Type K Thermocouple

Type K Thermocouple (Nickel-Chromium / Nickel-Alumel): The type K is the most frequently used type of thermocouple. It's inexpensive, accurate, dependable, and has a wide temperature range. The type K is often found in applications because of its comparative radiation hardness. Maximum continuous temperature is approximately 1,100C.

• 

Type K (chromel--alumel) is the most popular general-purpose thermocouple with a sensitivity of around 41 µV/°C. It is inexpensive, and a wide variety of probes can be found in its −200 °C to +1350 °C (−330 °F to +2460 °F) range. Type K was specified at a time when metallurgy was less advanced than it is now, and consequently, characteristics may vary between samples. One of the metals is magnetic; a feature of thermocouples made with magnetic material is that they experience a deviation when the material reaches its Curie point, which happens for type K thermocouples at around 185°C.

Type K, Wire Grades:
Thermocouple Grade Wire: -454°F to 2,300°F (-270°C to 1260°C)
Extension wire: 32°F to 392°F (0°C to 200°C)

Type K Accuracy (whichever is greater)
Standard: +/- 2.2C or +/- .75%

They function very well in oxidising atmospheres. If, however, a mostly reducing atmosphere (like hydrogen with a small amount of oxygen) comes into contact with the wires, the chromium from the chromel alloy oxidises. This decreases the emf output, and the thermocouple reads low. This phenomenon is referred to as green rot because of the colour of the metal that was affected.

• 

Although not necessarily distinctively green, the wire will develop a mottled silvery skin and become magnetic. An easy way to check for this problem is to see whether the two wires are magnetic (normally, chrome is non-magnetic).

Hydrogen in the air is the usual cause of green rot. At high temperatures, it can diffuse through an intact metal thermowell or solid metals. A cover of magnesium oxide covering the thermocouple will not keep the hydrogen out.

Green rot doesn't occur in atmospheres sufficiently full of oxygen, or oxygen-free. A sealed thermowell can be full of inert gas, or an oxygen scavenger (e.g. a sacrificial titanium wire) can be added. Alternately, additional oxygen can be introduced to the thermowell. A different option is using a changed thermocouple type for the atmospheres where green decay can happen; a type N thermocouple is an appropriate option. TYPE K THERMOCOUPLE (Chromel / Alumel)200°C into +1260°C / -328°F to +2300°F

This type of thermocouple should be protected with a metal or ceramic protection tube, especially in reducing atmospheres. When conditions are appropriate, in atmospheres, such as electric furnaces, tube security isn't always necessary; however, it is recommended for cleanliness and general mechanical protection. Type K will generally outlast Type J since the JP wire rapidly oxidises, particularly at higher temperatures.

Manual Temperature Calculation
A thermocouple circuit comprises of the two metal junctions, wire and connectors and a voltage measuring device. When the junctions are experiencing temperature difference, measurable current flows through the circuit. The output potential difference is related to the temperature differential. Since the measurement is relative, one of the temperatures must be known to compute an absolute temperature. In early days the thermocouple were compensated by keeping one junction in melting ice at 0 degree C. Now, one of the junctions, the"cold junction," is compensated using electronics to maintain a standard. Another junction, the"hot junction," is exposed to the environment to be measured.

• 

A Type K thermocouple can be connected to a voltmeter for easy data collection. In this case, the output is a voltage, and the reader has to convert the level to temperature using a conversion formula. The thermocouple can be connected to a data recorder to store behaviours and patterns. In these cases, software operation or a conversion circuit may be used to compute the temperature using the voltage output.

Composition of K-Type Thermocouple
Composition of Chromel 90% nickel and 10% chromium.
Composition of Alumel 95% nickel, 2% manganese, 2% aluminium and 1% silicon.

Typical Applications
This is the most popular thermocouple type that provides the broadest operating temperature range. Type K thermocouples generally will work in many applications because they are nickel and have good corrosion resistance.
• A Traditional alternative for high-temperature work.
• Suitable for use in oxidising or inert atmospheres at temperatures up to 1260°C (2300°F).
• Vulnerable to sulfur assault (refrain from exposing to sulfur-containing atmospheres).
• Perform best in oxidising atmospheres that are clean.
• Not suggested for use under partially oxidising conditions or when exposed to alternating cycles of oxidisation and reduction.

• 

TYPE K THERMOCOUPLE APPLICATIONS
Type K thermocouples are used for measurements in several unique types of environments like water, mild chemical solutions, gases and boiler areas. Engines, oil heaters and boilers are examples of areas. They are also used as thermometers in the food industry and hospitals.

