Development of an Autonomous Carbon Glider 

to Monitor Sea-Air CO2 Fluxes in the Chukchi Sea

Principal Investigator: Claudine Hauri1

Co-Investigators: Andrew McDonnell

2

, Peter Winsor2, Brita Irving2, Hank Statscewich

2

1

International Arctic Research Center, University of Alaska Fairbanks 

2

College of Fisheries and Ocean Sciences, University of Alaska Fairbank

Final Report 

OCS Study BOEM 2018-016

May 2018
Contact Information:

Email: UAF-CMI@alaska.edu

phone: 907.474.6782

Coastal Marine Institute

College of Fisheries and Ocean Sciences

University of Alaska Fairbanks

P. O. Box 757220

Fairbanks, AK 99775-7220

Administration of the University of Alaska Coastal Marine Institute is funded in part by the U.S. Department of the

Interior, Bureau of Ocean Energy Management (BOEM) through Cooperative Agreement M15AC00005 between

BOEM, Alaska Outer Continental Shelf Region, and the University of Alaska Fairbanks. This report, OCS Study 

BOEM 2018-016 is available through the Coastal Marine Institute, select federal depository libraries and 

electronically from https://www.boem.gov/BOEM-Newsroom/Library/Publications/Alaska-Scientific-and-TechnicalPublications.aspx

The views and conclusions contained in this document are those of the authors and should not be interpreted as

representing the opinions or policies of the U.S. Government. Mention of trade names or commercial products does not

constitute their endorsement by the U.S. Government.
ii

Table of Contents

List of Figures................................................................................................................................ iii

Abstract.......................................................................................................................................... iv

Introduction......................................................................................................................................1

Methods............................................................................................................................................3

pCO2 Sensor Prototypes ..............................................................................................................3

Prototype 1.0...........................................................................................................................3

Prototype 1.1...........................................................................................................................3

Prototype 2.0...........................................................................................................................3

Prototype 2.1...........................................................................................................................4

Prototype 2.2...........................................................................................................................4

Prototype 2.3...........................................................................................................................4

Prototype 2.4...........................................................................................................................4

Testing Equilibration Time .........................................................................................................4

Glider-Sensor Integration............................................................................................................6

Results and Discussion ....................................................................................................................7

Equilibration Experiments...........................................................................................................7 

Glider-Sensor Integration............................................................................................................7

Early Field Trials.........................................................................................................................8 

Development of Glider Hover Missions .....................................................................................8

Prototype 2 Field Trials and Improvements................................................................................9

Arctic deployment.................................................................................................................12

Final sea trials.......................................................................................................................15

Conclusions....................................................................................................................................21

Acknowledgments..........................................................................................................................21

Study Products...............................................................................................................................21

References......................................................................................................................................22
iii

List of Figures

Figure 1. Carbon glider in a tank at the UAF glider facilities during ballast testing.......................5

Figure 2: Carbon glider with prototype 1.1 and 2.1 pCO2 sensors..................................................5

Figure 3: Laboratory setup at the Alaska SeaLife Center in Seward, Alaska..................................6

Figure 4: Data from equilibration-time experiments at the Alaska SeaLife Center in Seward .......7

Figure 5: Time series of temperature, salinity, and pCO2 from initial field testing ........................8

Figure 6: Glider flight data showing the first successful glider hover mission on May 23, 2016, 

with controlled hover at the prescribed flight depths.......................................................................9

Figure 7: Profiles of temperature, salinity, and pCO2 from the saw-tooth carbon glider prototype 

2.1 mission during the August 2016 sea trials...............................................................................10

Figure 8: Image of the carbon glider prototype 2.1 at the surface during the August 2016 sea 

trials and profiles of temperature, salinity and pCO2 from a hovering mission. ...........................11

Figure 9: Picture of the wrinkled membrane from prototype 2.1 following successful sawtooth 

and hover missions during the August 2016 sea trials in...............................................................12

Figure 10: The carbon glider deployed near the R/V Sikuliaq in the Chukchi Sea in September 

2016................................................................................................................................................13

Figure 11: The carbon glider depth profile from a September 2016 mission................................14

Figure 12: Image of the perforated membrane from the flooded pCO2 sensor after deployment in 

the Chukchi Sea .............................................................................................................................14

Figure 13: Glider flight data showing a glider hover mission during the sea trials in August 2016 

with controlled hover at the prescribed flight depths.....................................................................16

Figure 14: Temperature, salinity, and pCO2 data from a mixed sawtooth/hover mission.............17

