def _read_coefficients(self):
        """Read & save the calibration coefficients"""
        coeff = self._read_register(_BME280_REGISTER_DIG_T1, 24)
        coeff = list(struct.unpack('<HhhHhhhhhhhh', bytes(coeff)))
        coeff = [float(i) for i in coeff]
        self._temp_calib = coeff[:3]
        self._pressure_calib = coeff[3:]

        self._humidity_calib = [0]*6
        self._humidity_calib[0] = self._read_byte(_BME280_REGISTER_DIG_H1)
        coeff = self._read_register(_BME280_REGISTER_DIG_H2, 7)
        coeff = list(struct.unpack('<hBBBBb', bytes(coeff)))
        self._humidity_calib[1] = float(coeff[0])
        self._humidity_calib[2] = float(coeff[1])
        self._humidity_calib[3] = float((coeff[2] << 4) |  (coeff[3] & 0xF))
        self._humidity_calib[4] = float((coeff[4] << 4) | (coeff[3] >> 4))
        self._humidity_calib[5] = float(coeff[5])
Read & save the calibration coefficients