Storing the previous temperature value and comparing it to the current value

Using Arduino Programming Questions
#1

Hi, I am new to the forum and pretty new to arduino as well.
I have spent a few days researching the forum and google to solve the issue I am having, but I haven’t been able to figure out a solution.

I am working on some rough ideas for a project and I am trying to get the general concept down before I move onto some more complicated aspects.

I would like to monitor a thermocouple and print the value if the previous temperature was greater than 30 degrees and the current temperature is less than or equal to 30 degrees.

I have tried this a few different ways, but to no avail.
The first method I tried was to use a boolean that became true if the temperature dropped below 30 degrees.
I tried to create logic that would print the temperature value if the boolean flag was previously false, and was now true.

I think the part that I am having problems with is establishing a way to store the previous value.

The code that I am posting now was written after I referenced another thread in the forum where someone was trying to store pressure values.

I can easily get the temperature value to print if it drops below a threshold, but when I introduce the logic to reference the previous value it does not work.

Any help would be greatly appreciated.

#include <Adafruit_MAX31856.h>


// Use software SPI: CS, DI, DO, CLK
// Defines pins as D10, D11, D12, D13
Adafruit_MAX31856 max = Adafruit_MAX31856(10, 11, 12, 13);


void setup()
{
  Serial.begin(115200);
  
 max.begin(); //Initializes sensor

 max.setThermocoupleType(MAX31856_TCTYPE_K);// Sets as a K Type Thermocouple

 max.readThermocoupleTemperature();
}

void loop()
{
 float temp = max.readThermocoupleTemperature();
float current_temp = temp;
float prev_temp = current_temp;


  if((current_temp<= 30)&&(prev_temp>30))

 {
  Serial.print("Oil Temp: ");
  Serial.println(max.readThermocoupleTemperature());
 
 
  }
  else {
  }
  
  // If Oil Temp is not less than 30 degrees check for Thermocouple faults
  uint8_t fault = max.readFault();
  {
  if((fault & MAX31856_FAULT_TCRANGE) ||(fault & MAX31856_FAULT_TCHIGH)||(fault & MAX31856_FAULT_TCLOW)||(fault & MAX31856_FAULT_OPEN))
    if(fault=true) // If any of the above fault conditions are present fault = true
    {
    Serial.println("Thermocouple Fault Detected"); // if fault = true display "Thermocouple Fault Detected" on Serial Monitor
// If fault is present print fault type in display
    }
    else{}
    delay(500); // else if no fault is present do nothing and wait for pre-determined amount of time
  }
  }
#2

Your code

  float temp = max.readThermocoupleTemperature();
  float current_temp = temp;
  float prev_temp = current_temp;

You need to save the temperature to the prev_temp variable before you read the temperature again and do the comparison. In your code the current and previous temperatures will always be the same because of the last line of code above.

#3

Thanks!
This seems to work like I want it to.

void loop()
{
 float temp = max.readThermocoupleTemperature();
float prev_temp = temp;

float current_temp = (max.readThermocoupleTemperature());

  if((current_temp<= 30)&&(prev_temp>30))

 {
  Serial.print("Oil Temp: ");
  Serial.println(max.readThermocoupleTemperature());
  //If boolean temp flag is true print oil temp on serial monitor
#4 
 float temp = max.readThermocoupleTemperature();
float prev_temp = temp;

float current_temp = (max.readThermocoupleTemperature());

NO !

Save the temperature that you read last time through loop() to prev_temp then read the temperature again and do the comparisons.

#5