How it Works: A Brief Guide to the Thermocouple Temperature Sensor

Thermocouples are electromotive devices used to measure temperature. They're highly accurate and durable. Since they can withstand high vibration, extreme temperatures, and high pressure, thermocouples are great for a wide range of uses.

A few applications for thermocouples include thermometers and vehicle diagnostics. Most boilers, ovens, and water heaters use thermocouples for temperature measuring as well. Typically, people who have worked in healthcare or manufacturing industries have used a thermocouple at least once.

But how does the thermocouple temperature sensor work? Continue reading to learn the answer to this question, as well as additional useful information on thermocouples.

The Seebeck effect is sometimes referred to as the thermoelectric effect. This is a scientific principle referring to the process where electric energy is created from converted thermal energy.

The Seebeck effect details the electrical voltage that happens when two different conductors are connected. This produced voltage varies according to temperature.

Thermocouple temperature sensor design involves two wires of dissimilar metals. Each of these wires will have different electrical properties at different temperatures.

At one end, these metal wires connect, so they maintain constant contact. The part where the two wires connect is called the measuring point.

The other end of the wires are connected to the voltage reader. This end is called the connection point.

When the temperature changes at the measuring point, the electron density of each metal wire shifts simultaneously. Measured at the connection point, this changing electron density is the voltage.

Contrary to popular belief, thermocouples don't measure absolute temperature. They measure the differential temperature between the measuring and connection points.

For this reason, thermocouples require cold junction compensation. This ensures the ambient temperature at the cold junction doesn't influence the measured result. With the inclusion of a cold junction, thermocouples can provide more accurate readings.

When choosing a thermocouple, the most important thing to look at is the temperature range you'll need for your specific applications. If you need something with the widest temperature range, the K-type thermocouple is your best choice.

T-type thermocouples are your best choice for use in lower temperature ranges. Most high-heat applications will do best with the N-type thermocouple. For extremely high heat applications, you may need a B-type thermocouple.

Stability should be considered if the thermocouple will be around chemicals or in abnormal or extreme conditions. Some industries and sectors need a specific thermocouple type. In these circumstances, it's best to consult an expert before making a decision.

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There are several common types of thermocouple temperature sensors. The differences in each type boil down to their optimal temperature ranges and the types of metals used. The most common thermocouple types are discussed more in-depth below.

A K-type thermocouple is a general-purpose, inexpensive thermocouple with great temperature precision used in a diverse range of industries, sectors, and applications. Their wide usage is thanks to a combination of being affordable to manufacture and producing accurate readings.

These thermocouples have an optimal temperature range of -200 and 1260 Celsius. Type K uses (Chromel / Alumel) wire as dissimilar metals as the conductors using Chromel as the positive wire and Alumel wire as the negative.

J-type thermocouples are the lowest cost to make. Unfortunately, the trade-off is that their lifespan is shortened if regularly exposed to excessive heat outside their optimal temperature range.

These thermocouples have an optimal temperature range of -40 to 750 Celsius. One of their wires is consists of iron, while the other is made from Constantan a copper-nickel alloy. Iron is the positive and Constantan is used for the negative leg.

N-type thermocouples are more costly to create than k-types. But, they provide better stability in radiation environments such as nuclear power applications. As such, the additional costs to manufacture are worthwhile in specific industries.

These thermocouples have an optimal temperature range of -270 to 1300 Celsius. The wires are created from a combination of nickel – chromium -silico, (Nicrosil) and the other is nickel -silicon-magnesium (Nisil). Nicrosil is the positive and Nisil is used for the negative leg.

T-types operate well in extreme cold. This is why they're often used in cryogenics and other situations where extremely low temperatures are typical.

These thermocouples operate in an ideal temperature range of -200 to 350 Celsius. Their wires are crafted from a copper-constantan combination. The wires are created from a combination of copper-constantan. Copper is the positive and constantan is used for the negative leg.

E-type thermocouple sensors offer higher accuracy and strength when compared to K-type and J-types. But they only do so at moderate temperatures, making them poor choices for extreme heat or cold applications.

These thermocouples operate in an ideal temperature range of -200 to 900 Celsius. One wire is created from a combination of nickel-chromium (chromel) and the other wire is made of constantan. Chromel is the positive and constantan is used for the negative leg.

S-type and R-type thermocouple sensors are grouped together because they're basically the same thing. The only difference is that R-types are essentially a more expensive version of the S-type. Both these thermocouple sensors are often used in pharmaceuticals and Bio-Tech industries.

Both these thermocouples have an optimal temperature range of 0 to 1450 Celsius. Both have one wire made of platinum-rhodium and one wire made of platinum. Platinum is the positive and rhodium is used for the negative leg.

B-type thermocouple temperature sensors can be used for the highest temperatures. They also provide the most accurate, stable readings in extreme heat conditions.

These thermocouples have an optimal temperature range of 0 to 1700 Celsius. Both wires are created from high-quality platinum-rhodium wire the positive consists of 30% platinum and the negative has 6% while the balance on each is rhodium.