Choosing a Thermocouple
Types of Thermocouples
| Platinum 10% Rhodium | Expensive, chemically stable, good mechanical properties, must be protected in use, not suitable for low temperature applications |
| Platinum 13% Rhodium | Expensive, chemically stable, good mechanical properties, may be used in oxidizing atmospheres, must be protected in use, not suitable for low temperature applications |
| Platinum 20% Rhodium | Very expensive, for use in temperature ranges beyond the capabilities of the above noble metals |
| Iron-Constantan | Low cost, suitable for oxidizing or reducing atmospheres. Subject to chemical attack. |
| Copper-Constantan | Suited to ambient temperatures, similar to the model above. |
| Chromel-Alumel | Higher temperature range (1000-2000°F). Can be used in an oxidizing atmosphere, is mechanically strong, and is resistant to chemical attack. |
| Chromel-Constantan | Very stable, not to be used in reducing atmospheres, subject to chemical attack. |
Types of Thermocouple Junctions
There are three types of thermocouple exposures available depending on the atmosphere of the intended application, intended life span of the thermocouple, process being measured, as well as a variety of other factors.
 1.) Exposed Junction The thermocouple wires are welded together and the insulation is sealed against penetration by liquid or gas. This type of thermocouple offers the least protection and is best suited to ambient atmospheres. |
 2.) Grounded Junction The thermocouple wires are welded together with (grounded to) the sheath. This provides more protection, and is used in cases where an exposed junction would not work due to harsh atmospheric conditions, or when longer life is required. |
 3.) Ungrounded Junction The thermocouple wires are welded together and are insulated from the sheath. This provides the most protection for the thermocouple elements and is recommended for the longest life. Different types of sheath materials (protection tubes) offer greater protection depending on the conditions. |
Factors to Consider
Choosing the correct thermocouple requires that you look at a number of factors. For example, a thermocouple which is only used only periodically may be less expensive, but it may have a shorter life span and require servicing more often. If the thermocouple is intended to be used for long periods of time without service it may be necessary to use a thicker gauge of wire and it may be necessary to use a design engineered for a greater degree of protection.Below are some general guidelines to consider when choosing a thermocouple. If you need more help making a decision our knowledgeable sales staff is available to recommend a design which will best suit your application.
- What are the maximum and minimum temperatures the thermocouple will see?
- Are there any cost limits?
- What error tolerances are necessary for your application?
- What is the furnace atmosphere?
- What is the expected life of the thermocouple?
- What is the time response required?
- Will the thermocouple be used continuously or periodically?
- Will the thermocouple see bending or flexing during it's life?
- What is the immersion depth?