ITS 90 facts for kids

The ITS 90 is a special way to measure temperature all around the world. It was created in 1990. Think of it as a rulebook for how to measure temperature very accurately. It helps scientists and engineers get the same temperature readings everywhere. This scale tries to be as perfect as possible, matching the true scientific temperature scale.

A Brief History of Temperature Scales

Measuring temperature accurately has always been important. The first worldwide temperature scale was made in 1927. People kept improving it over the years. A better version came out in 1948.

Then, in 1968, a new one called IPTS 68 was introduced. The 'P' in IPTS stood for 'practical'. In 1975, they added ways to measure very, very cold temperatures. Finally, since 1990, we have been using the ITS 90.

How the ITS 90 Works

The ITS 90 uses a list of special, unchanging temperatures. These are called "fixed points." Imagine them as exact markers on a temperature line. Scientists from different countries worked together to find these points.

The B.I.P.M. (which stands for Bureau International des Poids et Mesures) in Paris approved this list. This group makes sure measurements are the same everywhere.

Measuring Between Fixed Points

To measure temperatures between these fixed points, scientists use special tools. One common tool is a platinum resistance thermometer. This thermometer can measure temperatures up to 660 °C.

Before using it, the thermometer needs to be "calibrated." This means checking and adjusting it to make sure it's correct. Calibration is done at several of the fixed temperatures. This helps the thermometer give accurate readings across a wide range. The ITS 90 also provides clear steps and math formulas for how to do this.

Calibrating a Thermometer: An Example

Let's say you need a thermometer that works well from room temperature up to 400 °C. The ITS 90 gives you specific fixed points to use for calibration.

For example, at 0.01 °C, there's the "triple point of water." This is a very exact temperature where water can be solid, liquid, and gas all at once. For 231.928 °C, you use the freezing point of tin. For 419.527 °C, you use the freezing point of zinc.

Primary and Secondary Calibration

First, you measure the electrical resistance of your thermometer at these exact fixed temperatures. Then, you put this information into a special formula. This formula helps you figure out how your thermometer might be slightly off. It gives you "deviation coefficients" for the range from 0 to 420 °C.

Once you do this, your thermometer is accurately calibrated. This is called "primary calibration." Now you have a very precise thermometer that follows the ITS 90 rules.

You can then use your newly calibrated thermometer to check other thermometers. This is called "secondary calibration." It's a little less accurate than primary calibration. However, it's useful because you can do it at any temperature within your thermometer's range. This is because your calibrated thermometer can accurately measure temperatures between the fixed points.

For more information, you can visit the official site of BIPM.: http://www.bipm.org/en/publications/its-90.html