Thermocouples 101 – Everything You Need To Know

A thermocouple is probably one of the most popular temperature devices in use today. A basic model is relatively low cost, accurate, and can measure an extensive range of temperatures.

Find out:

• How does a thermocouple work
• Thermocouple junctions
• The laws of thermocouples
• What a thermopile is

How does a thermocouple work

A thermocouple is an electric device for measuring temperature. It is made up of two plates of dissimilar metals that are connected at one end to create a junction.

The first end is referred to as the hot junction and is kept on the surface or element where you want to measure temperature. The second one is the cold junction or reference junction which is placed at room temperature (lower temperature).

The Seeback effect states that the temperature difference between two dissimilar or different semiconductors or conductors generates a thermal voltage difference between two junctions.

So by measuring the voltage, you can calculate the temperature of the hot junction using thermocouple tables.

Different types of thermocouples

Here are some of the different types of thermocouples:

• Type K: It’s accurate, reliable, and inexpensive and this makes it very common.
• Type J: Has a shorter lifespan at higher temperatures and a smaller temperature range than type K, but is very common.
• Type B: Need protection, easily contaminated.
• Type N: For general use and has better resistance to sulfur and oxidation than Type K.
• Type S: Highly reproducible, laboratory standard. Need protection, easily contaminated.

The three thermocouple junctions

Thermocouple probes usually come in exposed, grounded, and ungrounded junction styles. Each of them has different characteristics and serves a different purpose.

Exposed junction thermocouples

They are for measuring the temperature of a gas. Since there’s no protective cover, this thermocouple junction is extremely fragile. No solid, liquid, or moisture contaminants should be present in the gas.

This thermocouple junction, however, does offer a faster response time as thermal energy doesn’t have to travel via a compacted insulation or metal sheath to impart energy to the junction.

Grounded junction thermocouples

The thermocouple wires are usually connected to the inside of the probe wall, which leads to good heat transfer from the outside, via the probe wall to the junction. That means that grounded junctions will have faster response times than insulated or grounded thermocouples.

Ungrounded junction thermocouples

The junction is detached from the probe wall and the response time is slower compared to the grounded style.

What are the laws of thermocouples?

• Laws of homogeneous circuits: It states that an electric current can’t flow in a circuit that is made up of one homogeneous metal when you apply only heat to it.

• Law of intermediate metals: It states that a third wire can be inserted into a thermocouple without changing or affecting the voltage reading if the new junctions with the third metal are at a uniform temperature or at the same temperature.

• Law of intermediate temperatures: It states that if thermocouples measure the temperature difference and produce a thermal EMF of X; it’ll remain at that number if a third material is introduced into the circuit. The law is extremely crucial to thermocouples since the cold junction of several thermocouples won’t be used at 32 degrees F. Sadly, the standardized electromotive force tables are normally only available at 32 degrees F as a reference temperature.

Note that there are also laws of additive voltages, which can be stated mathematically or in terms of voltage differences. To understand the laws better, you can check diagrams or setups on the internet that use constant temperature baths.

What is a thermopile?

A thermopile uses several thermocouples connected in parallel or in series. They are used for contactless temperature sensing and their function is to transfer the heat radiation produced from the object to a voltage output or output voltage.

With enough sensing junctions, they can generate useful voltages. For instance, a thermopile is often used to control shut-off valves in furnaces.