Thomas Seebeck and the Thermoelectric Effect

To boot, a thermocouple is indispensable to humanity. As the most widely used temperature sensor on the planet, it’s as indispensable as the tools we use every day which operate them. Think about it. We’re talking about thermostats, ovens, and steam kettles. And that’s just for starters. 

It’s true, thermocouples may not be on the radar of consumers and ordinary Americans. But if you talk about getting the temperature of the hottest materials on the planet, say the temperature of the insides of a volcano (1,250°C or 2,200°F), you’re going to find yourself in an extremely tight situation if you don’t make the most of a thermocouple. 

Quite simply, other temperature sensors will fail. For one, mom’s go-to fever mercury thermometer is bound to perish when confronted with hot lava — the mercury inside turned to gas. As science would have it, mercury boils at just 674°F (356°C), not even halfway through a volcano’s raging temperature. The same holds true at below-freezing temperatures. Your mercury thermometer will freeze solid, falling short. 

Examining the merits, therefore, of a thermocouple should bid us well. Knowing what makes it tick will allow us to use it as effectively as we need it to be.  Ultimately, we’re going to bump into the works of Thomas Johann Seebeck (1770 - 1831). By discovering the thermoelectric effect, the Baltic German scientist introduced to the world its most widely-used temperature sensor, the thermocouple. Becoming a more livable planet in the process. 

The Puzzle: Electricity Versus Heat

For centuries, the notion of electricity was introduced to us by natural means. Long before we have formal knowledge of the workings of electricity, ancient Egyptians (2750 BCE) were in awe of the shocks brought by the electric fish, calling the fish the “Thunderer of the Nile.” Today, millennia later, we have learned so much about electricity that we have used it to power our homes.

As for heat, evidence shows Homo erectus, our ancestors, made use of fire 1,000,000 years ago as a tool for survival Today, we refer to heat when we want to warm our winter nights. 

What was not clearly established was the direct relationship between the two, heat and electricity. But seeping through the cracks, clues seem to lead us to connect one to the other. For one, there’s conduction in science. For some time there, we’ve used the term conduction to refer to the propagation of both heat and electricity. 

And we even find that there are materials that conduct well for both scientific phenomena. The top of the list is metals. Iron, for one, conducts well for both electricity and heat. On the other hand, plastics don’t. 

Indeed, this could be a telltale sign of the connection between the two elements. Scientific research should tell us there must be a link between how metal conducts heat and electricity.  

To note, electricity rides on the backs of electrons, the super-tiny charged particles inside atoms. As such, electrons carrying electricity through the material are like an army of ants carrying leaves to their colony. By the same token, electrons that can conduct electricity and also piggyback heat. It’s no accident then that metals exhibit both electrical conductivity and heat conductivity. 

Thomas Seebeck: Connecting the Dots

Have you tried holding one piece of a long metal rod and putting the other end on fire? At first, your hand won’t feel the heat. Over time, however, you will have to let the rod go as the heat travels upwards. What you don’t know is that electricity travels to your hand along with all that heat. 

The first scientist to observe such a thermal-electric phenomenon was Alessandro Volta in 1794. Volta (1745 - 1827), an Italian pioneer of electricity is the inventor of the electric battery, not to mention the discoverer of the ever-useful methane gas, the main component of natural gas. The physicist to whom electrical quantity volt is named realized that “animal electricity” is produced when two distinct metals were connected to one another in series with the legs of a frog. And from this experiment was born the world’s first electric battery and source of direct current. 

But the thermoelectric effect was rediscovered in 1821 in greater detail by a Baltic German physicist named Thomas Johann Seebeck. Then, Seebeck observed that a compass needle would move when a temperature difference is applied to a closed-loop made by two distinct metals joined in two ends. At first, Seebeck called the phenomenon the “thermomagnetic effect” in reference to the magnetic field produced. Such a name oversight was rectified to the “thermoelectric effect” by Danish physicist Hans Christian Ørsted (1777 - 1851), the scientist who established that electric current created magnetic fields. 

Hot and Cold Junction

FIGURE A.

In essence, the components of a thermocouple are two different metals (two curves) joined together on their two ends. If something hot (thing to be measured) is placed on the HOT JUNCTION, while the other end is placed on something cold (COLD JUNCTION), voltage is bound to develop. A voltmeter measures this electrical difference. 

In time, the thermo-electric phenomenon would be called the Seebeck effect. And as he moved forward with his experiments, Seebeck uncovered more amazing discoveries. He noticed that there are certain specific circumstances for current to flow. 