Figure 15: Glider flight data showing a glider hover mission during the sea trials in August 2016

with controlled hover at the prescribed flight depths.....................................................................18

Figure 16: Temperature, salinity, and pCO2 data from a mixed sawtooth/hover mission.............19

Figure 17: Temperature, salinity, pCO2, backscatter, CDOM, and Chlorophyll-a data from glider 

missions after the pCO2 sensor was flooded..................................................................................20
iv

Abstract

The waters around Alaska are undergoing unprecedented environmental change including 

warming temperatures, freshening, extensive loss of sea ice, increased storm frequency and 

magnitude, elevated rates of coastal erosion, increased inputs of terrestrial organic matter, and 

ocean acidification. All of these factors could impact cycling of carbon in the Arctic. Continued 

development of oil and gas resources will result in increasing CO2 emissions and additional 

changes in ocean carbon chemistry. Traditional ship-based observations are operationally 

expensive and insufficient to provide the spatial and temporal coverage of dissolved CO2

measurements required to improve quantitative assessment and conceptual understanding of the 

region’s carbon cycle. The goal of this project was to develop and test a carbon glider unit that 

could autonomously measure spatial and temporal dissolved CO2 throughout the water column at 

high-resolution. The project included the design of a customized Pro-Oceanus pCO2 sensor, 

integration of the sensor’s physical, power, communication, and software systems with a

Teledyne Webb Research (TWR) Slocum Glider, and development of glider hover missions that 

allowed for full sensor equilibration and the use of sensors with slower response times. As a 

result of this project, we developed a carbon glider and brought it to a Technology Readiness 

Level 6 (TRL-6 per NASA) and demonstrated a capacity for pCO2 data collection during glider 

flight missions at sea. 
1

Introduction

Contrary to physical parameters that are routinely measured from a wide range of autonomous

platforms, biogeochemical properties such as pCO2 typically require ship-based observations, 

which do not provide the temporal and spatial resolution needed to further our understanding of 

ocean productivity, carbon cycling, and ocean acidification and its impact on climate. To address 

this challenge, we need to integrate and deploy maturing biogeochemical sensors with 

autonomous sampling platforms. Carbon sensors have been successfully integrated with 

remotely operated platforms such as wave gliders and drifters. However, these platforms cannot

measure carbon in subsurface waters where the spatial variability and vertical gradients of pCO2

are often large and dynamic. Subsurface carbon dioxide can strongly impact surface water 

properties and air-sea gas exchange via physical mixing. Further, subsurface concentrations are 

key indicators of biological production and ocean acidification. 

Our present knowledge gaps about the Chukchi Sea carbon cycle demonstrate the need for 

glider-based CO2 observational capabilities. Due to inherent chemical, physical and biological 

processes, the Chukchi Sea is especially suited to take up atmospheric CO2 in summer. This 

shallow shelf sea (<50 m) is fed with nutrient‐rich waters from the Pacific Ocean (Sambrotto et 

al., 1984; Coachman, 1986) and exposed to sustained solar radiation during sea‐ice free periods. 

These factors make the Chukchi Sea one of the most productive ecosystems in the world with 

primary production estimates in the range of ~470 g C m2

y‐1 (Springer and McRoy, 1993). High

rates of primary production in the summer drawdown CO2 at the surface (Pipko et al., 2002; 

Bates, 2006; Gao et al., 2012; Mathis and Questel, 2013), support large fluxes of organic carbon 

to a biomass‐rich benthic ecosystem (Grebmeier et al., 2006), increase levels of remineralized 

inorganic carbon (Pipko et al., 2002), and cause aragonite undersaturation in subsurface waters 

(Bates et al., 2013; Mathis and Questel, 2013). Subsequent circulation‐driven export of these 

carbon‐rich subsurface waters into the upper halocline of the Arctic Ocean (Anderson et al., 

2010) is thought to support a strong continental shelf carbon pump, with an estimated CO2 sink 

strength of 38 Tg C per year (Bates, 2006). 

Our recent findings suggest a more complex and somewhat contradictory description of the 

carbon dynamics in the Chukchi Sea. Frequent autumn storms disrupt stratification and bring 

CO2‐rich subsurface waters to the surface, thereby decreasing the strength of the carbon sink 

(Hauri et al., 2013). In October 2011, wide areas of surface waters were supersaturated with CO2

(reaching a maximum of pCO2=604 μatm), implying substantial seasonal CO2 outgassing to the 

atmosphere during windy conditions. These high surface pCO2 values and relatively high surface 

salinities (>31) were observed during or right after a storm that was strong enough to 

homogenize the shallow water column, indicating mixing of CO2 rich subsurface water to the 

surface. A low surface O2/Ar ratio (90% of saturation ratio) was a further sign of a well-mixed 

water column, indicating a strong respiratory signal along with the high pCO2 levels. Seasonality 

of the western Arctic and North Pacific atmospheric pressure and wind fields suggested that such 
2

wind-induced mixing events happen on an annual basis in autumn as the water column cools and 

overturns prior to late fall ice formation. 

