FixO3 Fixed point Open Ocean Observatories Network Grant ... · global temperatures to rise by at...

FixO3

Fixed point Open Ocean Observatories Network

Grant Agreement Number: 312463

The work described in this report has received funding from the European Union Seventh framework Programme (FP7/2007-2013).

Work Package 12

RESEARCH AND DEVELOPMENT ON CRITICAL

OBSERVATORY FUNCTIONS

Deliverable 12.5

Conclusion on the capabilities of the

different pH sensors

Lead beneficiary: ULPGC

Lead author: Melchor Gonzalez Dávila (ULPGC) [email protected]

Contributors: Vanessa Cardin (OGS), J. Magdalena Santana-Casiano (ULPGC),Yves Degres

(NKE), Per Hall and AndersTengberg (UGOT) and with the special contribution from

WP11 researchers Sue Hartman and Katsia Pabortsava (NOCS)

Work Package leader: Jean Francois Rolin, IFREMER, [email protected]

Due date: Project Month 42 (02-2017)

Dissemination level: RE

mailto:[email protected]

FixO3 Deliverable 12.5 / Conclusion on the capabilities of the different pH sensors

2

Contents 1. INTRODUCTION .......................................................................................................................... 3

1.1. Background and objectives ................................................................................................. 3

1.2. Aspects of this report ........................................................................................................... 4

2. RESULTS AND DISCUSSION ................................................................................................... 6

2.1 PAP-SO time series .................................................................................................................. 7

2.1.1 pH sensors on DY032 (July 2015-April 2016) ......................................................... 9

2.1.2. pH sensors on DY050 (April 2016-April 2017)....................................................... 14

2.2 Measuring pH in the Arctic Ocean: Colorimetric method or SeaFET? ............................ 17

2.3. Intercalibration exercise in the Koljo Fjord. ........................................................................ 18

2.4 Recent experiences from field measurements with pH optodes ...................................... 20

3. CONCLUSIONS AND OUTLOOK ........................................................................................... 23

4. REFERENCES ........................................................................................................................... 24

5. ACRONYMS ............................................................................................................................... 27


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1. INTRODUCTION

1.1. Background and objectives

Under a business-as-usual scenario, climate change is expected to be associated to an increase

of atmospheric carbon dioxide (CO2) concentration exceeding 500 parts per million and

global temperatures to rise by at least 2°C by 2050 to 2100, values that significantly exceed

those of at least the past 420,000 years during which most extant marine organisms evolved

(Collins and Knutti, 2013). The oceans have been recognized as an important sink of CO2

emissions from anthropogenic sources (e.g., Doney et al., 2009; Sabine et al., 2004). This is

in large part due to time series observations and models for estimating ocean–atmosphere

interactions. Dissolved CO2 in the ocean's surface produces carbonic acid that dissociates

releasing hydrogen ions. The net result of CO2 uptake is an increase in hydrogen ion

concentration – hence a decrease in pH – commonly referred to as ocean acidification (OA)

(Doney et al., 2009; Orr et al. 2005). Ocean pH has declined by 0.1 units compared with

pre-industrial measurements (Haugan and Drange 1996). Several reports projected that

continuous emissions of CO2 will cause a further reduction of pH between 0.3 and

0.6 units by the end of the century (Caldeira and Wickett 2005; Intergovernmental Panel

on Climate Change (IPCC) 2014; Joos et al. 2011).

The need for understanding the changing chemistry of the ocean and the impacts of OA on

marine ecosystems has catalysed the development of platforms and technologies capable

of observing the marine carbonate system at suitable space–time resolution (Reggiani et

al., 2016). Gathering the required field data using conventional sampling efforts (i.e.,

research cruises) is demanding in terms of costs and logistics, and, due to relatively poor

spatial and temporal coverage, do not have the required resolution for observing high

frequency or regionally-specific variability (Hofmann et al., 2011; McNeil and Sasse,

2016; Reggiani et al., 2016). The commercial availability of instruments for measuring the

seawater carbonate system variables – partial pressure of carbon dioxide (pCO2), pH, total

alkalinity (AT), and total dissolved inorganic carbon (CT) – is a testament of the maturity

of technology for underway and autonomous OA observations (Martz et al., 2015). In the

case of AT and CT, reliable and high quality measurements still rely on benchtop set-ups

for fulfilling “climate” level analytical figures of merit (Newton et al., 2014). In terms of

autonomous carbonate system observations, a promising approach comes from

simultaneous spectrophotometric detection of CT and pH, in which a three-week time-

series deployment of an autonomous system achieved ~4 μmol kg−1 and 0.0025 accuracy,

respectively (Wang. et al., 2015). pCO2 and pH sensors are the most developed, advanced

and used sensors.

Following this recognizing idea, FixO3 project included in its development an specific

work package, WP12– RESEARCH AND DEVELOPMENT ON CRITICAL

OBSERVATORY FUNCTIONS where the overall objective of this work package was to

enhance the capability of the FixO3

infrastructures to make very high quality observations

(Tasks 1-3) and to develop of new low energy consuming platform design in order to

http://link.springer.com.bibproxy.ulpgc.es/article/10.1007%2Fs11356-016-7863-y#CR77






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promote more sensors per platform and extension capacities (Task 4). The tasks 1 to 3

were focused on the enhancing of CO2 measurement, pH measurement and passive

acoustic practices.

