One of the first projects I did a couple years ago, was designing a custom freezer monitoring system. It's end purpose was monitoring -80°C freezer, but I tested them successfully in liquid nitrogen, down to -196°C.

The system communicates wirelessly with two gateways, by using a Zigbee mesh network between multiple nodes. The system works perfectly fine on one gateway, meaning the second gateway is purely redundant to keep up-time as high as possible.

Some of the collected live data is available at: https://grafana.xnorit.be/

A custom PCB was developed using KiCad. The system is powered through a micro-USB connection, as these are easier to solder than USB-C connectors. I used an XBEE3 zigbee MMT module for communication, combined with two small CR2032 batteries as temporary backup in case of a power failure.

A k-thermocouple is used for temperature sensing, which is connected to an MAX31855 thermocouple IC with integrated cold-junction compensation. Everything is read out by a custom programmed microcontroller, which sends all collected data to the XBEE3 over uart.

Optionally it's also possible to connect the module to a freezers alert port, which is basically a relay that triggers whenever the Freezer itself detects an issue. This provides an early warning, even before the temperature inside the freezer rises to much.

As this system is utilized in a very busy environment, I decided to go with a combination of aluminium shell and 3D printed covers for the IO. On the bottom are two neodinium magnets with each a pull-force of 3.3 kg, to keep the module neatly against the freezer.

I decided to use a mySQL database for data storage, combined with Grafana as GUI. I'm however in the process of merging all data in an Influx database for performance improvements.