We have recently published two new papers in the Journal of Experimental Biology detailing the results of field experiments we carried out. Our goal was to monitor the behavior and internal temperatures of sea mussels (Mytilus californianus) on the shoreline, and link their recent experiences to their physiological status. There can be substantial variation in the environmental experience of mussels sitting only a few centimeters away from each other, and this project was an attempt to catalog that variation for the first time in a wave-swept rocky intertidal environment. This project involved the development of a new datalogger system (MusselTracker) to allow us to attach individual sensors to live mussels and deploy them in the field for multiple weeks on battery power. The MusselTracker system is described in our papers, and on my GitHub site: https://github.com/millerlp/MusselTracker which also includes the design files and software to recreate the system. The two papers are:
Miller, L.P. and W.W. Dowd (2017). Multimodal in situ datalogging quantifies inter-individual variation in thermal experience and persistent origin effects on gaping behavior among intertidal mussels (Mytilus californianus). Journal of Experimental Biologyhttp://dx.doi.org/10.1242/jeb.164020
Gleason, L.U., L.P. Miller, J.R. Winnikoff, G.N. Somero, P.H. Yancey, D. Bratz, and W.W. Dowd (2017). Thermal history and gape of individual Mytilus californianus correlate with oxidative damage and thermoprotective osmolytes. Journal of Experimental Biology http://dx.doi.org/10.1242/jeb.168450
To accompany that information, here I outline the process of making the actual wired sensors that we attached to the mussel shell.
Here’s a pictorial walkthrough of the process I used to waterproof our accelerometer and hall effect sensors for our mussel tracking experiment.
The goal was to stick 30 of these little breakout sensor boards on the end of cables, and run the cable back through a waterproof bulkhead fitting into a waterproof box that housed a datalogger.
For cabling we settled on standard USB cable since we needed pieces that were between 12 and 24″ long. Cutting up 6′ long USB cables worked fine for this purpose. Monoprice.com has some of the cheapest prices on USB cables. The diameter of the cable will vary with the length of the cable and the application. These 6′ micro-USB cables have an outside diameter of about 3.9mm (0.15″), and have 4 28-gauge conductors (typically red, black, white, and green) along with a foil shield, a copper drain wire, and additional shielding wire outside the foil. For less than $2 a piece, they’re quite well made. There are smaller diameter cables available in shorter lengths, but cables that are longer than 6′ (1.8m) are usually made of larger diameter wire, around 0.2″ or larger.
The first step is to install the bulkhead fitting so that you don’t forget it later.
The bulkhead fittings we used were obtained from McMaster.com, and are referred to generically as nylon liquid-tight cord grips. These are made for 0.08 – 0.2″ diameter cable, part number 69915K46. They have a PG-7 thread on the base, which is essentially a 1/2″-20 UNF thread, but is not tapered and won’t be self-sealing like a tapered pipe thread. Larger versions of these fittings are available with tapered pipe threads. The fitting does come with an o-ring that seals against the flange in the middle of the body when screwed into your bulkhead, but we back this up by gluing them in place whenever possible.
A person with a lesser sense of humor would title the next section something like “Crimpin’ ain’t easy”, but this is a classy website, so I won’t do that.
For the cable to datalogger connection, we use JST PH-series connectors. A 1×4 male header is installed on the datalogger board, and the cable will get a matching 1×4 female connector. This connector is narrow enough to fit through the large hole you have to drill in the box for the bulkhead fitting. The crimp-on female ends are JST part number SPH-002T-P0.5S. They insert into the 1×4 plastic housing part number PHR-4.
That concludes the proximal end of the sensor cable. The remainder of the work involves installing the sensor board and waterproofing it.
Thermocouples. We also had thermocouple sensors on this project to measure temperature, which were built with 36-gauge K-type wire from Omega, with a PFA (Teflon-like) jacketing. To toughen these up, we covered them with 1/32″ heat shrink tubing, which increases the diameter of the lead, but makes them more resistant to abrasion.
The tips were twisted together with forceps and soldered to form the temperature-sensing junction that would be inserted in the mussel.
It’s very important to note that seawater can migrate up the wires of the thermocouple, inside the jacketing of the individual wires, and make its way into a watertight box. To combat this, you must coat the tip in something, whether that is adhesive-lined heat shrink, superglue, silicone, or polyurethane, in order to waterproof the tip.
To get these small thermocouple leads into our waterproof box, we used a 4-hole liquid-tight cord grip from McMaster, part number 7807K32, for 4 cords of 0.05-0.06″ diameter. The 1/32″ heat shrink makes the size of the thermocouple lead large enough to be snugly gripped by this bulkhead fitting.
That pile of breakout boards shown at the top of the post presented plenty of opportunities for bad solder joints to cause problems. During the assembly process for each sensor, I built a test rig on a breadboard that allowed me to test each individual breakout board with a simple pass-fail indication.
The board is run by an Arduino Pro Mini 3.3V board loaded with a simple program to test the hall effect sensor or the accelerometer/magnetometer sensor boards. The mounts for the two different boards are seen to the left of the Arduino, and they have little wires sticking up through the 4 holes where the sensor board sits. I could then simply slide the sensor board onto the 4 leads, and power up the Arduino. The Arduino would attempt to talk to both the the hall effect sensor and the accelerometer, and if it got reasonable responses it would turn the green LED on. If it didn’t get a response, it turned on the red LED. A set of pogo pins would have made this easier to slap the sensor boards onto, compared to the 4 wires sticking up, but this was a short-term project that didn’t warrant making a whole special testing PCB design.