Temperature measurement is a recurring action in science and technology. From laboratory research to industrial processes, the use of thermometric scales allows us to interpret and control thermal phenomena.
But what exactly are thermometric scales and what is their role in temperature measurement? This article explores the fundamentals, classifications and applications of these scales, with an educational and accessible approach.
What is a thermometer scale?
A thermometric scale is a reference system that relates a measurable physical property to temperature values.
These scales are based on physical phenomena that vary in a predictable way with changes in temperature, such as the expansion of liquids, the variation in electrical resistance or the pressure of a gas at constant volume.
It is important to distinguish between thermometer scales and temperature scales. Temperature scales, such as Celsius, Fahrenheit, and Kelvin, are standardized systems used to quantitatively express temperature.
Thermometric scales, on the other hand, are associated with the methods and instruments used to measure temperature.
Fundamentals of Thermometric Scales
Thermometric scales are fundamental tools for measuring temperature, based on the relationship between the physical properties of materials and temperature changes.
To understand how they work, it is necessary to know the principles that govern them, such as thermometric properties, calibration and reproducibility.
Thermometric properties
A thermometric property is a physical quantity that varies in a predictable manner with temperature. This allows it to be used as a basis for measuring this parameter.
Common examples of thermometric properties include the expansion of liquids, such as mercury or alcohol, which increases with heat, and the variation in pressure of a gas at constant volume.
Other examples include changes in the electrical resistance of conducting materials and thermal radiation emitted by hot bodies, used in devices such as pyrometers.
Calibration
Calibration of a thermometric scale is achieved by defining specific reference points, such as the freezing and boiling points of water, or the triple point of water, which corresponds to 273.16 K on the Kelvin scale .
These points allow precise and uniform values to be assigned to the measured temperatures.
Linearity and reproducibility
Furthermore, ideal thermometric scales are characterized by linearity and reproducibility . This means that the relationship between the measured property and the temperature is proportional and consistent, ensuring that the results are reliable and comparable, regardless of the instrument used.
Classification of thermometric scales
Thermometric scales are classified according to the physical property used to measure temperature.
The most common ones and their main characteristics are described below:
1. Based on dilation
Mercury and alcohol thermometers are the most representative examples of this category. These devices measure temperature by taking advantage of the expansion of a liquid contained in a capillary tube.
They are widely used in everyday applications due to their simplicity and reliability.
Mercury thermometers are inexpensive, easy to use and require little maintenance. However, their measuring range is restricted by the freezing and boiling points of the liquid used, making them unsuitable for extreme temperatures.
2. Based on the pressure of a gas
Constant volume gas thermometers work by measuring the pressure of a gas that varies linearly with temperature, as long as the volume is kept constant.
In general, they offer high precision and a very wide measuring range.
They are common in scientific research and in the calibration of other measuring instruments.
3. Based on electrical properties
In this category we find resistance thermometers (RTDs) and thermocouples. RTDs use the variation in electrical resistance in materials such as platinum, whose resistance increases predictably with temperature. Thermocouples, on the other hand, generate a voltage proportional to the temperature difference between two joined metals.
The advantages they have are that they are robust, have a wide temperature range and allow remote measurements.
They are used in industrial control, process monitoring and scientific experiments.
4. Based on thermal radiation
Pyrometers and infrared thermometers measure the thermal radiation emitted by hot bodies to determine their temperature.
These types of thermo-thermal scales do not require physical contact with the object being measured and are ideal for extremely high temperatures or moving objects.
They are used, among others, in the metallurgical industry, in quality control and in security systems.
5. Based on phase transitions
These scales are based on the temperatures of phase transitions, such as melting or solidification, which are very precise reference points. A prominent example is the triple point of water (273.16 K), used to calibrate the Kelvin scale.
These thermoelectric scales provide very high precision.
Relationship between thermometric scales and temperature scales
Thermometric scales allow temperature to be measured accurately, but the results must be expressed on a standardized temperature scale.
For example: A mercury thermometer calibrated in Celsius will give readings like 0 °C (freezing point of water) or 100 °C (boiling point of water). On the other hand, a resistance thermometer might be calibrated in Kelvin, with readings like 273.15 K for the freezing point of water.