code: public function getDeviceTemperatureScale($serial_number = NULL) {
          $serial_number = $this->getDefaultSerial($serial_number);
          return $this->last_status->device->{$serial_number}->temperature_scale;
      }
docs: Get the thermostat preferred scale: Celsius or Fahrenheit.
  
  @param string $serial_number The thermostat serial number. Defaults to the first device of the account.
  
  @return string 'F' or 'C'