Therm RTD is an addition to the Therm PID Controller family, with support for RTD temperature sensors. RTDs (or Resistance Temperature Detectors) use a coil of fine wire made from a material (usually platinum, copper or nickel) that has a very predictable temperature coefficient of resistance (or change in resistance as temperature changes). RTDs are generally more accurate and stable than thermocouples, and have a much greater range than thermistors – although they can tend to be more expensive than both.
The hardware and functionality of Therm RTD is very similar to the original Therm; the biggest change is the replacement of the thermocouple chip with a MAX31865 chip to support RTDs instead. This chip also supports thermistors; however, the reference resistor must be replaced with a larger one before using higher resistance thermistors. The OLED display, USB port, and solid state-relay driver are left unchanged, but the external power input has been removed. Removable mounting feet have also been added to assist in installation.
The new hardware comes with a firmware update that is compatible with both models. The menu system has been improved, with more extensive reboot and reset options. Issues with non-volatile memory have been fixed, allowing settings to be saved across reboots. Temperature display resolution has been increased for both C and F, and PID calculations are now performed using 1/10th degree precision. Source code for both the firmware and the hardware are available on the Protofusion repository.
Future updates are planned to add software-selectable reference resistors so that both RTDs and thermistors can be used without hardware modifications. Temperature profiles with support for multiple setpoints are also in the works to facilitate more complex temperature control applications.