Type K Thermocouple Color Code

• 

Temperature Range:
Melting Point, 2550°F (1400°C)
Accuracy (whichever is greater):
Deviations in the alloys can impact the accuracy of thermocouples. For type K thermocouples, the tolerance course one is given as ± 1.5 K between -40 and 375 °C. Deviations between thermocouples coming from precisely the production are very small, and a higher precision can be achieved by individual calibration.
Metallurgical changes can lead to a calibration drift of 1 to 2°C in a couple of hours, increasing to 5 °C with time. A special grade of Type K is available that can maintain up limit accuracy to ten times more than the grade.

TYPE K THERMOCOUPLE PROS AND CONS

TYPE K THERMOCOUPLE PROS AND CONS

Pros	Cons
Good linearity of emf to the measurement temperature.	Not acceptable for reducing air but will withstand metallic vapour.
Good resistance against oxidation under 1000°C (1600°F).	Aging of the emf feature, in comparison to thermocouples of noble materials (B, R, and S).
Most stable among thermocouple of inexpensive material.

WHY CHOOSE A TYPE K THERMOCOUPLE?
A coupling of Chromel and Alumel wires has a range of -270 °C to 1260 °C and an output signal of -6.4 into 54.9 mV over a maximum temperature range. This is one of the most considerable benefits of thermocouple type k over other thermocouples generally or other temperature transducers such as the thermistor or the resistance temperature detector (RTD).
Its capability to work in rugged environmental conditions and a variety of atmospheres makes it preference over other temperature transduction devices.
Thermocouple devices must use a suitable wire because different wires measure various temperature ranges. Type K is popular due to its wide temperature range. Of the four major thermocouple types, type K covers the broad range from−200°C to 1,260°C (approximately minus 328°F to 2,300° F).

WIRE PROTECTION OR INSULATION MATERIAL
Due to its reliability and accuracy, Type K is used widely at temperatures up to 1260°C (2300°F). It is good practice to protect this type of thermocouple with a ceramic tube. When other conditions are appropriate, in atmospheres, such as electric furnaces, tube protection is not always necessary; however, it is recommended for general preservation and cleanliness. Type K will generally outlast Type J because the (iron) wire rapidly oxidises, particularly at higher temperatures. When protected or isolated with ceramic beads or insulation material in reducing atmospheres, the life increases.

• 

Outer metal sheath (MGO) and Mineral insulation are used for the temp sensor.
We can use Type K from -20 to 1260°C (-6 to 2300°F). If the program is between 600 to 1100°F, we urge Type J or N because of short-range ordering that can lead to drift of +2° to +4°F in a few hours. Type K is stable in radioactive nuclear environments. For applications below 0°C (32°F), special alloy selections are often required.

• 

Thermocouple wire responsiveness and boundaries of error are concerns when selecting a type. Type K has a higher margin of error than other types of thermocouple wire; manufacturers that choose this type are usually willing to sacrifice precision for the range of sensitivity. Type K has a margin of error associated with a percentage of the temperature measured. It's roughly 0.75‰ or 2.2°C, whichever is greater. Type K has an exponentially increasing voltage; the differences in voltages makes it easier to measure accurate at higher temperatures. At freezing temperatures minus 260° C to minus 250° C K type thermocouple output differ only a few micro Volt for every degree Celsius. At extremely high temperatures around 1,350°C voltage is different about 3.3 hundredths of a millivolt per degree Celsius.

WIRE COATING
Coated wires display different reaction times in different media. Some test subject chemicals might damage probes and wires. A 1/4-inch wide, sheathed, ungrounded type K thermocouple reacts to temperature changes in water in about 2.25 seconds.

TYPE K THERMOCOUPLE JUNCTIONS
Grounded JunctionThis is the most frequent junction style. A thermocouple is grounded when the two thermocouple wires and the sheath are welded together to form 1 intersection in the probe tip. Grounded thermocouples have a very good response time because the thermocouple is currently making contact with the sheath. A drawback of this grounded thermocouple is that the thermocouple is more prone to interference. This is because the sheath frequently comes into contact with the surrounding area, providing a path for interference.

• 

Un-Grounded Junction
A thermocouple is grounded once the thermocouple wires are welded together, but they are insulated from the sheath. The wires are often separated by mineral insulation.

• 

Exposed Junction (or "bare wire thermocouples")
A thermocouple is exposed while the thermocouple wires are welded together and directly placed into the process. The response time is very quick, but vulnerable thermocouple wires are vulnerable to degradation and corrosion. This style is not recommended unless exposed junctions are required by your application.

• 

Thermocouple conductors come in a variety of sizes. Depending upon your application, the gauge will affect the thermocouple's functionality. The higher the gauge section, the more thermal mass of the thermocouple. Higher thermal mass reduces the response. The larger the gauge size, the higher the operating and the stability life. Conversely, a tinier gauge size will have a faster response, but may not provide stability or working life required.
thermocouple response time wire gauge

THERMOCOUPLE SHEATH
316 Stainless Steel
Maximum temperature: 1650. The best corrosion resistance of the austenitic stainless steel grades. They are used in the food and chemical industry. Carbide precipitation occurs at 900°F to 1600°F (482°C to 870°C), which makes it crucial to keep the working temperature below 900°F.