Subsurface waters with high pCO2 levels in bottom waters have been measured all across the 

Chukchi Sea (Pipko et al., 2002; Bates et al., 2013; Mathis and Questel, 2013). Hauri et al. 

(2013) hypothesized that vertically‐entrained CO2 enriched waters are a widespread pattern.

However, due to the high spatial and temporal variability of pCO2 in the region and limited data 

availability, a revised annual carbon budget for the Chukchi Sea and the Arctic Ocean was not 

proposed. These findings illustrate that the current knowledge of the carbon cycle in poorly 

sampled Arctic regions is strongly biased by extrapolations of temporally and spatially sparse 

observations. The US Coast Guard vessel Healy has added a continuous-underway surface pCO2

sensor, so surface pCO2 is now measured along transects through the Chukchi Sea approximately 

monthly. However, these surface transects do not give insight into the processes in the water 

column and at the seafloor, and they do not cover enough area to determine the carbon budget in 

this highly variable environment. Unfortunately, no oceanographic observation systems exist that 

can quantitatively collect carbon cycle data at the spatial and temporal scales necessary to 

accurately constrain regional carbon budgets and understand the processes that are controlling 

the carbon cycle. Moreover, ocean acidification and climate change are altering the chemical and 

physical environment in the ocean, and conventional observational techniques are not well suited 

to detect this change. For these reasons, the development of new autonomous methods to monitor 

the biogeochemical properties of the water column is a critical step in improving our 

understanding of carbon cycling in the oceans.

To fill this observational gap, we have developed a unique carbon glider that enables 

autonomous collection of pCO2 profiles throughout the water column, thereby increasing the 

spatial and temporal coverage of inorganic carbon measurements. In this study, the glider has 

gone through intensive research and development including design of a customized Pro-Oceanus 

Mini-Pro pCO2 sensor, integration of the sensor’s physical, communication, power, and software 

systems with the Teledyne Webb Research (TWR) Slocum Glider, and development of glider 

hover missions that allowed for full sensor equilibration and the use of sensors with slower 

response times. The custom Pro-Oceanus glider Mini-Pro sensor variant went through several 

iterative improvements from the base unit that is currently sold by Pro-Oceanus. The sensor's 

response time was cut down from approximately three minutes to about one minute, which 

makes it suitable for glider hover missions. Power requirements were decreased by nearly 50%

through hardware and mechanical changes to the sensor. 
3

Methods

In the vertical sense, a glider is a fast-moving platform that requires sensors that can respond 

quickly to a changing environment, especially in areas with strong stratification. Over the last 

few years, there has been an explosion in the number and capabilities of pCO2 sensors, based on 

a variety of technologies. The industry standard is a membrane-based sensor that allows carbon 

dioxide to diffuse through a membrane to the gas detector on the inside of the housing. However, 

for our prototype, off-the-shelf pCO2 sensors had equilibration times that were too slow 

(conventional Mini-Pro CO2, Pro-Oceanus) or the sensors were too big (HydroC, Contros). To

obtain a pCO2 sensor suitable for the glider application (i.e., fast enough equilibration time, small 

size, and lightweight), we partnered with the sensor manufacturer Pro-Oceanus to develop a 

modified version of their existing membrane-based Mini-Pro CO2 sensor.

pCO2 Sensor Prototypes

Prototype 1.0

The first objective for the sensor improvement was to reduce the equilibration time. Decreasing 

the equilibration time from three minutes (original Pro-Oceanus Mini-Pro model) to a few 

seconds was considered necessary to resolve the rapid changes in CO2 that are expected as the 

glider moves through the water column.

The original Pro-Oceanus Mini-Pro CO2 membrane has a surface area for gas exchange of ~12.5 

cm2 diameter. This conventional flat membrane was replaced with a hollow fiber module from 

Permselect (https://www.permselect.com). The hollow fiber module is a spindle of hundreds of 

small diameter silicon tubes leading to a surface area of one square meter, all contained in a 

compact form. The gas circulates inside the tubes to the infrared detector inside the main housing 

of the instrument. The ambient seawater that gets pumped through the hollow fiber module 

remains outside the tubes but is able to come to equilibrium with gas inside the tubes across a 

one square meter surface area. The alignment of the water outlet ports on the hollow fiber 

module was adjusted to a 120° angle with the gas ports (instead of 180°). This adjustment 

allowed for the gas ports to be closely aligned with the glider while the water outlets were still 

pointed vertically for gas bubbles to escape.

