GY-63_MS5611/libraries/MAX31855_RT

MAX31855_RT

Arduino library for MAX31855 chip for K type thermocouple

Description

The MAX38155 is a chip to convert the reading of a K-type thermocouple to a temperature. The working of thermocouples (TC) is based upon Seebeck effect. Different TC's have a different Seebeck Coefficient (SC) expressed in µV/°C. See http://www.analog.com/library/analogDialogue/archives/44-10/thermocouple.html

For every type of TC there exist an MAX31855 variant, this library is primary developed for the K-type sensor. However it has experimental support for all other types of TC's. See details below.

Library tested with breakout board

     +---------+
 Vin | o       |
 3Vo | o       |
 GND | o     O | Thermocouple
  D0 | o     O | Thermocouple
  CS | o       |
 CLK | o       |
     +---------+

Hardware SPI vs software SPI

Default pin connections (ESP32 has more options)

HW SPI	UNO	ESP32
CLOCKPIN	13	18
MISO	12	19
MOSI	11	23

Performance read() function, timing in us. (ESP32 @240MHz)

mode	clock	timing UNO	timing ESP32
HWSPI	32000000	ni	~15
HWSPI	16000000	~68	~16
HWSPI	4000000	~72	~23
HWSPI	1000000	~100	~51
HWSPI	500000	~128	~89
SWSPI	bit bang	~500	~17 (!)

Interface

To make a temperature reading call tc.read(). It returns the status of the read which is a value between 0..7 The function getStatus() returns the same status value.

Table: values returned from read() and getStatus()

value	Description	Action
0	OK
1	Thermocouple open circuit	check wiring
2	Thermocouple short to GND	check wiring
4	Thermocouple short to VCC	check wiring
7	Generic error
128	No read done yet	check wiring
129	No communication	check wiring

There are six functions to check the individual error conditions mentioned above. These make it easier to check them.

• openCircuit()
• shortToGND()
• shortToVCC()
• genericError()
• noRead()
• noCommunication()

After a tc.read() you can get the temperature with tc.getTemperature() and tc.getInternal() for the internal temperature of the chip / board itself.

Repeated calls to tc.getTemperature() will give the same value until a new tc.read(). The latter fetches a new value from the sensor. Note that if the tc.read() fails the value of tc.getTemperature() can become incorrect.

The library supports a fixed offset to calibrate the thermocouple. For this the functions tc.getOffset() and tc.setOffset(offset) are available. This offset is included in the tc.getTemperature() function.

As the tc object holds its last known temperature it is easy to determine the delta with the last known temperature, e.g. for trend analysis.

  float last = tc.getTemperature();
  int state  = tc.read();
  if (state == STATUS_OK)
  {
    float new  = tc.getTemperature();
    float delta = new - last;
    // process data
  }

The tc object keeps track of the last time tc.read() is called in the function tc.lastRead(). The time is tracked in millis(). This makes it easy to read the sensor at certain intervals.

if (millis() - tc.lastRead() >= interval)
{
  int state = tc.read();
  if (state == STATUS_OK)
  {
    float new = tc.getTemperature();
    // process read value.
  }
  else
  {
    // handle error
  }
}

GetRawData

The function tc.getRawData() allows you to get all the 32 bits raw data from the board, after the standard tc.read() call.

Example code can be found in the examples folder.

  int state = thermocouple.read();              
  uint32_t value = thermocouple.getRawData();  // Read the raw Data value from the module

Pull Up Resistor

To have proper working of the MAX31855 board, you need to add a pull-up resistor (e.g. 4K7 - 1K depending on wirelength) between the MISO pin (from constructor call) and the VCC (5Volt). This improves the signal quality and will allow you to detect if there is proper communication with the board. WIthout pull-up one might get random noise that could look like real data.

Note: the MISO pin can be different from each board, please refer to your board datasheet.

If the MAX31855 board is not connected tc.read() will return STATUS_NO_COMMUNICATION.

You can verify this by tc.getRawData() which will give 32 HIGH bits or 0xFFFFFFFF).

You can use a simple code to detect connection error board:

  uint8_t status = thermocouple.read();
  if (status == STATUS_NO_COMMUNICATION)
  {
    Serial.println("NO COMMUNICATION");
  }

or

  uint8_t status = thermocouple.read();
  if (thermocouple.getRawData() == 0xFFFFFFFF)
  {
    Serial.println("NO COMMUNICATION");
  }

Operation

See examples

Experimental part (to be tested)

NOTE: The support for other thermocouples is experimental use at your own risk.

The MAX31855 is designed for K type sensors. It essentially measures a voltage difference and converts this voltage using the Seebeck Coefficient (SC) to the temperature. As the SC is linear in its nature it is possible to replace the K-type TC with one of the other types of TC.

Datasheet Table 1, page 8 SC = Seebeck Coefficient

Sensor type	SC in µV/°C	Temp Range in °C	Material
E_TC	76.373	-270 to +1000	Constantan Chromel
J_TC	57.953	-210 to +1200	Constantan Iron
K_TC	41.276	-270 to +1372	Alumel Chromel
N_TC	36.256	-270 to +1300	Nisil Nicrosil
R_TC	10.506	-50 to +1768	Platinum Platinum/Rhodium
S_TC	9.587	+50 to +1768	Platinum Platinum/Rhodium
T_TC	52.18	-270 to +400	Constantan Copper

The core formula to calculate the temperature is (Datasheet page 8)

Vout = (41.276µV/°C) x (Temp_R - Temp_internal)

As we know the internal temperature and the returned temperature from the sensor the library can calculate the Vout measured (as the chip assumes that a K-type thermocouple is connected. Having that Vout we can redo the math for the actual thermocouple type and calculate the real temperature.

The library has two functions tc.setSeebeckCoefficient(factor) and tc.getSeebeckCoefficient() to get/set the Seebeck Coefficient (== thermocouple) to be used. One can adjust the values to improve the accuracy of the temperature read.

The tc.getTemperature() has implemented this algorithm, however as long as one does not set the Seebeck Coefficient it will use the K_TC as default.