In this report we will focus on Task 12.2, defined as the enhancement of pH sensors which

involved the Institutions ULPGC, OGS, NKE, UGOT with the implication of NOCS that

keeps running PAP-SO time series.

1.2. Aspects of this report

Quality pH measurements are critical for long-term ocean acidification research (precision

≤ 0.003 pH unit; OA-ON report; Newton et al., 2015) but challenging to obtain this data

with an automated instrument. The challenge is not only to find pH sensors with the

longevity, stability, reliability, and robustness required for deployments in any seawater

environment from coastal to oceanic, from Equatorial to Arctic and Antarctic, but also the

measurement method must be capable of accurate pH measurement in all such a

conditions. Moreover, long-term in situ pH measurements in the extreme conditions refer

to both surface and the deep ocean, where in this particular last area, it remains a challenge

for many oceanographers. Until recently, pH was measured by spot samples analysed with

pH indicator dye spectrophotometry for the most oceanographic reliable studies. These

intermittent pH measurements are incapable of providing sufficient spatiotemporal

resolution to understand the oceanic carbonate system and ocean acidification. As such,

the need for a robust and reliable in situ sea pH sensor is urgent.

Several automated pH systems have recently been developed based on the colorimetric

method (Assmann et al., 2011; Rérolle et al., 2012; González-Dávila et al., 2016). As

indicated by Rérolle et al (2016) an advantage of the colorimetric method is that the

temperature and salinity dependence of the (purified) pH indicator dye m-Cresol Purple

(mCP) is already well-characterised in the temperature (T ) and salinity (S) ranges 5 ≤ T ≤

35 °C and 20 ≤ S ≤ 40 (Liu et al., 2011). The colorimetric method can provide high quality

data (accuracy∼0.005 pH unit and precision ≤ 0.0005 pH unit, Carter et al ., 2013;

González-Dávila et al., 2016) in wide ranges of salinity and temperature with a negligible

drift over long periods of time (Rérolle et al., 2012; 2016; González-Dávila et al., 2016).

However, this method has the drawback that it consumes indicator, which, as it should be

purified to achieve high accuracy measurements (Yao et al., 2007), is currently not easy to

obtain. Additionally, colorimetric analysers are usually relatively complex, with pumps,

valves, mixer, and fragile optical components which make them expensive, power hungry

and potentially unreliable over long (>6 month) deployments if power is not provided by

external sources. Spectrophotometrically commercially available sensors are provided at

the present by SAMI company (Sunburst company, www.sunburstsensor.com) and by

SENSORLAB (www.sensorlab.es). The SAMI-pH has been in production for about 8 year.

They has participated in the Wendy Schmidt Ocean Health XPRIZE and using technology

http://www.sunburstsensor.com/

http://www.sensorlab.es/


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based on the SAMI-pH won both the affordability and accuracy purses in 2015. They are

currently bringing a smaller less-expensive version to market for shallow use (<5 m). The

SAMI-pH is the only one pH-spectrophotometric sensor able to be deployed in deeper

water (0-600 m depth) with a self power duration of 4300 measurements and a response

time of 3 minutes with a deployment life that can reach 1 year with a 2 hour resolution.

Long-term drift is lower than 0.001 pH units over 6 months. The SAMI-pH (contact James

Beck, [email protected]) has been used extensively by NOAA-PMEL

(www.pmel.noaa.gov) on their MAP-buoy program and also in the GIFT time series

station at the Strait of Gibraltar by the CSIC-Spain research Institute of Marine Science at

Andalucia (Flecha et al., 2015). The CSIC group at GIFT station has been using SAMI

sensors for several years. After many initial problems with the sensor with several

communications with Sunburst Company, the sensor is providing data at 360 m depth for

more than 3 years starting in 2012. The SENSORLAB company was established in 2011

after the European project EUROSites at the University of Las Palmas de Gran Canaria,

when the first submarine pH sensor was released following the benchtop system produced

in 2007. The pH Sensor has evolved from the initial SP101-SM version to the SP201-SM

one, where high accuracy, stability spectrophotometer and led light technology have been

included and peristaltic pumps have been removed. The Sensorlab-pH (contact Herve

Precheur, [email protected]) has a duration of 5000 measurements (with a 250 ml bag)

with a response time of only 55 sec and the last version it can be provided with an internal

battery that can be externally charged for autonomous pH measurements. The different

Sensorlab-pH sensors have been extensively used by ULPGC-QUIMA group at FixO3

sites ESTOC (European Station for Time series at the Ocean the Canary islands,

PLOCAN), PAP (Porcupine Abyssal Plane, NOCS), and the Cretan Sea E1-M3A Site

(HCMR) as part of WP 11 and in the Saronikos Gulf coastal buoy as part of JERICO

European Project (González-Dávila et al., 2016).

