## How Metals Glow and Expand! The History of Temperature Measurement- Visaya Weekly Episode 26

Hey, guys, this is Germán or at least the interpretation of my glow! By using a thermal imaging camera you are able to see the infrared radiation or thermal radiation that I’m emitting this way you can “see” my temperature.

And you can see this radiation because Everything that has a temperature above 0 Degrees Kelvin {-273.15 Degrees Celcius} glows.

The thing is that I’m glowing at frequencies so high that are not within our visible spectrum.

You need a camera like we are using right now to see this radiation and actually know my temperature!

### But how did we measure temperature before we had this kind of equipment?

How did we know if something was hot, or if it was getting hotter or cooler?

Expert Japanese swordsmiths were able to tell the exact temperature of a blade just by looking at the glow of the blade! This same technique was used in glass and pottery production for centuries.

The Italians, Galileo, and Santori were the first to try to measure the changes in temperature, but it was Daniel Gabriel Fahrenheit the first one to offer a precision thermometer.

First of all, what is a thermometer?

A thermometer is composed of two parts, a sensor and a scale.

The Fahrenheit thermometer uses a glass mercury bulb as a sensor. Fahrenheit observed that the increase in temperature by 1 degree will increase the volume of mercury by a known value.

This is how Fahrenheit scale came to be with 32 degrees as water melting point and 212 as its boiling point, with 180 steps between the melting and boiling points!

This was the first standard scale used in temperature measurement. The most common temperature scales are Celsius [˚C] and Kelvin [˚K]

The industry still uses the mercury thermometers, but there are two big limitations:

• First is that mercury thermometers are usually made from glass, that is a fragile material and sensors break quite easily,
• Second, it cannot be used in a lot of industrial applications since mercury has a limit of 356.73 °C that is the boiling point of Mercury.

### During the 19th century with the first attempts of process automation the bimetallic thermometer was introduced.

In this case, the sensor is metal strip made of 2 metals bonded together and coiled like the shell of a Snail. The difference of expansion coefficients makes this coil expand or contract.  you can measure temperature by translating this rotation into a polar scale.

Bimetallic thermometers are really inexpensive sensors and are still used in various applications, but they have a low accuracy [2 degrees˚C / ˚K].

Bimetallic thermometers have to be changed regularly since they are made of moving components.

Next week we will talk about temperature measurement based on electrical principles.

Thank you for watching.

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