Thermocouple Created by lady ada Last updated on 2017-11-12 06:14:57 PM UTC
Guide Contents Guide Contents Overview 2 3 Some Basic Stats This is for a K-type thermocouple with glass overbraiding 3 3 Wiring a Thermocouple Arduino Code Arduino Library Adding a Display CircuitPython Code Usage F.A.Q.
Overview A thermocouple is a kind of temperature sensor. Unlike semiconductor temperature sensors such as the TMP36 , thermocouples have no electronics inside them, they are simply made by welding together two metal wires. Because of a physical effect of two joined metals, there is a slight but measurable voltage across the wires that increases with temperature. The type of metals used affect the voltage range, cost and sensitivity, which is why we have a few different kinds of thermocouples.
Size: 24 gauge, 1 meter long (you can cut it down if desired) Price: $10 at the adafruit store Temperature range: -100°C to 500°C / -150 to 900°F (After this the glass overbraiding may be damaged) Output range: -6 to +20mV Precision: +-2°C Requires an amplifier such as MAX31855 Interface: MAX6675 (discontinued) MAX31855, or AD595 (analog) K Thermocouple Datasheet MAX6675 Datasheet MAX31855 Datasheet © Adafruit Industries https://learn.adafruit.
Wiring a Thermocouple As we mentioned before, trying to actually measure the voltage across the wires will be very difficult for most people, which is why we strongly suggest using a thermocouple interface chip. The nicest one we've seen so far is the MAX6675 (and its replacement version called the MAX31855) which unfortunately is only available in SOIC package. While not too difficult to solder, we nevertheless have in the shop a breakout board that is ready to go .
Arduino Code If you're using an AD595 interface chip, you can simply connect the voltage output to an analog input on your microcontroller and do some basic math to multiply the 10 mV/°C input into numerical output. If you're planning to use the MAX6675/MAX31855, there's a little more work to be done. First off, Vin and GND must connect to a 3-5V supply.
If you are using the MAX31855 v1.0 in a noisy environment, you may need to add a 0.01uF capacitor across the thermocouple leads. The MAX31855 does not support grounded thermocouples - if the sensor touches ground the chip will return an error Arduino Library If you have an older MAX6675 breakout, download the MAX6675 Arduino library code by going to the github page and clicking Download Source.
As you can see, its pretty simple to use the library, simply tell the sensor object what the clock, chip select and data pins are, then call readCelsius() or readFahrenheit() to get a floating point result. Adding a Display A common request is to have the temperature output onto a 'classic' character LCD such as the ones in this tutorial. © Adafruit Industries https://learn.adafruit.
For this wiring, we connected CLK to digital 3, CS to digital 4 and DO to digital 5. Once you get it working, you can change the pin connections in the sketch We have an example sketch for this as well. First get the LCD working by following our tutorial.
CircuitPython Code It's easy to use the MAX31855 sensor with CircuitPython and the Adafruit CircuitPython MAX31855 module. This module allows you to easily write Python code that reads the temperature from the thermocouple. First wire up a MAX31855 to your board exactly as shown on the previous pages for Arduino.
You can also download the adafruit_max31855.mpy from its releases page on Github. Before continuing make sure your board's lib folder or root filesystem has the adafruit_max31855.mpy, and adafruit_bus_device files and folders copied over. Next connect to the board's serial REPL so you are at the CircuitPython >>> prompt. Usage To demonstrate the usage of the sensor we'll initialize it and read the temperature.
F.A.Q. The temperature of my thermocouple seems to be backwards! If I heat up the probe the reported temperature goes down This is likely caused by the thermocouple wires being labled incorrectly. Try swapping the two thermocouple leads, even if yellow and red wires are in the right slots - we've seen some thermocouple where the wire colors are wrong.
thermocouples. Most thermocouple thermometers have the offset corrected in software which is what we suggest. See this guide for tips on calibration: Sensor Calibration For precision temperature measurement, we suggest a 1% Thermistor. How can I connect multiple thermocouples? You can connect as many MAX31855's as you have pins.
Project Examples Need ideas? Check out these projects! Jeelabs has a detailed walkthrough for a reflow controller (uses an AD595-type chip) © Adafruit Industries https://learn.adafruit.
Terran's PI conntrolled coffee roaster © Adafruit Industries https://learn.adafruit.
RocketNumberNine's reflow toaster project © Adafruit Industries https://learn.adafruit.
Downloads Datasheets & Files MAX31855 Datasheet MAX6675 Schematic and layout files can be found at GitHub MAX31855 Schematic and layout files can be found at GitHub Fritzing objects for both in the Adafruit Fritzing library Schematic Click to embiggen Fabrication Print Dimensions in Inches © Adafruit Industries https://learn.adafruit.
© Adafruit Industries Last Updated: 2017-11-12 06:14:57 PM UTC Page 18 of 18
