Post on 06-Oct-2020
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) melchor.gonzalez@ulpgc.es
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, jean.francois.rolin@ifremer.fr
Due date: Project Month 42 (02-2017)
Dissemination level: RE
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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
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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
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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, jim@sunburstsensor.com) 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, hp@sensorlab.eu) 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
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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
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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
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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
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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
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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.
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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
FixO3 Deliverable 12.5 / Conclusion on the capabilities of the different pH sensors
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
FixO3 Deliverable 12.5 / Conclusion on the capabilities of the different pH sensors
19
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).
FixO3 Deliverable 12.5 / Conclusion on the capabilities of the different pH sensors
20
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
FixO3 Deliverable 12.5 / Conclusion on the capabilities of the different pH sensors
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.
FixO3 Deliverable 12.5 / Conclusion on the capabilities of the different pH sensors
22
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.
FixO3 Deliverable 12.5 / Conclusion on the capabilities of the different pH sensors
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
FixO3 Deliverable 12.5 / Conclusion on the capabilities of the different pH sensors
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