# Thermometers and temperature scales

Article about Thermometers and temperature scales

1. Thermometers

The tool designed to measure temperature is a thermometer. There are many types of thermometers, but the working principle is the same. Usually, we use thermometric objects, the nature of matter that changes with temperature. If the temperature of the object changes, the shape, and size of the object also changes. Most thermometers use objects that can expand or shrink when the temperature changes. Thermometers that are often used consist of glass tubes, where there is alcohol or mercury in the center of the tube. When the temperature increases, alcohol or mercury in the container expands so that the length of the alcohol or mercury column increases. Conversely, when the temperature decreases, the length of the alcohol or mercury column decreases. On the outside of the glass tube, there are numbers which are the scale of the thermometer. The number shown by the upper end of the alcohol or mercury column states the value of the temperature of the object being measured.

Other types of thermometers commonly used are thermometers that use bimetallic sheets (two metals of different types and different expansion speeds). When the temperature increases, one metal experiences a greater expansion than the other metal. As a result, the pieces are curved. Usually, a bimetallic piece is a spiral, where one end of the chip remains, while the other end is connected to the scale indicator. When the temperature changes, the pointer will rotate. Thermometers that use bimetal sheets are typically used as air thermometers, room thermometers, oven thermometers etc. A thermometer, that is more accurate or precise, often uses the electrical properties of an object.

2. Thermometer Calibration

Thermometer calibration is the process of scaling a thermometer. Here are some steps to calibrate the thermometer. First, prepare a mercury thermometer or alcohol thermometer without a scale. Second, prepare ice. Third, prepare water. Fourth, prepare a water heater that can be used to heat water to boiling. Fifth, put ice and water into a container (water and ice have the same mass). Thereafter, put the thermometer in a container containing water and ice.

At first, the thermometer in contact with air so that the thermometer was hotter than ice. After being put in a container, the column length of mercury will be reduced because the mixture of water and ice is more cooling. Let until the column length of mercury does not change (the surface of mercury does not move). When the column length of mercury does not change, a mixture of ice cubes and water is in thermal equilibrium. Mark the position of the mercury column (mark the top end of the mercury column). This is the temperature of the ice point or the freezing point of water. Sixth, boil water using an electric heater. Insert the thermometer into a container containing heated water. When the water gets hotter or the water temperature increases, the column length of mercury also increases (the surface of mercury moves up). After water is boiling, the surface of the mercury stops moving. Mark the end of the mercury column. This is the temperature of the boiling point of the water or vapor point.

If you want to make a Celsius scale, the distance between the two marks is divided into 100 lines, where the distance between each line must be the same. The bottom sign = 0 oC, while the upper sign = 100 oC. See the figure on the side. Ice point temperature and vapor point temperature depend on air pressure. Therefore, a thermometer that is calibrated in a place with different air pressure will give different results. Mercury thermometers commonly used are calibrated at one atmospheric pressure so that the freezing point temperature of the water is 0 oC and the boiling point temperature is 100 oC. Regular thermometers such as mercury thermometers or alcohol thermometers are usually limited. The thermometer cannot be used to measure very low temperatures (lower than 0 oC) or very high temperatures (higher than 100 oC).

3. Temperature scales

For a thermometer to be used to measure temperature, it is necessary to set a temperature scale. Two temperature scales are often used, including the Celsius scale and the Fahrenheit scale. Another name for the Celsius scale is the centigrade scale. Centigrade = one hundred steps. The temperature scale that is critical in the field of science is the absolute scale or the Kelvin scale. Regarding the Kelvin scale will be discussed later. The fixed point of the Celsius scale and the Fahrenheit scale use the freezing point and boiling point of water. The freezing point of a substance is the temperature at which solid and liquid forms are in thermal equilibrium. Conversely, the boiling point of a substance is the temperature at which the liquid form and gas form are in thermal equilibrium. The freezing point and the boiling point constantly change with the air pressure, so the air pressure needs to be determined first. Usually, we use standard pressure, which is 1 atm (one atmosphere). The atmosphere is one unit of air pressure.

3.1 Celsius Scale

The fixed point over the Celsius scale uses the boiling point of pure water, while the fixed point below the Celsius scale uses the freezing point of pure water. The freezing temperature of pure water (also known as an ice point) is zero degrees Celsius (0 oC) and the temperature of the boiling point of pure water (also known as a vapor point) is one hundred degrees Celsius (100 oC). Between the ice and vapor points, there are 100 degrees. In thermometers that use the Celsius scale, a temperature scale lower than the ice point temperature is typically marked with a negative number.