RS485 Soil NPK Sensor Calibration

Hi everyone,

I'm currently working on a project that involves using an NPK sensor via RS485 Modbus communication. Based on the sensor's user manual, the calibration formula for the sensor is Y = AX + B,

where:
Y is the calibrated value (the output I need after calibration),
X is the raw reading from the sensor,
A is the coefficient (slope) for each parameter (N, P, K),
B is the calibration offset for each parameter (N, P, K).

Also, according to the manual, I can access the coefficient and calibration values from specific Modbus registers:

• Nitrogen coefficient (high byte: 0x04E8, low byte: 0x04E9),
• Nitrogen calibration value (0x04EA),
• Phosphorus and Potassium have similar registers for their coefficients and calibration values.

I am using an Arduino to read the raw sensor data and communicate with the sensor via Modbus. I’m a bit unsure about the best way to incorporate this calibration formula into my code. Could someone help me apply the calibration formula to adjust the sensor readings?

Code:

#include <SoftwareSerial.h>
#include <Wire.h>

// RE and DE Pins set the RS485 module
// to Receiver or Transmitter mode
#define RE 8
#define DE 7

// Modbus RTU requests for reading NPK values
const byte nitro[] = {0x01,0x03, 0x00, 0x1e, 0x00, 0x01, 0xe4, 0x0c};
const byte phos[] = {0x01,0x03, 0x00, 0x1f, 0x00, 0x01, 0xb5, 0xcc};
const byte pota[] = {0x01,0x03, 0x00, 0x20, 0x00, 0x01, 0x85, 0xc0};

// A variable used to store NPK values
byte values[11];

// Sets up a new SoftwareSerial object
// Digital pins 10 and 11 should be used with a Mega or Mega 2560
SoftwareSerial mod(2, 3);
//SoftwareSerial mod(10, 11);
 
void setup() {
  // Set the baud rate for the Serial port
  Serial.begin(9600);

  // Set the baud rate for the SerialSoftware object
  mod.begin(9600);

  // Define pin modes for RE and DE
  pinMode(RE, OUTPUT);
  pinMode(DE, OUTPUT);
  
  delay(500);
}
 
void loop() {
  // Read values
  byte val1,val2,val3;
  val1 = nitrogen();
  delay(250);
  val2 = phosphorous();
  delay(250);
  val3 = potassium();
  delay(250);

  // Print values to the serial monitor
  Serial.print("Nitrogen: ");
  Serial.print(val1);
  Serial.println(" mg/kg");
  Serial.print("Phosphorous: ");
  Serial.print(val2);
  Serial.println(" mg/kg");
  Serial.print("Potassium: ");
  Serial.print(val3);
  Serial.println(" mg/kg");
  
  delay(2000);
}
 
byte nitrogen(){
  digitalWrite(DE,HIGH);
  digitalWrite(RE,HIGH);
  delay(10);
  if(mod.write(nitro,sizeof(nitro))==8){
    digitalWrite(DE,LOW);
    digitalWrite(RE,LOW);
    for(byte i=0;i<7;i++){
    //Serial.print(mod.read(),HEX);
    values[i] = mod.read();
    Serial.print(values[i],HEX);
    }
    Serial.println();
  }
  return values[4];
}
 
byte phosphorous(){
  digitalWrite(DE,HIGH);
  digitalWrite(RE,HIGH);
  delay(10);
  if(mod.write(phos,sizeof(phos))==8){
    digitalWrite(DE,LOW);
    digitalWrite(RE,LOW);
    for(byte i=0;i<7;i++){
    //Serial.print(mod.read(),HEX);
    values[i] = mod.read();
    Serial.print(values[i],HEX);
    }
    Serial.println();
  }
  return values[4];
}
 
byte potassium(){
  digitalWrite(DE,HIGH);
  digitalWrite(RE,HIGH);
  delay(10);
  if(mod.write(pota,sizeof(pota))==8){
    digitalWrite(DE,LOW);
    digitalWrite(RE,LOW);
    for(byte i=0;i<7;i++){
    //Serial.print(mod.read(),HEX);
    values[i] = mod.read();
    Serial.print(values[i],HEX);
    }
    Serial.println();
  }
  return values[4];
}

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