Return the dew point temperature in ºC for the last measurement.

        For sensors implementing temperature and humidity values.
        Extracted from the HTU21D sensor spec sheet.

def dew_point_temperature(self):
        """Return the dew point temperature in ºC for the last measurement.

        For sensors implementing temperature and humidity values.
        Extracted from the HTU21D sensor spec sheet."""
        if self.sample_ok:
            temperature = self._get_value_opc_attr('temperature', 3)
            humidity = self._get_value_opc_attr('humidity', 3)
            if temperature is not None and humidity is not None:
                # Calc dew point temperature in celsius
                coef_a, coef_b, coef_c = 8.1332, 1762.39, 235.66
                part_press = 10 ** (coef_a - coef_b / (temperature + coef_c))
                dewp = - coef_c
                dewp -= coef_b / (log10(humidity * part_press / 100.) - coef_a)
                return dewp
        return None