A thermocouple is a thermoelectrical thermometer with two different electrical conductors which form an electrical junction. One electrical conductor is used wherever the temperature is to be measured & another one is set aside at a constant lower temperature. The main function of a thermocouple is to generate a temperature-dependent voltage due to the Seebeck effect. This effect can be occurred due to the temperature difference that is proportional to the variation between the two junction’s temperatures. Thermocouples are used broadly as temperature sensors. Thermocouples are available in different types like T-Type, R Type, J Type, K Type, Type B, Type N, S Type, and R Type. This article provides brief information on a K-type thermocouple, its working, and its applications.
What is a K-type Thermocouple?
Type K Thermocouple is the most commonly used thermocouple which includes two legs; positive leg or non-magnetic & negative leg or magnetic. In this type of Thermocouple, fixed base metal is utilized because it works at high temperatures & also provides a wide operating temperature range. One of the essential metals used in this type of Thermocouple is Nickel because it is magnetic within nature.
The working principle of the K-type thermocouple is based on the Seeback Effect. So this effect states that whenever a closed circuit is created by combining two different metals simply at two junctions & these junctions are maintained at various temperatures then an e.m.f or electromotive can be induced within the closed circuit.
K Type Thermocouple
Construction
The construction of K Type Thermocouple is; the positive leg of the thermocouple is composed of 90% nickel and 10%chromium whereas the negative leg is composed of 95% nickel, 2% manganese, 1% silicon & 2% aluminum. The sensitivity of this thermocouple is approximately 41µV/°C.
K Type Thermocouple Construction
In this type of thermocouple, two types of insulation are used; Ceramic beads and compacted mineral insulation. Ceramic bead insulation is mainly used because it is a lightweight insulating product. This insulation is made with high-purity-based alumino-silicate materials. This insulation has low thermal mass so it does not hold heat, has less thermal conductivity & is very effective because it can resist 1260 °C of high temperature.
After that, the compacted mineral insulation & external metal sheath-like Magnesium Oxide is used. The dielectric strength of this material is high; it reacts very quickly to changes in temperature & is extremely durable. The insulation of Magnesium Oxide is suggested for this kind of thermocouple whenever thermocouples are to be submerged within liquids, corrosive gases, high pressures, or high moisture.
Features & Specifications
The features and specifications of K type thermocouple include the following.
- This thermocouple has two legs; positive and negative.
- The continuous temperature range is 0 to + 1100oC.
- The short-term temperature range is – 180 to +1300oC.
- The materials used are; Chromel (10%Cr,90%Ni).
- The materials used are; Alumel (95%Ni, 1%Si, 2% Mg & 2% AI).
- Its sensitivity is 41uV/ oC, where uV is microvolts.
- Its tolerance class is ± 1.5 K between -40 & 375 °C.
- The K-type Thermocouple accuracy is usually a maximum of +/- 2.2°C (or) +/- 0.75%.
- Type K thermocouple includes Alumel and chromel conductors that reach the output necessities as stated within IEC 60584 or ANSI/ASTM E230.
K-type Thermocouple interfacing with Arduino
The K-type thermocouple interfacing with the Arduino Uno board is shown below. In this interfacing, we can learn how to interface K Type Thermocouple with the Arduino board. Here, a K-type thermocouple is a temperature sensor with a wide range of temperature from −200ºC – 1350ºC/−326 – 2300ºF. The required library needs to be installed for this interfacing and also a simple circuit helps in displaying the sensor readings on the Serial Monitor.
A MAX6675 thermocouple amplifier is used in this interfacing to acquire the temperature reading from the thermocouple. Here the temperature output with the thermocouple amplifier mainly depends on the read voltage over the reference junction. Thus, the voltage at the reference junction mainly depends on the temperature difference between the reference & the thermal junctions. Thus we need to recognize the temperature at the reference junction.
Generally, the MAX6675 thermocouple amplifier includes a temperature sensor, used to measure temperature at the reference junction & change the small voltage on the reference junction so that we can read it with a microcontroller. This thermocouple amplifier easily communicates with a microcontroller through the SPI communication protocol & the output data is in a 12-bit resolution.
