All about Thermocouples and EGT Probes

When you first start modding your car, especially with forced induction or nitrous, you might run across people telling you that you need an EGT gauge. You might not even know what these are or what they do, but pretty soon, you end up with one in your car. Great, now what?

Measuring temperatures in a race application is vital to the longevity of your engine or other parts of your car. EGTs that are too high can melt pistons, valve or valve guides, for example, so ensuring that these temperatures are reasonable is important! EGT probes are a specific application where a temperature sensor called a thermocouple is used.

Okay, What’s a Thermocouple?

A thermocouple is a simple concept – When a specific combination of metals are permanently attached to one another (usually by welding) they generate very small voltages that correspond to specific temperatures. There are different types of thermocouples made of these dissimilar metals, like J-type, K-type, and others, which offer different temperature ranges and have different. For automotive applications specifically, K-type thermocouples are used almost exclusively, because they’re able to measure from around -328F to around 2300F, which means that you can measure pretty much anything in or around an engine.

There are a few reasons to use k-type thermocouples

  • They’re relatively inexpensive.
  • They have a reasonably fast response time.
  • They have a wider temperature range than other sensors.
  • You can measure exhaust gases with them, there aren’t many sensors that will withstand that kind of heat.
  • It’s relatively easy to amplify the low voltage signal of the k-type thermocouple to a more measurable temperature range.
  • They are relatively tolerant of physical abuse.
  • They can be quite small.

But a few drawbacks when using thermocouples

  • They’re only relatively inexpensive. There are other temperature sensors are cheaper.
  • Their measurement speed is not as fast as some other sensors.
  • They only output on a millivolt scale, so an amplifier is usually required.
  • Because of their wide temperature range, they aren’t as accurate as other types of temperature sensors.

Despite these drawbacks, thermocouples are a great option to use to measure temperatures in an automobile.

So, where can I use a thermocouple?

  • EGTs – Measuring exhaust gas temperature is the most common usage of a thermocouple in an automobile.
  • CHT – There are companies that make special cylinder head temperature sensors that can be installed between the spark plug and head, or between a bolt head and cylinder head. These allow you to measure cylinder head temperatures, which is common in older iron head applications or in air-cooled applications.
  • Turbocharger air intake temperature – I use thermocouples to measure air intake temperature before the intercooler. Why? At higher boost levels, it’s not uncommon to see temperatures well above 350F, which is the max temperature that can be measured with a typical air intake temperature.
  • Oil/ATF temperature – These temperatures are low enough to be measured with other types of sensors, however, with a thermocouple amplifier, you end up with a linear, scaled 5V compatible output. Sometimes, this is just an easier signal to consume.
  • Brake rotor temperatures – There are companies that make consumable brake rotor thermocouples. These can be installed to scrape against the rotor, providing a way to measure rotor temp. Frankly, this is probably unnecessary because infrared sensors can do this job and do not wear out. It’s hard to recommend using them in this application, although they still exist.
  • Surface temperatures – Like other specialized thermocouples, companies make pad thermocouples with an adhesive backing that you can stick on any flat surface on your car. You can use these to measure manifold surface temperatures, cylinder heads, engine blocks or anything else with a flat surface.

There are more specifics than we can mention here in this technote – things like probe grounding, wire sheathing, and application specifics. If you have questions about these, please contact us and we can give you a hand determining what is best for your specific application!