Pax Instruments Inc is making an open source temperature datalogger. Charles made a great video going over the entire current prototype showing off the tech inside and reasoning behind many of the build and design decisions. The device is using a MCP3424 A/D, it has 18 bits of resolution so your measurements should be nice and accurate. There is also a lot of thermal management that has been done in the board design which should minimize the heating effects of the control and power electronics interfering with the measurement readings. They even designed a custom thermocouple connector since the ones on the market were not good enough for their product. Since this interface is very common I would have expected that you would have been able to get one very cheap all the way up the a military grade device that is built like a tank and would last 100 years.Â