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276 lines
8.9 KiB
Markdown
276 lines
8.9 KiB
Markdown
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[](https://github.com/marketplace/actions/arduino_ci)
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[](https://github.com/RobTillaart/MAX31855_RT/blob/master/LICENSE)
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[](https://github.com/RobTillaart/MAX31855_RT/releases)
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# MAX31855_RT
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Arduino library for MAX31855 chip for K type thermocouple.
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The library has experimental support for other types of thermocouples E, J, N, R, S, T
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## Description
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The MAX38155 is a chip to convert the reading of a K-type thermocouple to a temperature.
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The working of thermocouples (TC) is based upon Seebeck effect.
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Different TC's have a different Seebeck Coefficient (SC) expressed in µV/°C.
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See http://www.analog.com/library/analogDialogue/archives/44-10/thermocouple.html
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For every type of TC there exist an MAX31855 variant, this library is primary
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developed for the K-type sensor. However it has experimental support for all
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other types of TC's. See details below.
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Library tested with breakout board
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```
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+---------+
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Vin | o |
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3V3 | o |
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GND | o O | Thermocouple
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D0 | o O | Thermocouple
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CS | o |
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CLK | o |
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+---------+
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```
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## Hardware SPI vs software SPI
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Default pin connections. ESP32 can overrule with **setGPIOpins()**.
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| HW SPI | UNO | ESP32 VSPI | ESP32 HSPI | Notes
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|:---------|:-----:|:-------:|:-------:|:----------|
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| CLOCKPIN | 13 | 18 | 14 |
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| MISO | 12 | 19 | 12 |
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| MOSI | 11 | 23 | 13 | *not used...*
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| SELECT | eg. 4 | 5 | 15 | *can be others too.*
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Performance read() function, timing in us. (ESP32 @240MHz)
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| mode | clock | timing UNO | timing ESP32 | Notes
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|:-------|---------:|-----------:|-------------:|:----------|
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| HW SPI | 32000000 | ni | ~15 | *less reliable*
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| HW SPI | 16000000 | ~68 | ~16 |
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| HW SPI | 4000000 | ~72 | ~23 |
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| HW SPI | 1000000 | ~100 | ~51 |
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| HW SPI | 500000 | ~128 | ~89 |
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| SW SPI | bit bang | ~500 | ~17 (!) |
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## Interface
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### Constructor
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- **MAX31855(const uint8_t select)** create object and set select pin => hardware SPI
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- **MAX31855(const uint8_t sclk, const uint8_t select, const uint8_t miso)** create object, set clock, select and miso pin => software SPI
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### Hardware SPI
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To be used only if one needs a specific speed.
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- **void setSPIspeed(uint32_t speed)** set SPI transfer rate
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- **uint32_t getSPIspeed()** returns SPI transfer rate
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### ESP32 specific
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- **void selectHSPI()** must be called before **begin()**
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- **void selectVSPI()** must be called before **begin()**
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- **bool usesHSPI()**
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- **bool usesVSPI()**
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- **void setGPIOpins(uint8_t clk, uint8_t miso, uint8_t mosi, uint8_t select)** to overrule ESP32 default hardware pins
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### Reading
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To make a temperature reading call **read()**.
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It returns the status of the read which is a value between 0..7
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The function **getStatus()** returns the same status value.
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Table: values returned from **uint8_t read()** and **uint8_t getStatus()**
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| value | Description | Action |
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|:-----:|:--------------------------|:-------------|
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| 0 | OK | |
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| 1 | Thermocouple open circuit | check wiring |
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| 2 | Thermocouple short to GND | check wiring |
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| 4 | Thermocouple short to VCC | check wiring |
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| 7 | Generic error | |
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| 128 | No read done yet | check wiring |
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| 129 | No communication | check wiring |
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There are six functions to check the individual error conditions mentioned above.
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These make it easier to check them.
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- **bool openCircuit()**
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- **bool shortToGND()**
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- **bool shortToVCC()**
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- **bool genericError()**
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- **bool noRead()**
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- **bool noCommunication()**
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After a **uint8_t read()** you can get the temperature with **float getTemperature()**
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and **float getInternal()** for the internal temperature of the chip / board itself.
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Normally these are (almost) equal.
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Repeated calls to **getTemperature()** will give the same value until a new **read()**.
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The latter fetches a new value from the sensor. Note that if the **read()** fails
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the value of **getTemperature()** can become incorrect. So it is important to check
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the return value of **read()**.
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### Offset
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The library supports a fixed offset to calibrate the thermocouple.
