Mounting Thermocouples in Thermocouple Wells

Thermocouples are used extensively in industrial applications to measure the temperature of liquids, surfaces, and enclosed spaces. e.g., the thermocouple may be used to measure the temperature of liquids such as water, soup, cola, or any other liquid as it's heated during processing. As another example, surface temperatures of the barrel of a plastic injection molding machine or extruder must be constantly monitored to ensure that they remain at the proper temperature during processing. Enclosed areas such as the inside of a heat-treating oven or a drying oven for a spray-painting operation must also be constantly monitored. All of these applications and many others rely on the proper installation and positioning of the thermocouple for accurate temperature readings.

The operation of the thermocouple requires that its tip is placed directly in the liquid or space, or in direct contact with the surface that is being measured. Since many of these locations would destroy the tip of the thermocouple, a thermocouple well is used to provide a means of mounting the thermocouple without allowing it to be damaged. The thermocouple well is also called a thermowell and ill. 1 shows a thermocouple well and probe. Notice that the well provides a cavity where the thermocouple can be easily inserted. When the thermocouple is completely inserted into the cavity, its tip will come into contact with the body of the well and heat can be transmitted between the two very easily so that the thermocouple can accurately measure the temperature.

All wells are mounted so that their tips are directly in the liquid or environment and they are secured so that a complete seal is provided. Since the outside of the thermocouple well provides the seal, the thermocouple can be quickly removed and replaced from the cavity in the inside of the well without breaking the seal between the well and the environment. The tip of the thermocouple must always be in contact with the well.

In ill. 1 notice that the thermowell is identified by the letter D, and the thermocouple element is identified by the letter C. The section identified by B is the extension assembly and these can be any length to ensure that the tip of the well is in the exact location, and the head of the assembly can be where it's easily accessible for maintenance and calibration. The thermowell head is identified bv the letter A. The head has a cover that screws on or off to allow access to the thermocouple terminals. The cover provides protection for the head and the thermocouple terminals to keep them from becoming damaged or corroded by humidity or other caustic substances that may come into contact with the outside of the well.

Above: ill. 1 A thermowell that shows the thermocouple element inside. The extension section and sealed head are also shown. The letters indicate the parts of the thermowell. A is the sealed head, B is the extension section, C is the thermocouple element, and D is the thermowell. The unit above is from Omega.

ill. 2 (a and b)shows the thermocouple and the thermowell individually. The thermowell allows the thermocouple to be directly mounted in it by simply inserting the tip of the thermocouple completely into the well until it touches the bottom of the well. This type of well uses threads to provide a seal with the system. Thermowells are made from a variety of material such as stainless steel, Teflon (plastic), or Teflon-coated metal. The plastic-type thermowells are only usable in low-pressure applications with limited fluid flow, since they don't provide a seal that is as good as the metal types. The Teflon-coated and Teflon (plastic) types are useful in applications where the temperature of caustic acids, electroplating liquids, or other corrosives must be measured. Keep in mind that many foods such as tomatoes are acidic and will severely corrode typical metals. Stainless steel and plastic both provide a degree of corrosion protection and they are the types of material generally used for these applications.

Above: ill. 2 (a) A thermocouple ready to be inserted into a thermowell.

Above: ill. 2 (b) Various sizes of thermowells that are ready to be threaded into tanks (or similar vessels) to provide a liquid-tight seal. The thermocouple is located inside the thermowell.