First up, connecting two ends of the metals together won’t produce any current and the voltmeter won’t move a notch. Secondly, no current flowed if the ends of the two metals had the same temperature. 

Upon further experiments with other metals, the German scientist noted that different metal materials produced differing amounts of electricity.  Today, this electrical setup has been modified to using two strips of distinct metals of equal length joined together at their ends to create a loop. To produce electricity, one end must be dipped on a hot surface (e.g.., hot water) while the other end is dipped on something cold. To measure the current flow, a milli-voltmeter can be used. 

Please check figure A above for details. 

In understanding how thermocouples work, we must be aware of the variable. Right off the bat, the amount of electricity depends on two things: 

- Temperature difference (between the two junctions)

- Type of Metals used

Volta correctly deduced one doesn’t need the frog’s legs to produce electricity between two different metals (battery).  By the same token, know that you can have the Seebeck effect without the metal junctions. You just need the metal junctions in the actual use of thermocouples. 

How does the Seebeck Effect Materialize?

Heating of Metal

FIGURE B. Heating one end of a metal (fire) causes the cooler end to be negatively charged. 

At the onset, it may be hard for us to wrap around our head on the Seebeck effect. We have that traditional notion that they are separate forms of energy. Electricity, being viewed as a flow of charge, and heat as the movement of molecules vibrating vigorously are two distinct entities. Indeed, many traditional pundits limited by their schools of training (e.g., thermodynamics, electricity) may fail to see the connection between the two, becoming shortsighted in the process. They define each one as independent of one another. 

Science, of course, is a process. As aforementioned, even Thomas Seebeck himself failed to see the electric current involved in the thermoelectric effect. The German scientist thought the phenomenon involved a magnetic field produced by heat calling it the “thermomagnetic effect”. But his Danish contemporary scientist (physicist and chemist) Hans Christian Ørsted intervened to straighten things out. 

Indeed, there’s an undeniable link between thermal conduction and electrical conduction that happens in metals. It’s no accident that these two phenomena occur in the material. When a substance is heated, its molecules get excited. It vibrates. Think of heat as music and the molecule movement as dancing. The hotter the heat becomes the faster the molecules dance as the faster beat of the music becomes. All that movement causes molecules to bump into nearby molecules causing a transfer of energy from one end of the metal to the other. 

When this happens, electrons, the negatively-charged particles of an atom, spread out. Now, when we hit one end of the metal, electrons there diffuse and flow towards the cooler end. The more heat, the greater the transfer of electrons. In the process, electricity is created with the hotter end of the rod as more positively charged while the cooler end becomes more negatively charged. In short, the Seebeck effect is induced. 

How about the thermocouple? How do two distinct metals forming a close loop produce the Seebeck effect? Is it the same application?

For starters, you are using two great conductors of electricity, metal alloys, in the thermocouple setup. That means these alloys conduct both heat and electricity well, a requirement to produce the Seebeck effect. Using insulators such as plastic won’t cut it as electrons are not free to move around. In short, they are a bad conductor of electricity and heat. 

Secondly, another requirement for a thermocouple is there must be two distinct metals used to form the loop. The means both have different heat conductivity levels. So if you put together a rod of copper with a rod of iron, electrons will flow from iron to copper. 

But if you join iron and copper in a closed loop with two junctions, the voltage produced will cancel each other out. One end of the junction will produce a positive voltage while the other will produce an equivalent negative voltage. 

However, if one junction gets hotter than the other, electrons on this hotter side will spread out faster there. As a result, there will be a voltage difference between the two junctions. The amount of which will be dependent on the temperature difference between the two junctures. That is how the Seebeck effect gets applied in thermocouples. 

Calibration: Starting Thermocouples Right

The voltage output of a thermocouple is very small; it’s in millivolts. How much the voltage increases will show you how much the temperature has increased. However, you need to know how much change in voltage will happen for every degree of change in temperature. This process is called calibration. 

Take note that each thermocouple needs to be calibrated for efficient use. As each one has different characteristics depending on the metal alloys used, due diligence must be observed to produce reliable results. As such, you need to plot the thermocouple’s voltage-temperature curve or rate of change. 

To calibrate, you need to figure out the working formula for a particular metal-junction. Think of it as marking a thermometer’s scale. Only after you calibrate a thermocouple can you use it to measure any temperature you wish. 

Do this using a Thermo bath container. Fill it with water and turn it on. Next, heat this water to 30°C. Then connect the two leads of a multimeter to one of the ends of the thermocouple. The resulting voltage reading should be 1 microvolt. 

