<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def temperature_series(self, unit='kelvin'):
 """Returns the temperature time series relative to the meteostation, in the form of a list of tuples, each one containing the couple timestamp-value :param unit: the unit of measure for the temperature values. May be among: '*kelvin*' (default), '*celsius*' or '*fahrenheit*' :type unit: str :returns: a list of tuples :raises: ValueError when invalid values are provided for the unit of measure """

        if unit not in ('kelvin', 'celsius', 'fahrenheit'):
            raise ValueError("Invalid value for parameter 'unit'")
        result = []
        for tstamp in self._station_history.get_measurements():
            t = self._station_history.get_measurements()[tstamp]['temperature']
            if unit == 'kelvin':
                temp = t
            if unit == 'celsius':
                temp = temputils.kelvin_to_celsius(t)
            if unit == 'fahrenheit':
                temp = temputils.kelvin_to_fahrenheit(t)
            result.append((tstamp, temp))
        return result