Thermocouples

Thermocouples

Electricity from Hot Metal!

Thermocouples are amazingly simple devices that produce electricity from nothing but hot metal. You’ve likely heard of them before as they’re commonly used used to measure temperature. Take two wires of different metals, then heat them at a point where they’re attached together. Voila! You can measure a voltage across the unheated ends of the wires.

This voltage and current are quite small, however, so don’t get too excited. You’re not going to be recharging your Tesla with a couple of warm paper clips or anything. The voltage is just large enough to use as a signal. You can put a bunch of thermocouples together and do some other interesting things though. 

We’ll first talk about how thermocouples work and why they are so useful for temperature measurement. Then we can extrapolate some interesting applications beyond a single set of two wires.

How do Thermocouples Work?

We already stated that Thermocouples are really just two wires made of different metals that are connected at one end.  There’s nothing too novel in that, but the “different metals” part hints at something interesting.  Lets explore:

Back around the early 1800’s, it was discovered that heating a piece of metal at one end would create a measurable voltage difference.  Interesting!  This is because outer valence electrons in the atoms of a metal are able to migrate through the material (electrical conduction), and they do so when heat is applied.  Thermal energy means vibration on an atomic scale.  Fewer electrons occupy the space with all the increased jostling going on inside the hot end of the material.  The added thermal energy also helps the electrons to jump to higher excited energy states (think “wider orbits”) around the atomic nuclei which makes it easier to escape the atom.  Long distance relationships are hard.  In contrast, the cooler end of the wire is a bit more relaxed and can take in the electrons looking for refuge.  The result is more electrons hanging out on the cool side and fewer on the warm side.  Since an electron is a negatively charged particle, we can restate this to say a more negative charge on the cool side and a positive (less negative) charge on the warm side.  That’s a voltage difference!

One warm wire does not a battery make.  The state described above is a material in equilibrium.  The warm region of the metal has exactly the number of electrons that makes it stable and the cooler end has what it wants.  It will look to reject any additional electrons added to it and attract in any deficits were some of the electrons to be forcibly removed.

Metals of different elements have different numbers of outer shell or valence electrons that are able to freely move from atom to atom.  Due to the size of the atoms, attractive and repulsive subatomic forces between different quantities of charged particles, and other seemingly incomprehensible magic, materials behave differently.  Gold (79) and mercury (80) are two metal elements directly next to each other on the period table.  This means they only differ by one proton/neutron/electron set.  That one little difference makes one a brilliant yellow shining solid people love to implant in their mouths and earlobes, and the other is a heavy greyish poisonous liquid.  The relative density of electrons at a given temperature is a more subtle difference and that is the important one for thermocouples.

When two wires are set side-by-side, joined together at one end, and heated at the joint, the electrons will tend to disperse away from the heated end.  The electrical charge at the tip of both wires will be identical because they are conductive metals that are coupled together.  Away from that point on the opposite end of the wire, you’ll have a relatively higher number of electrons which means a negative charge. 

If both wires are the same elemental metal, the exact same charge will be developed.  However, if the metals are different, the electrons will disperse differently.  The charge will still be more negative on the cool end versus the warm end of the wire though one wire will have a more negative charge than the other.  A voltage difference can be measured across the two cold ends of the wires. 

Example:

Wire ‘A’ has an electrical charge difference of 0 (hot) to -5 (cold)

Wire ‘B’ has an electrical charge difference of 0 (hot) to -3 (cold)

The net difference between wire ‘A’ and ‘B’ is (-5) – (-3) = -2 net difference

It is important that the cold ends of the wires are at the same temperature.  If we know that cold end reference temperature along with what material each wire is made of, we can reference the measured voltage to a data chart and know the exact temperature at the hot tip of the wires.  This is a thermocouple.

What are the Advantages of Thermocouples?

Thermocouples are easy to construct.  They are small and flexible.  In fact, thermocouples work best when they are made of very thin wire so that it absorbs very little thermal energy when measuring the temperature at its tip.  Their low profile also makes them least disruptive and very quickly responsive to changes.  They can be bonded inside machine components to precisely measure the internal temperatures at critical points.  The length of the wires has a negligible impact on the reading as long as other electrical interference is avoided.