The data output on this prototype was not comma separated, causing communication issues with 

the glider. The firmware was programmed for a 10-second startup and a 2-second sampling rate.

Prototype 1.1

The SBE 5M pump was upgraded to a stronger pump (SBE 5P @ 4500 rpm) to provide an 

adequate flow rate through the hollow fiber module (>1 L/min). The firmware was updated to 

support comma separated data output.

Prototype 2.0

The second iteration of the pCO2 sensor featured the conventional, but enlarged disc membrane. 

The pressure housing of the Mini-Pro CO2 was modified to accommodate a disc membrane head
4

with an 80 cm2 surface area as commonly used in the Pro-Oceanus Pro-CV CO2 sensor. Using 

the larger, flat membrane, prototype 2 had decreased power consumption and was less 

susceptible to clogging than prototype 1.1 while still maintaining a shorter equilibration time (~1

min) than the original Pro-Oceanus sensor. Furthermore, moving away from the hollow fiber 

module extended the depth range of the carbon glider to 200 m.

This prototype contained a new logger/controller board with firmware supporting a slower 

startup (>15 s) and less frequent sampling rate (4 s).

Prototype 2.1

This prototype was outfitted with a logger/controller board containing the firmware of prototype 

1.1 (startup=10 s, sampling rate=2 s). 

Prototype 2.2

To better accommodate the disc membrane stability during quick pressure changes, Pro-Oceanus 

developed a new water-pumped head with a waterside protective screen along the membrane. 

This new unit was installed prior to the Chukchi Sea deployment. Weeklong laboratory testing of 

the carbon glider showed that the sensor sporadically produced faulty data, leading to a time out 

of the data stream and prompt termination of the communication between the sensor and the 

glider, essentially aborting the mission.

Prototype 2.3

This prototype contained a new logger/controller board with a repaired pressure sensor. The 

firmware contained a two-second startup with an initial test data stream to prevent data stream 

delays and communication issues between sensor and glider.

Prototype 2.4

The membrane was backed with a silver filter to help prevent failures. A 9600 baud rate was 

hard-coded to prevent communication issues.

Testing Equilibration Time

The response time and stability of prototype 1.0 over a longer time period were determined with 

a series of laboratory experiments at the UAF laboratory facilities in Fairbanks (Figure 1). The 

initial experiments revealed that the SBE 5M pump recommended by Pro-Oceanus was not 

strong enough to provide an adequate flow rate (>1 L/min) through the hollow fiber module. The 

SBE 5M pump was upgraded to a stronger pump (SBE 5P @ 4500 rpm), which was 

subsequently tested at the Alaska SeaLife Center in Seward, Alaska (Figures 2 and 3). The sensor 

was run continuously for one week and pCO2, flow rate, and pressure were closely monitored. 

Thirty-seven individual perturbation experiments were conducted to test the response time of the 

sensor in a rapidly changing environment. During each experiment, water supply to the sensor 

was changed from ambient to high CO2 (~1000 to 2000 ppm) or vice versa. A response lag time 

was determined by noting the difference between the time that the water supply was changed and 
5

the initial detection of a perturbation by the sensor. In addition, the response time constant was 

computed by fitting an asymptotic exponential curve to the pCO2 time series.

Figure 1. Carbon glider in a tank at the UAF glider facilities during ballast testing.

Figure 2. Carbon glider with prototype 1.1 (upper panel) and 2.1 (lower panel) pCO2 sensors. The 

prototype 1.1 configuration (upper panel) includes an SBE 5P pump.
6

Figure 3. Laboratory setup at the Alaska SeaLife Center in Seward, Alaska. Inset: High particle 

concentration clogged the hollow fiber module after one week.

