Engineer Thomas Knutsen has shared the design for a programmable thermocouple meter and data logger, created for use with a small freezer converted into an Arduino-powered thermal test chamber a few years ago.
"Some time back (2016 time frame) I built a test chamber out of an inexpensive freezer, after having several project that required taking some thermal data over the expected useable range," Knutsen explains. "An Arduino was fixed above the power supply, and a small piece of Veroboard with some relays and driver transistors to switch the power to the pump motor and to the heater."
"The program running on the microcontroller is a fuzzy logic implementation with a 2°C hysteresis. There is also some timing logic to not turn on the pump before the back pressure has bled through, or the fuse will blow. I started out with some SCRs but the cheap eBay SCR I bought had too much loss before it caught fire."
With a test chamber made, Knutsen moved on to finding the best way to log the temperatures within — and ended up with a custom-built programmable thermocouple meter and data logger, built around a Microchip SAM D21 microcontroller.
"The design is done primarily as a programmable instrument, but it has a OLED display, so it can show the current temperature," Knutsen notes. "The programming uses SCPI, the same type of programming strings that most newer (1990 forward) instruments use. The meter consists of a Empyrean microcontroller from Etherkit and a couple of MAX31855 thermocouple amplifiers."
"The enclosure used is a cheap eBay case, 100x75x35mm. For this I have selected to mill out new font and back panels. There are 2 PCBs that interlock on this design, a main PCB that fits the enclosure and holds most of the parts and a display PCB for fixing the display in the correct position. There is a planned extension header (8 pole connector) for a 2. board of thermocouple amplifiers. This way the meter can measure up to 4 points in the same meter."
Knutsen has released board design files and software source code on GitHub under an unspecified license, but warns there are improvements which could be made: "The voltage regulator [...] should be replaced with a small switch mode regulator to 5V and a linear regulator down to 3.3V," he adds, "[and] another point of improvement is that USB is finky at best. It would be better to replace this with a network connection."
More information is available on Knutsen's blog.