code: def to_si(self, values, from_unit):
          
          if from_unit in self._si_units:
              return values, from_unit
          elif from_unit == 'degF-hours':
              return self.to_unit(values, 'degC-hours', from_unit), 'degC-hours'
          else:
              return self.to_unit(values, 'degC-days', from_unit), 'degC-days'
docs: Return values in SI and the units to which the values have been converted.