This is a project to build a robo-limpet, which is a temperature sensor made from an iButton and a limpet shell of about 5 cm in length and 1 cm deep. These devices are designed to approximate and measure the temperature that living limpets experience in areas wherever the sensor is placed. Since limpets primarily live in rocky intertidal areas, these sensors are waterproof and durable enough to last several months in the pounding surf. I did this project as a part of my master's thesis where the sensors were used to compare the differences in temperature on opposing sides of large boulders. This work almost exactly follows the design developed by Fernando Lima, Ph.D., which was published in the following open access article:
Lima, F.P., and D.S. Wethey. 2009. Robolimits: measuring intertidal body temperatures using biomimetic loggers. Limnology and Oceanography: Methods 7:347-353.
Step 1: Disassemble the IButtons
Using 4" end nipper pliers, snip a little bit of the rounded edge of the iButton in 8 places. Then use the pliers to peal back the edge of the outer casing, making sure you don't damage the circuit board. Once the casing is completely pealed back the cap will fall off and the entire sensor along with its battery can be removed. Discard the casing after recording the serial number off the cap.
Step 2: Remove Circuit Board and Solder
Slide the circuit board out of the sensor before doing any soldering. I didn't do this with my first one and I'm pretty sure the heat destroyed the sensor. The heat doesn't seem to affect the battery though, so, I left it inside the plastic frame.
I then soldered some small sections of wire which I took out of an old phone cord. There are three contacts. The left is negative, the middle is positive and the right is the I/O contact. I soldered these wires to the negative and I/O contacts. See the Lima & Wethey article for an excellent depiction of this.
Step 3: Solder on Constantan Wire
Trim down the wires and solder on some longer sections of constantan wire. The constantan is necessary to resist corrosion when the robo-limpet is exposed to saltwater.
Once cooled, slide the circuit board back into place.
Step 4: Drill Holes in Shell
Using a Dremel high speed cutter (#569), drill two holes in the posterior end (flatter side) of the limpet shell. These holes should be just big enough to slide the constantan wire through.
Step 5: Fit Sensor to Shell
Now fit the sensor into the shell. I started by sliding the constantan wires through the hole, and marked their position once they were in far enough for the entire sensor to fit inside. Then slide the wires back out and use pliers to bend them at approximately 90-100 degree angle. Bending them while they are inside the shell will probably break the shell.
Everything should fit inside and nothing should stick out beyond the depth of the shell. It may same some adjustment of the wires and a couple times I had to re-solder them.
Now is a good time to test the temperature sensors to make sure they still work.
To test the sensor, use a pair of alligator clips connected to a phone wire which then connects to a RJ-11-to-USB adapter. These adapters can be purchased from most suppliers of iButtons. Here, I connected the alligator clips to the red and green wires
Step 6: Pouring the Scotchcast
Now for the fun part.
Prop the limpet shells on their back, toothpicks worked just fine here to make sure they stay level. The constantan electrodes also helped to support and keep the edge of the shell level.
Mix 3M Scotchcast 2130 Flame Retardant Compound as specified on the package.
Clearly, its easy to make a huge mess with this stuff. Two main lessons learned:
1) Before mixing, the scotchcast has the consistency of cold molasses. After mixing, it warms up and is viscous like oil coming out of a warm engine.
When you're ready to pour, I recommend cutting a fairly small hole in the scotchcast to give you more control over the pouring process.
2) Although purchasing the larger 21.7 oz bag may make economic sense, you will probably only need the 7.6 oz bag for every 10-15 of these sensors.
Fill the shells completely and be sure nothing is protruding through the scotchcast.
Since I made a mess of things, I had to scrape off a lot of scotchcast that had gotten onto the outside of the shells.
Let them sit overnight and verify that the sensors still work in the morning. All 10 of the sensors I poured here, worked just fine when it was done.
Step 7: Deploying Robo-limpets
Make sure you are familiar with and meet any permitting requirements for your state or area before doing this part.
Once they are ready to deploy, use A-788 Z-Spar Splash Zone Compound to attach the sensor to a boulder. I used a paint stirrer to mix about a tablespoon of part A and part B of the Z-spar in a little disposable container. The Z-spar was then applied to the bottom of the robo-limpet using my fingers while wearing a glove. I used enough Z-spar to cover the entire bottom of the shell, then pushed it into place on the boulder.
I scrapped away any excess using a popsicle stick.
To download the data, just attach the alligator clips to the robo-limpet and the USB adapter to a laptop.
Step 8: Bye
Bye for now.
If you have any questions, please feel free to ask.