AIM Technology Inc.

Advanced Inprocess Monitoring Systems

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Thermocouple Specifications

Other Specifications or information pertinent to the design

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What is a Thermocouple? A thermocouple is a simple, mechanically robust, electrically versatile and inexpensive temperature sensor. Thermocouples can be used over a wide range of temperatures for monitoring and process control applications. A thermocouple consists of two dissimilar metal wires, joined at one end. When the joined end is heated there is a continuous flow of current. The net open circuit voltage (Seebeck voltage) is a function of the junction temperature and the composition of the two metals. As a result this means that when the junction of the two metals is heated, or cooled, a voltage is produced that can be correlated back to the temperature. Ungrounded: In an ungrounded probe, the thermocouple junction is detached from the probe wall. Response time is slower than the grounded style, but the ungrounded offers electrical isolation. Sheath type depends on the instrumentation. If there is any chance that there may be a reference to ground (common in controllers with nonisolated inputs), then an ungrounded probe is required. If the instrument is a handheld meter, then a grounded probe can almost always be used.

Thermocouple Types Thermocouples are available in different combinations of metals. The most common are the “Base Metal” thermocouples known as Types J, K, T, E and N. There are also high temperature combinations known as Noble Metal thermocouples - Types R, S, C and GB. Thermocouples are capable of operating reliably over a wide range of temperatures -200° to 1250°C (-328° to 2282°F). Reference EnvirOptics “Thermocouple Chart Type Selector” below:

Sheath Types Based on the application temperature range to be monitored, and the corrosive nature of the environment, a wide variety of protective sheath types are available. Choosing the proper sheath will protect the thermocouple, which will result in a temperature sensor which will be reproducible and have reliable and sustainable performance. Sheathed thermocouple probes are available with one of three junction types: grounded, ungrounded, or exposed.Grounded: At the tip of a grounded junction probe, the thermocouple wires are physically attached to the inside of the probe wall. This results in good heat transfer from the outside through the probe wall to the thermocouple junction. Ungrounded: In an ungrounded probe, the thermocouple junction is detached from the probe wall. Response time is slower than the grounded style, but the ungrounded offers electrical isolation. Sheath type depends on the instrumentation. If there is any chance that there may be a reference to ground (common in controllers with nonisolated inputs), then an ungrounded probe is required. If the instrument is a handheld meter, then a grounded probe can almost always be used.

Sheath Material Temperature Graph Thermocouple Temperature Limit for different Sheath Materials

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