Glider-Sensor Integration

The mechanical and electrical aspects of the CO2 sensor glider integration were completed in 

collaboration with Teledyne Webb Research (TWR) and Pro-Oceanus. The integration involved 

designing the mounting bracket, writing integration software to support communication and 

power supply between the sensor and the glider, and adding an outlet port to connect the sensor 

to the glider electronics. TWR integrated the sensor package (Glider Mini-Pro CO2 prototype and 

SBE 5P @ 4500 rmp pump) in a nicely streamlined position conducive to glider deployment, 

flight, and recovery.
7

Results and Discussion

Equilibration Experiments

Laboratory tests suggested that the equilibration time of the prototype 1.1 pCO2 sensor was short

enough to be suitable for the glider application. Response times typically ranged from 10–20 s at 

flow rates greater than 1.5 L/min and increased to ~30 s for flow rates less than 1 L/min (Figure 

4). However, over the course of the response time experiments, the high suspended sediment 

concentration in the flow-through seawater sourced from just offshore the Alaska SeaLife Center 

reduced the flow rate through the hollow fiber module (Figure 3 inset). Over the first five days of 

testing, the flow rate dropped slowly from 5.5 L/min to 2.16 L/min, which suggested that use of 

the 1.1 sensor prototype would limit carbon glider mission durations to about one week to avoid 

the effects of the obstruction of the hollow fiber module by suspended sediments.

Figure 4: Data from equilibration-time experiments at the Alaska SeaLife Center in Seward. Each line 

represents a separate test.

Glider-Sensor Integration

The power supply from the glider to the pump and pCO2 sensor worked well, water flow through 

the hollow fiber module was strong, and the glider was well ballasted. However, an infrequent

parsing error interrupted the communication between the pCO2 sensor and glider. This problem 

was eliminated by updating the sensors firmware to support comma separated data output 

(prototype 1.1).
8

Early Field Trials

Initial field testing of the carbon glider showed that the integration of the Teledyne Webb 

Slocum Glider, the Pro-Oceanus Mini-Pro pCO2 sensor (prototype 1.1), and a Seabird 5P 

submersible pump (Figure 2, upper panel) was successful. The glider operated as expected 

during several dive and climb cycles, and the components successfully communicated with each 

other to record pressure, temperature, salinity, backscatter, Chlorophyll-a, CDOM 

concentrations, and the partial pressure of carbon dioxide (pCO2) data throughout the water 

column (Figure 5). We were able to verify that the addition of the pCO2 sensor and pump, which 

directed seawater through the hollow fiber module, did not significantly alter the flight 

characteristics of the glider.

Figure 5: Time series of temperature, salinity, and pCO2 from initial field testing (backscatter, CDOM, 

and Chlorophyll-a are not shown). The initial dives to 40 m sampled temperature, salinity, backscatter, 

CDOM, and Chlorophyll-a continuously throughout the water column; whereas, the Mini-pro pCO2

sensor was set to sample only during the downcast in order to conserve battery power for the Chukchi Sea 

missions.

During the first sea trials, we encountered a leakage issue with the hollow fiber module that was 

added to the Mini-Pro to accelerate its equilibration time. Data suggested that the hollow fiber 

module failed on the second mission at pressures within our intended working depths. This issue, 

combined with findings from our laboratory testing that indicated the hollow fiber module was 

susceptible to clogging and required a high-powered pump (both of which shorten mission 

duration), helped us determine that the hollow fiber module was not suitable for the intended 

glider application. 

Development of Glider Hover Missions

We constructed glider flight control missions to accommodate a pCO2 sensor with a longer 

equilibration time. During these missions, the glider hovered at various prescribed depths for a 

preset time (Figure 6). The testing and refinement of the hover missions was, in itself, a success. 
9

Hover missions for gliders are uncommon and had to develop and refine the technique and create 

mission program files for the glider to execute. The ability to hover a glider is a powerful new 

tool for use with sensors that require an equilibration period.

Figure 6: Glider flight data showing the first successful glider hover mission on May 23, 2016, with 

controlled hover at the prescribed flight depths. Depth (top), ballast pumped (second from top), pitch 

(third from top) and glider battery position (bottom) are plotted as a function of time. Black lines ( “m”)

designate measured values, and blue lines ( “c”) designate commanded value.

Prototype 2 Field Trials and Improvements 

In close collaboration with the pCO2 sensor manufacturer, Pro-Oceanus, we modified our sensor 

configuration to work without the hollow fiber module (see prototype 2.0 section above). Testing

the carbon glider with prototype 2.0 (Figure 2, lower panel) produced reasonable pCO2 data 

strings in the laboratory; however, a subsequent glider mission at sea produced zeros and error 

readings. Troubleshooting showed prototype 2.0 required a longer startup time than the glider 

software allowed, leading to communication issues. This problem, caused by small changes ProOceanus made to the logger/controller board, was resolved by installing a new logger/controller 

board (prototype 2.1) containing the firmware of prototype 1.1. The new board was tested in the 

laboratory in combination with the glider. During these tests, the glider actively logged the data 

(rather than simply live streaming to the computer), which ensured that the laboratory tests were 

representative of a true glider mission at sea.