Then, take one junction of the thermocouple and place it in the thermobath. Let it stabilize. This happens when the voltage on the multimeter doesn’t fluctuate anymore. Put the resulting voltage on record. 

Increase the temperature in the thermobath to 5°C making sure you put the resulting voltage on record. Continue the process, incrementing temperature by 5°C and recording. Until you reach 60°C.

After which, take note of the room temperature. Refer to the voltage chart of your thermocouple type within room temperature. Take note, for instance, type K should read 1 millivolt at room temperature of 25°C.

By using the curve-fitting method, zero in on the line that best fits your acquired data. The resulting slope should be your guide in determining the voltage increase for each corresponding temperature degree increase. For type K thermocouple, for example, it’s usually 40 microvolts per degree Celsius change in measured temperature. 

How do You Apply Thermocouples in Real-life Applications? 

 

Thermocouple Circuit Diagram

FIGURE C.

A thermocouple product sample and circuit diagram. 

In real-life use, we cater to only one junction of the thermocouple: the tip measuring a material’s unknown temperature. This is especially true with commercially-available thermocouples. Take note that the voltage produced by a thermocouple circuit (check above) is very minimal, in millivolts. Thus, an electronic voltage amplifier is used to enlarge this voltage allowing better measurement. 

Know that not all thermocouples are the same. As distinct metal alloys have different melting points and characteristics, unique metal alloy combinations are bound to behave differently. Over time, distinct metal alloy combinations have been assigned by letter type. It’s paramount therefore that you get to see for yourself which particular type of thermocouple best fits your application. 

Some of the most popular metals used for thermocouples include aluminum, chromium, copper, iron, nickel, and rhodium. More often than not, a particular type is chosen because of its accurate behavior at a particular temperature range. 

Thermocouple Types and Performance Table

Type

Legs Composition

Temperature Range (° F)

Peculiarities

B

Platinum 30% Rhodium/

Platinum 6% Rhodium

2500 up to 3100

Low electrical output; not suitable for lower temperatures

E

Nickel-chromium/

Constantan

32 up to 1600

More stable than K-type with a higher degree of accuracy

J

Iron/Constantan

32 up to 1400

Widely used; cheapest

K

Nickel-chromium/

Nickel-aluminum

-328 up to 2300

Most widely used

N

Nicrosil/ Nisil

-454 up to 2372

Widely used; Better performance than K type

R

Platinum 13% Rhodium / Platinum

1600 up to 2640

Similar to type-S but better stability

Platinum 10% Rhodium / Platinum

1600 up to 2640

Can be used in very high temperatures but need protection from moisture 

T

Copper/Constantan

-75 up to 700

Widely used; Very stable best for very low temperature


What are the Top Industries Using Thermocouple? 

Thermocouples are heaven-sent. Think about it. When it comes to temperature range, no probe-type sensor can measure as wide as these two-junction sensors. Though thermistors and RTDs have their distinct advantages, they fall short in delivering temperature readings in extremely hot and extremely cold environments. Here are just some of the industries that use thermocouples: 

Thermocouple Types and Top Industries Using Them

Thermocouple Type

Industry

Use

Type B

Steel and Iron Industry

Monitor temperatures throughout the steel-making process

Type C 

- Space industry

- Nuclear reactors

- Industrial heating

- High-pressure research

Vacuum furnaces with extremely high temperatures

Type E

- Cryogenic

- Pharmaceutical

- Chemical applications

Sub-zero, inert or oxidizing applications

Type J

  (Same as Type E)

Can be used exposed or unexposed

Type K

Most widely used

Used in many environment types: engines, boilers, heaters, hospital thermometer, food industry

Type N

Nuclear applications

(stabler than Type K)

Used in a vacuum and controlled environments

Type R/Type S

- Semiconductor industry

- Glass manufacturing

Used as control sensors

Type T

Environmental applications

Food monitoring


Truly, it’s amazing how much thermocouples have served man over the years. Even better,  they’re spot-on when fielded in automated measurements as they are self-powered and generate electric currents. And what a convenience that is. Instead of having to measure temperatures regularly at intervals, you can just connect them to a monitoring circuit (e.g., computer) and collect results over time. 

To top it all, the thermocouple’s simple design makes it cheaper to manufacture en masse. It’s basically a couple of metal strips joined together. And voila! Indeed, we are forever thankful for the brilliance of Baltic German scientist Thomas Johann Seebeck. As inexpensive and as wide are its uses, humanity has progressed a lot with a thermocouple in tow. 


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