Thermocouple types: understanding the differences

There are different thermocouple types available consisting of different metal and calibration combinations. A thermocouple is a sensor used to measure temperature. It contains two different metals joined together at one end. When the metals are heated or cooled, a voltage is created.

Recognizing the four most common thermocouple types

There are four different thermocouple sensors that are common calibrations. The most common are: J, K, T and E. There are also high temperature thermocouple types: R, S. C and GB. Each calibration is characterized by a different temperature range and environment, even though the maximum temperature varies according to the diameter of the wire used in the thermocouple. The calibration of each thermocouple type is characterized by different temperature ranges. The maximum range of each thermocouple type is also limited by the thermocouple wire. You cannot count on a thin, thermocouple wire to reach the expected temperature range.

Choosing a thermocouple for a specific application

Each thermocouple type measures in brad temperature ranges. They are also ruggedly constructed from almost any material that is suitable for a specific application. This is part of the reason for the popularity the surround the thermocouple type sensors. Here are a few guidelines to assist you in choosing the right thermocouple type:

• Access the temperature range

• Consider the chemical resistance of the different thermocouple type sensors

• Observe abrasion and vibration resistance of each thermocouple device

• Individual installation requirements for each thermocouple type (to make sure the thermocouple you choose is compatible with your equipment look closely at the size of the installation holes

Choosing the right thermocouple junction type

You can find a thermocouple type enclosed in sheath with on of the three different junction types: grounded, ungrounded or exposed. The grounded junction probe type has thermocouple wires that are connected to the inside of the probe wall. This provides good heat transfer from the exterior into the probe wall straight to the thermocouple junction. When you are searching for the right thermocouple type, realize that when there is an ungrounded probe, the thermocouple junction is not attached to the probe wall. This means that the response time is a lot slower than the grounded style, but the ungrounded thermocouple type can offer electrical isolation. The thermocouple type in the exposed junction style will poke out from the tip of the sheath covering which exposes it to the environment. This thermocouple sensor has the best response time, but it should only be used for dry, non pressurized applications.

What is so important about the thermocouple's response time?

The response time for a time constant is the time a sensor needs in order to reach a 63.2 percent increase in temperature under certain conditions. You must have five time constants for a sensor to receive 100 percent of the temperature increase. Naturally an exposed junction thermocouple type is going to respond the fastest, but overall the maximum temperature will more than likely wind up being lower.