How do we measure temperature?

This is a dumb question because the answer is to use a thermometer. However, there's a lot more to it than that because of digital techniques, and there are many things to consider when making a measurement of temperature.

The idea of measuring temperature goes back to at least the 17th century. At first they used a column of air with water to show how temperature expands or contracts the air. Then, some guy by the name of Francesco Sagredo put a scale on it. Eventually, they realized that such a device is also a barometer because it's affected by air pressure. Later on they used a sealed tube with alcohol as the indicator. Alcohol thermometers are still used to measure temperatures below the freezing point of mercury: -38 C. Most thermometers use mercury because it doesn't boil until 356 C. You can deduce from this that measuring high temperatures is done with something other than a mercury thermometer.

When we talk about temperature we can't omit taking about the two main scales: Fahrenheit and Celsius. These were two eighteenth century guys that were involved in temperature measurement. Fahrenheit, a German physicist and engineer, made thermometers with mercury because it has a high coefficient of expansion. Celsius was a Swedish astronomer who had the good sense to make the freezing point of water 0 C and the boiling point 100 C. Fahrenheit made the freezing point of water 32 F and boiling point 212 F. The Celsius scale is uniformly used in science.

In order to measure temperatures above the boiling point of mercury, inventors turned to another physical idea. They fastened two dissimilar metals together and created a bimetallic thermometer. The way this works is that one metal expands more per degree than the other and bends the fastened strip. This is a purely mechanical method--No batteries needed. This is the way that thermostats worked up until recently when they were replaced by thermocouples, thermistors or RTD's.

Thermocouples are the most widely used temperature measurement idea. This is based on using two alloys that are fastened at points and generate a voltage when one end of the thermocouple is subjected to a higher temperature. The most common type is composed of chromel (an alloy of mostly nickel and chromium) and alumel (an alloy of nickel, magnesium and aluminum with silicon). This type of thermocouple can detect temperatures from -200 C to 1350 C. Thermocouples are used in furnaces and other applications in which high temperatures may be encountered. Of course, one needs a voltage meter that calibrated as to temperatures.

A thermistor is what its name suggests: it's a special type of resistor that sensitive to temperature. In other words, it changes its resistance with temperature. RTD's or resistance temperature detectors work much the same way but thermistors are ceramics or polymers and RTD's are metals. RTD's are more useful because they have a much larger useful temperature range than thermistors.

There are other more exotic methods of measuring temperature. An infrared sensor device is able to detect the temperature of an object or surface by focusing the infrared radiation emitted by the object or surface onto an infra red sensor. These devices typically use a red laser pointer to help the user know where it's pointed. This type of device is commonly called a pyrometer and is typically hand held so that it can be used remotely. Pyrometers find use in a number of industries where it would be inconvenient or dangerous to use any other method.

Thanks for reading.