An alternative method is potentiometric pH determination using either glass electrodes

(Seiter and DeGrandpre, 2001) or ion-sensitive field-effect transistors (ISFET)

(Shitashima et al., 2002). Potentiometric sensors have the key advantages of being

relatively small, not consuming reagents, and having fast response times allowing for

continuous high frequency measurements. On the other hand, these sensors can drift over

time and require co-localised discrete samples to correct possible calibration offset

(Bresnahan et al., 2014). Potentiometric sensors can also be sensitive to salinity and

temperature changes. The Honeywell Durafet and the recently developed ISFET-based pH

sensor (SeaFET) have been demonstrated to have great stability over 9 months of

deployment at surface seawater (Bresnahan et al., 2014; Martz et al., 2010). The use of the

SeaFET pH sensors is increasing rapidly worldwide because of its ease of use and good

performance in seawater (Bresnahan et al., 2014; Hofmann et al., 2011). The ISFET offers

many advantages over current pH monitoring technologies and has been recently adapted

for deep ocean monitoring on carousels, profiling floats and moorings. According to

Branham et al. (2016), in order to maintain the accuracy of the sensor over the temperature

and pressure range found within deep-ocean environment, the liquid/gel Ag/AgCl

reference electrode used in the original Durafet design was replaced with a solid state


http://www.pmel.noaa.gov/



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Ag/AgCl reference electrode. The use of a solid state reference electrode and a custom

designed pressure tolerant housing for the Durafet chip extended the pressure rating of the

sensor from 100 to 3000 psi. The solid state Ag/AgCl reference electrode also required

conditioning in seawater for approximately 1 week prior to deployment to maintain the

accuracy of the sensor in natural seawater. Once calibrated and conditioned the Deep Sea

Durafet (DSD) is stable and accurate for multiple months to years. The DSD analytical

performance was validated during the recent Wendt Schmitt Ocean Health Xprize, where

they were awarded second place in accuracy after a side-by-side field competition between

3 other finalists. The Durafet sensor is also utilized for the Southern Ocean Carbon and

Climate Observations and Modeling Program (SOCCOM, e.g.

www.soccom.princeton.edu). To date, over 50 DSD enabled profiling floats have been

deployed in the southern ocean and continuously stream in situ pH data to scientist around

the world. Finally, the DSD was also deployed on a profiling float near the Hawaiian

Islands and has been taking in situ ocean pH measurements daily for nearly two years.

A recent and very promising design is the Aanderaa pH optode based on the successful

and commercially available Aanderaa (www.aanderaa.com) O2 and pCO2 optodes. The

sensor housing is made of titanium, rated to 6000 m water depth (12000 m is optional),

with a diameter of 36 mm and a total length of 86 mm. This housing includes an

optical/sensing part, a temperature sensor placed close to the foil, and the necessary

electronics (a microprocessor with digital signal processing capacity). This design

combined with a multipoint calibration provides internal data processing with temperature

compensation of the signal. Power consumption of these sensors is as low as 0.004A or

about 80 mW at 5 s sampling and 7 mW at 1 min sampling frequency. Response time (τ63)

is between 30 sec (at 40°C) and 3 min (at 0°C) in a non-pumped mode. Auto-start upon

powering up is embedded into software. Sensors could be used in polled mode or output

data at pre-set interval. The measurement principle of this optode-type sensor is based on

the different response of a sensing fluorophore when is in contact with water with different

pH. The embedded DLR (Dual Lifetime Referencing) material exhibits a pH dependent

fluorescence change, which is detected as a phase shift value of returning modulated red

light. Detection of pH is done within the sensing membrane patented by Presens

(www.presens.de).

2. RESULTS AND DISCUSSION

As part of WP11, different pH sensors have been installed in several FixO3 sites.

Moreover, inside WP12, an intercalibration exercise was carried out in the cabled Koljo

Fjord observatory, operated by UGOT, on the West Coast of Sweden that included

together with 8 pCO2 sensors that was the main focus of the exercise, 7 pH sensors.

During the deployment fouling was significant and affected some of the pH sensors. We

will start this report presenting results from PAP site, where long term results of both

http://www.soccom.princeton.edu/

http://www.presens.de/


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Durafet and spectrophotometric sensors have been obtained, and then we will include

some relevant results about the use of the same pair of sensors in an Artic cruise by

Rérolle et al. (2016). The most important results from the Koljo Fjord intercalibration

exercise have already been presented and discussed in D12.2 report, and, therefore, only

some aspects will be presented here. Data from a new promising sensor, the pH optode

sensor, will be included in the last session. As an important action inside FixO3,

community building inside WP11 in close relation with WP12 mobilized several partners

on pH assessment. Four different sites were selected among those integrated inside FixO3,

including the Mediterranean sea (the E1-M3A site, Figure 1A), the subtropical North

Atlantic (ESTOC site, Figure 1B), the North Atlantic high productive area (PAP-SO site)

and the very high productive region of a fiord (Koljo Fjord site) where strong

collaboration among the responsible of each site, the sensor providing groups and SMOs

worked together in order to achieve the highest

confident values. This kind of work put

in value the importance of communication among groups, initial phases of organization,

the different type of sensor processes and integration needs, and the role played by the

high specialized technicians in installing, integrating and communication software

development in order to get any sensor in an unattended buoy system. FixO3 has reached a

high level of technician specialization after the many deployments under different

environments and summer schools.

Figure 1. A) Buoy deployed at the E1-M3A site by the HCMR including a Sensorlab pH SP101-SM

sensor provided by ULPGC. B) Buoy deployed at the ESTOC site by PLOCAN including a

Sensorlab pH SP201-SM sensor provided by ULPGC.

