Thermocouple on an Arduino Uno

// Thermocouple: a K-type thermocouple on a MAX6675 board, read twice a
// second with Adafruit's max6675 library.
//
//   MAX6675  SCK -> 6, CS -> 5, SO -> 4, VCC -> 5 V, GND -> GND
//
// A thermocouple is two different metals joined at the tip. The junction
// makes a few microvolts per degree, far too little for the Uno's ADC, so
// the MAX6675 measures it, corrects for the temperature of its own terminals
// and sends the result as a 16-bit word: pull CS low and clock out sixteen
// bits. The temperature is bits 14 to 3, a quarter degree a count.
//
// The chip converts while CS is high and a conversion takes up to 0.22 s.
// Reading faster than that returns the same number again, which is why
// every MAX6675 sketch waits between reads.
//
// Things to try. Drag the slider on the board and watch the reading follow.
// Then open the board's properties and untick probe: the open thermocouple
// reads as NAN, and the sketch says so.

#include <max6675.h>

const int SCK_PIN = 6;
const int CS_PIN = 5;
const int SO_PIN = 4;

MAX6675 thermocouple(SCK_PIN, CS_PIN, SO_PIN);

void setup() {
  Serial.begin(9600);
  Serial.println("MAX6675 ready");
  // Give the chip time to finish its first conversion.
  delay(500);
}

// Hundredths printed as a whole number and two decimals, which keeps the
// float printing code out of the sketch.
void printHundredths(long h) {
  if (h < 0) {
    Serial.print('-');
    h = -h;
  }
  Serial.print(h / 100);
  Serial.print('.');
  if (h % 100 < 10) Serial.print('0');
  Serial.print(h % 100);
}

void loop() {
  float celsius = thermocouple.readCelsius();
  if (isnan(celsius)) {
    Serial.println("No thermocouple");
  } else {
    Serial.print("C = ");
    printHundredths((long)(celsius * 100));
    Serial.print("  F = ");
    printHundredths((long)(celsius * 180) + 3200);
    Serial.println();
  }
  delay(500);
}