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For this the functions **float getOffset()** and **void setOffset(float offset)** are available.
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This offset is "added" in the **getTemperature()** function.
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Note the offset used is a float, so decimals can be used.
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### Delta analysis
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As the **tc** object holds its last known temperature it is easy to determine the delta
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with the last known temperature, e.g. for trend analysis.
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```cpp
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float last = tc.getTemperature();
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int state = tc.read();
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if (state == STATUS_OK)
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{
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float new = tc.getTemperature();
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float delta = new - last;
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// process data
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}
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```
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### Last time read
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The **tc** object keeps track of the last time **read()** is called in the function **uint32_t lastRead()**.
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The time is tracked in **millis()**. This makes it easy to read the sensor at certain intervals.
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```cpp
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if (millis() - tc.lastRead() >= interval)
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{
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int state = tc.read();
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if (state == STATUS_OK)
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{
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float new = tc.getTemperature();
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// process read value.
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}
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else
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{
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// handle error
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}
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}
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```
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### GetRawData
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The function **uint32_t getRawData()** allows you to get all the 32 bits raw data from the board,
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after the standard **uint8_t tc.read()** call.
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Example code can be found in the examples folder.
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```cpp
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int state = thermocouple.read();
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uint32_t value = thermocouple.getRawData(); // Read the raw Data value from the module
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```
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This allows one to compact the measurement e.g. for storage or sending over a network.
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## Pull Up Resistor
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To have proper working of the MAX31855 board, you need to add a pull-up resistor
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(e.g. 4K7 - 1K depending on wirelength) between the MISO pin (from constructor call) and the
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VCC (5Volt). This improves the signal quality and will allow you to detect if there is
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proper communication with the board. WIthout pull-up one might get random noise that could
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look like real data.
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**Note:** the MISO pin can be different from each board, please refer to your board datasheet.
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If the MAX31855 board is not connected **tc.read()** will return **STATUS_NO_COMMUNICATION**.
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You can verify this by **tc.getRawData()** which will give 32 HIGH bits or 0xFFFFFFFF).
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You can use a simple code to detect connection error board:
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```cpp
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uint8_t status = thermocouple.read();
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if (status == STATUS_NO_COMMUNICATION)
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{
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Serial.println("NO COMMUNICATION");
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}
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```
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or
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```cpp
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uint8_t status = thermocouple.read();
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if (thermocouple.getRawData() == 0xFFFFFFFF)
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{
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Serial.println("NO COMMUNICATION");
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}
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```
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## Operation
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See examples
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## Experimental part (to be tested)
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(to be tested)
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**NOTE:**
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The support for other thermocouples is experimental **use at your own risk**.
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The MAX31855 is designed for K type sensors. It essentially measures a
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voltage difference and converts this voltage using the Seebeck Coefficient (SC)
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to the temperature. As the SC is linear in its nature it is possible
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to replace the K-type TC with one of the other types of TC.
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Datasheet Table 1, page 8 SC = Seebeck Coefficient
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| Sensor type | SC in µV/°C | Temp Range in °C | Material |
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|:-----------:|:------------|:-----------------|:--------------------------|
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| E_TC | 76.373 | -270 to +1000 | Constantan Chromel |
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| J_TC | 57.953 | -210 to +1200 | Constantan Iron |
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| K_TC | 41.276 | -270 to +1372 | Alumel Chromel |
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| N_TC | 36.256 | -270 to +1300 | Nisil Nicrosil |
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| R_TC | 10.506 | -50 to +1768 | Platinum Platinum/Rhodium |
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| S_TC | 9.587 | +50 to +1768 | Platinum Platinum/Rhodium |
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| T_TC | 52.18 | -270 to +400 | Constantan Copper |
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The core formula to calculate the temperature is (Datasheet page 8)
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```
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Vout = (41.276µV/°C) x (Temp_R - Temp_internal)
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```
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As we know the internal temperature and the returned temperature from the sensor
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the library can calculate the Vout measured (as the chip assumes that a K-type
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thermocouple is connected.
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Having that Vout we can redo the math for the actual thermocouple type and
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calculate the real temperature.
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The library has two functions **setSeebeckCoefficient(float factor)** and
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**float getSeebeckCoefficient()**
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to get/set the Seebeck Coefficient (== thermocouple) to be used.
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One can adjust the values to improve the accuracy of the temperature read.
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The **float getTemperature()** has implemented this algorithm, however as long
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as one does not set the Seebeck Coefficient it will use the K_TC as default.
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