Measuring and recording the plastron temperature of Eastern Box Turtles using re-constructed Thermochron iButtons
Box turtles use their carapaces and plastrons for thermoregulation by basking in full sunlight and using the soil as a heatsink when in resting forms. In my study turtles, I measure carapace temperatures using an infrared thermometer, and plastron temperatures using a thermister probe at the soil/plastron interface while in a form, but this is only done once every 3 or 4 days, and only once per each visit.
Dallas (Maxim) Thermochron Ibuttons can be programmed to record temperatures at any interval up to 256 minutes and can be read and re-launched nearly an unlimited number of times over the 10 year lifetime of the internal lithium battery.
Ibuttons are about the size a stack of 4 dimes and are used elsewhere in turtle research, primarily (if sometimes used improperly to record air temps) to record soil/carapace temperatures of a turtle during hibernation. (See website)
iButtons fail after about 10 years years of use due to their internal lithium battery failure. They can be re-purposed if the battery can be replaced.
Eastern Box turtles have behaviors convenient to this project. Box turtles don't drag their plastrons while walking, rarely walk backwards, and many male box turtles have deep concave depressions in their plastrons. Small information labels have been used on my turtles for many years, by super-gluing them in the concave where they survive for several years before falling off.
An off-the-shelf iButton thermochron is tiny, but still too thick to be mounted inside even the deepest concave on the turtle's plastron. The circuit board (minus battery) inside the stainless steel container is smaller diameter than a dime and about the same thickness. The logger can be re-constructed so that its thickness, with new battery, is less than the concave depth of many male box turtles.
The best way to disassemble the steel case is with a hobby Dremel tool using a small stone rotary bit. Manually using a flat file will also work.
Grind all the way around the rim (the hat brim) of the steel case, and lift off the thin metal cover exposing the battery and the black plastic insulating grommet.
Pull out the battery. It's not soldered to the circuit board, and won't be re-used. With the battery removed, the circuit board can then be seen at the bottom, held in place by simple pressure of the black plastic grommet (spacer). The flexible grommet is not glued in place, and can be lifted out, but be careful not to damage any parts on circuit board. The board will just fall out once the plastic grommet is removed.
Any size 3 volt lithium battery can be used as a replacement, but for this application a 1.6mm or 2.0mm thick battery is best. The battery must come with solder tabs, since the stainless steel cases on lithium coin cells can't be soldered very easily
This logger uses 2 wires to communicate with the software. One wire is called Data the other is Ground. They are both needed for programming and later for data download. This is likely to be done in the field with a portable computer (or an old Palm III computer with adapter) so the 2 wires need to be accessible but also protected from damage. They also need to be tiny; the thickness of the battery or less.
A two-pin connector (see photo) seems to work best but must be mounted so it is not damaged between uses. No tiny plug-in jack found so far is small enough to be used, and would almost certainly have problems with dirt clogging between uses.
The battery tabs need to be soldered to the gold connectors located on the component side of the circuit board. The tabs can be bent into any configuration necessary, but for this project, the battery needs to be mounted nearly right beside the circuit board, with just a slight bend so all fits inside the turtle's plastron concave without protruding above.
At the same time, the two communications wires need to be soldered, one to the battery negative, and the other to the bottom side of the circuit board. All soldering needs to be done quickly, and cooled quickly, so the components are not damaged.
The completed unit will be attached using 5-minute epoxy, and completely covered with epoxy to protect it from moisture and physical damage. But my experience shows that the epoxy is somewhat conductive before it sets. It's best to coat everything with lacquer (finger nail polish) and let to dry completely, before use.
FYI; the 5 minute epoxy will be hard when it is fully set, but will start to soften in 30 or 60 days. This is good when it comes time to remove the epoxied item (iButton, transmitter, etc.), but it must be inspected often so as not to loose the logger.
FYI2; 5-minute epoxy can be shaped while it is still soft using wooden sticks, etc. and even using fingers. Covering the epoxy with plastic flagging first allows the epoxy to be shaped and flattened with fingers, and the flagging will pull off easily AFTER the epoxy sets. Usually this is done on the next visit, but I sometimes leave the pieces of flagging on when attaching a transmitter and write the date and turtle number on it with a broad tipped sharpie. When the flagging falls offand found later, that location is recorded as a turtle location.
