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Document number: BST-BME280-DS001-23 Revision_1.23_012022

8. Appendix A: Alternative compensation formulas

8.1 Compensation formulas in double precision floating point

Please note that it is strongly advised to use the API available from Bosch Sensortec to perform

readout and compensation. If this is not wanted, the code below can be applied at the user’s risk. Both

pressure and temperature values are expected to be received in 20 bit format, positive, stored in a 32

bit signed integer. Humidity is expected to be received in 16 bit format, positive, stored in a 32 bit

signed integer.

The variable t_fine (signed 32 bit) carries a fine resolution temperature value over to the pressure

compensation formula and could be implemented as a global variable.

The data type “BME280_S32_t” should define a 32 bit signed integer variable type and could usually

be defined as “long signed int”. The revision of the code is rev. 1.1 (pressure and temperature) and

rev. 1.0 (humidity).

Compensating the measurement value with double precision gives the best possible accuracy but is

only recommended for PC applications.

// Returns temperature in DegC, double precision. Output value of “51.23” equals 51.23 DegC.

// t_fine carries fine temperature as global value

BME280_S32_t t_fine;

double BME280_compensate_T_double(BME280_S32_t adc_T)

{

double var1, var2, T;

var1 = (((double)adc_T)/16384.0 – ((double)dig_T1)/1024.0) * ((double)dig_T2);

var2 = ((((double)adc_T)/131072.0 – ((double)dig_T1)/8192.0) *

(((double)adc_T)/131072.0 – ((double) dig_T1)/8192.0)) * ((double)dig_T3);

t_fine = (BME280_S32_t)(var1 + var2);

T = (var1 + var2) / 5120.0;

return T;

}

// Returns pressure in Pa as double. Output value of “96386.2” equals 96386.2 Pa = 963.862 hPa

double BME280_compensate_P_double(BME280_S32_t adc_P)

{

double var1, var2, p;

var1 = ((double)t_fine/2.0) – 64000.0;

var2 = var1 * var1 * ((double)dig_P6) / 32768.0;

var2 = var2 + var1 * ((double)dig_P5) * 2.0;

var2 = (var2/4.0)+(((double)dig_P4) * 65536.0);

var1 = (((double)dig_P3) * var1 * var1 / 524288.0 + ((double)dig_P2) * var1) / 524288.0;

var1 = (1.0 + var1 / 32768.0)*((double)dig_P1);

if (var1 == 0.0)

{

return 0; // avoid exception caused by division by zero

}

p = 1048576.0 – (double)adc_P;

p = (p – (var2 / 4096.0)) * 6250.0 / var1;

var1 = ((double)dig_P9) * p * p / 2147483648.0;

var2 = p * ((double)dig_P8) / 32768.0;

p = p + (var1 + var2 + ((double)dig_P7)) / 16.0;

return p;

}

// Returns humidity in %rH as as double. Output value of “46.332” represents

46.332 %rH

double bme280_compensate_H_double(BME280_S32_t adc_H);

{

double var_H;

var_H = (((double)t_fine) – 76800.0);

var_H = (adc_H – (((double)dig_H4) * 64.0 + ((double)dig_H5) / 16384.0 *

var_H)) * (((double)dig_H2) / 65536.0 * (1.0 + ((double)dig_H6) /

67108864.0 * var_H *

(1.0 + ((double)dig_H3) / 67108864.0 * var_H)));

var_H = var_H * (1.0 – ((double)dig_H1) * var_H / 524288.0);

if (var_H > 100.0)

var_H = 100.0;

else if (var_H < 0.0)

var_H = 0.0;

return var_H;

}

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