The value of an variable changes if i open the serial monitor

Projects made with PlatformIO
,
1

when i open the serial monitor the value of a variable that is altered every 800ms gets an offset.

The code is for my fish tank. Currently i have added a temperature and a tds sensor and i output it on a TFT display.

Now: if the serial monitor is closed the variable tdsvalue value is printed with like “715 ppm” on the screen. If i open the serial monitor the value changes to like “515 ppm”. Is there a reason why the serial monitor could alter the value?

The whole code looks like this:

#include <Arduino.h>
#include <TFT_eSPI.h> // Hardware-specific library
#include <SPI.h>
#include <OneWire.h>
#include <DallasTemperature.h>

//##########################################################
// TDS Sensor
//##########################################################
#define TdsSensorPin 4
#define VREF 3.3      
#define SCOUNT  30           
int analogBuffer[SCOUNT];    
int analogBufferTemp[SCOUNT];
int analogBufferIndex = 0,copyIndex = 0;
float averageVoltage = 0,tdsValue = 0;
int getMedianNum(int bArray[], int iFilterLen);
unsigned long TDSTimepoint = millis();
unsigned long printTDSTimepoint = millis();

//##########################################################
// Temperatur Sensor
//##########################################################
const int oneWireBus = 18;     
OneWire oneWire(oneWireBus);
DallasTemperature sensors(&oneWire);
float temperature;
unsigned long TempTimepoint = millis();

//##########################################################
// TFT Display
//##########################################################
TFT_eSPI tft = TFT_eSPI();       
unsigned long currentMillis, seconds, minutes, hours, days;
void printTFT(float &temp);

//##########################################################
// Main Code
//##########################################################

bool print_out = false;
void setup(void) {
  Serial.begin(9600); 
  //########################################################
  // TDS Sensor Setup
  //########################################################
  pinMode(TdsSensorPin,INPUT);

  //########################################################
  // TFT Display Setup
  //########################################################
  tft.init();
  tft.setRotation(3);
}

void loop() {
  //########################################################
  // Temperatur Sensor loop
  //########################################################
  if(millis()-TempTimepoint > 500U)     //every 40 milliseconds,read the analog value from the ADC
  {
    TempTimepoint = millis();
    sensors.requestTemperatures(); 
    temperature = sensors.getTempCByIndex(0);
    print_out=true;
  }
  //########################################################
  // TDS Sensor loop
  //########################################################

  if(millis()-TDSTimepoint > 40U)     //every 40 milliseconds,read the analog value from the ADC
  {
    TDSTimepoint = millis();
    analogBuffer[analogBufferIndex] = analogRead(TdsSensorPin);    //read the analog value and store into the buffer
    analogBufferIndex++;
    if(analogBufferIndex == SCOUNT) 
    {
      analogBufferIndex = 0;
    }
  }   

  if(millis()-printTDSTimepoint > 800U)
  {
     printTDSTimepoint = millis();
     for(copyIndex=0;copyIndex<SCOUNT;copyIndex++)
       analogBufferTemp[copyIndex]= analogBuffer[copyIndex];
     averageVoltage = getMedianNum(analogBufferTemp,SCOUNT) * (float)VREF / 1024.0; // read the analog value more stable by the median filtering algorithm, and convert to voltage value
     float compensationCoefficient=1.0+0.02*(temperature-25.0);    //temperature compensation formula: fFinalResult(25^C) = fFinalResult(current)/(1.0+0.02*(fTP-25.0));
     float compensationVolatge=averageVoltage/compensationCoefficient;  //temperature compensation
     tdsValue=(133.42*compensationVolatge*compensationVolatge*compensationVolatge - 255.86*compensationVolatge*compensationVolatge + 857.39*compensationVolatge)*0.5; //convert voltage value to tds value
     Serial.print("TDS Value:");
     Serial.print(tdsValue,0);
     Serial.println("ppm");
     print_out=true;
  }

  //########################################################
  // TFT Display loop
  //########################################################
  if(print_out == true)
  {
    printTFT(temperature);
    print_out = false;
  }

}

int getMedianNum(int bArray[], int iFilterLen) 
{
      int bTab[iFilterLen];
      for (byte i = 0; i<iFilterLen; i++)
      bTab[i] = bArray[i];
      int i, j, bTemp;
      for (j = 0; j < iFilterLen - 1; j++) 
      {
        for (i = 0; i < iFilterLen - j - 1; i++) 
        {
          if (bTab[i] > bTab[i + 1]) 
          {
            bTemp = bTab[i];
            bTab[i] = bTab[i + 1];
            bTab[i + 1] = bTemp;
          }
        }
      }
      if ((iFilterLen & 1) > 0)
        bTemp = bTab[(iFilterLen - 1) / 2];
      else
        bTemp = (bTab[iFilterLen / 2] + bTab[iFilterLen / 2 - 1]) / 2;
      return bTemp;
}

void printTFT(float &temp){
  currentMillis = millis();
  seconds = currentMillis / 1000;
  minutes = seconds / 60;
  hours = minutes / 60;
  days = hours / 24;

  currentMillis %= 1000;
  seconds %= 60;
  minutes %= 60;
  hours %= 24;

  tft.fillScreen(TFT_BLACK);
  tft.setCursor(0, 0, 2);
  tft.setTextColor(TFT_WHITE,TFT_BLACK);  
  tft.setTextSize(1);
  
  tft.print(temperature);
  tft.println(" `C");
  
  tft.print(tdsValue);
  tft.println(" ppm");

  tft.print(days);
  tft.print(' ');
  if (days < 10)
    tft.print('0');
  tft.print(hours);
  tft.print(':');
  if (minutes < 10)
    tft.print('0');
  tft.print(minutes);
  tft.print(':');
  if (seconds < 10)
    tft.print('0');
  tft.print(seconds);
}