2.1 PAP-SO time series

Two Satlantic SeaFET sensors (S/N 105 at 30m and S/N 111 at 1m) were deployed at PAP-

SO in July 2014 during ME108 and were successfully recovered on 24.06.2015. The sensor

slot of the SeaFET-111 was covered in biofilm; the surface of the connector was also

corroded right below the pins. These however did not affect the functioning of the SeaFET-

A B


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111. The data was successfully downloaded from the internal memory of both instruments.

The SeaFET-105 was recording data from 11.07.2014 till 24.06.2015, while the SeaFET-111

stopped logging the data on 02.05.2015 which may be related to corrosion of the connector.

The raw data from both sensors are shown in Figure .

Figure 2: Uncorrected data from SeaFET S/N 105 and 111 pH sensors deployed during 2014-2015.

The performance of the SeaFET S/N 105 and 111 pH sensors was tested using Tris Buffer

#26 solution. Note, the buffer solution was opened on 20.06.2015, which could bias the

outcome of the calibration. The results are summarised in Table 1: Post-deployment check of

the SeaFET 105 and 111 pH sensors against Tris buffer #26 solution (on DY032).

Table 1: Post-deployment check of the SeaFET 105 and 111 pH sensors against Tris buffer

#26 solution

Date S/N T[°C] pH internal pH external Tris#26


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27.06.2015 105 21.0 8.478±0.0005 8.170±0.0029 8.22

27.06.2015 111 21.1 8.161±0.0087 8.199±0.0246 8.217

2.1.1 pH sensors on DY032 (July 2015-April 2016)

The set of pH sensors at PAP1 deployment include a Sensor Lab SP101-Sm pH sensor

(ULPGC) along with two Satlantic SeaFET pH sensors (s/n 63 at 30m depth and S/N 257 at

1m depth). The SP101 was calibrated before being received by NOC and checked and

serviced in Southampton before the cruise began.

The SeaFET pH sensors (S/N 063 and 257) were tested in the lab at NOC and on-board RRS

Discovery using TRIS buffer #26 solution of a known pH. The sensors were sampling in the

CONTINUOUS mode. The sensors were allowed to warm up for approximately 2 hours

before the readings were logged. Temperature was recorded at the start and end of the

calibration test to calculate the pH of the TRIS buffer solution (Equation provided by the

manufacturer). The results of the calibration test are summarised in Table 2.

Table 2: Results of the pre-deployment bench calibration of SeaFETs 063 and 257 against

Tris buffer #26 solution

Date S/N T [°C] pH internal pH

external

pH

Tris#26

19.05.2015 63 20.1 8.211±0.013 8.270±0.022 8.24

19.05.2015 257 20.1 8.217±0.016 8.240±0.016 8.24

20.06.2015 63 22.0 8.177±0.002 8.129±0.007 8.188

20.06.2015 257 21.9 8.223±0.005 8.223±0.030 8.191

The Sensor Lab SP101-Sm pH sensor was attached to the buoy keel. The SensorLab pH

sensor is powered through the buoy and takes a reading each time it is powered. It is set along

with a Satlantic SeaFET pH sensor (s/n 257). A second SeaFET (s/n 63) is fitted in the sensor

frame at 30m, and both SeaFETs are powered from dedicated battery packs in separate

housings. The SeaFET sensors can also be powered from the hub or from an internal small

battery pack. They were set up to sample in periodic mode with a sampling interval of 30


10

min, producing 3 Frames per burst (output of 3 samples, each is an average of 10 readings)

and creating a Daily log ASCII file. SeaFET s/n 63 in the sensor frame was configured to

sample at 23 minutes past the hour (1380 sec offset) while s/n 257 on the buoy samples at 27

minutes past the hour (1680 sec offset).

After deployment, it became apparent that SeaFET s/n 257 on the buoy is only sampling once

per hour suggesting that a mistake was made in the configuration of this sensor. Note that this

sampling regime cannot be changed remotely.

The sensors were recovered on DY032 in April 2016. In comparison to previous years the

system was extremely biofouled after 10 months of deployment, especially at the buoy. This

caused sensors to fail working properly or not at all. The SeaFET sensor on the buoy was

sampling data for the entire deployment but the internal-external measurement diverged when

checked in the laboratory probably due to extreme biofouling. The sensor Lab SP101-Sm pH

sensor failed intermittently through winter. On 2016/01/13 it failed to take sensible pH data

but was still sending good MicroCAT data. The extreme biofouling on the sensor area may

explain the failure.

For the SeaFET sensor on the frame (at 30m) data started to scatter on March 2015. Some

biofouling was noticed but the main problem was a need for more frequent calibration. The

SeaFET sensors S/N 063 (frame) and 257 (buoy) deployed during DY032 in June 2015 were

successfully recovered on 25.04.2016. The uncorrected data collected by SeaFETs is shown

in Figure 3.

The sensor slot of both instruments was covered with biofilm (Figure 4).

Figure 3 - pH and temperature data collected by (A) SeaFET-063 (B) and SeaFET-257. The pH value

measured by internal and external sensors are in red and blue, respectively. Temperature values are

shown in green.