The field trials to test sensor prototype 2.1 took place in August 2016. Conditions in 

Resurrection Bay were not favorable for glider missions. Unexpectedly low surface salinities of 
10

18 salinity units led to a density gradient twice as large as the glider could handle in the upper 40 

m of the water column at our regular field test sites. The glider was deployed in the Gulf of 

Alaska, about 25 nm offshore from Seward, at surface salinities of 26 and a manageable density 

gradient of 6 sigma units. The deployment site was in the Alaska Coastal Current, which exposed 

the glider and boat to a large swell and strong drift. These difficult oceanographic conditions 

caused a delay in the glider operations and limited the number of deployments we were able to 

accomplish. However, we were able to conduct the necessary tethered glider safety missions 

(making sure the glider can operate in the conditions), two saw-tooth dives (Figure 7), and one 

hovering mission (Figure 8). The glider successfully logged pCO2 data from prototype 2.1 during 

these missions. 

The first double sawtooth mission (to 40 m) demonstrated that the glider was able to dive and 

climb at a steady rate while maintaining course and logging pCO2 data and other ancillary data 

(Figure 7). The pCO2 profile shows the low to high gradient expected with low salinity and high 

primary production measured at the surface, and less freshwater and primary production and 

more respiration of organic matter at 40 m depth. As expected from the response time of the 

pCO2 instrument, there was some hysteresis observed in the profiles, with the maximum pCO2

values observed 67 seconds after the maximum depths were reached.

Figure 7: Profiles of temperature (upper panel), salinity (mid panel), and pCO2 (lower panel) from the 

saw-tooth carbon glider prototype 2.1 mission during the August sea trials.
11

The hover mission was conducted with programmed hover depths every 5 m between 5 and 30 m 

and at 40 m (Figure 8). The glider executed this hovering by dynamically adjusting the battery 

position such that the glider maintained a level pitch. Meanwhile, the buoyancy was dynamically 

adjusted to bring the glider to the desired depth. At the 15 and 20 m hover depths, the glider was 

essentially neutrally buoyant and required very little buoyancy adjustment. At other depths, 

significant buoyancy adjustments were required to reach the desired depths. These difficult 

conditions pointed out the need to improve the glider’s hovering capabilities by refining the 

flight control in response to the local conditions. pCO2 stabilized within a few ppm during the 

hover missions, which demonstrates that short hover missions can achieve equilibration of the 

sensor and construction of pCO2 profiles in the water column, a first for an autonomous glider.

Figure 8: Image of the carbon glider prototype 2.1 at the surface during the August 2016 sea trials (a) with 

profiles of temperature (b), salinity (c), and pCO2 (d) from a hovering mission. Hover stops were 

programmed every 5 m between 5 and 30 m and at 40 m.
12

The August sea trials also demonstrated that the water-pumped head, designed by Pro-Oceanus 

for our second generation Mini-Pro pCO2 sensor, had the potential to damage the flat semipermeable membrane. The semi-permeable membrane is used as an interface for the 

equilibration of headspace gas with the surrounding water and is the only barrier preventing 

water from penetrating the sensor housing and electronics. The repeated pressure cycling during 

dives and climbs pushed the membrane against the water-pumped head, slightly wrinkling it and 

potentially flooding the sensor (Figure 9). 

Figure 9: Picture of the wrinkled membrane from prototype 2.1 following successful sawtooth and hover 

missions during the August 2016 sea trials.

Arctic deployment

Prototype 2.3 was deployed and recovered twice during the 2016 September cruise of the R/V 

Sikuliaq (Figure 10). The first deployment attempt on September 11 was aborted after a failure in 

the glider’s science bay. Following an initial dive to 5 m, the glider began aborting the mission 

because it lost communication with its science computer, having blown a fuse on the science 

board. The second deployment was on September 20 at 71.61°N, 163.77°W. Before this second 

deployment, the glider was sitting outside for over 12 hours in 0°C weather. On-deck tests the 

night before and an hour before deployment reported pCO2 values ranging from -2000 to 0 ppm. 

The glider was checked out following the standard deployment procedure and sent on an extra 

check-out mission to dive to 20 m with 2-minute safety stops at 5 m at the start and end. On this 

initial hover mission, the glider sank through the water column during its attempt to hover at 5

m. The poor hover behavior was attributed to the glider being ballasted too heavy for the surface 

density of 1022.7 kg/m3

.
13

Figure 10: The carbon glider deployed near the R/V Sikuliaq in the Chukchi Sea in September 2016.