MAX6675 Amplifier
The MAX6675 thermocouple amplifier features mainly include the following.
- Its operating temperature ranges from -20 to 85ºCentigrade.
- Digital conversion for k-type thermocouple o/p can be done directly.
- It has Cold-junction compensation.
- It has a simple serial interface compatible with SPI.
- Its operating voltage ranges from 3.0 – 5.5 volts.
- Resolves temperatures to 0.25ºCentigrade and allows readings at 1024ºC or 1875ºF.
By using this interfacing, you can get temperature readings from your k-type thermocouple. This interfacing simply reads the temperature & also displays it on the Serial Monitor of Arduino IDE. The required components to make this interfacing mainly include; a K-type thermocouple using a MAX6675 amplifier, an Arduino Uno board, and female-to-male jumper wires. The MAX6675 amplifier connections to the Arduino uno board are shown in the following interfacing diagram.
K Type Thermocouple interfacing with Arduino Uno
The MAX6675 amplifier connections with the Arduino uno board is shown in the following interfacing diagram. So the connections of this interfacing follow as;
- The VCC pin of the MAX6675 thermocouple amplifier is connected to the 5V pin of the Arduino Uno board.
- The GND pin of the MAX6675 thermocouple amplifier is connected to the GND pin of the Arduino Uno board. The SCK pin of
- MAX6675 thermocouple amplifier is connected to Pin 6 like the SCK pin of the Arduino Uno board.
- The SO pin of the MAX6675 thermocouple amplifier is connected to Pin 4 like the SO pin of the Arduino Uno board.
- The CS pin of the MAX6675 thermocouple amplifier is connected to Pin 5 like the CS pin of the Arduino Uno board.
To obtain a temperature reading from thermocouple with the MAX6675 amplifier, usually, there are different libraries available. But here we utilize the max6675 amplifier library from Adafruit. To install this library into Arduino IDE, you need to follow these steps.
- First Arduino IDE needs to Open. For that, go to Sketch => Include Library => Manage Libraries. The Library Manager must open.
- After that, need to Search for “max6675 amplifier within the search box, and then the library needs to install it from Adafruit.
Code
Obtaining temperature readings from the thermocouple using the Arduino Uno board is very simple. So the library gives an example to obtain the temperature & displays the outputs on the Arduino IDE Serial monitor. The required code for this interfacing is shown below.
#include “max6675.h” //Include the Library//
int thermoCS = 8;
int thermoDO = 9;
int thermoCLK = 13;
MAX6675 thermocouple(thermoCLK, thermoDO, thermoCS);
void setup()
{
Serial.begin(9600);
Serial.println(“MAX6675 test”); // wait for MAX chip to stabilize//
delay(500);
}
void loop() { // basic readout test, now print the current temperature//
Serial.print(“F = “);
Serial.println(thermocouple.readFahrenheit());
Serial.print(“C = “);
Serial.println(thermocouple.readCelsius()); // For the MAX6675 to update, you must delay AT LEAST 250ms between reads!
delay(1000);
}
First, need to define the pins to interface with the MAX6675. After that create a MAX6675 amplifier object known as a thermocouple on the pins which we have previously defined.
In the setup() function, the Serial Monitor needs to be initialized at a 9600 baud rate. Next in the loop() function, we read the temperature & display it above the Serial Monitor. Here the library offers a technique to read the temperature reading in Celsius & a technique to read the temperature within Fahrenheit degrees.
Once the above code is uploaded, the Serial Monitor needs to be opened at a 9600 baud rate. For every second, new temperature readings will be displayed on the Serial Monitor.
Temperature Ranges
The temperature ranges of the K-type thermocouple are listed below.
- The max suggested temperature for #8 AWG wire gage with 0.128 diameter is 1280°C/2300°F.
- The max suggested temperature for #14 AWG wire gage with 0.064 diameter is 1090°C/2000°F.
