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SeaFET pH Sensor

SeaFET™ Ocean pH Sensor

The SeaFET™ Ocean pH sensor is an ion selective field effect transistor (ISFET) type sensor for accurate long-term pH measurements in salt water.

The SeaFET™ reports pH determined potentiometrically in two different ways.  The ISFET potential is measured against a reference electrode bearing a liquid junction (internal reference) and against a solid state reference electrode without a liquid junction (external reference).  This approach provides the user with the ability to quality assess instrument performance and ultimately achieve a greater understanding of the state of acid/base equilibria in seawater.

The SeaFET™ Ocean pH sensor has been used extensively for:

  • ocean acidification research
  • coral reef research
  • coastal marine biology
  • environmental monitoring.

The SeaFET™ Ocean pH Sensor was developed by Dr. Kenneth Johnson of the Monterey Bay Aquarium Research Institute (MBARI) and Dr. Todd Martz of the Scripps Institution of Oceanography, University of California San Diego.  We collaborated with MBARI and Scripps to make the instrument commercially available to the research community. SeaFET™ sensors have now been deployed in many locations, including Canada, USA, France, UK, Australia and China.

The sensing element of the SeaFET™ is an ion sensitive field effect transistor (ISFET).  This class of device has been used for pH sensing in industrial processes, food processing, clinical analysis and environmental monitoring. The advantages of the ISFET include robustness, stability and precision that make it suitable for ocean pH measurement at low pressure.

Replacing a SeaFET™ Ocean pH Sensor Battery (Note this is an older model)

Click on the Image to Watch the Video


Measurement Range 6.5 - 9.0 pH
Initial Accuracy ± 0.05 pH
Typical Stability 0.003 pH/month
Precision 0.004 pH
Internal Memory 4 GB                                                                        
Supply Voltage Range  6 - 18 VDC
Communication RS-232, USB
Weight (with batteries) 5.4 kg in air; 0.1 kg in water
Dimensions (Housing) Diameter 508 mm (20") x 114 mm (4.8")
Maximum Depth 50 m
Temperature Range 0 to 50 deg C


The list below includes (as applicable) the current product brochure, manual, and quick guide; software manual(s); and application notes.

The SeaFET  and SeapHOx Ocean pH Sensors come complete with SeaFETCom, an interactive graphic software application.

Version 2.0.3 released September 28, 2017
SeaFETCom-2.0.3-b115-x86.exe for Windows 7/8/10

Caution: SeaFET cannot operate without a valid firmware. When upgrading the firmware, the existing firmware is erased, and the new firmware is written to memory.

Version 3.8.0 released October 30, 2017 for Windows 7/8/10

Will the SeaFET™ work in high latitude regions?

Yes, provided the SeaFETTM sensing elements remain ice-free, the instrument works across the broad temperature range of 0 to 50 deg C. The original SeaFETTM prototypes developed at MBARI and SCRIPPS were deployed extensively in the Antarctic for several studies. For example times series data, please see Matson et al. 2011.

How often does the SeaFET™ need to be calibrated?

The stability of the SeaFETTM Ocean pH sensor is expected to be 0.005 pH units on the timescale of weeks to months (Martz et al. 2010). At minimum, the SeaFETTM should be calibrated yearly.  The body of data collected by the community of SeaFETTM users generally suggests that the stability of the ISFET-based pH measurement offers an improvement by orders of magnitude when compared to glass electrode based pH sensors.

Wherever possible the SeaFETTM should be deployed in association with a water sampling program in order to collect water for spectrophotometric pH determination or another external measurement technique.  Additionally, coincident measurement of multiple carbonate system parameters, allow the stability of the SeaFETTM to be assessed.  These approaches have been carried out by a number of researchers over the past several years.


Which pH scale is relevant to the SeaFET™ Ocean pH sensor?

The SeaFETTM reports pH measurements on the total scale. Values of pH reported on the total scale represent the effective hydrogen ion concentration with contributions from both free hydrogen ions and those which have reacted with sulfate.


Can the SeaFET™ pH sensor be deployed with a pump?

Yes, the SeaFET™ pH sensor comes with a flow through cell with fittings that can be plumbed with a pump for flow-through operations. For in situ deployments, we recommend the SBE 5P submersible pump.

How should I store my SeaFET sensor?

It is crucial to store the SeaFET sensors with clean seawater in the wet cap. If seawater is not available, artificial seawater will suffice. The internal KCl reference gel in the DuraFET will become dry if left out of water, resulting in calibration drift and potentially causing irreversible damage. Do not store the SeaFET with freshwater in the wet cap, as this will change the chemistry of the external reference electrode.


If storing or shipping the SeaFET, disconnect the batteries in SeaFETCOM:

  • Connect the SeaFET to SeaFETCom
  • Navigate to Sensor > Advanced > Disconnect for Storage.


This will disconnect the isolated battery pack from the external reference sensor to retain battery life.

Can I integrate the SeaFET™ with a CTD?

SeaFETTM serial number 200 and higher can be integrated with an SBE 37-SMP-ODO to form a SeapHOxTM. When properly configured, the SeaFET will automatically correct pH with CTD temperature and salinity. If a 37-SMP-ODO is not available, or if you SeaFET is below serial number 200, the SeaFETTM records time-stamped data internally that can be matched with CTD data during post processing for temperature and salinity corrections.

Do I need accurate salinity and temperature data to correct the SeaFET™ pH output?

The acquisition of conductivity/salinity data alongside the SeaFETTM is recommended. The acquisition of temperature data is not necessary as the SeaFETTM performs temperature measurements. Typically, conductivity and temperature measurements are combined in standard sensor packages. One can use an external temperature measurement calibrated to a relevant scale to confirm the accuracy of the SeaFETTM temperature measurement.

The proximity of the two sensors, as well as, the abruptness of spatial gradients in temperature are both important considerations.

Flow Cell

It is essential that the SeaFET™ sensing elements remain immersed in seawater at all times.  For this reason, the SeaFET™ is equipped with a resealable flow cell. The flow cell can be fitted with brass penetrating fittings and copper-nickel tubes to which one can attach flexible tubing or hoses for flow-through applications (Image shows one fitting attached and one not).



Bio-fouling Guard

The SeaFET™ bio-fouling guard is a copper based passive fouling prevention guard. The anti-fouling guard provides a reliable and affordable approach to increase deployment time, decrease operating costs, and collect high quality data for mooring applications. 



Internal Battery Pack (For earlier models)

The SeaFET™ internal battery pack contains seven Alkaline D-Cell batteries connected in series to provide nominal 10.5 V. The nominal capacity, 19.8 mAh specified by the battery manufacturer is attained under ideal operating conditions.




Alkaline Battery Pack

Reliable and user-friendly Alkaline Battery Packs are available in 102 Ah and 51 Ah capacities. The design of the internal battery compartment allows the user to easily change the D-Cell batteries with off-the-shelf replacements. The battery packs consist of an anodized aluminum pressure case, a D-cell battery compartment and a removable end-cap.