New SIMB3 MicroCTD add-on

Add ocean salinity measurements to SIMB3

As a partnership with the Alfred Wegner Institute, we've developed a prototype conductivity, temperature, and depth (CTD) package for SIMB3. This add-on collects the three main state variables required to calculate ocean salinity.

Design Motivation and Constraints

A CTD add-on, or at least an electrical conductivity add-on, has been at the top of our most-requested feature list for at least two years. In the past, it‘s a challenge I’ve avoided tackling because of the difficulty of sourcing parts that I felt would properly integrate into SIMB3. Commercially available CTD packages and components are often physically bulky, power hungry, and extraordinarily expensive. Given the ethos of SIMB3 as a small form-factor, low-power and easily deployed instrument, I didn’t feel that any off the shelf options would work well.

A couple of things changed this last year. First, Christian Haas at AWI reinitiated the idea of adding conductivity measurements as he was interested in deploying several SIMB3s in the marginal ice zone during springtime conditions. His use case mainly involves determining relative changes in upper-ocean salinity, and is less about measuring the the absolute value of salinity to a high-degree of accuracy. Relaxing the need for oceanographic-grade accuracies opened up several options for us.

The second change was my discovery of Atlas Scientific, which is a NYC company that produces an amazing line of sensors/circuits that are primarily marketed to the hydroponics industry. All of their sensors are small, power-efficient, and their documentation is some of the best that I’ve ever seen. On top of that, they offer support circuitry that integrates directly with Arduino, making electrical integration into SIMB3 straight forward. Considering the above, I decided to design our own CTD for SIMB3 using a combination of components from Atlas Scientific and Adadruit.

Sensor Selection

The sensors chosen for the package are the K-1 Mini EC probe, the A-10 Analog Pressure Transducer, and the Micro PT-1000 Temperature Sensor. Each were chosen based on a combination of their size, electrical requirements, and their accuracy and measurement range.

Two of the three sensors are digital and thus integrated directly into our existing SIMB3 hardware through our I2C line. The pressure transducer required an additional analog-to-digital conversion step before it could be integrated. The reported precision of this sensor is dictated primarily by the ADC, which in this case is 16-bit.

Now, you might be asking “why do you need a pressure sensor when you’re only doing ’moored’ measurements, i.e., not depth casts where pressure changes are significant?”. Great question, and there are two answers. 1) Christian wanted the ability to tell if SIMB3 shifted vertically during deployment, and 2) I thought it would be way cooler to make a MicroCTD rather than a MicroCT 😛.

Hardware Design and Placement

The hardware used to mount the CTD components was modeled and 3D printed in PLA plastic. Within SIMB3, the package mounts at the bottom of the lower section and is sealed with potting compound to maintain the necessary waterproof barrier. Location at the bottom of the lower section places the package approximately 1.5 meters below freeboard (distance below the ice will obviously vary with ice thickness). The entire package fits in your hand and adds just a few hundred grams to the SIMB3 weight.

Figure 1: Location of the MicroCTD package at the bottom of the SIMB3 lower section as seen during assembly. The white plastic portions visible get potted to form a waterproof barrier.

Data and Equipped Buoys

Addition of the MicroCTD adds three columns to the SIMB3 datasheet; CTD conductivity, CTD temperature, and CTD pressure (depth). The columns are represented with intuitive names as ctd_conductivity, ctd_temperature, and ctd_pressure, respectively. These are also the field names used for queries made through our REST API.

Currently, there are three SIMB3s equipped with the first three MicroCTDs that we have ever built. The buoys are:

  • AWI 2022 #1
  • AWI 2022 #2
  • AWI 2022 #3

They are set to deploy during Summer of 2022 and we are excited to see the data they return! Once they activate, data will be available instantly via their respective dashboards. I'll also note that these buoys are not equipped with our standard-issue Bruncin temperature string, so the datasheet will look much smaller than it normally does.

Limitations and Future Work

The development of the first-generation SIMB3 MicroCTD represents a long-awaited step forward for the SIMB3 program. Of course, we'll learn a lot about the survivability and reliability of this package once the above buoys are deployed, but I'm optimistic that it will go smoothly. Assuming they last, it will serve as an excellent validation of Atlas Scientific products for use in Arctic conditions. They offer a wide range of other sensors, including sensors for measuring pH, oxidation-reduction potential, and dissolved oxygen. All of these would easily integrate into SIMB3.

I'll also note that while this package has currently (as of June 2022) only been installed on a non-standard SIMB3 (without a temperature string), it will work just as well on a fully-equipped SIMB3. Given the demand, this is something we can develop.