The sensor package is a little bit less complicated than the OpenCTD control unit, but it has several tricky bits that make it a bit more technically challenging. For normal CTDs, I shave off about $40 from the price by building my own pressure sensor breakouts for the board-mounted pressure chip, but for a student kit that needs to be accessible to high school students, I prefer to use an off-the-shelf solution from SparkFun. 

Rounding out the package is a graphite conductivity probe from Atlas Scientific and three DS18B20 temperature sensors in a stainless steel cladding. Waterproofing heat shrink and heat shrink solder connectors for the connector round out the package. 

One of the nice things about having plugged away at this project for so long is that there are multiple ways to build just about every part of it, but that creates a challenge for making a standardized kit. There has to be One True Build for the kit OpenCTD, which means there will always be a tradeoff. Working with students also means that every step of the build has to fit neatly into a clean, 45-minute window, which is why I went with heat shrink solder connectors to turn 2 steps into 1. 

Of course, we also have a comprehensive build guide and the bill of materials has sourcing for all the alternatives, so there's always the option to go you own way.