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The data collected over a year of deployment

was successfully downloaded from the internal

memory of both instruments. The SeaFET-063

(from 30m) was recording data from 22.05.2015

till 25.04.2015, while the SeaFET-257 (from 1

m) was recording from 19.05.2015 to

25.04.2016.

Upon recovery, the performance of the sensors

was tested using the same set of CRMs as for

SeaFETs deployed during DY050. The results

of post-deployment calibration are summarised

in Table 2.

The offset between the measured pH and CRM values are shown in Figure 5 (A, C). Due to a

relatively large offset observed for both SeaFETs, a repeated test with CRMs was conducted

Figure 4 - Biofouling on the sensor

probe of SeaFET 257


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Figure 5: Results of the post-deployment calibration tests for (A,B) SeaFET 063 and (B, C) SeaFET

257. The columns show the difference between the pH of CRMs and the values measured by internal

(red) and external (blue) pH sensors of the SeaFETs.

Results from the Sensor Lab pH sensor are shown in Figure 6 and 7 for pH in total scale at in

situ sea surface temperature (SST) and salinity (SSS) (microcat seabird SBE37) and

computed total dissolved inorganic carbon (CT, µmol kg-1

) using total alkalinity values from

salinity (Lee et al., 2006)


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Figure 6 – pH, temperature and salinity data collected by Sensorlab pH 101 sensor. The pH value

measured is in black. Temperature values are shown in red and salinity in blue.

Figure 7 – pH and computed dissolved inorganic carbon (CT(pH,AT(S)) from data collected by Sensorlab

pH 101 sensor. The pH value measured is in black and the computed CT is shown in red.

Data from Sensorlab were strongly correlated with both the pH (r>0.9) value measured by

internal and external sensors of the SeaFET-257 at the surface, but in particular with the pH

provided by the external sensor. Data from Sensorlab were not affected by biofouling as the

seawater is pumped from out of the frame area by the internal peristaltic pump. Also, the

variability observed in short time periods were strongly correlated with both SST and SSS,

showing the high variability in the physical processes affecting the seawater properties at the


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location of PAP (Hartman et al., 2014).

2.1.2. pH sensors on DY050 (April 2016-April 2017)

The pH sensors deployed April 2016 (on DY050) include a Sensor Lab pH sensor on loan

from ULPGC on Gran Canaria along with two Satlantic SeaFET pH sensors (s/n 105 at 30m

depth and S/N 111 at 1m depth). The SP101 was calibrated before being received by NOC

and checked and serviced in Southampton before the cruise began by ULPGC responsible.

However, it is powered through the telemetry system from the batteries of the buoy. Due to

power failure on the buoy, it will not be operational during this deployment. Likewise we will

not receive real time data from the SeaFET sensors although they should continue to record

data internally.

The SeaFET pH sensors for deployment April 2016 (S/N 105 and 111) were calibrated in the

lab at NOC and on-board RRS Discovery using a set of Certified Reference Materials (CRMs)

of known pH values (Batch 128, 146, and 151). The sensors were sampling in the

CONTINUOUS mode during calibration. The sensors were warmed up for approximately 2

hours (to stabilise internal temperature of the sensor) before the steady readings were logged.

Temperature was recorded with a thermometer at the beginning and end of the calibration test

and the pH of CRM was calculated using CO2Sys_v2.1 macro. The results of the calibration

test are summarised in 3. The offsets between the aim CRM values and pH measured by the

SeaFETs 105 and 111 are shown in Figure 8 and 9.

Test SeaFET

S/N pH internal pH external

Temperature

(ºC) CRM pH

Pre-deployment calibration 10.02.2016

CRM Batch 140 105 7.864±0.001 7.932±0.001 20.1 7.929

CRM Batch 128 105 7.956±0.001 8.085±0.001 20.3 7.999

CRM Batch 140 111 7.859±0.008 7.936±0.009 20.4 7.929

CRM Batch 128 111 7.955±0.008 8.105±0.009 20.6 7.995

On-board pre-deployment calibration 19.04.2016

CRM Batch 128 105 7.862±0.001 7.9812±0.002 21.6 7.98

CRM Batch 146 105 7.922±0.001 8.088±0.002 21.1 7.963

CRM Batch 151 105 7.865±0.0004 8.059±0.001 21.2 7.915


15

CRM Batch 128 111 7.950±0.006 8.039±0.008 20.1 8.002

CRM Batch 146 111 7.986±0.002 8.141±0.004 20.3 7.975

CRM Batch 151 111 7.928±0.001 8.111±0.001 20.5 7.963

On-board post -deployment calibration 1 25.05.2016

CRM Batch 128 63 8.256±0.007 7.639±0.007 22.7 7.964

CRM Batch 146 63 8.468±0.012 7.862±0.041 22.8 7.938

CRM Batch 151 63 8.499±0.002 7.898±0.025 22.9 7.890

CRM Batch 128 257 7.772±0.014 7.724±0.016 22.9 7.961

CRM Batch 146 257 7.878±0.010 7.888±0.010 22.7 7.939

CRM Batch 151 257 7.854±0.011 7.881±0.013 23.1 7.887

On-board post -deployment calibration 2 28.05.2016

CRM Batch 128 63 8.540±0.014 7.917±0.001 22.3 7.970

CRM Batch 146 63 8.619±0.001 8.007±0.001 22.6 7.940

CRM Batch 151 63 8.571±0.002 7.963±0.002 22.3 7.899

CRM Batch 128 63 8.061±0.054 8.083±0.060 22.5 7.967

CRM Batch 146 63 7.929±0.013 7.991±0.013 22.8 7.938

CRM Batch 151 63 7.859±0.012 7.938±0.010 22.7 7.893

Table 3- Summary of pre- and post-deployment calibration tests for Satlantic SeaFET pH