How do you Read a Thermocouple?

The difficult way to read a thermocouple is to measure the voltage with a very sensitive multimeter.  Then look up that value in a reference table.  The easy way is to use a circuit or reader devoted to the purpose of reading thermocouples.  It has the reference data table built right in.  Many digital multimeters include this feature.  The circuit is a combination of a signal amplifier and software or dedicated integrated circuit that converts the voltage signal to a temperature value.  The reader circuit must be configured for a specific set of wire materials though, or it will not be accurate.  Luckily, there are standardized pairs of materials widely available on the market and commonly referenced worldwide.

Standardized Thermocouple Materials

Thermocouple materials are standardized and referenced by single letter codes such a “K” or “J”.  These are the two most common.  Thermocouples made of other less common materials are better suited for alternate temperature ranges, higher precision, or improved corrosion resistance.  Often, these use brittle or very expensive materials.  “K” type thermocouples are inexpensive and cover a very wide temperature range.

More Power!

Thermocouples generate a voltage on the order of a few microvolts.  A microvolt is just 0.000001 volts.  It works fine as a signal for measuring temperature, but isn’t really useful for doing any work.  Now, string a bunch of thermocouples together in series and you add those voltages together.  You pile them together to get a “thermopile”.  Thermopiles convert the net difference in temperature between the hot and cold ends into a useful voltage on the order of hundreds of millivolts.  That’s more in the range of usefulness.  They use the same principles as Peltier Elements to convert directly between heat flux and electricity.

Thermopiles are most commonly used in gas burner safety controls.  Many gas stovetops, fireplaces, turkey fryers, patio heaters and the like use a thermopile to maintain a gas burner in the open position.  They usually require a valve to be pushed open manually and held in place while the gas is ignited. 

A thermopile is located directly in the burner and wired to the supply valve so that the hot flame generates enough electricity to maintain the valve in the open position.  If the flame ever goes out for any reason, the thermopile stops producing a voltage and the valve is released back to the closed position automatically.  This safety valve prevents an uncontrolled buildup of unburned propane or natural gas that could otherwise have explosive consequences.

Even More Power!!!

So…  what if you pile a massive amount of thermocouples together?  Like a mega thermopile?  Hmmm…

Thermoelectric Generators take advantage of the tiny incremental thermocouple voltages added together on a monumental scale.  While they still struggle to produce much power with an efficiency below 10%, they can still be useful.  In automotive applications, exhaust gas is hot and is essentially “waste heat” energy lost from combustion.  Thermoelectric generators have been developed to harness some of this lost energy and produce up to 750W (about 1hp) of useful power.  While these incredibly durable solid-state devices are useful, they tend to require high cost materials.  For now, that stifles commercial viability although materials scientists are continually pushing for improvements.

One application where these are particularly well-suited is in outer space.  Since the devices are solid state (no moving parts), orientation and acceleration have no impact on them.  They brush off the massive G-forces at launch, and low or no gravity are of no consequence to them.  Also, space is very cold.  A temperature gradient can easily be produced given that a heat source is available. 

Deep space probes have been using radioisotope thermoelectric generators for just this reason.  They are essentially solid state radioactive batteries where a hot radioactive decaying core is wrapped with thermocouples.  It is a very reliable system with no moving parts.  In fact, Perseverance, the rover that just landed on the surface of Mars uses an RTG for some of its supply power.

Thermocouples are simple reliable devices that generate electricity from a difference in heat.  They work exceedingly well as sensors to measure temperature at precise points.  The expanded applications derived from thermocouples are useful in everything from backyard cookout safety to space exploration. 

Are thermocouples, thermopiles, or thermoelectric generators a key solution to one of your product development challenges?

CDN Inc. is a product design and engineering firm that can adapt easily to your project needs; engineering, industrial design, prototyping & manufacturing.

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