This repository contains code to run a 8-channel thermocouple datalogger based on Arduino-type hardware. Currently there are 2 revisions, A and B, which differ in the type of thermocouple-to-digital chip they use. Revision A uses the older MAX31855K chip, while Revision B uses the more recent MAX31856 that effectively superseded the MAX31855 series. Revision A isn't recommended for new builds.

The directory TempTracker contains the primary files used to run a datalogger board, while the testing directory contains various utilities used to test the basic functions of a datalogger board or to help calibrate a board.

• https://github.com/millerlp/TClib2
• https://github.com/millerlp/Adafruit_MAX31856 - fork of the original Adafruit_MAX31856 library, mainly changed to return NAN temperature values automatically if there is an error.
• https://github.com/millerlp/RTClib - to run the onboard real time clock chip DS3231M
• https://github.com/greiman/SdFat - for reading/writing SD cards
• https://github.com/greiman/SSD1306Ascii - for writing to OLED displays

Eagle .sch and .brd design files are provided in the Eagle_files directory, along with images of the designs.

See the file Thermocouple_datalogger_parts.xlsx in the Parts directory for a list of all of the chips and other parts needed to build a board. There are separate sheets for Revision A and B, be careful to use the appropriate sheet. The majority of the parts should be available from a single vendor such as Digikey.com, but the OLED display screens and thermocouple connectors need to be sourced elsewhere.

For the OLED displays, any 128x64 pixel I2C SSD1306-based board should work (typically 0.96" size). These are available from several vendors on sites like Amazon or Ebay. Be aware that there are I2C and SPI versions available, but this datalogger board will only communicate via I2C, so buying the 4-pin I2C version of the display is sensible.

The thermocouple "miniature" connectors will need to be sourced from a specialized vendor. The board is designed around Omega Engineering's PCC-SMP-V connectors, which come in a minimum size batch of 5, so you'll need two batches to get 8. If you don't forsee needing to ever unplug a thermocouple, you could solder the leads directly the board, forgoing the Omega connector.

The MAX3185x thermocouple chips are relatively accurate from the factory, and for most purposes this will be sufficient, if you don't need absolute accuracy less than 1-2C. If you require sub-degree accurracy, calibration will be necessary, and this means calibrating both the datalogger board and the individual thermocouple leads you use.

Calibrating the datalogger board actually requires calibrating each of the 8 onboard MAX31856 chips separately, using a separate thermocouple calibrator such as the Omega Engineering CL3515R. These devices are capable of outputting a millivolt signal equivalent to what the relevant thermocouple type (K, T, J etc) would put out at a specified temperature. The calibrator comes with a thermocouple cable that plugs directly into the thermocouple plugs on the datalogger board, mimicing a real thermocouple. The program Calibration_routine_OLED_RevB (or the RevA version) is meant to be used with the external calibrator. After generating a set of calibration data for a set of known temperatures, you can use the observed temperatures (recorded by the datalogger) and the known temperatures (from the calibrator) to fit a straight-line regression, and get the intercept (aka offset) and slope of the relationship between observed and expected readings.

The program Store_calibrations.ino can then be used to upload the full set of 8 offset values and 8 slope values for the 8 MAX3185x chips to the datalogger's EEPROM onboard memory. The datalogger board will then have these calibration values available to use every time it boots up. Typically I find that an individual MAX31855K or MAX31856 chip has a slope very near to 1.0 (ideal), but the intercept (aka offset) may often be +/- 0.5 to 1C off from ideal (0C offset). Therefore, calibration is useful if you want the 8 individual channels on a board to be directly comparable to each other at the sub-degree level. The nominal temperature resolution of a MAX31855K chip is 0.25C (14-bit A-D converter) and the MAX31856 is 0.0078125 (19-bit A-D converter), but chip-to-chip variation from the factory can easily swamp that and only get you within +/- 2C of the true temperature in the normal biological range of temperatures. Careful calibration and housing the datalogger board in a thermally stable container should get you within 0.1C across channels.

The thermocouple-to-digital chips rely on an internal temperature sensor in the chip itself to measure the "cold junction" temperature. This temperature is ideally equivalent to the temperature at the point where the thermocouple plugs into the board and makes the transition from special thermocouple wire to the plain copper of the circuit board (which is a thermocouple junction by definition). An accurate measurement of the distant ("hot") end of the thermocouple probe relies on the MAX3185x chip also knowing what the cold junction temperature is quite accurately. If the MAX3185x chip internal temperature is a few degrees different than the thermocouple plug on the board, the registered thermocouple temperature will also be off. Ideally, a block of material that is thermally conductive (an "isothermal block") would be used to connect all 8 of the MAX3185x chips and their respective thermocouple plugs, and thus keep all of those points at the same temperature. A bar of aluminum, copper, or brass, mounted so that it sits across the top of all the MAX3185x chips and also rests against the thermocouple plug terminals would be best, although these should not all be in electrical contact. To thermally couple the various parts without creating short circuits, adhesive thermal gap filler (often used to attach heat sinks to circuit boards and chips) can be used. This material is electrically non-conductive, but does a reasonable job of allowing heat to transfer across it, and has the added benefit of filling small gaps that might exist between an isothermal block and the various parts due to height differences or variation in how flat the chips sit on the board. The RevB board has two holes on the edges of the board positioned to help bolt an isothermal block across the tops of the MAX31856 chips.

After provisioning an isothermal block, it would also be useful to then enclose the datalogger board in a housing that will stay relatively temperature-stable, or at least won't change temperature rapidly at different places. Surrounding the board in insulating styrofoam may be desirable, and then placing that inside a plastic or metal housing that only exposes the front of the thermocouple connectors to the outside air. The RevB board is designed so that panel-mount buttons can be attached to wire leads and plugged into the board (via standard 0.1" headers), so that the buttons can be mounted to a housing. The 2 OLED displays can be attached via 4-wire cables in the same manner. The rear/bottom edge of the board (opposite the thermocouple connectors) will need to be accessible through a housing wall so that the serial port, micro SD card slot, and 2.1mm DC power plug are accessible.