Right now you can get a T400 on Kickkstarter https://www.kickstarter.com/projects/1880050573/pax-instruments-t400-temperature-datalogger
The Pax Instruments T400 datalogger is an open source four-channel thermocouple temperature datalogger based on the Arduino™ Leonardo platform. It is ready to use out of the box with the features you want most. Measurements can be logged to MicoSD card, printed to serial port, and graphed. The T400 is a great tool for anything from live thermal process monitoring in the lab to long-term environmental data collection in the field.
Here's a schematic of the 0.7 hardware.
The Pax Instruments T400 datalogger is designed to be out of the box ready for professionals and hobbyists alike. If you need a temperature datalogger that works every time, this is the device for you.
Open source spirit
The hardware and software design files are available to you at no cost to use, modify, or redistribute. This allows you and others to extend the devices capabilities or tailor it to your specific application.
Arduino™-compatible hardware means while hacking on the platform you will be able leverage the work of others while sharing your own work with large community of hackers and makers. Sharing is caring.
Hardware MicroSD slot
Readings can be saved to a microSD card in standard CSV format for processing in Microsoft Excel, LibreOffice, or your favorite data analysis tool.
USB serial port
Readings can captured live via the USB serial port. This is perfect for live process monitoring in lab experiments or connecting to an internet-enabled device.
Thermocouples connect via standard mini thermocouple connectors. The T400 is compatible with a wide variety of K-type thermocouple sensor types from stainless steel probes to rolling surface-contact sensors.
The MCP9800 temperature sensor is used for cold junction compensation.
The MCP3424 analog-to-digital converter measures the voltage produced by each thermocouple.
The DS3231 real time clock is used to trigger readings. Between readings the device is put into low power sleep mode. The RTC wakes up the unit to take a reading. This gives a longer battery life.
The T400 uses a standard BL-5C battery. This is great for battery replacement in the field.
The generous 132×64 LCD is capable of displaying the current temperature for each thermocouple as well as a graph of the most recent readings.
The T400 runs on the ATmega32u4 AVR processor.