Thermocouple
Created by lady ada
Last updated on 2016-01-15 09:00:17 PM EST
Guide Contents
Guide Contents
Overview
Some Basic Stats
This is for a K-type thermocouple with glass overbraiding
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3
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4
Wiring a Thermocouple
Using a Thermocouple
Arduino Library
Adding a Display
F.A.Q.
Project Examples
Downloads
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18
© Adafruit
Industries
https://learn.adafruit.com/thermocouple
Page 2 of 18
Overview
A thermocouple is a kind of temperature sensor.
Unlike
semiconductor temperature sensors such as the TMP36
(http://adafru.it/ckT)
,
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. The main
improvement of using a thermocouple over a semiconductor sensor or thermistor is that the
temperature range is very much increased. For example, the TMP36 can go from -50 to 150°C,
after that the chip itself can be damaged. Common thermocouples on the other hand, can go from -
200°C to 1350°C (K type) and there are ones that can go above 2300°C!
Thermocouples are often used in HVAC systems, heaters and boilers, kilns, etc. There are a few
different kinds but this tutorial will discuss K type, which are very common and easier to interface
with.
© Adafruit
Industries
https://learn.adafruit.com/thermocouple
Page 3 of 18
One difficulty in using them is that the voltage to be measured is very small, with changes of about
50 uV per °C (a uV is 1/1000000 Volts). While it is possible to read these voltages using a clean
power supply and nice op-amps, there are other complications such as a non-linear response (its
not always 50uV/°C) and cold-temperature compensation (the effect measured is only a differential
and there must be a reference, just as ground is a reference for voltage). For that reason, we
suggest only using an interface chip that will do the heavy lifting for you, allow you to easily integrate
the sensor without as much pain. In this tutorial we will use a MAX6675 K-thermocouple interface
chip which doesn't even require an ADC, spitting out a nice digital data signal of the temperature.
Some Basic Stats
This is for a K-type thermocouple with glass overbraiding
Size:
24 gauge, 1 meter long (you can cut it down if desired)
Price:
$10 at the adafruit store
(http://adafru.it/aIH)
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
(http://adafru.it/269)
Interface:
MAX6675
(http://adafru.it/aLd)
(discontinued)
MAX31855
(http://adafru.it/269),
or
AD595
(http://adafru.it/aLe)
(analog)
K Thermocouple Datasheet
(http://adafru.it/kFe)
MAX6675 Datasheet
(http://adafru.it/aLg)
MAX31855 Datasheet
(http://adafru.it/aLh)
© Adafruit
Industries
https://learn.adafruit.com/thermocouple
Page 4 of 18
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
(http://adafru.it/269)
.
First thing to determine is which wire is which. As you recall, thermocouples are made by welding
together two wires, the chip reads the voltage difference between the two. One is the negative (for
K-type its made of Alumel) and the other positive (ditto, Chromel). Luckily the wires are color coded,
and almost all of the time you'll find the Alumel is red and the Chromel is yellow.
Connect the leads as required to your amplifier:
The MAX6675 and MAX31855 thermocouple amplifiers are not compatible with grounded
thermocouples.
We've seen some K type thermocouples where the leads were marked incorrectly, so if you
find that the thermocouple temperature goes down instead of up when heated, try swapping
the red and yellow wires
© Adafruit
Industries
https://learn.adafruit.com/thermocouple
Page 5 of 18