316L Stainless Steel
Maximum working temperature: 1650°F (900°C). Same as 316 SS (04) but low carbon variant allows for better welding and fabrication.

304 Stainless Steel
Maximum temperature: 1650°F (900°C). Most widely used temperature sheath material and extensively used in food, refreshment, drug and other industries where corrosion protection is required.
Carbide precipitation occurs at 900°F to 1600°F (482°C to 870°C), which makes it crucial to keep the working temperature below 900°F.

304L Stainless Steel
Maximum temperature: 1650°F (900° C). The low carbon version of 304 SST (02). Lowering carbon content permits this material to be fused and melted in the 900 to 1600°F (480 to 870°C) range without damage to corrosion resistance.

310 Stainless Steel
Maximum temperature: 2100° F (1150° C). Corrosion and mechanical resistance, similar to but better than 304 SS. Very good heat resistance. This alloy includes 25% chromium, 20% nickel. Not, as ductile as 304 SS.

321 Stainless Steel
Maximum temperature: 1600°F (870°C). Similar to 304 SS except ceramic stabilised for intergranular corrosion. This metal is intended to overcome susceptibility to carbon precipitation in the 900 to 1600°F (480 to 870°C) range. Used in aerospace and chemical applications.

446 Stainless Steel
Maximum temperature: 2100°F (1150°C). Ferritic stainless steel has great resistance to sulfurous atmospheres at high temperatures. Excellent corrosion resistance to nitric acid, sulfuric acid and most alkalies. 27 content gives this alloy the highest heat resistance of any ferritic stainless steel.

Inconel 600
Maximum temperature: 2150°F (1175°C). Most used thermocouple sheath material. Good high-temperature strength, corrosion resistance. Resistance to stress corrosion cracking and also resistance to oxidation at elevated temperatures. Do not use in sulfur-bearing environments. Excellent in nitriding environments.

Inconel 601
Maximum temperature: 2150°F (1175°C) constant, 2300°F (1260°C) intermittent. Similar to Alloy 600 with the attachment of aluminium for excellent oxidation resistance. They are designed for high-temperature corrosion resistance. This substance is good in carburising environments and has excellent creep rupture strength. Don't use in vacuum furnaces! Susceptive to intergranular attack by extended heating in 1000 to 1400°F (540 to 760°C) temperature range.

Inconel 800
Maximum temperature: 2000°F (1095°C). Widely used as heater sheath material. Minimal usage in thermocouples. Superior to Alloy 600 in sulfur, cyanide salts and other salt environment. Susceptible to intergranular attack in some programs by exposure to the temperature range of 1000 to 1400°F (540 to 7607deg C).

TYPE K THERMOCOUPLE SIGNIFICANCE
Like all thermocouples, they're inexpensive, have a quick response time, are modest in size and are reliable. They can accurately measure extreme temperatures. Depending on the manufacturer, these range from −270° to 1,370° degrees C or Celsius, with mistakes within 0.5 to 2 degrees C. They have a sensitivity that's roughly 41 microvolts per degree C.
Generally the implementation of K types at temperatures above 550 °C. To restrict excessive error; the recommended usage is in oxidising or completely inert atmospheres utilising a range of −200° to 1,260° C.
All thermocouples have some disadvantages. They have to be calibrated before use. Their output signals are tiny, and so they could have a problem with noise. They are prone to stress, strain and rust as they age. K types,are no different.

TYPE K THERMOCOUPLE CONSIDERATIONS
Type K thermocouples are only stable for short periods at certain temperatures, and they often drift in a positive direction. The extent of this drift is determined by the temperature cycles. By way of instance, at 1,093° C, their readings may be off by up to five degrees. Cyclical or alternate exposure below 371° and above 760° C yields unstable measurements. Prolonged exposure from 427° to 649°C makes them age. The chromel component is subject to what's called"green rot." When this happens, the chromium turns corroded and green and becomes oxidised. This occurs in decreased oxygen environments from 815° to 1,040°C. Such environments are called reducing, and K-type thermocouples shouldn't be used in cyclically or reducing oxidising and reducing atmospheres. They should not be used in such environments because they will become brittle and break quickly. The presence of chromium makes them unsuitable for vacuums as this may lead to vaporisation.

TYPE K WORKAROUND
The problems could be minimised by using them within the recommended temperatures and environments. Calibration, installing them with the correct connectors and wires, and utilising compensation circuits serve as aids. K types constructed to reduce the errors include the ones which are insulated, pre-aged or are annealed above their temperatures. Some users also take care to replace them. Others switch to type N, that was specially constructed to be an improvement over K.