Connect your Arduino to your computer with the SD card reader attached. If you have a micro SD card reader on your computer you could skip this step but it's worth knowing how to do it this way. The sketch looks for a file of a particular name on the SD card and reads it out and prints it to the Serial Monitor if it's there.

Step 1:

Grab the code and give it a try.

 
/*
  Read microSD Card

  This is specifically for reading a .txt file off a microSD card using the SD board
  and the Arduino 

  The libraries in question are 
   documented here... https://www.arduino.cc/en/Reference/SD
   documented here...  https://www.arduino.cc/en/Reference/SPI
  
     SD card attached to SPI bus as follows:
 ** MOSI - pin 11
 ** MISO - pin 12
 ** CLK - pin 13
 ** CS - pin 10

  It is an important skill for an STEM worker to be able to understand and
  use a tool that they don't fully understand. This is personallly difficult
  for me because I want to know how it happens but sometimes you just gotta
  go with the flow and this is one of those times.

  I got the very lightly commented code below from 
  https://electronicshobbyists.com/arduino-sd-card-shield-tutorial-arduino-data-logger/
  and I am thankful to be able to do so. There are many almost identical
  versions of this code so it is clearly in the public domain.

  I will add my own thoughts or perhaps insights to the comments with my initals
  so that you know to be cautious about them.
  
 */
 // these #include statements make the functions defined in these libraries
 // accessible to the writer of this sketch. See the documentation links
 // above for detailed descriptions of all functions. Notice that any function
 // used from these libraries starts with SD.'function' or SPI.'function'. You
 // can tell the IDE recognizes them because of the color change.
    #include <SD.h>
    #include <SPI.h>

 // Creates an object that is of a type File and gives it a name much
 // like defining an int or float (needs more clarity) -- BE
    File sdcard_file;
    bool removeFile = false; // if you wish to erase file after reading set to true

// NOTE: data file name must be manually edited on lines 77, 86, 103, 106
    
 // the CS pin manages the back and forth flow of information to the card as 
 // part of the Serial Protocol Interface (SPI) method of communication. - BE
    int CS_pin = 10; 

 // this test sketch has been set up to complete a short set of writes and 
 // reads to the connected SD card. This is not actually looping as you would
 // need in a data logger. This is once and done which is a useful feature of
 // the void setup() call.
 
    void setup() {

 // Opens the serial monitor so we can watch the write/read process
     Serial.begin(9600); //Setting baudrate at 9600
     while (!Serial) {
       ; // wait for serial port to connect. Needed for native USB port only
     }
     pinMode(CS_pin, OUTPUT);//declaring CS pin as output pin

 // SD.begin initiallizes the library and the card and returns TRUE if 
 // successful. Argument articulates the control pin. Default pin depends
 // on board.
     if (SD.begin(10))
     {
     Serial.println("SD card is initialized and it is ready to use");
     } else
     {
     Serial.println("SD card is not initialized");
     }
// Check to see if the file exists: This is a potentially useful tool when reading an
// SD card.
     if (!SD.exists("datalog.txt")) {
       Serial.println("data file doesn't exist.");
       Serial.println("You need to edit the read sketch");
     } else {
       Serial.println("data file exists.");

// This opens a file called mydata.txt on the card in the READ mode.See microSDTest.ino
// for additional comments. 

       sdcard_file = SD.open("datalog.txt", FILE_READ);
       if (sdcard_file) {
         Serial.println("Reading from the file");
         Serial.println();
         while (sdcard_file.available()) {
         Serial.write(sdcard_file.read());  
         }
         sdcard_file.close();
         Serial.println("Done with Read");
       }
       else {
         Serial.println("Failed to open the file");
       }

       // delete the file if removeFile = true
       if (removeFile) {
         Serial.println("Removing data file...");
         SD.remove("datalog.txt");
       }
       // verify that file is gone
       if (SD.exists("datalog.txt")) {
         Serial.println("data file still exists.");
       } else {
         Serial.println("data file is gone.");
       }
     }
}

    void loop() {
    //Nothing in the loop
     }
        

Step 2: Things to try!

You can select the output of the data reader from the serial monitor and copy/paste it into a .txt file on your computer. If you have a microSD card reader on your computer you can just copy the file from the SD card onto your computer.

Below is a sample of the data file that I read using this sketch. The temperature sensor had been outside in the sun on a hot day and was truly up to 40C. This is a 10 min slice of the cooling down process inside. Easy to drop into a file or Excel to plot.

Temperature (C)

0, 40.33

1, 38.87

2, 36.43

3, 34.47

4, 33.50

5, 32.52

6, 31.54

7, 31.05

8, 31.05

9, 30.57