www.esrl.noaa.gov/gmd/ccl/reg.guide.html

Installation on VOS ships

Equilibrator drain

Some marine growth protection systems alter pHDon’t take the chief’s word that the water is unalteredValidate!

Keep dirt out of gas systems!

Quick ID of metric andimperial Swagelok

Rules for tightening Swagelok

• 1/16” and 1/8” – ¾ turn past finger tight

• 1/4” and 3/8” – 1 ¼ turns past finger tight

• When remaking a fitting only tighten a tiny bit past finger tight (1/16 of a turn max).

• Fittings can only be remade a limited number of times before they become over-swaged

Overtightening

• If the tube is stuck in the fitting it has been overtightened – cut off the end and remake with new ferrules

• If the nut does not spin freely on the fitting it has, at some point, been overtightened so much that the fitting body has been expanded – throw away the fitting body as well

Properly made 1/8” fitting

¾ turns after finger tight

Over-tightened fitting

It took a bit of force to pull the tube from the fitting body – cut the tube and remake with new ferrules

Massively overtightened fitting

Had to use pliers to pull the tube from the fitting. Fitting body has expanded making the nut hard to turn – replace everything!

Trying to put the tube from the last photos on the previous slide into a new, undamaged, fitting body.

It won’t fit because the tube end has been forced to expand radially.

Trick to be able to remake fitting more times

• Instructions say to push the tubing all the way into the fitting before making it up (tightening the nut to compress the ferrule)

• If you pull the tube back VERY SLIGHTLY (0.5 – 1 mm) before tightening, it will allow the connection to be remade several times before the end of the tubing gets jammed into the fitting body.

• This will create a small dead volume – do not do it in chromatography or inside a system where it should never have to be undone.

• Always do a visual check that the tube was not pulled back too far –eg, make up ½ turn, undo to check ferrule position then do it up with the final ¼ turn.

Valco fittings

Pipe Threads

Thread identification guide:https://www.swagelok.com/downloads/webcatalogs/en/MS-13-77.pdf