16

Figure 8: Results of the calibration tests for (A) SeaFET-105 and (B) SeaFET-111 pH sensors

conducted in the land laboratory. The columns show the difference between the pH of certified

reference materials (CRMs) and the values measured by internal (red) and external (blue) pH sensors

of the SeaFETs.

Figure 9: Results of the pre-deployment calibration tests for (A) SeaFET-105 and (B)

SeaFET-111 pH sensors. The columns show the difference between the pH of CRMs and the

values measured by internal (red) and external (blue) pH sensors of the SeaFETs.

The SeaFET sensors are programmed to take samples every 30min. They are connected to

internal batteries and external batteries. At the frame, the SeaFET 105 is connected to an

Ocean Sonics battery with 150Ah and at the buoy the SeaFET 111 is also powered by a Pro-

Oceanus 268Ah.


17

A distinctive characteristic of the SeaFET is that it requires an uninterrupted and isolated

source of power to keep the sensing element conditioned. Because the power consumption of

the isolated battery power is 10uA in operation and 1.1mA in standby, the isolated battery

will last more than 15/0.0011 = 13636h = 568d and thus the consumption for keeping the

elements conditioned is not a limitation.

On the frame, SeaFET was set up to sample in periodic mode with a sampling interval of 30

min and 1380 sec offset (23 min past the hour), producing 3 Frames per burst (output of 3

samples, each is an average of 10 readings) and creating a DAILY log ASCII file. On the

buoy, SeaFET was set up to sample in PERIODIC mode with a sampling interval of 30 min

and 1620 sec offset (27 min past the hour), producing 3 Frames per burst (output of 3

samples, each is an average of 10 readings) and creating a DAILY log ASCII file. Note that

the sampling regimes cannot be changed remotely.

In the new deployment schedule for 2017 at PAP, the new Sensorlab 201SM will be included

in the surface buoy.

2.2 Measuring pH in the Arctic Ocean: Colorimetric method or SeaFET?

In a recent publication by Rérolle et al. (2016) in Methods in Oceanography, a

spectrophotometric pH sensor and a SeaFET instruments were set up in parallel on the on-

board underway seawater supply for 65 days, enabling comparison in various conditions in

the Arctic Ocean. It should be noticed that in this case the systems were not deployed in an

unattended buoy but in a research vessel where cleaning, checking the functioning,

calibration and other maintenance aspects could be done in situ.

The SeaFET sensor, as indicated above, combines one Durafet pH sensor and two different

reference electrodes referred to as internal reference and external reference by the

manufacturer. The external reference is sensitive to salinity and is more stable than the

internal one, which is not sensitive to salinity within the typical seawater conditions (30–

36) (Bresnahan et al., 2014). Due to this fact, it is used to define the SeaFET as a SeaFET

sensing pair (Durafet + Internal reference and Durafet+External reference electrodes)

referred as SeaFET_int and SeaFET_ext. In the publication by Rérolle et al. (2016), it is

indicated that short term precision of the two SeaFET electrodes and the colorimetric

analyser was 0.0005 and 0.0010, respectively. When the full set of data were corrected, the

correlation between the data from both sensors was high (r > 0.9) but significant

discrepancies with mean residuals (MR) higher than 0.010 pH unit were observed in some

areas. The two sensors compared well in open ocean water (area 1) with residuals

pH_SeaFET_int − pH_SeaFET_ext = 0.002 ± 0.005 but the comparison worsened with

residuals about an order of magnitude higher in ice-covered areas (area 2) (positive

residuals) and ice-free but more coastal water (area 3) (negative residuals). Data from

SeaFET_int compared well with the colorimetric data from open ocean water in Area1 and

the ice-covered Area2, with residuals between 0.000 ± 0.007 and 0.002 ± 0.011 pH unit. In


18

Area3, the SeaFET_int data differed from the colorimetric data, with similar discrepancies

to those observed between SeaFET_int and SeaFET_ext. Finally, the data from

SeaFET_ext compared well with the colorimetric data in the open water regions in Area1

and Area3 (absolute residuals ≤ 0.002±0.016) whereas large discrepancies were observed

in the ice-covered Area2 (residuals = −0.026 ± 0.016). The authors explain the

discrepancies comparing the residuals with salinity and temperature values concluding that

large part of the residuals between the two SeaFET datasets and between SeaFET_ext and

the colorimetric analyser is due to the salinity distribution in the entire transect and in the

ice-covered area (Area 2). In the area closer to Asia coastal region (Area 3), temperature

seems to be the predominant factor. The SeaFET_int data quality appears to worsen over

time with the discrepancy between SeaFET_int and SeaFET_ext increasing from 0.010 to

more than 0.020 pH unit in a week. A similar sensor drift was observed for the sensor

SeaFET-063 deployed at 30 m at PAP (Figure 2) and has also been observed by Bresnahan

et al. (2014), attributed to deterioration of the internal reference electrode. As conclusion,

the authors indicate that both type of sensors provide high quality pH data when ISFET pH

with the internal reference electrode is used, as the external reference electrode did not

perform well in the ice-covered area. However, long term stability of the instrument in

such environment also requires further investigation.

