Four-point Calibration for Gravity Analog Dissolved Oxygen Sensor?

Hi all!

I'd like to use a four-point calibration method for the oxygen meter instead of a two-point calibration to increase the accuracy of the measurements. I already have my measurements at 4 different temperatures but I'm not sure how to modify the formula in the code. Here is all the code before the setup. The formula I want to modify is the last bit of code. Thanks!

#include <Arduino.h>
 

#define DO_PIN A1
 

#define VREF 5000 //VREF (mv)

#define ADC_RES 1024 //ADC Resolution
 

//Single-point calibration Mode=0

//Two-point calibration Mode=1

#define TWO_POINT_CALIBRATION 0
 

#define READ_TEMP (25) //Current water temperature ℃, Or temperature sensor function
 

//Single point calibration needs to be filled CAL1_V and CAL1_T

#define CAL1_V (1600) //mv

#define CAL1_T (25) //℃

//Two-point calibration needs to be filled CAL2_V and CAL2_T

//CAL1 High temperature point, CAL2 Low temperature point

#define CAL2_V (1300) //mv

#define CAL2_T (15) //℃
 

const uint16_t DO_Table[41] = {

14460, 14220, 13820, 13440, 13090, 12740, 12420, 12110, 11810, 11530,

11260, 11010, 10770, 10530, 10300, 10080, 9860, 9660, 9460, 9270,

9080, 8900, 8730, 8570, 8410, 8250, 8110, 7960, 7820, 7690,

7560, 7430, 7300, 7180, 7070, 6950, 6840, 6730, 6630, 6530, 6410};
 

uint8_t Temperaturet;

uint16_t ADC_Raw;

uint16_t ADC_Voltage;

uint16_t DO;
 

int16_t readDO(uint32_t voltage_mv, uint8_t temperature_c)

{

#if TWO_POINT_CALIBRATION == 0

uint16_t V_saturation = (uint32_t)CAL1_V + (uint32_t)35 * temperature_c - (uint32_t)CAL1_T * 35;

return (voltage_mv * DO_Table[temperature_c] / V_saturation);

#else

uint16_t V_saturation = (int16_t)((int8_t)temperature_c - CAL2_T) * ((uint16_t)CAL1_V - CAL2_V) / ((uint8_t)CAL1_T - CAL2_T) + CAL2_V;

return (voltage_mv * DO_Table[temperature_c] / V_saturation);

#endif

}