Introduction
Thermocouple is made by two different metal wires (thermoelements) joined at one end called measuring end (also called as junction end). Another end is either joined or not joined, called as reference end of which temperature is known.
The temperature difference between these two ends produce voltage difference and this voltage is used to measure the temperature.
Generally, thermocouples are used for measuring temperature over large range i.e. -270 to 3000 degree Celsius.
Thermocouples are widely used in industries, which includes measuring temperature of burner, diesel engines, furnace etc.
How Thermocouple Works?
Thermocouple Working Principle
Thermocouples consist of two dissimilar metals (wires), metal A and metal B. These metals are joined at an end called as measuring junction, while the other end is called as reference point as shown in above figure.
Note that measuring Junction point is used to measure the temperature. The reference point in figure is a known temperature.
As per Seebeck effect, thermoelectric voltage is generated which is proportional to the temperature difference between two junctions. This voltage can be measured at reference point.
The Seebeck effect states that when two different or unlike metals are joined together at two junctions, an electromotive force (EMF) is generated at the two junctions.
Types of Thermocouple
The thermocouple types are depending upon certain combinations and alloys.
The selection of combinations is driven by cost, availability, convenience, melting point, chemical property, stability and, output.
Different thermocouples are suitable for different types of
applications.
The following table shows different types of thermocouples, their temperature, material used, and, their colors.
Table: Types of thermocouple
Types | Temperature | Positive leg | Negative leg | Color code |
Type B | 0ºC to 1700ºC | Platinum-30% Rhodium | Platinum-6% Rhodium | Gray & Red |
Type E | -100º to 1000ºC | Chromel | Constantan | Purple & Red |
Type J | 0 to 750 ºC | Iron | Constantan | White & Red |
Type K | -100 to 1300 ºC | Chromel | Alumel | Yellow & Red |
Type N | -230 to 1300 ºC | Nicrosil | Nisil | Orange & Red |
Type R | 0 to 1600 ºC | Platinum-13% Rhodium | Platinum | Black & Red |
Type S | 0 to 1600 ºC | Platinum-10% Rhodium | Platinum | Black & Red |
Type T | -200 to 350 ºC | Copper | Constantan | Blue & Red |
As the thermocouples generate voltage in uV range, we need to amplify it using instrumentation amplifier.
There are many instrumentation amplifiers specially made for thermocouple metal types available in IC form. e.g. AD594, AD595 etc.
The reference point is usually compensated in integrated circuit (IC) for industrial applications.
The cold junction temperature is compensated by producing a voltage equal to the thermocouple voltage at 0°C.
Specification of K-Type Thermocouple
Temperature range: -270°C to 1300°C
Temperature measurement accuracy: ±2% or ±2°C
Thermal response time: <1 second
Insulation resistance: >100 MΩ
Diameter: 0.5-1mm
Material: Chromel-Alumel
Voltage output: 40uV/°C
Linearity: ±1%
Alternate options for K-Type Thermocouple
E Type thermocouple
J Type thermocouple
N Type thermocouple
RTDs (Resistance Temperature Detectors)
Thermistors (NTC or PTC)
List of amplifiers for thermocouple temp measurement
MAX31855: This is a thermocouple-to-digital converter IC that can be used with K-type thermocouples. It has a 14-bit resolution and can measure temperatures up to +1800°C.
MAX6675: This is another thermocouple-to-digital converter IC that can be used with K-type thermocouples. It has a 12-bit resolution and can measure temperatures up to +700°C.
AD8495: This is a thermocouple amplifier IC that can be used with K-type thermocouples. It has a low offset voltage and can measure temperatures up to +900°C.
AD595: This is another thermocouple amplifier IC that can be used with various types of thermocouples. It has a high gain and can measure temperatures up to +600°C.
K-Type Thermocouple Interfacing with Arduino
K-Type Thermocouple Temperature measurement Code for Arduino
const int thermocouple_input = A1; /* AD595 O/P pin */
voidsetup(){
Serial.begin(9600); /* Define baud rate for serial communication */
}
voidloop(){
int adc_val;
float temperature;
adc_val = analogRead(thermocouple_input);
temperature = ( ((adc_val * 4.88) - 0.0027 ) / 10.0 );
Serial.print("Temperature = ");
Serial.print(temperature);
Serial.print("\n\n");
delay(1000);
}
The code reads an analog signal from an AD595 thermocouple module connected to pin A1 of the Arduino board. It then calculates the temperature based on the analog value and prints it out to the serial monitor every second.
The output on the serial monitor will show the current temperature in Celsius (°C).
Commentaires