enjoyneering / max31855

Arduino library for 14-bit MAX31855 K-thermocouple to digital converter

MAX31855

This is an Arduino library for 14-bit MAX31855 K-thermocouple to digital converter with 12-bit cold junction compensation & maximum sampling rate about 10Hz. Supports all sensor features. Can work with hardware or software emulated 3-wire SPI.

• Maximum power supply voltage is 3.6v
• K-type thermocouples have an absolute accuracy of around ±2°C..±6°C.
• Measurement tempereture range -200°C..+700°C ±2°C or -270°C..+1372°C ±6°C with 0.25°C resolution/increment.
• Maximun SPI bus speed 5Mhz
• Cold junction compensation range -40°C..+125° ±3°C with 0.062°C resolution/increment. Optimal performance of cold junction compensation happends when the thermocouple cold junction & the MAX31855 are at the same temperature. Avoid placing heat-generating devices or components near the converter because this may produce an errors.
• It is STRONGLY recommended to add a 10nF/0.01mF capacitor, placed across the T+ and T- pins, to filter noise/false readings on the thermocouple lines.

**Library returns 2000, if thermocouple open/shorted or a communication error has occurred

Supports:

• Arduino AVR
• Arduino ESP8266
• Arduino ESP32
• Arduino STM32

Beware of a fake MAX31855 K-Thermocouple Sensor Module

max31855's Issues

In the case we read ZERO, IC could be faulty or not connected!

Today i got the case, i burned an max31855 and got an nice 0°C reading. After many trial and error i used my logic-analyzer to check whats going on, and the maxims DO-PIN have bin LOW all the time.
After reading the datasheet again, there is only one very very rare case, when you can read out (all) ZERO, only when Thermocouple-Temp. AND Junction-Temp. is 0°C. My personal point-of-view is, that the MAX31855 is mostly used in cases where thermocouple-temp is going over 100°C and junction-temp not lower then 10°C, the case that both are truly read out 0°C is lower then winning in lottery. So i think we can ignore this case as a good reading. A faulty, not proper connected or fake chip should be a more common case.

So i think, it would be an good idea to handle this case in MAX31855::detectThermocouple

// MAX31855.cpp
uint8_t MAX31855::detectThermocouple(int32_t rawValue)
{
  if (rawValue == MAX31855_FORCE_READ_DATA) rawValue = readRawData();

  if (bitRead(rawValue, 16) == 1)
  {
    if      (bitRead(rawValue, 2) == 1) return MAX31855_THERMOCOUPLE_SHORT_TO_VCC;
    else if (bitRead(rawValue, 1) == 1) return MAX31855_THERMOCOUPLE_SHORT_TO_GND;
    else if (bitRead(rawValue, 0) == 1) return MAX31855_THERMOCOUPLE_NOT_CONNECTED;
    else                                return MAX31855_THERMOCOUPLE_UNKNOWN;
  }
  #ifndef MAX31855_CAN_BE_ZERO
    if (rawValue == 0) return MAX31855_READ_FAIL;
  #endif
  return MAX31855_THERMOCOUPLE_OK;
}

// MAX31855.h
#define MAX31855_THERMOCOUPLE_OK            0
#define MAX31855_THERMOCOUPLE_SHORT_TO_VCC  1
#define MAX31855_THERMOCOUPLE_SHORT_TO_GND  2
#define MAX31855_THERMOCOUPLE_NOT_CONNECTED 3
#define MAX31855_THERMOCOUPLE_UNKNOWN       4
#define MAX31855_READ_FAIL       5

Filter Capacitor

I added a comment that I later realized was not correct. I have withdrawn that.

Add linearization to MAX31855

Hi,
Can you add linearization to the library? This shouldn't be too complicated - https://github.com/heypete/MAX31855-Linearization/tree/master/MAX31855_Linearization

F("string") at blue pill stm32 boards hangs it

Hi!

First of all -thanks for the code.

Code works on Blue Pill, but only after I remove println(F("string")) structures. Otherwise I do not have hardware USB serial debug(and nothing happens at all).
I recommend reworking those lines.

Best regards,
Fedor.

Missing "virtual" declaration in MAX31855.h

Hi - Great work.

In order to get the Soft SPI version to work the "virtual" declaration of:

int32_t readRawData(void);

In MAX21855.h line 95 must be added to be:

virtual int32_t readRawData(void);

Otherwise the bit bang method of the derived class (MAX31855soft) is not called from the parent class when reading data.

BR

Stego72

Temperature Readings About 2 X Too High

I'm using your unmodified "MAX31855_sw_SPI_Demo" sketch with a MAX31855 and a NodeMCU-32S (ESP32). The only change I have made to the sketch is:

MAX31855soft myMAX31855(5, 19, 23); //for ESP8266 use D3, D4, D7

I'm getting a temperature reading on both "getColdJunctionTemperature()" and "getTemperature()" of nearly 2x what it should be.

I have checked the thermocouple with a dedicated thermocouple measuring meter, and it is correctly showing the TC at room temperature (about 28 degrees C right now). But with the same thermocouple and environment, the internal and external readings from your library are both giving me around 67 degrees C.

I have also swapped the Adafruit MAX31855 library into my sketch, and it also reports around 28 degrees from the same circuit, which is correct.

What could be causing a high-temperature reading of both the internal and external temperatures from your library?