Thermocouple Type K: Type K (Chromel®–Alumel®) is the most common general-purpose thermocouple.
It is inexpensive, and a wide variety of probes are available in its −200 °C to 1350 °C range. Chromel® alloys are made from nickel-chromium. Alumel® thermocouples are based on nickel and aluminium, although small percentages of manganese and silicon are usually present.
Type K thermocouples are used for measurements in many different types of environments such as water, mild chemical solutions, gases, and dry areas. They may be found engines, oil heaters, and boilers. They are also used as thermometers in hospitals and the food industry.
Type K is the most popular thermocouple, and as with all thermocouples, they have some disadvantages: they need to be calibrated carefully before usage, their output signals are very small, and they may have a problem with noise. They are also prone to stress, strain, and corrosion, particularly as they age. Type K, however, has special problems associated with their usage.
Type K thermocouples are more generally used at temperatures above 540° C. To limit excessive error, the recommended usage is in oxidising or completely inert atmospheres with a range of -200 to 1260°C.
Type K thermocouples are only stable for short periods at certain temperatures, after which they tend to drift in a positive direction. The size of the drift is dependent on the temperature. For example, at 1093°C, their readings may drift as much as five degrees. Alternate or cyclical exposure below 371 and above 760°C yields unstable measurements. Prolonged exposure from 427 – 649°C makes them age faster.
One of the constituent metals in the Chromel®–nickel thermocouple is magnetic. A characteristic of thermocouples made with magnetic material is that they undergo a step change in output when the magnetic material reaches its Curie point (around 354°C for type K thermocouples). The Chromel® element is subject to what is known as ‘green rot’. This occurs in reduced oxygen environments from 815 – 1040°C. Such depleted oxygen environments are called reducing, and type K thermocouples should never be used in either reducing or cyclically oxidising and reducing atmospheres. Also, they should not be used in sulphurous environments because they will become brittle and break rapidly. The presence of chromium makes them unsuitable for vacuums, except for short periods of time. This is because vaporisation may occur.
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