- The max suggested temperature for #20 AWG wire gage with 0.032 diameter is 980°C/1800°F.
- The max suggested temperature for #24 AWG wire gage with 0.020 diameter is 870°C/1600°F.
- The max suggested temperature for #28 AWG wire gage with 0.013 diameter is 870°C/1600°F.
- The max suggested temperature for #30 AWG wire gage with 0.010 diameter is 760°C/1400°F.
K Type Thermocouple Output Voltage
The output voltage of K type thermocouple for different temperatures is shown below. Here the output at 0°C/32°F mainly depends on a reference Junction.
- For 0°C temperature, the output voltage is 0.000 mV.
- For 100°C temperature, the output voltage is 4.096 mV.
- For 200°C temperature, the output voltage is 8.138 mV
- For 300°C temperature, the output voltage is 12.209 mV.
- For 400°C temperature, the output voltage is 16.397 mV.
- For 500°C temperature, the output voltage is 20.644 mV.
- For 600°C temperature, the output voltage is 24.905 mV.
- For 700°C temperature, the output voltage is 29.129 mV.
- For 700°C temperature, the output voltage is 33.275 mV.
- For 900°C temperature, the output voltage is 37.326 mV.
- For 1000°C temperature, the output voltage is 41.276 mV.
- For 1100°C temperature, the output voltage is 45.119 mV.
- For 1200°C temperature, the output voltage is 48.838 mV
Advantages & Disadvantages
The advantages of a k k-type thermocouple include the following.
- The K-type thermocouple is the most commonly used thermocouple.
- This thermocouple is not expensive, reliable, and accurate & it has an extensive range of temperatures.
- As compared to other types of thermocouples, this thermocouple can work in different atmospheres and also in rugged environmental conditions.
- These are very durable and no resistance problems occur within the lead wire.
- This thermocouple has a fast response and is available in small sizes.
- Type K thermocouple has a longer life as compared to type J because the iron wire in type J quickly oxidizes particularly at high temperatures.
- Generally used at temperatures above 540 degrees C.
- These types of thermocouples are extensively used because of their wide range of temperature, versatility & accuracy.
- These are extremely robust & versatile, particularly when using a mineral-insulated cable.
- These thermocouples have simple assembly & convenient replacement.
- It has good seismic performance.
- It has a fast thermal response time.
- This thermocouple service life is long.
- The temperature resistance of this thermocouple is high equal to 1300 degrees;
- Its pressure resistance is good and its mechanical strength is high.
The disadvantages of k-type thermocouples include the following.
- The signal of this thermocouple is not linear.
- These are susceptible to corrosion, electromagnetic interference, and drift over time.
- As compared to RTDs, these are less accurate.
- These are not appropriate for vacuum applications because of chromium vaporization within the positive element.
- These thermocouples should not be utilized in a Sulphuric environment because both elements will corrode quickly & the negative element will ultimately fail due to becoming fragile.
K-type Thermocouple Applications
The applications of k-type thermocouples include the following.
- The type K thermocouple is usually found in nuclear applications due to its effective radiation hardness.
- These are used in Iron and Steel industries for monitoring temperature and chemistry during the steel-making procedure
- These are used to test heating appliance security.
- These are used to test associated temperatures with process plants like petroleum refineries & chemical production.
- These thermocouples are applicable in different kinds of environments like water, gases, dry areas & mild chemical solutions.
- These are found in the places of oil heaters, boilers, engines, etc.
- These thermocouples can be used as thermometers in the food industry & hospitals.
Thus, this is an overview of a K-type thermocouple, their working, specifications, interfacing, and their applications. The K thermocouple is composed of metal wires within Ni-Cr (Nickel-Chromium) (+) & Ni-Al (Nickel-Aluminum) (-). It is the most commonly used thermocouple in the industrial sector because of its wide range of temperatures, low cost, good corrosion resistance and they are Ni (nickel) based. These types of thermocouples are suggested for neutral atmospheres or continuous oxidizing which are used frequently above 538 °C. Here is a question for you, what is J type thermocouple?