2.3. Intercalibration exercise in the Koljo Fjord.

As one of the main objective of FixO3 and within WP12, task 12.1, Enhancement of CO2

measurements, the Koljo Fjord Cabled Observatory (www.emso.eu) was proposed as the

place to conduct the experiment. Even the project is related with open ocean observatories,

the capabilities of this cabled observatory was initially considered suitable for this kind of

studies. The site in the fjord (58.22825 N, 11.57400 E) was set in April 2011 at 42 m

depth, and it has been operational at this location since then. During the exercise, 4

different technologies (fluorometry, colourimetry, electrochemical and ISFET-based) from

5 different manufacturers for pH measurement were used. The main characteristics of the

sensors are

1. pH ISFET sensor, brought into the exercise by Dr. Kiminori Shitashima, an

external partner representing University of Kyushu, Japan. The sensor is custom-

made units based on ion-sensitive field-effect transistor (ISFET) technology, as

indicated above. The pH units were calibrated on a total hydrogen ion scale (pHT)

using artificial seawater buffers (Tris and AMP) prior to the deployment. No

antifouling protection was provided.

2. pH electrodes, were brought into the exercise by Dr. Kiminori Shitashima, an

external partner representing University of Kyushu, Japan. The submersible glass

electrode pH sensors, model SPS-14, were manufactured by Kimoto Electric Co.,

LTD. The electrochemical pH units are based in a pressure-balanced glass-electrode.

Calibration of the pH units was done on a total hydrogen ion scale (pHT) using


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artificial seawater buffers (Tris and AMP) prior to the deployment. No antifouling

protection was provided.

3. EXO2® pH electrode was kindly supplied by YSI Inc. (www.exowater.com), an

external partner, for demonstration purposes. The measurement principle of this

electrochemical pH electrode is, as indicated above, a Glass combination electrode.

EXO2 measures pH with two electrodes combined in the same probe: one for

hydrogen ions and one as a reference. The sensor was calibrated on a total

hydrogen ion scale (pHT) using artificial seawater buffers (Tris and AMP) prior to

the deployment. This calibration was later used to convert raw data (mV and T)

into pHT. A mechanical wiper was used to clean the sensing surfaces every 6h. In

addition, a sensor guard made of copper was mounted to protect the sensors from

fouling on unprotected surfaces on the sensors, those that were not wiped.

4. Aanderaa Seaguard pH optode sensor. The new, non-commercialized pH optode

from Aanderaa uses fluorescent dual lifetime detection of pH. It is based on a

similar principle as that for pCO2 sensor. When CO2 diffuses into a gas permeable

foil, the pH in an embedded buffer will change, subsequently changing the

fluorescence of the sensitive indicator. This design combined with a multipoint

calibration provides internal data processing with temperature compensation of the

signal. No antifouling protection was provided.

5. The Sensor Lab SP101-Sm pH colorimetric sensor was attached to the frame. The

SensLab pH sensor was powered through the buoy. It should be indicated that no data

were recorded. The turbid waters of the Koljo Fjord caused clogging of the inlet

tubing, which caused leakage and failure of the instruments to operate in its usual

manner.

As indicated in D12.2 report, the Koljo Fjord conditions were not the best for the pH

sensors tested on the intercalibration conditions. The pH data from the electrochemical pH

(Kimoto Inc and EXO2) sensors were within ±0.02-0.03 pH units for the first two weeks

of the deployment (Figure 10). After that the unprotected from fouling electrodes A and B

started to show higher amplitude of daily variations, clearly providing pH values for the

affected biofouled seawater but not for the Fjord bulk one. The ISFET sensors seemed to

follow the trend shown by the electrochemical units, but also got heavily fouled after two

weeks in the highly productive waters of the fjord. Aanderaa pH optode data provided data

that drifted over time resulting in unrepresentative data. Offset between the reference

values (yellow circles) and the EXO2 electrode (red line) at three occasions was 0.009,

0.065 and 0.01, pointing on rather trustworthy pH data recorded by EXO2 (Figure 10).

The results for the electrochemical sensors considered in this study are presented in Figure

10 (after D12.2 report).


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Figure 10: Results of the several electrochemical pH sensors deployment in the intercalibration

exercise at the Koljo Fjord, during April-June 2014.

Biofouling in the fjord affected all the sensors after some weeks, being only the ones with

biofouling protection (EXO2 sensors) providing data that could be used when regular use

of an external calibration procedure is considered. This made this kind of sensors difficult

to use in unattended buoy systems.

2.4 Recent experiences from field measurements with pH optodes

pH optodes participated in the Koljoefjord intercalibration exercise but did not do well

there mainly because of two reasons. The first was that they were exposed to sunlight,

which at that time could irreversible bleach the sensing material. The second reason was

that these sensors, due to the open structure of the sensing foil, appears to be more

sensitive to fouling than other optodes.