The glider was sent on its next mission after reviewing the science and engineering data files 

from the test dives. Due to the response time of prototype 2.3, we developed novel missions to 

allow the glider to hover at various depths through the water column in order to get accurate 

pCO2 data. Since the glider can only make headway during its standard “yo” or “sawtooth” 

missions, we had to develop a secondary mission that would allow us to make forward

movement as well as perform the required safety stops. The intended mission had the glider do a 

safety stop, perform ten dives to 35 m, do another safety stop, then surface after approximately 

two hours. 

The glider began its mission September 20th at 21:45 UTC and did not resurface until September 

21st at 03:37 UTC, 5.86 hours after diving. The sensor must have been partially flooded at

mission start because the glider was not able to do the safety stop or yo properly (Figure 11). 

During each of the ten dives, the glider attempted to climb but would complete early and sink 

back to 37 m. The safety stop was the last behavior the glider was supposed to run in the 

mission; instead, it sank to the seafloor at a depth of 42.3 m. The glider aborted after 30 minutes 

because it was not able to change depth. After four hours of attempting to surface while sitting 

on the seafloor, the glider began its emergency procedure to drop a 1 kg weight and within nine

minutes of dropping the weight surfaced. 
14

Figure 11. The carbon glider depth profile from a September 2016 mission.

Science data were not collected during the mission because the Mini-Pro pCO2 sensor caused a 

fatal error at the start of the mission. After removing the water-pumped head, small indentations 

were visible on the semi-permeable membrane. Inspection of the pCO2 sensor by Pro-Oceanus 

confirmed that the sensor flooded as a result of the perforated membrane (Figure 12). The failure 

was due to a missing O-ring between the membrane and mesh in the pressure head, which may 

have come loose during shipment.

Figure 12: Image of the perforated membrane from the flooded pCO2 sensor after deployment in the 

Chukchi Sea.
15

Final sea trials

Final sea trials took place in May 2017, just south of Caines Head in Resurrection Bay. 

Deployments were made from the small vessel Dora . The goals for these trials were to fly the 

glider for consecutive days, ground-truth the pCO2, salinity, and temperature data with CTD 

casts, and proof the overall concept of the newly developed unit.

The carbon glider experienced another blow out of the F2 fuse on the science board during 

laboratory tests between field trials. Since this also happened during the Chukchi Sea 

deployment, it appears an infrequent high-power demand by the pCO2 sensor causes this failure. 

Additional bench top tests also showed that the sensor suddenly changed baud rate from 9600 to 

19200, causing communication issues with the glider. The carbon glider was run in the 

laboratory tank for nearly 20 hours without re-occurrence of these issues before being deployed

for final sea trials.

When the glider first deployed into the water for the sea trial, the sensor baud rate switched back 

to 19200 after checking out fine minutes before. After this initial problem, the deployment, 

recovery, and mission executions were very smooth. The data from these missions showed that 

the glider stayed +/- 2.5 m of the set point while hovering. While the glider was able to stabilize 

its pitch, it had a hard time maintaining buoyancy (Figures 13 and 15). The pCO2 profiles 

suggest that either the 7-minute long hovers were too short for the sensor to equilibrate or the 

glider went in and out of different water masses while hovering (Figures 14 and 16). Note that 

salinity data is not corrected for sensor lag or thermal lag effects. This glider has an unpumped

CTD and relied on forward motion to increase response times. It would take significant time to 

redo processing scripts to account for these hover missions, which is outside the scope of this 

project.
 Figure 13: Glider flight data showing a hover mission during the sea trials in May 2017 with controlled hover at the pre

 (top), ballast pumped (second from top), pitch (third from top) and glider battery position (bottom) are plotted as a func

 designates measured value, whereas “c” designates commanded value.

16
17

 

 

Figure 14: Temperature, salinity, and pCO2 data from mixed sawtooth/hover mission 1. The upper left 

panel shows temperature (blue), salinity (red), and density (black) as a function of pressure. pCO2 is 

shown as a function of time (middle left panel) and as a function of pressure and time (lower left panel). 

The right figures show temperature (upper), salinity (middle), and density (lower) as a function of time 

and pressure. 
Figure 15: Glider flight data showing a hover mission during the sea trials in May 2017 with controlled hover at the pre

(top), ballast pumped (second from top), pitch (third from top) and glider battery position (bottom) are plotted as a func

designates measured value, whereas “c” designates commanded value.