For the Wendy Schmidt Ocean Health XPRIZE Aanderaa and YSI (USA) formed the

Xylem team and entered with updated and better calibrated pH optodes. The Xylem team

was selected as one of the five finalists and participated in the final deep-water trials off

Hawaii.

Several field trials have recently been carried out with these sensors including on an

autonomous vehicle (http://www.sailbuoy.no) in the

Arctic (Figure 11).

Figure 11: A “Sailbuoy” is deployed in the Arctic, N.

Svalbard, during the summer of 2016. In the bulb, at the


21

lower part of the keel, sensors to measure pH, pCO2, O2, Salinity and Temperature are mounted.

Data from this mission are presented more in detail in 12.3 Long-term deployment of

pCO2. After an initial stabilisation time of about 1 day the pH optode showed a clear

correlation with O2 and anti-correlation with pCO2 and had a resolution of about 0.005 pH

units. These data are at the moment in the process of quality control and evaluation at the

University of Bergen, Norway.

In the Koljoefjord (part of the EMSO network http://www.emso-eu.org/site/ocean-

observatories.html) another pH optode was deployed in September 2016 about 8 m below

the surface. This deployment is mainly focused on to test combinations of environmentally

friendly antifouling methods. Other sensors included on the same SeaGuard platform were

measuring Salinity, Temp, O2, Currents and Wave/Tide (real time data are available at:

http://koljofjord.marine.gu.se).

In Figure 12 is a combined plot of pH and O2 for the period since deployment. As can be

seen the pH optode appears to be stable and there is a clear expected correlation between

pH and O2 but the absolute values of pH are too low. With reference data there are ways of

shifting the pH calibration curve up. The resolution of the pH optode in this case was

about 0.005 pH units.

http://www.emso-eu.org/site/ocean-observatories.html

http://www.emso-eu.org/site/ocean-observatories.html

http://koljofjord.marine.gu.se/


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Figure 12: pH and O2 data from the Koljoefjord (September 1, 2016 – February 22, 2017) about 8

m below surface.

Within the frames of FixO3 another pH optode was deployed in April 2015 in the Adriatic

Sea at the E2-M3A site (Figure 13). This deployment is part of an on-going TNA that is

named COMBO. Recovery of this equipment is expected during the first months of 2017.


23

¨

Figure 13: Buoy with SeaGuardII instrument for TNA COMBO is being deployed in the Adriatic

Sea in April 2015.

The pH optode technology has the potential to become an attractive alternative to other

technologies because of robustness, compact size, low power consumption, high-pressure

resistance (12 000 m), low pressure hysteresis, no need for reagents and high (0.0005 pH

units) resolution but calibration and foil stability issues prior to deployment should be

improved.

3. CONCLUSIONS AND OUTLOOK

1. The spectrophotometric pH sensors present the advantage that the instrument

calibration can be tested in the laboratory prior to deployment. Regular checks are

always advisable during deployment but not imperative.

2. The spectrophotometric pH measurements are reproducible in the long term.

3. It is recognized that purified mCP indicator dye, although difficult and expensive to

obtain, should be used as this has been carefully characterised under extensive

temperature and salinity range and produces reliable data independent on the pH

sensor.

FixO3 TNA COMBO, E2-M3A buoy with: • SeaGuardII: Doppler Current Profiling Sensor,

Pres/Temp, Sal/Temp/Cond, O2/Temp, pCO2/

Temp, pH/Temp. Internal battery & Storage, 30 min logging

• Deep water SeaGuard: Zink anode moved away from Cond/Sal & O2 optode cable connected


24

4. The spectrophotometric pH sensors use valves and pumps with power requirements

that can be problematic when internal batteries are used in unattended buoys

without extra power supply or deep deployments.

5. Among the different commercially available spectrophotometric pH sensors, only

SAMI-pH is indicated for 0 to 600 m depth.

6. SeaFET instrument has been used in different and extreme conditions from low

temperature to ice-covered areas and from surface to 3000 m depth with high

resolution and low power consumption.

7. When using the SeaFET instrument, the regular use of an independent procedure,

to re-calibrate SeaFET at in situ conditions and monitor sensor drift is strongly

recommended. This is particularly critical for long term deployments.

8. The pH optode technology because of low power consumption, high-pressure

resistance (12 000 m), low no need for reagents and high resolution appear as a

promising sensors when calibration and foil stability issues prior to deployment are

improved.

9. For any sensor selected for your site, check the instrument is calibrated and set up

for the expected deployment conditions.

10. FixO3 experiments demonstrate the interest to intercalibrate on several sites and

configuration the promising pH sensors. This effort is to be continued in further

projects as key to the build Research Infrastructure capacities addressing OA such

as in EMSO, the continuation of FixO3.

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5. ACRONYMS

FixO3 Fixed point Open Ocean Observatories Network

pHT pH in total proton scale at the in situ temperature, salinity and pressure.

AT Total alkalinity in µmol kg-1 seawater

CT Total dissolved inorganic carbon in µmol kg-1 seawater

OA Ocean Acidification

ISFET ion-sensitive field-effect transistors

SeaFET ISFET-based pH sensor

CRM Certified Reference Material

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