18
19

 

Figure 16: Temperature, salinity, and pCO2 data from a mixed sawtooth/hover mission 2. The upper left 

panel shows temperature (blue), salinity (red), and density (black) as a function of pressure. pCO2 is 

shown as a function of time (middle left panel) and as a function of pressure and time (lower left panel). 

The right figures show temperature (upper), salinity (middle), and density (lower) as a function of time 

and pressure.

After two days of successful hover and yo missions, the carbon glider was checked out for its 

first overnight mission. The pCO2 sensor switched baud rates mid/end of its first 2-hour mission. 

Since the baud rate cannot be changed over iridium, the sensor was switched off, and the carbon 

glider continued with its missions without collecting pCO2 data. Upon recovery of the carbon 

glider, the pCO2 sensor was found partially flooded (<118 ml of seawater). The sensor was 

gutted and outfitted with a new board, new O-rings, and new membrane. The new board was 

outfitted with a code to fix the baud rate issue. The carbon glider was redeployed and collected 

pCO2 data for another few hours. Then, all missions produced "delayed ERROR (558)" errors, 

without causing aborts. In addition, science sensors and pressure cut out randomly after the last 

mission with pCO2 data. Examination of the sensor showed that an impulse connection failure 

was the reason for these issues. A pCO2 profile collected by the carbon glider before the 

complete failure shows very long response times (Figure 17), suggesting that the pump failed 

due to the defect impulse connector not supplying the sensor with adequate through-flow to 

decrease its equilibration time.
20

Figure 17: Temperature (upper left), salinity (middle left), pCO2 (lower left), backscatter (upper right), 

CDOM (middle right), and Chlorophyll-a (lower right) data from glider missions after the pCO2 sensor 

was flooded.

Problems identified through the final sea trials included both glider and sensor issues. The hovers 

were not very good, in large part because the glider software will have to be updated to account 

for tight station-keeping requirements. Hover time was increased by several minutes over the 

August 2016 trials, which added to the problem of the glider holding position. Mini-Pro related 

problems included a partially flooded sensor, slower response time than in previous versions, a 

defective impulse connector, and ongoing issues with blown fuses (has happened twice with 

carbon glider, but uncommon for other glider operations)
21

Conclusions

The main goal of this project was the development of a carbon glider to autonomously acquire

high-resolution spatial and temporal data of dissolved CO2 concentrations throughout the water 

column. Initially, we aimed to reduce the equilibration time of the Mini-Pro pCO2 sensor to 

accommodate measurements of fast pCO2 changes as the glider moves through the water 

column. However, since we were not able to reduce the equilibration time to <1 minute, we 

developed glider hover missions. We have made significant advancements with the hover flight 

profiles, but more work is required to reduce the need for dynamic adjustments to the glider 

buoyancy, a change that would save energy and further extend the mission duration. While the 

membrane set-up of the pCO2 sensor went through several iterations, it is currently still not 

strong enough to withstand the rapid pressure changes that the sensor experiences during glider 

missions. Once this issue is resolved, the performance of the carbon glider needs to be 

thoroughly evaluated with in situ pCO2 sampling. Furthermore, longer test missions need to be 

conducted in order to understand the power budget. However, due to the high power 

consumption of the pCO2 sensor and the SBE 5P pump, mission longevity is estimated to about 

one week. Upgrade to a lithium powered glider would increase the mission length and, therefore,

the safety of the carbon glider substantially.

As a result of this project, we developed a carbon glider and brought it to a Technology 

Readiness Level 6 (TRL-6 per NASA), with demonstrated capacity of pCO2 data collection 

during glider flight missions at sea. However, as discussed above, issues remain to be solved 

before the carbon glider can be advanced to a fully operational level.

Acknowledgments

We are very grateful to the Seward Marine Center and the Alaska SeaLife Center for 

accommodating our experiments, bench top testing, and pre-cruise preparation work. Many 

thanks also go to Dora Captains Mike and John. We would also like to thank Alutiiq Pride

Shellfish Hatchery for letting us test the carbon glider in their tank and for sharing ‘Burkulator’ 

data with us.

Study Products

Results from this study were presented at the Alaska Marine Science Symposium in 2017 and 

annual Coastal Marine Institute meetings. The raw data from the carbon glider missions was 

archived at the National Oceanic and Atmospheric Administration’s National Centers for 

Environmental Information (NOAA NCEI). Information on this project can be found at 

http://mather.sfos.uaf.edu/artlab/projects/CarbonGlider/. 
22

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